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  Introductory Information on
Astron Power Supply
Components and
Troubleshooting Tips

Compiled from a number of different sources
by Mike Morris WA6ILQ
Maintained by Robert Meister WA1MIK
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If your Astron is going into current limit at random times, this comment from an email sent by Ron Rogers WW8RR is relevant:

A VERY common cause of random current limit shutdown in linear Astron supplies is due to the manufacturing process: a bad solder joint on the collector tab of one of the pass transistors. During manufacture they solder the buss wire from collector to collector. If you take a pair of pliers and grab the buss wire next to the solder joint at each transistor and pull on it, you will most likely find one that will simply pull off.

All it takes is for one of those transistors all wired in parallel to have a bad collector‑to‑buss solder joint and all of the source current from the supply tries to flow through the Emitter-Base junction of that one transistor that has the bad solder joint. Bingo !!! Immediate current limit mode!!

From yet another email to repeater-builder:

Another common failure (these supplies are full of them) is due to the use of parallel‑wired diode bridge rectifiers. A good example is on the RS‑35 supply schematics. They use two packaged bridge rectifiers that are wired in parallel with pieces of #10 wire tying them together with no attempt at current equalization. To use a Martha Stewart term, this is A Bad Thing, engineering-wise; at least the designer used current balancing / equalizing resistors on the pass transistors. If you look at the schematic you will see that only the positive halves of the bridge rectifier(s) are used; the negative ends are left floating. Unless the two bridges are absolutely identical, or at lease very closely balanced, one will hog most / all of the current and the other will just sit there and watch. Eventually one diode will short or open, and that often takes the other one along with it. This causes the supply's primary fuse to blow. If your supply instantly blows the primary fuse (and it's not the MOV, referenced later in this article), then you need to unsolder the two heavy transformer wires (frequently colored yellow) from the bridge rectifier terminals and then see if a new fuse blows. If not, check for a shorted main filter capacitor and shorted pass transistors. If all checks out OK, chances are high that one or more diodes have shorted. They have to be completely unwired to be properly checked. I bought several 1,000V 50 Amp diode bridges for about US$5 each as replacements and just use one of those in place of the two parts in parallel in RS‑35s. The ones I have here are labeled KEST KBPC 5010 or KEST KBPC‑5010 and are still being sold on eBay as of this writing (October 2007). Click here for the KBPC-5010 data sheet.

Here's a photo of the inside of a dual-rectifier Astron RS‑35.

Here's a photo of the 50 amp replacement. It's the same physical size as one of the existing rectifiers.
Both photos above are courtesy of Robert Meister WA1MIK and were taken to illustrate the above situation.

Astron used different transistors at different times in their history. Don't be surprised if the ones in your supply are not listed in the table below (and if they are not, please let us know what you find). Just Google the part number and you will find the info somewhere on the web.

From an email to repeater-builder from an amateur that works in the power supply design field...

Next to the regulator chip, the pass transistors are the heart of the supply. Astron started out with 2N3055 devices, which are good for 10 amps per paralleled device and used 2N3771s in the higher current units. When the price on the 2N3771 devices dropped enough they switched to that device completely.

For those that want details, here you are:

(see note)
at IC
Power Dissipation
in watts
DigiKey Price
(mid 2006)
2N3055 10 60 20-70 @ 4a 75 $2
30 40 15-60 @ 15a 150 $2.50
2N3772 20 60 15 @ 10a 150 $2.10
2N3773 16 140 50-60 @ 8a 150 $2.25
2N5301 30 40 15-60 @ 15a 200 $4
2N5302 30 60 15-60 @ 15a 200 $4
2N5686 50 80 15-60 @ 25a 300 $8
Note that The maximum collector current specified above is only valid as long as the internal transistor chip temperature is less than the rated maximum temperature for the device. Also note that the higher-numbered 2N3773 has a maximum current of about 1/2 that of the lower-numbered 2N3771.

Except for the diameter of the pins on the 2N5686, all the devices listed in the table above are drop‑in replacements with better performance for a very reasonable price.

One of the two pass transistors on the heat sink of an RS‑20.
The "9549" is a date code and indicates that the
transistor was made in the 49th week of 1995.

Back to the email:

The "book" rated dissipation of a power transistor is noted in the book as having been measured at at 25°C (77° F). As the temperature goes up, the power dissipation ability goes down. During long heavy load situations (like a long repeater transmitter key-down session) the internal chip temperature of the pass transistors will be a lot hotter than the case, which will be hotter than the heat sink. The characteristic that causes a time lag between internal chip temperature rising and the heat sink temperature rising is called thermal resistance. Think of the situation as including a "heat pipe" between the source (the transistors) and the destination (the heat sink) - like water, the larger the diameter of the pipe, the greater the quantity flowing, and the lower the thermal resistance, the faster the transistor can dump the heat into the heat sink. You can't do anything about the thermal resistance between the internal chip and the case of the power transistor (i.e. the internal construction of the power transistor), but you can lower the thermal resistance between the transistor case and the heat sink with an application of the proper type and amount of heat sink compound. And you can add a fan blowing air across the heat sink to help it shed the heat. Note that due to the density of the air a fan on a heat sink at a mountaintop site at 5,000 feet won't cool as well as the same fan on the same heat sink at sea level, fortunately mountaintop sites are generally cooler than at sea level.
More notes on fans below, including a link to very interesting article on the effectiveness of a fan by John Huggins KX4O.
The 2N3055s, 2N3771s and 2N3772s have been found in known-stock Astrons (as was said above the 2N3055 is found only in the earliest and smallest units and Astron switched completely to the 2N3771 at some point). The 2N3773s, 2N5301s, 2N5302s and ECG181s have been found in used ones bought at swap meets. The 2N3773s may have been stock (if they were replacements they were very nicely done, and I couldn't tell), the 2N5301s and 2N5302s were obvious field replacements.

My personal rule on rectifier diodes and pass transistors is at-least-double-the-rated-current of the supply. I use the 2N5686 exclusively as a replacement in the larger supplies as not one supply I have rebuilt with them (over 20) has EVER come back to haunt me (at least for a pass transistor problem). Yes, in most cases they are total overkill and they do cost more, but the price difference on four new devices for an RS‑35 is around $25, even if the originals are 2N3055s or 2N3773s. If you find 2N3055s in your used supply and don't want to go to 2N5686s at least replace them with 2N3773s, and preferrably 2N3771s. The 3773s provides 60% more current capacity than a 2N3055 and twice the power dissipation for an additional 25 cents each. If you use 2N3771s (which is what Astron themselves uses) that's triple the current for an additional 50 cents each.
And think about it: If you go cheap what is a future power supply failure, the down time, a round trip to the repeater site (don't forget to consider the price of vehicle wear and tear plus the gasoline) and ANOTHER power supply (or repeater) rebuild going to cost? What is your time worth?

Note from Mike WA6ILQ:
I read an article in the local newspaper in 2008 that was discussing the cost of the daily commute to work. The author was driving just under 10 miles per day as his round trip to work and back. He totaled up gasoline, maintenance, auto insurance, and figured a 10-year depreciation on the difference between the price of the new vehicle and what he could get for it 10 years later. His total came out to right around 45 cents per mile. (FYI, in early 2008 the IRS allowed 50.5 cents per mile for business use of a personally owned vehicle. It's probably higher now.)

Back to the email:

Don't mix the transistor types !!! The emitter ballast resistors do their balancing act only when the transistors are identical. Always replace a dead pass transistor with an identical part number, or if you can't find an exact match, then replace all of them as a group. And don't go down in ratings - if you find 2N3771s do not replace them with 2N3773s or 2N3055s. My own short rule of thumb: Use only the 2N3771s or 2N5686s - and use all of one or the other.

If you have to replace all of the pass transistors in a supply, and if the old ones are 2N3055s or 2N3773s do yourself a favor and buy something larger ‑ at least 2N3771s (also known as the ECG181) and preferably 2N5686s. Note that you can probably use fewer 2N5686 devices than the 2N3771s that came with the supply.

Note from Mike WA6ILQ:
Note that on some models it may not be worth the upgrade parts or the effort to install them . The RS‑7 uses a single 2N3771, a 30 amp device. The RS‑12 uses (used?) a pair of 2N3771s (the dual transisitors are obviously a holdover from the days of 2N3055 pass transistors as each one was only good for 10 amps)... so the stock RS‑12 has 60 amps worth of pass transistors for a supply they rated at 12 amps (but is only good for 5-6 amps continuous)... On the RS‑7 and RS‑12 (and similar low current supplies), at least, save your modification time and money.

Back to the email:

If you find one bad pass transistor make sure you measure the resistance of all of the emitter ballast resistors (the current balancing resistors) and compare each against the others. If even one is different, replace ALL of them with new 5% (or better) units - that way you have identical values (after all, they are supposed to evenly distribute or balance the current, and can't if they are not absolutely identical in resistance). Using all new from the same batch is the simplest practical way to get identical values.

Don't forget to use some good beryllium based thermal compound (the thick white stuff that is the consistency of axle grease), but don't go overboard - you want just enough to put a thin layer between the transistor and the insulator, and again between the insulator and the heat sink. All you are doing is eliminating any air pockets. Note that beryllium compounds are known to be human carcinogens when inhaled, fortunately the greasy consistency of the heat sink compound prevents any airborne dust, but you still want to keep it off your skin (i.e. use a surgical glove or gloves for protection, and maybe a popsicle stick to spread it thin). You can usually beg a pair or two of rubber or nitrile gloves from paramedics, ambulance attendants, nurses, doctors or veterinarians. Non-sterile gloves are available at many auto parts suppliers and tool stores (like Harbor Freight, but they are usually of thin, low grade rubber or nitrle). See this web page for more details on beryllium. See this page from the CDC. See this Wikipeida page on the results of beryllium exposure.
I'm not trying to scare you - it's safe as long as you read the directions, use your head, and be careful.

If you can't find the beryllium based thick white stuff then use the "Arctic Silver" that is made for use between the CPU chips and the heat sinks in high-end PCs.

As long as I am inside the Astron I also do the following:
  1. Some Astron models use stud mount rectifier diodes, others use one or two rectifier blocks in parallel. The ones that use dual rectifier blocks get them replaced with a single rectifier block rated at least 2 times the maximum current of the supply, if not 3 or 4 times. Yes, I'll use a 50 amp bridge in a 12 amp supply. It's cheap and I won't have to worry about it ever again. Some of the larger supplies that use a pair of rectifier blocks get converted to 75 amp or 100 amp stud‑mount diodes. Astron uses two 40 amp diodes in parallel on each side of the transformer in an RS‑70, personally I feel that's not enough headroom (My rule on rectifiers is at-least-double-the-rated-current), and besides, they aren't using any current balancing components on the diode pairs. In the last RS-70 I rebuilt I used two 200 amp diodes (only because the surplus store was out of the 150 amp ones), and because I used one on each side of the transformer there was no balancing required.

  2. I add a 0.1 µf capacitor rated at 100 volts (or better) from the positive terminal of the large bridge rectifier to ground.

  3. I add three 0.01µf 600 volt (or better) bypass caps - one across the AC line and one more from each side of the AC line to ground, and I use the gap-cap version of the bypass cap if I can have them in stock (see If you are going to google it then note that Vishay calls it a "gap-kap". Search for the Vishay 0.01µf S103M69Z5U283L0R. I've found a single cheap ceramic bypass cap across the mains power line in some of the Astrons I've worked on, most don't have it. And since the Astron is on my bench because it has problems, I assume the existing bypass cap (if any) is damaged and it gets trashed.

  4. I add three 150 volt Metal-Oxide Varistors (MOVs) - one in parallel with each of the gap-caps mentioned above (areas with 220 / 240 volt AC mains will need higher voltage devices). Some Astrons already have one MOV (across the power line) already in place. Due to the normal failure mode of a varistor (see the "Regarding MOVs" paragraph below), and due to the reason that the Astron is on my bench is because it has problems, I assume the existing varistor(s) (if there are any) is (are) toasted and trash it (them). See the MOV note below for the preferred part number.

  5. I add the missing compensation capacitor (470pf or 500pf) - see the 723 data sheet and the text above.

  6. I test the connection from the chassis to the ground prong of the power plug. Use your ohm meter on the x1 scale to test between the case to the ground of the AC mains power plug (the round prong in the USA). It should show zero ohms. I've worked on several Astrons that arrived with a non-functional safety ground wire in the power cord. I scrape away the paint and add a star washer under the power cord green wire lug. On one I found the ground lug crimped onto an insulated wire (i.e. they hadn't stripped the wire before they crimped the lug onto it). In each case the housing / chassis was floating above mains power safety ground. Remember the power plug ground wire is a SAFETY ground, and your survivors will appreciate it if you don't scrimp on the safety. Make it a good solid ground. And remember that the "green" wire in the power cord may be green with a yellow stripe or stripes.

    Update: Some Astrons use a multipoint terminal strip with the green wire soldered to the grounded lug. See the photos in the article on the index page that is titled "Reducing Inrush (Surge) Current on Astron Power Supplies". The final test is the same, make sure the resistance between the case and to the round (ground) pin of the AC mains power plug is zero ohms.

  7. Some Astrons have the negative side of the DC output grounded to the case, some float it, some have a resistor. You want the negative side floating. Again, use your ohm meter to test between the negative terminal on the case to the ground (round prong) of the AC power plug.
    Note from Mike WA6ILQ: See the paragraph further down the page that starts with "Negative side grounded to the cabinet", especially the note on this topic from Eric Lemmon WB6FLY.

  8. I add more split or star washers under the pass transistor mounting screws. I've seen thermal cycling loosen them. As long as you are inside the housing I would add split washers under the filter capacitor screw heads as well. In several cases I've found multiple filter capacitor connection screws only finger tight.
    Note from Mike WA6ILQ: The voltage regulator board is mounted to the filter capacitor in the higher amperage supplies by the screws that are the terminals of one of the filter capacitors (see the RS-35 internals photo above). It wouldn't hurt to put an additional star washer between the board and the top of the capacitor (i.e. two washers, one for each capacitor screw). That's a simple pressure connection and if it gets loose it causes problems. There's an email below that describes a random crowbar situation caused by a missing star washer at that location.

  9. Most of the time I relocate the voltage adjustment pot to the front panel, mounted under the voltmeter (if it is a metered supply). The voltage adjustment is a carbon linear pot. I simply mount a screwdriver-adjust pot (of the proper resistance and of the type that has a locking nut on the adjustment), and run wires to the pads where the regulator board voltage pot used to be. I use a three conductor shielded wire for this (that's three wires plus the shield), and ground the board end of the shield. By the way, the vertical tab on the left side of the pot in the photo is an anti-rotation feature - it prevents the pot from rotating in the front panel. When you mount that style of pot you need to drill two holes - a large one for the center shaft and a second one just large enough to clear the anti-rotation tab. And sometimes the anti-rotation tab is taller than the front panel is thick, you may have to shorten it a little (a pair of diagonal cutters and a flat file to take the burrs off works just fine for this).

  10. On the VS series, and on the supplies where I relocate the voltage adjust pot to the front panel I add three ferrite beads, one on each lead of the front panel potentiometer(s) right at the PC board (but not on the shield). If necessary a drop of hot melt glue holds the bead in place. Some of my Astrons are at broadcast sites, and I don't need high RF levels confusing the voltage regulators, the overcurrent foldback circuit, or the crowbar circuit.

    Note from Mike WA6ILQ: The internal voltage adjust pot in the Astron can adjust the voltage up to the point where the crowbar trips. A better way of wiring a front panel pot into the Astron regulator board is described in the Robert W. Meister WA1MIK article titled "Adding Voltage and Current Adjust Pots to an Astron 13.8V Linear Supply".
    Astron supplies have a non-adjustable current limit feature that can be made adjustable. The WA1MIK article describes how to do that as well. I highly recommend adding both to your workbench and radio site Astrons. Use the above mentioned shielded wire and ferrite beads.

    Note that Astron BB series supplies are adjusted differently, as the load they see is a combination of the repeater and the battery... and if the mains power has just been restored after a long outage an Astron with a stock Current Limit circuit that is charging a large battery is going to let the battery suck some serious charging current and the Astron will burn itself up. If you have a large battery bank then you will want to add the Current Limit potentiometer and limit the total current to something that the supply can deliver in continuous duty mode for many, many hours - perhaps as low as 15 amps from an RS-35. In that situation a fan on the heat sink is mandatory.

    And one quirk with an Astron connected to a battery without a series Shottky diode (to prevent backfeeding the supply): always turn the PS "on" before attaching it to the battery.

  11. I add an IEC style power cord fitting (available for free from a dead PC power supply). Yes, it requires a bit of sheet metal work, but it's worth it to prevent tripping over a dangling power cord again. I still have scars from the rough concrete on the steps of the repeater building. The IEC connector may not be appropriate for the high current units (i.e. an RS‑50 or RS‑75) unless you can find a high current IEC and a really heavy duty wire IEC power cord... something like #12 or #10 sized copper inside the jacket... the last thing you want is for someone to slip up and use a leftover wimpy #20 or #18 cord originally made for someone's PC monitor. And label the power supply end of the #12 or #10 cord as where it goes - you don't want some newbie who is helping out on his first repeater site visit and has his head in the back of the rack accidentally swapping the #12 sized cord for the main repeater RS-70 with the #18 cord that feeds the link radio RS-12.

    If you do chose to add an IEC power connector you need to make sure that the current rating of the IEC connector is at least twice the measured AC current draw of the supply at max DC load. Then you get to make sure that it is wired properly - see this drawing. Some IEC connectors have dots on the pins: brown is the international standard for hot, blue for neutral, and green or yellow for ground. Some manufacturers use molded-in lettering, "L" for line, "N" for neutral, "G" or "E" for ground or earth. Some even use both the molded letters and the color dots.
    Update from Mike WA6ILQ: Astron is now shipping some models with an IEC power connector already installed.

  12. A lot of Astrons have the incoming power hot lead feed the fuse holder, then the front panel power switch then the load (the transformer primary). Some have the hot lead feed the switch, then the fuse holder then the load. The second wiring method is preferred. Make sure the fuse holder is wired properly - the center pin is supposed to be fed by the hot lead and the outer sleeve of the fuse holder (nearest the case) goes to the load (the transformer primary). This is another life safety issue where your survivors will thank you. The idea is that when you remove the fuse from the holder the current source (the center pin) connection is broken first.

    In other words, the sequence of flow for the power should be from the hot wire in the power cord (or the hot pin of the IEC power connector) to one side of the AC power switch, then from the other side of the switch to the tip of the fuse holder, and from the barrel of the fuse holder to the power transformer primary. The other side of the power transformer should go to the neutral wire in the power cord (or to the neutral pin of the IEC connector).

  13. Most of the Astrons use a rocker-style power switch with an internal neon lamp to indicate when the power is on. After a while the neon lamp starts to flicker and finally dies from old age. I add a diffused (for wide viewing angle) bright green LED to the front panel, with an appropriate series resistor, and wired across the +12vDC output. Or you can do as one friend did and carefully take apart the switch and replace the dead neon lamp with a bright red diffused LED, then reassemble it. Add the appropriate external series resistor for 120vAC and you are done.

Regarding MOVs, remember that they have a finite life and do wear out. This is because when the voltage across a MOV reaches the breakover point, the MOV avalanches and conducts therby shorting the excess impulse energy into heat within the body of the MOV itself. If it draws enough current the MOV just blows the input fuse in the device. If a MOV absorbs a spike too big to digest they just short out. In that situation the shorted MOV just blows the new fuse as soon as you replace it.

As I said above, if the voltage spike is not very energetic the MOV just dissipates it as heat. The problem is, if the MOV gets hot enough, the internal heat affects the MOVs internal characteristics - and the effect is that the avalanche (breakover) voltage (the threshold voltage) increases with each hit it takes. The next impulse comes along and less of it gets shunted. If a MOV sees enough action, the threshold voltage goes up to where it is effectively an open circuit. This slow change of the MOV threshold voltage may take years. As you would expect, when the MOV is effectively open the equipment protection is compromised to zero and you won't know it, but physically the MOV still looks perfectly good.

This failure mode is why the common PC "surge protection" power strips are a joke and a delusion - usually all that is inside is a single unlabeled, bottom-of-the-quality-barrel MOV (it may not even be the right voltage MOV, in multiple cases I've found a 260v MOV (i.e. one made for a 240 volt mains voltage) in a low-end chinese made "surge protected" power strip sold for 120vAC service in the USA. The Tripp-Lite "Isobar" units are much, much better - and I use one as the rack cabinet power entry protection at every repeater site. MOVs are cheap, however, and if they are the correct voltage are better than no protection at all. Just make them your secondary layer of protection, and plan on replacing all three of the MOVs every so often, especially if the power line feeding the power supply has taken a lightning strike (at which point you replace all three, plus the gap-caps and then go looking for further damage).

Another common MOV failure mode is where the MOV just plain shorts out and vaporizes the fuse and frequently itself. I've seen MOVs reduced to two bare leads waving in the breeze and bits of red plastic scattered around the inside of the power supply case. But the supply was repairable, and went back into service, and 8 years later (as I write this paragraph in mid-2009) is still in service.

When you purchase new MOVs you need to make sure that you pick the parameters properly. A MOV rated at 130 VAC is suitable for a 120 VAC circuit only if its tolerance is tight, say plus or minus 2%. You won't find that tolerance or quality at Radio Shack (their part 276-568, and the last time I checked they were over for $2 each). If your line voltage is a little high then it may be a good idea to install new MOVs rated at 140 VAC with a 10% tolerance. Mouser (and others) stock them. I use the Littlefuse V130LA20A, which is available from Mouser for under 50 cents each (late 2009 price). In the power supplies that have them Astron uses a V150LA10AP, which costs $0.34 in the summer of 2012. A 650kB PDF data sheet for these parts can be downloaded here.

You would think that installation of the MOVs and gap-caps could be as simple as soldering them right to the pins on the back of the IEC socket that you installed in the back of the case, and yes, you can do that, it's just that the plastic structure of many cheap IEC connectors melt from soldering iron heat. Rather than unsolder and solder replacements on a supply that is headed for an unmanned hilltop, it might be a better idea to connect the MOVs and gap-kaps via a multi position screw terminal block bolted to the inside of the cabinet just above the IEC socket. This allows you to replace them without warming up a soldering iron. By the way, the dead-short failure mode of the MOVs generates a LOT of heat, so mount that terminal block where a long burst of heat that is much hotter than a soldering iron isn't going to destroy an expensive part - like the 100,000µF filter cap in an RS-50.
As an aside, this failure mode inside a plastic cased low end "surge arrest" or "surge protect" power strip is an invitation to a major fire - especially if that power strip is sitting on a flammable surface, like on a carpeted or wooden floor in a residence. Don't forget that most plastics are petroleum based and are a very flammable fuel when melted (like from heat). This is yet another reason why I like the metal cased Tripp-Lite "Isobar" power strips.

Note from Mike WA6ILQ:
One of the RS-20 Astrons that I own was purchased at a ham swap meet simply becasue it was tagged with a simple note in big block letters on a 3x5 inch file card taped to the top of the case... "NFG, blows fuses". I paid $5 for it, took it home, popped the cover off, poked around with a VOM for 5 minutes, cut out the the old MOV with some side cutters and put in a new fuse. The supply worked just fine. I did most of the critical mods listed on this page (add 1 new compensation cap, one new .01µf cap from the positive side of the rectifier to the negative side, three new V130LA20A MOVs, three new Vishay gap caps, floated the negative output of the supply, verified the chassis ground (the green / 3rd wire in the power cord), added split washers under the pass transistor mounting screws and under the filter capacitor screw heads, added a wide diffused and obnoxiously bright green LED to the 12 volt output, etc.) and the unit is now the primary supply for a set of base station radios at a Red Cross chapter building. The supply runs a set of a UHF Maxtrac, a VHF Maxtrac, and a low band Maxtrac for the 47.42 MHz nationwide channel.

If the Astron I'm rebuilding is going to be powering a continuous duty load (i.e. at a repeater site) I add a voltmeter, an ammeter, and if it's a high duty cycle load (i.e. a busy repeater) a 24vDC fan (or two 12vDC fans in series) blowing air across the heat sink... If it's a busy repeater with large supply (with a wide heat sink) I'll use definitely use two. A 24vDC fan operated on 12vDC moves enough air to keep the supply heat sink cool and will last a lot longer than a 12v fan running at normal speed. To save the fans from running 24x7 you could use a simple SPST snap-action thermostatic switch mounted on the heat sink. Just drill two small holes in a flat area (even on one of the heatsink fins), and mount the switch with a dab of thermal grease to ensure a good heat transfer. Another option to switch the power to the fan(s) is a PTT-triggered timer so that the fans run only when the repeater is actually in use. Many repeater controllers have digital outputs that can be used to control other things, including a fan or a set of fans. If your controller does, and the programming supports it, you could have it ignore a kerchunk (i.e. a transmission of less than 30 seconds), yet turn othe fan(s) on when the repeater becomes active (perhaps more than 60 seconds). If your repeater controller has analog inputs and allows you to measure temperatures and allows you to take action based on them then you can measure the power supply heat sink, the transmitter heat sink (or both), then control the fans based on that.

Note from Eric Lemmon WB6FLY:
One good choice of snap action switch is the Cantherm Corp. R2005015 normally-open thermostat that closes at 50 degrees C (about 122 degrees F). When attached to a heat-sink fin, it turns the fan on when necessary, and keeps it on until the heat sink cools below about 100 degrees F (right around body temperature). This particular switch is available from Digi-Key as Catalog Number 317-1094-ND, for about $9 each (2005 price).

More on fans below, including a link to very interesting article on the effectiveness of a fan by John Huggins KX4O.

Don't forget that the power transformer and rectifier block need cooling as well. Depending on the duty cycle you may want to add an additional fan blowing air through the Astron housing. Note that the bridge rectifier is going to drop about 0.7v to 0.8v at anything up to the full load current. Let's say that you have an RS‑50 loaded to 50% current. 25 amps times 0.7 volts is 17.5 watts, and that's only at half of the maximum current. 17 watts is a LOT of heat for a small-package bridge rectifier to dissipate, especially with nothing but convection cooling inside a sealed metal box. As easy as it is to do, add a little dab of white beryllium thermal grease under the rectifier block and let the bottom plate of the supply shed some of the heat, or remove the epoxy case unit and install a couple of heavy duty stud-mount diodes into the inside of the rear heat sink.
Since the power transformer itself is not 100 percent efficient it will generate some heat as well. If the outer case of the supply is so hot that you can't put place the palm of your hand on it full time then it's worth punching a 3 inch or 4 inch diameter hole in the top and bottom of the power supply case, put copper screening over the holes and use a 24v fan to force some air flow over the internal components (punching one of the holes in one side of the lid and the other in the other side of the bottom results in cross-wise air flow at zero cost). If cabinet clearances are tight (i.e. no room above or below, like in most rack mounted repeater systems), I'll punch both ends of the Astron cabinet, add two pieces of copper screening and mount the internal fan on the outside over the intake hole (in general, pushing air into a cabinet cools better than sucking air out). Don't forget a finger guard of some form.
(end of the email)

Some notes and comments on the above email from Mike WA6ILQ:
a) If you end up modifying the wiring in the AC side (to change the sequence, or to add gap-kaps, MOVs or an IEC connector) it wouldn't hurt to add a couple of snap-on ferrite chokes... just snap one on the hot wire and a second on the neutral wire, both just inside the back wall of the cabinet.

b) Copper screening is stocked by many model airplane / hobby stores and you will find that it is easy to solder a grounding pigtail to it. Copper screening is very soft, be gentle with it. If you can't find it locally then check the Georgia Copper web site. They sell "copper mesh" in 12 inch by 12 inch (30cm by 30cm) squares - but the minimum shipping cost is enough that you will want to pool an order from several people and / or several projects.

c) Remember that fans don't cool anything, they just move air. Any particular fan only cools if the air that it moves is sourced from a lower temperature. Make sure that your "cooling fan" has a source of cooler air.

d) A constantly operating fan can pull a lot of dust and dirt into the equipment, especially if it's pulling from floor level. Besides, the fan is ineffective until the device(s) get(s) warmer than the air around it, so starting the fan immediately upon key-up is a waste. In my opinion, a snap-action thermal switch mounted on the heat sink (even if it's just on a fin) is the simplest and most practical means of controlling a fan, and a repeater controller timer is the second simplest. If you are going to mount a fan to push some air through the Astron cabinet then you are probably also adding a fan to blow air across the heat sink. Just wire the fan motors in parallel.

e) If you are going to add a fan (or two) to a repeater site power supply (actually, any device) make sure you use a new name-brand ball bearing fan - cheap fans use brass or bronze bushings, cheaper fans use plastic bushings, good fans use metal ball bearings and are worth the extra money. Fans with cylindrical or tapered needle bearings are even better (i.e. mil-spec quality) but not too common and when they are found are usually expensive.

f) This goes hand-in-hand with the previous comment. Don't buy surplus fans for an unmanned repeater site. Bite the bullet and buy new quality ball bearing fans. I don't need to make yet another hill trip to replace a failed used fan. Think about what piece of hardware a fan failure is going to affect - how much equipment will you have to replace? PA decks are not cheap. Power supplies are not cheap, and when they fail they might take something else with them. Save the questionable used fans for something that is easy to get to and fix. I've seen surplus fans advertised as "new old stock" where the seller took a used fan, cleaned it up, popped the bearing cap, added fresh grease, and then replaced the cap. No, thank you. As was said in the above email, "If you go cheap what is a future power supply failure, the down time, a round trip to the repeater site (don't forget to consider the price of vehicle wear and tear plus the gasoline) and ANOTHER power supply rebuild going to cost? What is your time worth?"

g) John Huggins KX4O did some research and measured the effectiveness of a fan at various distances from a heat source, then wrote an interesting article.

h) In the time from when this article was initially written and the time of an update Robert Meister WA1MIK wrote two highly relevant modification articles that I recommend that you consider for your radio site and workbench Astrons:
1) Reducing Inrush (Surge) Current on Astron Power Supplies
2) Adding Voltage and Current Adjust Pots to an Astron 13.8V Linear Supply

Several items in the email above can be done at almost zero parts cost. Adding a dab of heat sink compound under the rectifier block(s) is almost free as well. Adding the compensation cap in item 5 is mandatory for repeater sites or customer sites, and optional for the workbench.

All Electronics is a good reputable mail order surplus source for a wide variety of goodies, including #12 IEC cords, voltmeters, ammeters and fans.
Disclaimer: I have no financial interest in All Electronics, I'm just an occasional retail customer, and I've never been dissatisifed with the products or service.

Troubleshooting and Repairs:

When you are repairing or modifying a power supply - of any brand - you really don't want to use a real radio, etc. as a test load. There are several failure modes that would put the full unregulated voltage (as much as 20-24vDC in an Astron) across the load. It's much better to have a made-for-the-job test load, and folks have used things like old car headlights that have one filament burned out, etc. I once saw a test load made up of four old headlights, with four pull-push headlight switches and a single ammeter. The owner had bought the bulbs from a junkyard for 50 cents each. The particular bulbs he had purchased had two good filaments, and each switch controlled the low beam and high beam, just like you would expect a headlight switch to do, but on only one lamp per switch.
Tony King W4ZT had a need for a test load and created a web page about it. I was sent a PDF of it, which can be found here. Later I found the original web page mentioned above.

Transformer Failures:
From another email to repeater-builder describing another failure mode, and the fix:

Most of the Astron designs have a single secondary winding with three taps for a total of 5 wires, with the center section being the heavy (high current) wire and the two outer sections use much thinner wire (it is just for powering the voltage regulator board). See the diagram below:

-----------------    -----------------   Thin wire (to the voltage regulator board)
                 )  (
                 )  (
                 )  (  Thin wire section of the secondary winding
                 )  (
Primary winding  )  (
                 )   -----------------   Thick wire (high current) to main rectifiers
                 )  (
                 )  (
                 )  (  Thick wire section of the secondary winding
                 )  (
                 )  (
On the 120/240   )   -----------------   Center tap (thick wire)
models the       )  (
primary is in    )  (
two sections     )  (  Thick wire section of the secondary winding
that are in      )  (
parallel for     )  (
120vAC and in    )   -----------------   Thick wire (high current) to main rectifiers
series for       )  (
240vAC           )  (
                 )  (  Thin wire section of the secondary winding
                 )  (
                 )  (
-----------------    -----------------   Thin wire (to the voltage regulator board)

I've seen two supplies with one of the outer sections opened up. There is no way to do a stock repair of it short of a new transformer (or having the old transformer rewound). The inexpensive method to salvage the supply is to abandon the thin-wire section and move the two thin wires from the regulator board to the secondary of an added small separate 24vAC transformer. There is plenty of room inside the cabinet for it. Do the next guy a favor and leave a note inside the cabinet as to why the extra transformer is there.

Drifting Output Voltage:
From another email:

...a recent problem I had with an Astron RS-20A:

The symptom: low output (7-10 volts) and varying
I replaced the 723 regulator chip and the TIP29 base drive transistor with no effect.
I replaced all of the diodes on the regulator board with no effect.
I checked the pass transistors. Both were good.
I finally changed the .01µF cap that is connected to pin 13 of the 723 chip and that fixed it.
Evidently the cap was breaking down or leaking internally and causing the 723 to vary the output voltage.

Random Crowbar Tripping:
From another email:

...although the auto reset circuit seemed to help, we finally found the real issue in the RM-35M supply at the repeater site. As this may affect other units in the field I wanted to pass along what we found:

On this particular power supply the screw they used to fasten the regulator board to the top of the filter capacitor was too long and bottomed out before snugging up the connection. That in turn caused a bad high resistance connection that overheated and caused the intermittent triggering of the over voltage circuit. Although I had pounded on the unit, it never showed up on the bench. It finally got hot enough to burn the board and damage the capacitor. I would highly suggest that readers check their supply and if they find a similar situation that they add an inside or outside star style lock washer between the board on the first ring terminal and another between the two ring terminals. That should take enough slop out of it not to bottom the screws and cause a flaky connection.

AC Mains Fusing:
Most of the Astron schematics show a fuse in the AC mains side of the transformer. Some of them do not specify slow-blow or fast blow. You will want to use a slow-blow fuse rated at the mains voltage (or greater) when powering a highly inductive load, like a big transformer or motor. There are "32 volt" fuses made for low voltage circuits (i.e. 12v automotive, 24v industrial electronics, etc). Don't use them - you want one rated at 125 volts (mains voltage in the USA) or 250 volts (in localities where 220 or 240 volt circuits are used). Linear supplies draw a lot of current during the first few cycles of the AC voltage after being switched on as the capacitors charge. If there really is a short circuit the excessive current draw will blow the slow-blow fuse rather quickly.

The fuse that is came in my stock RS‑35M supply (probably 15 years old) is a Littelfuse part number 326008. That is an 8 amp, 250 volt, Slo-Blo fuse with a ceramic body, also known as type 3AB (the common 3AG is a glass barreled fuse). The labeling on the back of the supply just says "8A" without saing fast or slow blow. Some lower rated supplies use a 5 amp or even smaller fuse. If someone else is going to be servicing your repeater it wouldn't hurt to add a label reading "Use 8 amp slow blow ceramic fuses ONLY" (replace the "8A" with the appropriage amperage for your supply), and put a few fuses into the on‑site toolbox. Or put some fuses in a zip-lock bag along with a magnet and use the magnet to stick the bag to the front, top or side of the steel power suply housing. On a bench supply you could even jumper out the fuse and replace the on-off switch on the front panel with a toggle-switch style slow‑blow AC circuit breaker of the correct amperage. There are different breaker packaging styles:  (photo 1, photo 2, and photo 3)... pick one that you like.
Or add a breaker to the DC side of the supply - boating supply houses like West Marine and some RV supply houses stock 12vDC breakers. You can occasionally buy a DC breaker or two from small aircraft maintenance shops - check your local community airport as the repair shops occasionally have fuselages that they scavenge parts from - but watch th evoltage as some are 12v and some are 24v. Do not be offended if they ask you to sign a release as they won't want the liability of the part going back into aircraft service, and no matter what you say, they can't be sure. And don't be offended if they say that they can't touch that old fuselage in back - sometimes there is a legal issue with an old airplane. The FBO (fixed base operator) at the local community airport near where I lived had one stashed in the weeds behind the hangar for several years. It had an FAA hold on it for almost five years due to an accident investigation.

My RM-60 / RM-60M uses a 10 amp, 250V, ceramic body, slow-blow AC fuse. The local parts house stocks it as a Buss (also known as Bussmann) MDA-10, MDA-10R or MDA-10-R). The trailing R indicates RoHS construction (lead-free). Littelfuse calls it a part number 326010, and some catalogs add a leading zero (i.e. 0326010).

Eric Lemmon WB6FLY pointed out another problem with AC mains power connectors:

The RS-70M power supply is marketed as being able to supply 57 amperes continuously, and 70 amperes intermittently. The only problem with that statement is that the measured AC input current exceeds 12 amperes at loads above 50 amperes DC. Why is this important? Because the RS-70M is shipped with a 12 Amp fuse, and the fuse holder is marked with that value. Twelve amperes happens to be the maximum current that can be drawn from a NEMA 5-15R outlet - the "standard" power outlet found in homes. This 12 amp limit is specified in Article 210.21(B)(2) of NFPA70, the USA National Electrical Code. This NEC limit is what caused the "vacuum cleaner current wars" to top out at 12 amperes of "cleaning power." Since the common, parallel-blade plug used vacuum cleaners and other home appliances is intended to plug into the standard NEMA 5-15R receptacle, the appliance makers cannot legally market any product that draws more than 12 amperes and uses a 15 amp plug. Hence the 12 amp fuse that is shipped in the Astron RS-70.

Note that while Eric mentions the RS-70 / RS-70M, the same is true on the VS-70 /VS-70M.
So what do the owners of the RS-70 / RS-70M / VS-70 / VS-70M supplies do? Since those supplies draw about 16 amps from the power line (Eric measured 15.82 amps) under a full 70 amp load you need to replace the power cord with a 3 conductor #12 (minimum) line cord terminated with a 20 amp NEMA 5-20P plug and that needs to plug into a 20 amp NEMA 5-20 outlet (and you need to check that the wire inside the walls is at least #12, if not larger! I've seen situations where someone replaced the 15 amp duplex outlet with a 20 amp unit and didn't upgrade the wiring... I've also seen situations where a homeowner added a circuit on his own and used #14 wire - which per the NEC is maxed out at 15 amps and that wire size is also illegal on outlet circuits). The fuse in the back of the supply should be upgraded to at least a 16 ampere 250 volt, slow-blow fuse. 16 amp fuses are special order if available at all, you will probably end up using a 20 amp fuse.

Diagrams and photos of the various NEMA outlets can be found at this web site:

Contact Information:

The author can be contacted at: his-callsign // at // repeater-builder // dot // com.

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This page originally created in August 2000 by Kevin Custer W3KKC.
Totally rewritten 14-Oct-2004 by Mike Morris WA6ILQ.
Split into several pieces 13-Jan-2018 by Robert W. Meister WA1MIK.
Copyright © 2000 and and date of last update by

This web page, the hand-coded HTML on it, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.