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Experienced Jag/7100 techs: RX sens problem?

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ElroyJetson

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Board types "174", "179", and "191".

Problem found on multiple examples of each of these boards: Low RX sensitivity.

Case 1: About 15 dB loss in signal strength. Loss measured by means of inputting a
calibrated signal into the front end via an antenna port adapter, and using the signal
strength meter in the radio's FCC menu.


Case 2: Same as above but the loss is about 45 dB.


Unfortunately I lack an RF probe so I can't accurately read the signal strengths at
various points in the front end.


Have you seen this problem before and do you know of a typical cause and cure?


Elroy
 

MMIC

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That FCC menu might be out of calibration as there is an alignment procedure for that in the tracking data. Are you hearing a lot of static on the radio when you reduce your signal to about -95 in case 1 and about -70 in case 2 to verify that the radio is actually desentitized by those factors?
 

ElroyJetson

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I've verified a lack of sensitivity, as described. My signal generator is a Rohde & Schwarz SMY 02 and it's in recent calibration. The radio closes squelch at its own INDICATED -120 dBM range (approximately) per the FCC menu option, but at that point, the
generator's output is a good 15 dB above that for the better radio and a good 45 dB above
that for the worse radio.

-60 dBM reference signals input into the two radios show as -75 dBM for the better radio
and -105 dBM for the worse one.


Elroy
 

JungleJim

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Does it happen on wide band channels as well as narrow/NPSPAC? If one bandwidth works and not the other it could be the corresponding 2nd IF crystal filter.

Otherwise, I have seen the first IF mixer module go bad. It's usually from a significant drop (or the sudden stop). If you're feeling brave enough to lift the can off the module by heating the solder on one side and bending it over, you may find that one of the leads to the transformer inside has a cracked solder joint. You'll need a good magnifying glass to see it. Reflow the solder and it should work good as new.

Those are the only problems I have found.
 

ElroyJetson

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Success. Replaced the mixer module and sensitivity is now normal in any mode. But the radio does need a full alignment as the tracking data was corrupted.

That's fine. It give me a reason to go through the full procedure as described in the service manual.


Elroy
 

ElroyJetson

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Another obstacle: The ref oscillator is off a bit. The test frequency of 851.000 MHz actually reads out on a freshly calibrated HP frequency counter as 850.992774 MHz. That's over 7 KHz off, and obviously that's WAY off!

So I attempted to adjust the master oscillator reference via the appropriate adjustment in
radio tracking.

It didn't work. Full range adjustments of that value, from 0 to 1023, yielded a shift of
about 20 Hz. And that's nowhere near what it should have done.

I did try different master oscillators. They all gave the same results.

What could cause the oscillator warp to be almost completely ineffective?

I don't know yet. Got any ideas, people?


Elroy
\
 

JungleJim

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dividing 851 MHz by 19.2 MHz (reference oscillator freq) yields a multiple of 44.3. Dividing your error of 7kHz by 44.3 would give an error of about 158 Hz on the reference oscillator. Use an RF sniffer on your frequency counter and see if the oscillator is off by this amount (and in the same direction i.e. low). If it is, you have a bad oscillator. If the oscillator is on or near and adjusts with the tracking data then it is ok.

Also, check the voltages on the oscillator. Pin 1 is Vcc at +5V, pin 2 is the output, pin 3 is ground and pin 4 is the control and should be between +1 and +5 V and will change with the programmed tracking value. If is doesn't vary with the tracking data, look toward the Patti (counter part to the SLIC) IC# U500.

Incidentally, on the direct FM models the AFC is "summed" together with the TX baseband (CG/DCG, audio, data, etc.)

One that gets me every once in a while is solder blobs. Check everywhere and carefully.
 

ElroyJetson

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I'll check into that, thanks. But I've had a total of three different oscillators in this radio, on the off chance that it was an oscillator problem, and the results were identical. Frequency
didn't change by enough to be worth mentioning. So I suspect that Patti's on vacation,
or getting senile. I'll check the control line out and see what's up.

I've checked out the radio in other respects, and other than the fact it's off frequency, it's working very well, with absolutely remarkable RX sensitivity. It'll open squelch at -129 dBM,
and that's seriously sensitive! But that much sensitivity implies that it may not have the best rejection specs.


Elroy
 
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