Re: 8642B displays A6A1 transient failure F3 when audio peaks at FM mod input
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I have two 8642As, one working and the other has had A6 issues since I got it, FM modulation never worked. Recently it developed A12 unlock issues also. I am guessing some of the varactors may be bad, but I have not? taken it apart yet.
I went through the parts lists descriptions and found all the 60 V
Varactors. I looked at available data and I believe the following will
be suitable replacements.
1N5139A 6.8 pF, C2V/C60V 2.7 Min, Q 350 Min 10 pF, C2V/C60V 2.8 Min, Q 300 Min 1N5142A 15 pF, C2V/C60V 2.8 Min, Q 250 Min 1N5146A 33 pF, C2V/C60V 3.2 Min, Q 200 Min 1N5148A 47 pF, C2V/C60V 3.2 Min, Q 200 Min
I've obtained some of these from military surplus distributors. After I work on the unit, I will post here and will probably have extra parts. It will probably be several months, as my lab is torn apart for a home remodel.
In any case, I will post here with any updates. Any suggestions are also welcome.
Regards,
Mark
On Tue, Nov 6, 2018 at 12:19 PM Brian and Beth Miller < bmiller297@...> wrote: Jim is correct. Switching from AC coupled FM to DC coupled FM causes the FM PLL to unlock and incur a frequency error. This error is mitigated summing a DC voltage onto the FM VCO tune line. The counter is used to determine the unlocked frequency error which is then used to set the value of the compensating DC voltage. ? Brian (designer of the 8642 FM PLL)
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