R2600 Radio Communications Analyzer Repair

or how to jump start with RF electronics


The start of all desaster

For no particular reason, I bought this particular pretty looking piece of electronics at our favourite electronics scrap yard
Old and used Motorola R2600B unit
(click on image for larger view)

Brains OK, but without arms and legs

Its description said something like "I've tested a little bit of the functions, which appear to work, other functions are not plausible to me. So it is sold as defective". After receiving and unboxing, I've learned pretty fast, what this meant in reality:
  • The unit powered up running a quite lengthy self calibration procedure
  • When the self calibration finished, there were no error messages and I was able to play around with the various menus and settings
  • A quick check in generate mode showed that there was some kind of a signal at the gen out port, but neither of the expected frequency nor amplitude
  • Receiving didn't work either
  • Baseband audio analysis worked, using the baseband signal input
  • So now I was the proud owner of a pretty useless Radio Test Set, since none of the RF functions worked. First I set the unit aside and thinked about scrapping or reselling it. After a few days, I took the challenge and decided to see if I would be able to repair this thing.

    RF Ratsnest

    Opening the box, I was pretty fast persuaded that I'd have to find a service manual before attempting to repair anything:
    Look at the beautiful SMB RF cables above the RF card cage
    (click on image for larger view)
    Lots of neat SMB cables route lots of unknown RF signals from one RF PCB to another. I've no idea where to start without any knowledge of the signal routing.

    Something to look forward for

    Finding useful information turned out to be somewhat difficult. I found an internet forum, search for R2600 where people exchanged firmware and option EEPROM images, which looked interesting to me. I decided to download these images and keep them for later experiments, once the unit would be in a more useful working state. But no service manual, only some hints that a service manual existed.

    The Service Manual

    Still searching around for the manual, I finally discovered the notation "RLN-4120B" beeing the service manuals order number. Using this designator with your favourite search engine, I found an offer from Canada, some kind of ancient Motorola manual compilation on a DVD for a reasonable price. I ordered that DVD and got the RLN-4120 manual in advance as a download link short after placing the order. Cool service, the DVD arrived some two weeks later and contained various other old motorola manuals additionally to my needed service manual.

    As it is quite usual with such old equipment, the service manual contained full schematics and parts lists of the box. For each board there is also a fine description of how it works. Having this comprehensive information, it was easy for me to discover that I'm in the need of some RF instrument (preferably a spectrum analyzer) to do the basic diagnostics. Your average scope simply won't show anything if you try to follow a 1.7GHz signal. Luckily I was able to borrow such an instrument (an Advantest R3131 9kHz ... 3GHz spectrum analyzer) from work.

    Discover the missing 1st LO

    Having the manual, the SMB RF cables started to make sense to me:
    RF card cage with cableing diagram

    So I needed some point to start the basic diagnosis. Looking at the generate mode block diagram:
    generate mode signal flow

    I found it easy to pull one SMB cable after another and compare the real signal (using the R3131) to the expectation. Pretty fast I discovered that the high synthesizer module didn't output the 1st GEN LO signal. There was some signal visible, but not at the expected frequenzy and at least 20dB lower level than expected. Other signals, like the LO SYNTH RF from the low synthesizer module looked fine.

    A closer look at the high synth

    Next step: I removed the high synthesizer module from the card cage.
    High synthesizer module

    Pretty much shielding, the right two SMB connectors carry the 1st GEN LO. In the first place I soldered some wires to the relevant power supply rails, re-seated the board into the card cage and checked the supply volatages. They were OK.

    Next I suspected the VCO module to malfunction, since this is the source of the bad signal.
    High synthesizer block diagram

    I had to remove the VCOs shield to gain access to the signals around the VCO module.
    VCO shield removed

    The VCO is in its own shielded box inside the shielded box inside the shielded card cage. I didn't look inside the the VCO box, maybe there's more shielding? Looking at the schematic page concerning this part of the high synthesizer module, I started to check the supply and control voltages of the VCO module, using the same technique of soldering wires and re-seating the board into its box again.

    VCO schematic page

    The supply voltage was OK, the control voltage was stuck at one end. At some frequency setting the range selector input of the VCO (Pin 4) changed its level and the signal at J1 changed frequency. Carefully checking not to destroy any driving circuit, I did override the control voltage input using an external volatage source, and look here: The output frequency moved around proportional to the control voltage. Exactly what one would expect of a perfectly working VCO. The VCO module was OK, phew!.

    Not too many parts left to check: The MMIC gain blocks AR3 and AR5. I soldered wires to their bias voltages (collector of Q5 and Q6) and found one of them not plausible (way to high). Logical conclusion after checking the transitor: The MMIC must be defective. No MAR-8 lingering around in my workshop, but some other RF gain block samples left over from past RF experiments, having similar gain and frequency specs, but other case and bias requirements. I temporarily mounted this part and tested the R2600 functions again: Everything worked as expected. I was able to generate plausible signals, receiving a local FM radio station worked after discovering and replacing the blown fuse inside the ANT connector. So the fault was located and the unit seemed to be alive again.

    I decided to order some spare MAR-8 MMIC to have an original replacement for the broken amplifier. Ordering this part turned out to be somewhat difficult, not beeing available at the usual suspects, finally I've found the german Amateurfunker Magazine web shop having them in stock. When they arrived, I replaced the part, soldered the shield, screwed the module into the card cage, did a short test: Yes everything was fine.

    A short excursion

    Having downloaded the firmware and EEPROM images for the CPU board, I started investigating the new firmware and options. I had to burn four old style UV-erasable EPROMs and a 93C46 EEPROM, which turned out not be as easy as believed, since my good old ALL-03 didn't work anymore. Just for the records: I didn't repair that one, I found an colleague at work having an old EPROM burner that I could borrow. So I replaced the EPROMs and the EEPROM just to discover that the R2600 didn't work anymore. There was some kind of a heavily distorted signal at gen out and no useful receiving. I suspected either the new firmware or the new EEPROM content to somehow change the way the software accessed the hardware (maybe another hardware revisision has some bits in some register changed or whatever), and changed the EPROMS and EEPROM back to their originals, which I kept just in case I'd need them. And yes: everything worked again. But I was wrong with my suspect, as I discovered later.

    Return to main path

    A few days later, I turned on the unit and after a few ten seconds it turned off by itself. What's wrong now? Retrying changed the behaviour from turning off after a some ten seconds to a few seconds to immediately. At least I managed to check the internal supply voltages, and found many of them exceeding their nomimal voltages. The 12V rails was OK, all other voltages were too high. After reading the manual, now the power supply part, I was clear of the cause of the rapid turn-off: There's an overvoltage protection circuit that shuts down the power supply in case the 5V supply rises. This is pretty useful, otherwise the electronics would have been literally cooked by the excessive supply levels. So this little protection circuit saves the life of e.g. the CPU board.

    I did some checking to confirm my suspect: Powering the electronics from a bunch of lab power supplies revealed that everything was fine with my R2600, now I'd just have to repair the power supply.

    Next excursion

    Beeing quickend by this, I re-tested the new firmware EPROMs and option EEPROM, and tadaa: the R2600 powered by the lab power supplies worked like a charm, providing me two new functions: tracking generator and extended spectrum analyzer. So I suspected the former malfunction of the new firmware caused by the power supply, which later turned out to be wrong.

    And now for something completely different

    The repair of the power supply.
    power supply

    Checking the output voltages in idle and lightly loaded state showed nothing suspicious. Each of the 5 output voltages was perfectly in its specified range. Increasing the load on the +12V rails caused the supply to shut down at some point, caused by a rising voltage on the 5V rail. First I suspected the electrolytic capacitors of beeing old and lost ESR. I did replace them with not so nicely fitting ones (see them on the photo) and checked the old ones for capacity and ESR. Since the behavior of the power supply board didn't change with the replacement caps, and the old ones still looked very fine in terms of their electrical properties, I decided to put the old ones in agail, just because it looks better.

    Looking at the schematics, one can see that all of the output voltages were derived from a single transformer using an isolated forward converter scheme with coupled filter inductors.
    power supply diagram

    So basically any voltage in the output circuit is coupled to any other voltage. You just cannot easily use your scope to find partially defective parts. Each particular waveform on any output voltage looks very like any other voltage, just other amplitude. So I started removing rectifier diodes until the problem disappeared. Yes, at some point (all rectifiers but the +12V rail removed) the problem disappeared :-). Didn't know much more than before. It turned out that replacing all of them (except the 40V rails, since I didn't have suitable dioded) solved the issue for now.

    Rectifiers
    Don't know which one, but one of these must be bad in a bad way. Looking at them using the diode test of your favourite multimeter doesn't show out the bad one.

    Dazed and confused, part 2

    You might imagine me beeing somewhat confused: After swapping all the diodes and then reassembling the whole unit, it didn't work proper. I had the same wellknown malfunction showing nonsense generator output again. Re-activating the whole bunch of lab power supplies didn't change a thing. So all my theories, what would be causing the problem (unstable power supply, wrong firmware) were proven wrong now.

    So I started to think about what action was in common to the occurence of the failure: De- and re-assembly of the power supply or CPU board and opening several ribbon cable connections. Beeing known as prone to failures, I checked all of the ribbon cables and their connectors: No failures found, not even a hint of a slightly bad contact.

    Both actions include the task of removing the rear panel. For some reason, possibly simply because there wasn't sufficient free space left anywhere else, the people who built this thing decided to put the reference oscillator on the back panel. It is connected by a small ribbon cable and a SMB RF cable to the main unit. And finally I discovered the reason for the misbehaviour exactly there:
    Reference oscillator output connector

    That little beast went slightly loose while disassembling the rear panel and sometime got contact again while I assembled it back and sometimes it did not contact proper. There's no real need to open this connector when disassembling the rear panel, so I didn't and never noticed that it wasn't plugged in proper. Pushing this connector a little bit solved the issue in a fraction of a second.

    Restoring long term memory

    Replacing the memory backup battery with a fresh one takes care of the lengthy self calibration after power on. Luckily enough, someone had removed the old and flat battery right before it could spill its corrosive contents over the CPU board.

    Finally (really finally?) working

    Now there's a (more or less) fine Gen Out signal
    GEN out spectrum
    The generator out level was set to 0dBm, the -1.2dBm reading at 450MHz looks plausible for the used RG174 cable.

    Receiving (and listening to) a local FM radio station
    monitoring local radio station

    Receiving an FM test signal:
    viewing a FM signal

    To be continued ...

    A fews days later, I discovered this: After some time, the unit starts to "turn the dial by a ghosts hand" (e.g. set the cursor to a digit of the monitor frequency setting, and the set frequency increases magically). Another noticed symptoms were an unexpected restart and a sudden shut off (remember the power supply issue?) of the unit. I suspect, there's something left to do on the power supply board ...

    Back to Wunderkis.de (German language only) ... und ein Zaehlpixel hab ich auch :-)