Sharp gf9191 (and 9292) right chan in stereo issue

[docs]

As described in this old post

Sharp GF-9191

I thought I would go ahead and post a thread here while I wait for a service manual in the hopes that someone else has encountered - and fixed - my problem before. I'm restoring a Sharp GF-303SG that is basically the Japanese domestic version of the Sharp GF-9191. Everything works on the...

boomboxery.com

Unfortunately no solution was provided.

The right channel works in mono and wide but is distorted in stereo mode only and on all functions.

Things done.
Pots cleaned.
Traced audio and tested voltages from power amp IC's back to the mono/stereo/wide switch.
Tested components, caps and resistors and checked in on the transistors around the mono/stereo/wide switch.
I noticed there is a higher than usual voltage at q115 (phase shifter) base and emitter but after desoldering the base and emitter legs, there was still 6.7v at base pad when it should be 2.7v. Shrug.
Tried switching q115 and q116 over to see if this little transistor amp was the issue but the problem stayed in the right channel. The issue seems likely to be a capacitor issue since on powering up, sound is present then very quickly quietens and distorts.
I make the assumption that since the volume adjusts perfectly in mono/wide modes that the volume pots are functioning accordingly.

Anyone any clue or resolved this issue before? I can't figure put what components are used in stereo mode but not in mono or wide.

 

[Reli]

I've owned 6 of those models and all but 1 had problems with that switch

 

[docs]

Just rebuilt the switch and it is good.
There must be some component which stereo uses that wide and mono doesn't.

Looking at the 9191 schematic, collector of q115 and q116 are tied to switch SW101-G which sits on the side of the FM switch and is a muting switch. There are also a couple of resistors built into the board which need checking but I think that this would affect both not just the right channel?

 

[docs]

Not surprising this issue isn't well documented, it is not easy to resolve.

 

[docs]

The only discrepancy I can find is in q115, wide amp, collector has 9.6v, base is at 6.8v and emitter at 8-9v. Base and emitter are way over voltages. Unable to determine why.
I swapped q115 and q116 to see if issue would move to left channel. It didn't.

 

[hopey]

Does this happen with headphones, each channel has a separate return contact in the socket.

 

[docs]

Yes identical in headphones. I used the workaround of connecting two right channel points together which restores stereo sound in the right channel and works.
They go through the sliders and loudness switch which all checked out when testing but for some reason jumping here fixes the issue. I guess there is an open circuit somewhere along the route but I couldn't find it either on this volume board or the switch board which those wires goto.
Blue wire....

 

[caution]

Yeah there's probably a partial break somewhere. Those routes and resistors printed with silver and carbon inks could crack from flexing and age, and might lose their connection to the bottom side, I think the vias are just painted with silver ink too. You made the right call