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Thread: Spectra - Reference Oscillator off-frequency

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    Default Spectra - Reference Oscillator off-frequency

    Hey Guys,

    I am having serious issues with one of my radios. I just can't properly align the Reference Oscillator. The radio tester always shows +1.03 kHz upwards and I can't get it to a lower value. We are speaking about a J-split VHF radio. I have replaced all the boards (even the control head) to a known good radio. I have figured out that the Command Board is somehow faulty. Works nicely, but I can't set the Ref.Osc. The difference is always +1.03 kHz.

    I have pulled the faulty command board from the radio and I have put it into a K-split VHF radio (what was aligned properly before with its own, good working command board). And guess what? The difference was +1.03 kHz! I have tried the same with an UHF R-split radio and put the faulty command board into that one too. Voil' ! The difference was +1.03 kHz upwards and could not be lowered to properly set the Ref.Osc.

    I am totally confused now. A command board? Screwing up the Ref.Osc. setting? I have never seen that before!

    Any comments on this?


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    I moved your post out of the 63-post "Astro Spectra Upgrades" thread, and into its own, in the proper section of the forum. Good luck with getting it resolved.

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    Did you actually measure the Ref Tune DC voltage from the command board A/D converter going to the Ref Osc? Does it change?

    "radio tester" ?????

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    I have had several of the TCXO modules go bad in Spectra/Astro Spectra radios. Simply desolder the metal can from the RF board, (4 pins) and replace with another. The symptom is you can tune, but not enough to bring it on frequency. I have also seen others that were just plain dead, too.

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    The Ref Tune DC voltage changes, but won't frop below a value, so I can not tune properly the ref. osc. itself. It can be adjusted between cca. +1.03 kHz and cca. +2.5 kHz. But just won't go below +1.03 kHz.

    There is nothing wrong with the RF board itself. I have tried several RF boards but the issue was the same. The problem was only gone when I replaced the command board itself. So I think there is nothing wrong with the RF board. According to the Detailed Service Manual (DSM), the command board regulates the power for the VCO / Vocon / RF board. So if the command board is faulty, everything is screwed up.

    But I haven't found the error on the command board yet.

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    Yup, more than likely a failed TCXO, oscillator has gone out of spec. I usually replace them, and fix the broken one with a hammer to keep them separate from the good ones....
    "God as my witness" - Jeremy Dewitte - Felon

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    Can you tell me a part number for the so-called TXCO on the RF board? I'll double check it tonight. Is it Y350?

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    What is the dc range?

    I ran into one that had a faulty diff amp. Look for caps out of range and one IC output off spec while running. It will be easily apparent if you don't think its the TCXO. WATCH where you put the voltage probe.....
    "God as my witness" - Jeremy Dewitte - Felon

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    I have already ordered a replacement command board, but I want to repair this faulty one. Currently I am at my workplace, so I can only test it when I get home. I have a capacitor tester, I will measure all the capacitors' values. At the moment I can not remember the DC voltage range of the tuning voltage, but I will take a look in the afternoon at home.

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    Y350 is a 4-pole crystal filter, consisting of two independent 2-pole crystal filters contained in a single
    package. The filter package has a polarization mark located on the top to ensure proper installation.
    Y350 supplies the 109.65 MHz IF selectivity and its output passes through a matching network and
    then goes to ABACUS II IC (U301) pin 30.
    (For HRN6014D, HRN6020C, HRN6019C, HRN4009D, HRN4010C and later RF board kits, FL350
    and FL351 are 2-pole crystal filters which supply the 109.65 IF selectivity. The output passes
    through a matching network and goes to the ABACUS II IC (U301), pin 30.)
    "God as my witness" - Jeremy Dewitte - Felon

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    Read the detailed service manual section on the command board, section 3.2. It should point you in the right direction for trouble shooting. Keep in mind that this little sucker has a lot of dependencies with the VOCON, so care is needed when probing circuits to make sure your readings are correct.
    "God as my witness" - Jeremy Dewitte - Felon

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    Thanks for the advise, I'll carefully read the manual again and try to probe the voltages on the command board. Damn! Why are these radios unknown here?! I wanna live in the US

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    Come on over! We're always open!
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    The TCXO is easy to locate, it's a rectangular metal box about 1" x 1/2" x about 1/4" tall in one corner of the board. It's usually got a white sticker on it labeled "Motorola" and the frequency, "16.8000 MHz" or the like. I'm not sure of the exact part number but it's fairly obvious which one it is. Since it's only 4 pins the easiest way to troubleshoot them is to just try them. They were the same as the ones used in earlier radios, like Systems Sabers, etc. So they are fairly common.

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    That one is not faulty. I am 100% sure about that: I have tried three different RF boards from known good working VHF Astro Spectra radios. That was the first thing I've tried.

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    Sorry, I missed the bit about having tried others, and that you had isolated it to the command board. That is very weird. There is not a lot going on on that board, it's mostly audio and I/O handling. Troubleshooting that would probably amount to checking the small transistors and diodes, and if they are OK, throw it away, because it's probably one of the big custom ASIC chips on there, and they are made of unobtainum.

    Actually, don't throw it away if it's a later model, it has a valuable zener diode on it (13V?) that is used in fixing the dreaded FAIL 01/90-UCM-Related-Screwup.

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    I am very suspicious that U502 is causing all the issues. I think U502 doesn't have the proper voltage feed, so it is driving the reference oscillator in a dumb way. I'll have to do further probings on the panel when I have some time today evening or tomorrow evening.

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    I had some time this evening and I played a little with the command board.

    After some experiment it seems that U502 is the faulty one. On U502's pin13 the reference voltage is constantly changing. The next step: take a close look at Q521 (gets hot).

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    Do you have another command board? Those things are like empty beer cans over here....

    Q521 may be getting hot BECAUSE of U502, also. Don't forget the relationship there. If you have the skillset, changing both parts is not that hard, if you can get them. Maybe someone can check online and see if they are still available before I get into the office in the morning, but otherwise, if you figure out what you need if you can't get it over there, there are several people on this board that can get it here and ship it to you if you are hell bent on repairing the board.
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    Yepp, I have noticed the relationship between the two parts. Q521 can be easily replaced by another transistor NPN type of BDxxx (can't remember the number now). It is cheap here, cca. 1 USD is the price or less. That might not be a problem....but the U502? Ehhh......I don't know the replacement type for that one. Unfortunately we have no replacement parts or boards. The "nearest" shopping source is the USA