SX838 - trouble wiht FM section

[Bill S]

I have a recapped SX838 that I've replaced all caps and problem trannies but I haven't touched the FM board. A while back I had a mysterious thump that would happen every once in a while after the unit had been playing for a while. It was like the sound would cut out for a split second and then, THUMP - through both speakers. I never did isolate it but now I suspect it may have only been happening while listening to FM.

Today, the FM dropped completely. No sound at all and tuning meter was right in the middle, even when I turned the dial. I waited a few minutes and cycled through all the switches but still nothing. I played a CD and it worked perfectly. Tried the FM again and still nothing. About 15 minutes later, I switched back to FM and then it was back. Seems to me like a problem on the FM board. Any ideas before I open it up???

 

[larryderouin]

Sounds like it's heat related to a transistor. Have you some canned air??

Open it up and locate the board. Check DC Voltage on PIN 20 on the AWE-041 Tuner Assy, and Pin 14 on the POWER SUPPLY. Both should be 13V +/-1V @ pin 20(AWE-041). Note the voltage in your next post for each pin.

Turn the unit on and operate it with the top ON(just laying on top to keep heat in, you want to simulate the actual operating conditions. Take voltage readings above BEFORE AND AS SOON as possible after it dies on FM. If the voltages are OK continue on to the transistors. When the FM drops out, give each a very short burst of canned air. Use the straw and direct it right at the top. Keep can upright to keep from frosting transistor. Again very short bursts(less than a second). Wait 30 seconds or so before moving on to the next transistor. If the FM comes back on shortly after a burst, that's quite possibly your problem child. let it heat back up and repeat to verify.

Q11 and Q12 on the TUNER ASSY are listed as 2sa763p but have 2sa725 listed as an alternate part. Check the board. If they are 2sa725, replace them. If they haven't taken a dump, they will (these you replace without fail on any board, as the exception to the rule.). But you said FM dies completely. Have you checked AM??? Please do so. If it also dies, then check the VOLTAGES again on pins 14 and 20(as above). Mark has a better grasp of the tuner than I do, and can tell you what, where, how, when, etc.

But this should get you started.

 

[JURB]

Most likely the voltage regulator feeding the tuner. That is all not push pull and the thump was likely DC dropping when the power supply dropped out. then it comes back of course.

I didn't look it up, but the point is to just get to it when it is failed and see if there is power going to the tuner system. Of course being intermittent it might be a bitch. Try a hair dryer and maybe some freeze spray. Of course where you apply that might tell you where the problem is without even checking voltages.

 

[markthefixer]

Deoxit the FM function switch sections first. Thoroughly. Because you never mentioned it.

If when it cuts out AND there's +13v on tuner pin 20, look at the FM function switched power (+13v) to the FM tuner front end on pin 3 of the tuner. IF there is +13v there and still no sound,follow the +13v in to R57 (by the bottom of T6, the tall silver can) and look at the other side of R57, which is a power filter (R57 47 ohms, C42 100uf) for the FM stereo's output transistor amplifier section of Q11, Q12, Q13 & Q14 .

 

[Bill S]

Thanks for the tips on troubleshooting. I have the unit in the garage right now. It was a balmy 30 degrees when it last did this. I'll pull the unit and check it out in the house and report back if I can get it do do it for me when it's on the bench.