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  Introductory Information on
Astron Power Supply
Grounding and Metering

Compiled from a number of different sources
by Mike Morris WA6ILQ
Maintained by Robert Meister WA1MIK
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Is the negative side grounded to the cabinet?

Note that on some models of the Astron supplies the DC output is floating from the case and on other models the negative output is connected to the case. A good example of this is to look at the RS‑35 schematics from 1987 and compare them to those from 1991. Personally, any supply that is not owned by me and goes across my bench gets a P-Touch label stating either "Negative side is floating from the case" or "Negative side is grounded to the case". And Astron isn't consistent - some are grounded, some are floating, and some are in between - the Astron RS-50 / RM-50 schematic that is dated March 1996 shows a 3K resistor from negative to chassis. The RM-50 schematic that is dated January 2000 shows a hard ground. And just to be different, I've had an RM-50 on my bench that had the negative side floating, and I know the history of that supply - I can definitely state that is how it came from the factory. My personally owned supplies all have the negative side floating from the case - even if they didn't come that way.

Eric Lemmon WB6FLY ran into this situation and wrote about it on the repeater-builder yahoogroup:

It has come to my attention that Astron has a built-in design flaw that may cause problems for some repeater operators.

I discovered this when I replaced a suspect Pyramid power supply at my mountaintop 220 MHz repeater with an Astron SS-12 switching power supply. When I got back home, and in a very quiet environment, I was shocked to hear a very prominent 60 Hz hum on the 220 carrier. Since this switching power supply uses a switching frequency up in the 40 kHz range, I could not understand how there could be 60 Hz hum!

The very next day, I took a known-good DuraComm switching power supply with me and returned to the repeater site and exchanged the two units. This time, I moved some distance away from the repeater building and tested with a handheld to ensure that the carrier was hum-free, and it was. I could not detect the hum on the first trip because the electrical equipment next to the repeater is quite noisy.

Once I got the Astron power supply on the bench, the cause of the hum was obvious: The negative output terminal was grounded internally! Although most large Astron power supplies such as the RS-20, RS-35, SS-25, and SS-30 have a black jumper wire between the negative terminal and the case, the SS-12 uses a trace on the PC board to make the connection. I then e-mailed Astron Tech Support and received a schematic of the unit, along with advice as to where the offending trace was located. A quick bit of work with an Xacto-style hobby knife cut the trace, and floated the negative output lead. Problem solved!

Astron seems to be the only power supply brand that routinely grounds the negative lead; none of my units made by DuraComm, Samlex, Astec, or Pyramid have this connection. The hum was caused by a ground loop injecting 60 Hz into the DC source feeding the repeater. Since the radio, duplexer, and antenna feedline is always solidly grounded for surge protection, that means that the DC power source is grounded in more than one place- a very bad idea. I have modified all my Astron power supplies- both linear and switching- to remove any internal connections to ground at the negative DC output terminal.

I had a similar problem several months ago on my 6m repeater, which had a recurring problem with controller lockup. After I swapped the Astron RS-35M power supply out and put in a DuraComm supply, the problems went away. As you might expect, the Astron RS-35M was causing a ground loop, but this time it didn't cause audible hum. It did, however, corrupt some of the data signals going to the controller.

I strongly suggest that owners of an Astron power supply make a simple test with a V-O-M. With the output connections open and the power supply unplugged, measure the resistance between the grounding prong of the AC plug and the negative DC output terminal. If the reading is in the megohms, fine. If it is a short, you know what to do...

73, Eric Lemmon WB6FLY
Note that some Astrons have a resistor from negative to case, so the measurment that Eric refers to may not be a short... it may be a value in the range of a few thousand ohms.
And depending on the circumstances the hum may be either 60 Hz or 120 Hz...

Eric provided some photos of the PC board in the SS-12, and the board is probably similar to other Astron switching supplies:       Photo 1       Photo 2       Photo 3

This 700kB PDF article supports the ground bonding issue.


If you have a meterless Astron and have the front panel space for a meter or two I highly recommend adding at least a voltmenter if not a current meter as well. BUT... Don't even bother asking Astron if the factory metering option can be added later on... One day around 1999 or 2000 I was visiting a client in Irvine and used the opportunity to stop in at Astron to pick up a RS‑20A and RS‑35M schematic, and casually asked if I could buy the lower half sheet metal of an RS‑20M plus the meters to upgrade my RS‑20A to an M series. Yes, but the price was over 2/3 of the cost of a new supply - plus shipping! ("Sorry, you'll have to pay in advance, and we'll have to ship the parts, they aren't in stock") For that price I can buy a matching pair of surplus meters and cut the holes myself. By the way, adding the metering to most of the Astron designs is not hard - cutting the meter holes in the front panel is the hardest part. Acquiring matching voltmeters and ammeters is easy. For the wiring just refer to the metered version of your supply or of a similar model. Personally, I tend to use analog meters as they are much easier, they are quite adequate for the job (high precision is not needed) and some digital meters can't measure their own power source.

Tony King W4ZT bought a used astron that had both meters "stuck". He replaced them with digital meters and created a web page about installing digital meters. I was sent a PDF of it, which can be found here. Later I found the original web page mentioned above.
The ammeter he bought required an isolated power source, and the article includes a schematic and photo of the isolated 5vDC source that he build on perfboard.
There is another Tony King article on a power supply test load further down on this page.
By the way, digital ammeters that do not require the isolated supply can be found.

From another email to repeater-builder on power supply metering:

The typical Astron power supply ammeter is NOT a load current meter in the classic sense - it is calibrated in amps but is wired as a voltmeter, and actually measures the voltage drop across one of pass transistor emitter ballast resistors (also called an emitter swamping resistor, or a load balancing resistor). This technique only works properly if all of the pass transistors are absolutely identical (not just the same part number) AND all of the emitter ballast resistors are exactly the same resistance (not just the same marked value). Then and only then is the voltage drop across the one resistor directly proportional to the entire load. Neither the pass transistors nor the emitter resistors are that closely matched, so the resulting displayed current value is only approximate.

That said, who really needs that accuracy in a low-priced test bench supply? The Astron method of reading the voltage drop across the ballast resistor that carries 1/2, 1/4, or 1/8 of the total current is "good enough" for any rough measurement - in many cases all you are doing is looking for a reasonable value, for a peak or a dip and the internal Astron metering is fine for that. If you need better, you are probably not using the internal metering in an Astron. You have three options:

More on power supply metering, from an email to repeater-builder from Robert Meister WA1MIK:

There are plenty of 3-1/2 digit LED and LCD meters that can be purchased from electronics and surplus businesses. For example, Marlin P. Jones & Associates has several in the US$9-$12 range. Make sure that the meter you buy will work with a common ground for its power input and meter input. Not all meters can do that. In particular, many of those that require a 9V supply often state they can't measure their own power supply. Get one that operates on 5V and add a LM78M05, LM7805 (or similar) dedicated regulator to run the meter(s) off the Astron's internal unregulated DC supply across the main filter capacitor. The 5v regulator can be mounted almost anywhere.

You may also be content with a small analog meter. There are plenty of 0-15vDC meters for under US$10 that will do quite nicely. You can also use just about any meter you have and add an appropriate resistor in series to give you the scale you want; Astron themselves uses 1mA DC meters in their supplies for both voltage and current. See any of the power supply "M" schematics for the details. Or use a 0-5v meter and "stack" it on top of a 10vDC reference, then calibrate it as 10v at the bottom end and 15v at the top.

For test purposes I disconnected one end of both of the two meters on an RS‑35M and connected a Fluke digital multimeter, a 10k resistor, and a 10v power supply all in series. Both meters went to just about full scale, and the DMM read 995uA, so these are definitely 1mADC full scale meters.

There's about 15k ohms in series with the voltmeter. This is accomplished with a small trimpot soldered in series with one terminal on the back of the meter. I suspect the full-scale resistance is in the 20-25k ohm range.

There's about 360 ohms in series with the ammeter on my supply. This is also done with a small pot soldered in series with one terminal on the back of the meter. I suspect the resistance is in the 500-1000 ohm range, but it will definitely vary depending on the current rating of the power supply.

I calibrated my supply by setting the voltage to 14.00 on an external meter and adjusting the Astron's voltmeter to 14 volts. I then hooked a pair of 1.0 ohm 250 Watt resistors in parallel across the output terminals, and adjusted the Astron's ammeter for 28 amps. I had to work fast, those resistors get real hot in a hurry (with almost 400 watts being dissipated).

Robert later expanded on these ideas with a full article on adding meters to an Astron. For more info on the method of stacking a voltmeter on top of a fixed reference to get a meter that starts at a reference voltage just google the term "expanded scale" and "voltmeter".

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.