Synthesized MICOR Station Modification

By Greg Carttar (ex-WAĜLCZ)
3rd St. R & D Production Services

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Revision 03/12/14

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finished-station-1.jpg (139381 bytes)
Finished MICOR Station modification on a C64RXB3106
( 75 Watt UHF, DVP Transparent, tone remote control, voting )

station.jpg (156847 bytes)
Nearly finished MICOR Station modification on a C73RTB3106DT
( 110 Watt VHF intermittent duty, Non-DVP, PL, tone remote control, voting )
Built from a re-jumpered C73RTB3106E VHF Base Station
This photo also shows a 3-channel voting comparator and one
UHF link receiver

Why did we do this?

This project began as an experiment to come up with a frequency-agile station to back up several repeaters which are installed in a truck that we use on our special event communications projects.

We purchased a complete metropolitan-scale UHF system from Salt Lake City which included a number of B64RXB3106 (75W, Upright, DVP transparent, tone remote control) repeaters. These repeaters were also configured for voting repeater operation. Originally, we were going to part out the MICORS, but the more we got to know them, the more we fell in love with them. We then bought the voted DVP stations from Honolulu PD. We love these old dogs. We've got 18 in use now, and we want more.

And, we are MICOR purists. We feel that these stations are some of the most reliable and bulletproof machines in the world today. (20 years from now, try to get parts for an MTR2000......I don't think so). They are serviceable to this day.

It turned into a really good working upgrade for our stations.


How Motorola did it in those days......

One of the repeaters in the system was configured as a switchable multi-channel unit which backed up the others. It had an SP modification which added an additional sub-chassis that contained multiple channel elements, some buffers, and a switching harness. An audio patch bay was provided that would allow the agency to sub out any receiver, transmitter, or whole repeater which failed, quickly.

We considered modifying the other stations for similar operation.

Too many channel elements

Unfortunately, our licenses in sites and cities we go to do not always allow us to use the same set of frequencies. We were faced with having to have a huge selection of channel elements to accommodate a wide variety of licenses; and a pretty massive amount of re-alignment at different event sites. (The MICOR only provides an 800kHz range between lowest and highest frequency without re-alignment).

In disaster restoration, who knows what frequencies we'll need to cover.

I contacted several engineer/consultants regarding the custom design of synthesized exciters and receivers, or the possibility of replacing channel elements with some sort of synthesizer. It just got more and more problematic (and expensive) , since we needed to retain PL (tone squelch) capability and actually wanted to add DPL capability. We also did not want to eliminate entirely the potential capability of doing DVP.

DVP requires direct FM modulation and switching of PL circuitry in and out since DVP is incompatible with any coded squelch.

In the final analysis, it all came down to frequency stability, reliability, and reversibility (what if it didn't work and we had to put them back?)

We needed commercial stability, commercial modulation control, and for all its mysterious modules and myriad signal routings on the backplane, the MICOR station is incredibly flexible, adaptable, and reliable.

Last but certainly not least, we weren't ready  to spend $20,000+ each on Quantars (no matter how much we'd love to).

This modification project is not for the faint-hearted or those who cannot read schematics and trace signal flows, or for those who refuse to use the manual. Don't take this on if you do not have the books. This mod is not a way to resurrect a corpse that does not work. It is rather a mod to modernize a venerable, tough, and reliable machine.

No offer of support or elmering is expressed or implied unless we build a station for you.

These modified machines are rock solid, stable, and the RF is as pure as the driven snow.

Your machine has to work in stock configuration before you modify it.



We looked at a lot of options. We considered using SyntorX9000 front ends. We looked at several portables including MT1000's (see below). We considered Maxtrac or Radius Mobile synthesizers.

We decided to use Motorola HT50/P100 portables as exciters and receivers. They are synthesized, are capable of both PL and DPL encode and decode, already have good audio processing and deviation limiting, and provide a direct VCO modulation path. The VCO deviation is programmable. They have excellent transmit spectral purity. The receivers are real hot (typically .2 usable or better) and although they require some additional front end filtering (more on that later) they are pretty stable. They are capable of 12.5kHz channel spacings and have commercial frequency stability.

We specifically settled on the HT50/P100 radio because it runs on 10 volts and the station provides up to 3 amps of 9.6 volts (the 9.6 supply is fused at 4 amps). This is a tough, reliable little radio with a cast aluminum structure which is the heatsink and supports the board assembly, and is from the time when everything in the Motorola line was tough and well built. The HT50/P100 radio has selectable power out, either 1 watt or 4 watts; although there is a low-power model which is 1-watt out. They are either 2 channel or 6 channel and the channel select switch is very simple and easy to hack into, as is the transmit power control switch. On the low power setting, the UHF radio will key-up for hours at a stretch without warming up to any significant degree, although the VHF radio final gets a little warm. With a little bit of additional heat-sinking (more on that later) it will run at full power for hours as well.

The chassis of the radio is fully accessible when removed from the case, and the screw-on antenna can easily be replaced with a BNC connector.

The radio is not so highly integrated that it is impossible to get to important points in the circuitry.

We have experimented with using MT1000's instead, since they also run on 10 volts, and have played with integrating the circuitry of an enhanced display MVA...... but that's another article. They are more integrated and a little tougher to deal with.

It is at least theoretically possible to consider doing this mod with Maxtrac or Radius Mobile decks, but we have not tried it.

We had lots of P100's to sacrifice, but didn't want to take any mobiles out of service.

I suppose that you could use any brand of portable you want, provided that it puts out no more than 1 watt to drive the power amp, and provided that you can get PL detect or COR from the receive radio.


So What You're Really Doing Is.......

So basically what we're doing from the RF standpoint is putting a power amp on a portable for the transmit side, and an external antenna on a portable with additional front end filtering for the receive side; and tying them into the signal processing circuitry in the station chassis.

We're eliminating the exciter, channel element, IPA, and tripler in the UHF transmitter, the exciter in the VHF transmitter; and the whole receive deck in the receiver. We're keeping the excellent squelch operation of the MICOR audio/squelch board, but using COR/PL detect from the portable receiver.

With this modification, we have preserved all the functionality (voting, tone control, failure revert, PA protection, isolation, wireline interface, robust mechanical integration, other options) and flexibility of the MICOR station and the modification is 100% reversible back to original condition if you wish to.

In use, receiver re-alignment is reduced to simply peaking up the front end filter, and there is no transmitter re-alignment. Just re-zap the programming of the receiver and exciter and go.

In our application using 6-channel radios, we have a high probability of being able to set up channels so that a new site simply involves changing channels and peaking the helical filter and the transmit combiner.

In a ham application, this is an easy way to back up multiple machines at the same site, provided that all the frequencies are within 800kHz or so (max channel spacing for the helical front end). In a ham application, there will be some alignment of the portable to get it down into the ham bands. I know that a P200/HT600 and an MT1000 will go down there, but I have not tried it with a P100/HT50.

Make no are still going to have to have real good cavities for 2-meter operation at 600kHz offsets.

This Article

This article is specifically aimed at those who wish to upgrade a stock MICOR repeater with synthesized exciters and receivers. We cannot be of help on a stripped machine. Kevin Custer's repeater-builder website is the best place to get information on using stripped machines, and is the ultimate reference for same.

This modification article deals specifically with the B or C64RCB3106 unified chassis. (Upright or Compa, 75W, UHF, tone control station. It can also apply to a RXB (Securenet), but there are a couple of the DVP modules you will have to either bypass or leave in. (If you have an RXB, we'll swap you.) It does not deal with a CR Station, you are on your own there.

The basic unified chassis for a repeater is the same for VHF, UHF, and 800.

This mod can also be used with a 30 watt station, which has one of two power amp configurations, either a separate power amp with fewer power blocks in it, OR an intermittent duty PA which is mounted in the unified chassis.

We have not done this conversion to a non-unified chassis nor have we explored it; but this is basically about the RF sections so it should not be too much different.

As of this writing, we are doing a mod to a VHF station, and it's pretty painless so far. The one we're doing is an intermittent duty, unified chassis. You can tell an intermittent duty VHF repeater because it looks like it has the butt end of a mobile sticking out of the chassis. (It does) The included photos of the VHF mod are rough, because we haven't finished it yet, it's still burning in. Works great though.

As I said earlier, If you are going to leave the stock modules in your station, get a companion applications manual. Bite the bullet and buy the books. Kevin Custer recommends stripping all the modules except the station control module. If you are ever going to consider voting your station, leave the stock modules in (squelch gate, station control, line driver). The voting encoder module will drop in with very little grief and very little external stuff. However, if you have a community repeater model MICOR, forget about voting it. You can do it, but it's a pain.

If you decide to vote your repeater, you will want the squelch gate so that the station will revert to ICR (In cabinet repeat) in the event of a comparator failure. Also, if you vote it and your comparator is remotely located, you will want to keep your guard tone and f1/PL modules so the comparator can control the station with tones. Find a 4-wire line driver module (it does not have to be a Spectra-tac Line Driver)

Lastly, if you want to make a repeater "channel agile" for backup, you may want to hunt up some wild card or 4-frequency control modules and you'll need the F1/PL module to go with them.


Regarding Reverse Burst, squelch behavior, and squelch gates -
I prefer to use the stations in stock configuration, because we only use Motorola portables and mobiles which generate proper reverse burst to eliminate squelch tails. Don't get me started on Maxon, Kenwood, Vertex, etc.;  but they don't generate reverse burst. My event clients will not hear of using them.

I like squelch gates, when a station has a squelch gate, it will make it possible to enable the stock repeater configuration, and I don't find them that difficult to set up. I'm not challenging anyone's views on this, but this is my purpose and hence the reason for the way my mod is done.  Since we run voting systems, we need the ability for the station to recover from a failure of the voting comparator or the audio links. The squelch gate provides that capability. More on this below.

This mod will generate reverse burst on transmit if you are using PL, and there is no way to defeat it unless you use non-Motorola exciter radios.

To each his own.  :-)


External Ham Controllers and Kevin Custer's Mods

If, however, you want to use an external ham controller, stick with Kevin's directions about the modules. If you pull all your modules, Call or email us...we want them. Kevin's mod should not have any effect on the RF aspects of our mod, except that we use the channel element ground signal to key the exciter, so you want the variation that does not lock the channel element on. In our version, if the exciter keys, it's gonna transmit.

Do the RF part, ignore all the modules and signal flow part.

The MICOR Unified Chassis Station

Many others on the repeater-builder website have discussed how the MICOR station RF section works and so I'm not going to re-hash it, but suffice it to say that VHF and UHF stations are built around a VHF exciter which either drives the PA directly (in VHF),  drives a tripler/amp/IPA and PA (in UHF) , or a tripler/doubler/ IPA/PA in 800MHz and 900MHz. The transmitter is simply composed of 50ohm blocks culminating in a power amp. Modulation audio is applied directly to the channel element after being bandpassed and deviation limited. The digital DVP or NRZ pocsag bitstream is applied directly to the channel element. The station transmits direct FM, not phase modulation.

The power amp has a really good power control board and antenna network which provides protection and transmitter isolation (in UHF, no circulator in VHF). The tripler in the UHF station is sometimes problematic, and if it fails it's a pain to fix; and parts are getting scarce. It only takes about 800mw to drive the power amp to full output if it's healthy in either band.

The receiver section is a band specific receiver deck  which has a very good, very sharp helical front end and an excellent discriminator which outputs wideband audio to an excellent audio/squelch board.

The MICOR Station differs from the mobile in that the transmitter and receiver each have their own channel elements, and are totally independent. There are also differences in the audio/squelch board. The channel elements are different from the mobile elements, with the receive element having an AFC control pin, and the transmit element having a direct modulation input pin. By the way, a 3-pin receive element will work fine in these...just ignore the AFC or defeat it.

You could probably do a variation of this mod to a MICOR mobile for a simple repeater. There is no significant difference in the RF core of the radio except for heatsinking of the final.




Take a deep breath.....

......and get ready to tear your station apart. You do not even have to un-rack it or disconnect anything from it. Get a MICOR station manual and a control and applications manual for your specific model (some discussion of different MICOR models here)  from Motorola Parts. They are also often available on eBay.  (The standard control and applications manual is 68P81025E60 and explains everything in the backplane and modules for a standard Base/Repeater).

The UHF DVP manual ( 68P81034E25-C ) includes RF, signal processing, backplane and DVP modules.

The PURC manual ( UHF 68P81060E70-B ) includes all the paging modules. 

The 800MHz trunking repeater ( 68P81038E85-B ) includes most of the modules of interest. The companion manual is 68P81031E45

You can extrapolate between VHF, UHF, and 800MHz repeaters -- the primary differences are multiplier stages: VHF-none, UHF-tripler, 800MHz-tripler-doubler.

Here we go.......

First, test your station in stock configuration. Make sure it will transmit and receive and that all the audio and control paths are intact. If it has missing modules, fill them for the test. It won't transmit without a station control, and either an F1/PL-Guard Tone pair OR a DC transfer module. Using your Control and Applications manual, check the jumpering on all the modules and the backplane (jumpering of the modules is detailed at the end of this article). If it is a RA (base) station, go ahead and re-jumper it for repeater operation and get it working before you mod it. The C & A manual tells you everything you have to know. Study it and get familiar with how this equipment works. There is also information on Repeater-Builder about re-jumpering the interconnect boards for repeater use.   You can use the debugging instructions at the end of this article to help debug your stock station before you modifiy it.

The logic signals and audio paths go back and forth, back and forth between the modules. There's a good reason for it (flexibility). YOU NEED THE MANUALS.

Make sure that the transmitter will make rated power. If you have doubts and suspect that the tripler may be bad (in UHF ), use one of your UHF portables to directly drive the PA for a test. Use the low power (1 watt) position on the portable, and tie the control line of the PA to +12v. Terminate the PA through a wattmeter into a proper dummy load that will take 125 watts or more and BRIEFLY key the portable just long enough to get a power reading. These power amps will make as much as 125 watts or more of power with 1 watt of drive with the control line tied to 12V. Under these conditions, the power amp has no protection and is straining its guts out, don't overdo it. If the PA is healthy, don't worry about the exciter and tripler, we're going to lose them. If you cannot get at least 90 watts, the power amp may be lame.

The same holds true for the VHF amp. With the control line tied to 12V, only key it long enough to get the power reading.

Power it down.

1. Remove all the shields from the transmitter and receiver compartments. Keep the outermost (sheetmetal) shields.

2. In the UHF station, remove the exciter board, first bandpass filter, and tripler.   Take the little cable that runs from the tripler output to the BNC in the front panel of the transmit compartment and remove it from the little bracket and set it aside. Save the little BNC bracket. Remove the bracket frame that held the exciter board. There are screws in the top, bottom, side, and rear of the exciter compartment that need to come out. Leave the interconnect PC board in the bottom of the exciter compartment in place. It requires some maneuvering to get the bracket out.

If it was a base station and has a T/R relay, keep it in case you want to do the high/low power battery revert option.

In the VHF station, remove the exciter and the bandpass filter. Keep the bandpass filter. Remove the exciter bracket frame.

3. Remove the receiver RF board. In the UHF receiver, remove the helical front end assembly from the receiver board by taking out all the screws that go through the PC board. After the screws are out, it unplugs from the PC board.Set it aside. Don't re-assemble it yet, it will get modified. If your station has a receive PL board, pull the reed but leave the board installed on the audio/squelch board so you still have PL filtering. PL decode will be done in the new receiver deck. If you are not going to use PL, we'll deal with that later.

The high-pass filtering in the new exciter will filter PL out quite nicely, so if you want to pull the PL board, go ahead. If you do pull it, we want it.

In the VHF receiver board, the helical filter is attached in basically the same way, but the connection from the output of the filter to the PC board is a little flying lead.


4. You will need for the UHF Mod:

2 - UHF 2 or 6 channel P100 or HT50 portables. You don't need antennas, chargers, or any of that stuff. Try to get the high/low power selectable models, but the low-power only model will work fine.

2 - very high quality chassis mount BNC connectors (get amphenols, NOT radio shack blister packs) with ground lugs and solder cup center conductors.

1 - very high quality bulkhead mount double female BNC (again, get amphenol),

1 - RG142 or equivalent male BNC to male BNC jumper, 8" or so long.

1 - RG142 or equivalent male BNC to bare end jumper, 8" or so long.

For the VHF Mod:

2 - VHF 2 or 6 channel P100 or HT50 portables, high/low power switching models preferred.

1 - 8" or so long RCA to BNC jumper made from good coax (142 might be a little stiff)

1 - 8" or so BNC to bare end jumper

For either Mod:

1 - Right angle BNC adapter (radio shack will work for this)

1 - 25K or 10K  pot

1 - 1mfd or so NP cap, 15V or better rating. (we used a .22)

1 - 2N2222 transistor and some kind of little PC board to mount it on. (We used a very small radio shack experimeter board) You are going to build Kevin's little 2N2222 level shifter/inverter circuit to convert the TTL level audio enable signal (COS and/or PL decode) from the receiver deck to 9.6V positive logic for the station audio squelch board.
cosswitch-new.gif (4038 bytes)

Some Motorola board pins, about 8 of them. (will vary with how you do it). You can scrounge these from the exciter or receiver boards, or from junk boards.

A few feet of small flexible single conductor shielded wire and a few feet of small stranded hook up wire. This wire is going to have to solder on to some REALLY small points on the new exciter and receiver PC boards. We used ribbon cable except for the power wires to the exciter deck and the shielded runs (discriminator audio and TX audio).

Some lightweight solid copper sheet, just a little.

A big soldering iron or gun (300 watts or better), you will be soldering to castings and the chassis.

An HT50/P100 programming interface, HT50/P100 RSS software, and a really slow DOS computer to run it on (33MHz or slower). An old 286 is perfect. Bite the bullet and buy the software, a proper interface, and a RIB from Motorola.

A service monitor and spectrum analyzer. Service monitor needs to have variable PL/DPL deviation independent of the test tone deviation, and the ability to switch it off while leaving the RF on.

A reasonable DVM.

An audio signal generator.

An amplified speaker of some sort for tracing audio signals. Radio shack makes a dandy little one.

A precision true RMS audio level meter capable of reading in DBm.



Continue to Exciter and Receiver Radio Prep