SX-838 FM muting issue

loweran

loweran

Super Member
All,

I just recapped an SX-838. Everything is working great, except the FM Muting has no affect. The tune is very touchy too, but it pulls in stations and locks in well. I think it may need an alignment.

Any ideas about the muting issue?
 
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It is hard to say exactly what you did to throw it off. I suspect you installed the wrong component or got a ecap polarity wrong or bad solder joint or broke a wire.
pin 21 on awe-041 is the muting drive. Its o/p DCV is dependent on input RF signal.
it comes from ha1137 pin 12, it goes through S23(FM mute) to ha1137 pin 6 (audio mute input) and directly to the mute ckt awe-041 pins16,17 and the stereo decoder vco killer, pin 14. check the operation of these. check the switch operation first, clean if necessary.
You can check the discriminator in the test procedure step 5, it is the lower core, do not adjust the upper core as that affects thd and you need proper test equipment for that.
basically with no input signal, the afc voltage on ha1137 pin 7 should be the same as the reference voltage on pin 10, nom 5.6VDC. check out the ha1137 data sheet, lots of Japanese :)
good luck.
 
Thanks for all the good info. I will check it out when I have some time. Its not that big of a deal since I doubt the tuner will get much use anyway. I am fairly certain that all the caps are installed correctly. I will double check that too. I re-caped the power supply, protection board, eq amp, control amp, mic board and the power amp. I replaced all the known problematic transistors and regreased the outputs while I was at it. I did not touch the tuner.
 
How was the tuning and the muting working BEFORE the work was done - IMPORTANT!!!
Was it "touchy" ? Was the muting working?
 
yeah I wonder what touchy means too, I call touchy, something like the var cap is not smooth in operation and the center tuning meter is jumpy as you move the var cap slightly. Jumpy could be a mechanical issue as well.
 
I will blow it out and clean the brass shaft per the instructions on this site. We will see if that fixes it.
 
Go down to Home Depot and Get some CRC QD Electronics cleaner. It's non residue and works good to clean up the tuning fins and gets the crap off them and the ground points at the shaft. Then wet the end of a paper match or toothpick with DeOxit and touch it to the shaft where the ground wiper touches the shaft on each section. Exercise the tuner back and forth at least 10-20 times.
 
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