Pioneer SX-950 - Low FM Volume

[Leokarma]

My SX-950 has a strange problem. It works well on all inputs i.e. Aux, Tape, Phono with normal volume except FM. In FM mode the volume is about 40~50% of what it should be. Tuner section works fine as far as tuning to the station is concerned. It tunes to all stations and has good signal strength with good stereo separation but the volume is about 50%. Can someone help?

Just fyi I have most basic tools required for repair.

 

[tsd71]

Deoxit on all the selector switches,AM-FM-PHONO-AUX, Plus all the flip switches, and push button switches on the front panel. Best to remove the faceplate to get to these.

 

[markthefixer]

Make SURE nothing is inadvertently connected to the FM DET OUT jack and pulling the signal down.

to test:
On the back panel, connect a cable between FM DET OUT and the right channel tape 2 play.
Have it play FM stereo, then cut in tape loop 2 to switch to right channel only playing the raw FM signal.
See if that is louder.
That signal goes DIRECTLY to the FM stereo decoder chip.
The fm stereo decoder chip's outputs of L & R go through a low pass filter
and then through a muting circuit VERY similar to a protect type action
BUT it uses transistors.

Most failures would affect one channel and not the other.
A muting circuit Q12 problem would affect both channel's muting transistors (Q10, Q11) equally

a quick and dirty test / fix would be to remove the muting transistors (Q10, Q11).'
Just make sure that any extra transistor leads below the cuts are NOT touching anything.


edit C57 in the Q12 circuitry is a 0.1uf 25v CSSA cap - one of the dreaded sky blue sanyo caps that fail into resistors. In this circuit it could bias q12 instead of acting as a time delay. If that cap is clipped the time delay is eliminated, but I'm NOT sure if that would fail the mute on or off permanently - I expect failed off mute (thus allowing FM sound) but I wouldn't bet ANYTHING on it. If it failed permanently muted, removing q10 and q11 would get the sound back. Don't cut any wires with power on, and wait a few minutes after power is turned off.

Of course you can just replace C57 if so equipped.

 

[Leokarma]

Deoxit on all the selector switches,AM-FM-PHONO-AUX, Plus all the flip switches, and push button switches on the front panel. Best to remove the faceplate to get to these.

Thanks for quick reply. I've already done that. Faceplate removed, all the boards (tone control, flat amplifier) were removed from front and thoroughly sprayed with DeOxit. All switches including input selector was also sprayed with DeOxit.

 

[Leokarma]

Make SURE nothing is inadvertently connected to the FM DET OUT jack and pulling the signal down.

to test:
On the back panel, connect a cable between FM DET OUT and the right channel tape 2 play.
Have it play FM stereo, then cut in tape loop 2 to switch to right channel only playing the raw FM signal.
See if that is louder.
That signal goes DIRECTLY to the FM stereo decoder chip.
The fm stereo decoder chip's outputs of L & R go through a low pass filter
and then through a muting circuit VERY similar to a protect type action
BUT it uses transistors.

Most failures would affect one channel and not the other.
A muting circuit Q12 problem would affect both channel's muting transistors (Q10, Q11) equally

a quick and dirty test / fix would be to remove the muting transistors (Q10, Q11).'
Just make sure that any extra transistor leads below the cuts are NOT touching anything.


edit C57 in the Q12 circuitry is a 0.1uf 25v CSSA cap - one of the dreaded sky blue sanyo caps that fail into resistors. In this circuit it could bias q12 instead of acting as a time delay. If that cap is clipped the time delay is eliminated, but I'm NOT sure if that would fail the mute on or off permanently - I expect failed off mute (thus allowing FM sound) but I wouldn't bet ANYTHING on it. If it failed permanently muted, removing q10 and q11 would get the sound back. Don't cut any wires with power on, and wait a few minutes after power is turned off.

Of course you can just replace C57 if so equipped.

- Yes connecting cable from FM DET OUT via tape loop does work. Volume on both the channel are loud and normal. So I guess the problem is with the muting circuit ?

- Are you suggesting that I completely remove Q10 and Q11 transistors from the board and test the receiver ?

- Do you think problem could be due to Q10, Q11, Q12 and/or C57. Should I check and replace these as required ?


-

 

[markthefixer]

So the FM DET OUT is a normal volume. And the FM stereo has a reduced volume?

Just c57, can you replace it? just plain replace it. One from Radio Shack should suffice. Ceramic disk caps would be the SECOND choice, electrolytic caps the LAST resort.

RadioShack 0.1uF 50V 10% PC-Mount Capacitor, Catalog #: 2721069 $1.99

 

[Leokarma]

So the FM DET OUT is a normal volume. And the FM stereo has a reduced volume?

Just c57, can you replace it? just plain replace it. One from Radio Shack should suffice. Ceramic disk caps would be the SECOND choice, electrolytic caps the LAST resort.

RadioShack 0.1uF 50V 10% PC-Mount Capacitor, Catalog #: 2721069 $1.99

Yes FM DET OUT is normal Volume.

I'll replace C57 tomorrow and report.....

 

[rcs16]

- Are you suggesting that I completely remove Q10 and Q11 transistors from the board and test the receiver ?

Yes, that will isolate the muting ckt to determine if it the cause.

- Do you think problem could be due to Q10, Q11, Q12 and/or C57. Should I check and replace these as required ?

That is the idea.
Lets hope it is C57 a good possibility, if you are lucky. Not so easy to get at. C65,66 are also possible causes I think.

 

[Leokarma]

Success.......Replaced C57 and everything is working the way it's suppose to. Thanks Mark.....You are the best.....

Also thanks to rcs16 and tsd71 (Sorry I don't know your names)

Happy holidays to all.....

 

[tsd71]

Good deal! Merry Christmas! Mark is uncanny finding quick solutions. Merry Christmas MTF!

 

[rcs16]

way to go guys, mtf has a knack for sure.

 

[jerrycpor]

Love AK! Had this same exact issue this afternoon; went through the above trouble shooting on a customers' 950. Replaced C57 with a .1uf 63v Kemet film and viola, problem solved. Not the easiest cap to get to, but this fix solved the problem.

Two year old thread, but thanks MTF for the relatively easy fix!