SONY STR-7035 - Dead Right Channel

[JB8]

After cleaning pots and pushbuttons thoroughly (and multiple times), I'm still not getting any sound from the right channel via headphones or speaker outs. Cleaning the volume/balance pot increased the output of the good channel significantly, but the right channel is still dead. Right channel will only make a tiny static pop when toggling the speaker selector switch. If I push the balance hard right and turn it up I get a very faint signal, but still only out of the left channel speaker, so a little bleed. In looking at the input RCA's on the back panel I suspect this unit was exposed to smoke, there's some gummy residue present.

(When I got the unit several months ago it had no power cord and a few lights were out, so I'm just now tackling the bad channel)

I've started probing and checking voltages, etc., including the STK amp mods where I'm getting some significant variances across the two channels. The left channel matches the schematics fairly closely, but the right channel is producing some wonky numbers. I can provide the voltages if it will be helpful.

Two questions:

1) Should I assume the wonky STK-056 numbers indicate a bad amp module, or could a bad cap or transistor in the PS or amp section be contributing to the numbers?
2) If the consensus is a bad amp mod, there are several replacements available, primarily on the big 3rd party auction site. While I'd love to find a SONY replacement, are there any amp mod brands I should stay away from?

This unit was pretty cheap, it's super dirty still and perhaps not worth further investment, but I'd love to give it a second life. Any assistance here is much appreciated, as always!

 

[Binkman]

Any assistance here is much appreciated, as always!

Haven't looked at the SM but generally 1st check is output transistors possibly their driver transistors, too.
Throw in cold solder joint check and any fusible type resistors.

https://www.hifiengine.com/manual_library/sony/str-7035.shtml

 

[50 Green]

Don't overlook a thorough cleaning of your function switch. The actual switching is done by sliders on the board, which are challenging to get deoxit into the right location. Finding a hole or other access, then tipping the receiver whichever way is needed to get the deoxit to flow to the contacts is the best way I've found. You can get an idea of the contacts' locations by the pins on the bottom of the board. Once applied, work the switch well. I follow the deoxit with a little faderlube.

The speaker switch is the same type of slider. It's worthwhile to give both a good cleaning before going after other possible causes.

You don't need Sony branded STKs , but almost everything you're going to find on that auction site will be a fake.

 

[JB8]

Thank you both for your suggestions here, much appreciated!

I've checked voltages on nearly every transistor and haven't found anything totally out of whack or shorted. The thing is littered with 2SC631's and 632's, so if and when I've been on the fence I've swapped in 1845's, but of course none of the changes got my right channel to sing, yet. So far I've replaced Q503, 504, 553, 554.

The function and speaker switch cleaning was a brilliant idea and something I hadn't dug in on, so thank you for pointing me in the direction of those very unique slider switches. I've given them both a dose of D5 and didn't get a hint of right channel improvement even after exercising the heck out of the switches.

I am, however, still intrigued by the vol/bal alps pot, it only has two obvious holes to squirt the deoxit into, but the center sections appear almost sealed. Before any cleaning of the pots at all I had no right channel and a super weak left, but after cleaning what I could of the volume knob the left channel is now significantly stronger. This is to say, feels like I've only really cleaned half of the switch since its such a fortress. Anything I'm missing? Should I just pull the switch and see if there are ports on the bottom?

I've included pics of the pot, which transistors have been replaced (circled in red) and voltages on the STK's (in blue). Green arrow points to the primary suspect but I'm not smart enough to connect the dots and pinpoint the actual issue.

 

[JB8]

Any ideas on what could be causing such low voltage at pin 4 on the right channel amp mod? I know I mentioned trying to better clean the vol/bal pot, but the voltage discrepancy on the module has got to be contributing to the issue. If pins 1 and 3 are close to spec and similar across both channels, but 4 on the right channel is out of whack, is it safe to assume the module is getting proper power and that it's likely failed? Or is there some other source that would regulate or provide the 12V at that test point that I'm overlooking?

@dr*audio , I know you're pretty darn savvy with STK mods as well, any thoughts on this wonky voltage on pin 4?

Again, apologies on my lame signal tracing, still lots to learn!

 

[JB8]

Keepin it going here! I pulled the faceplate then removed the vol/bal pot/housing, super easy access on the side. Cleaned thoroughly, no change.

 

[50 Green]

Keepin it going here! I pulled the faceplate then removed the vol/bal pot/housing, super easy access on the side. Cleaned thoroughly, no change.

View attachment 2516960

I was going to suggest giving that a try. Too bad it made no change.
Signs seem to be pointing toward your STK module.

Have you checked R653 and C654 / C655? is there continuity across CB651? Across the parallel inductor / resistor?

If all of those check out OK, I'd suspect the STK. You could always try swapping the STKs, but if the right one was taken out by some other fault, you risk killing the other one (as well as risk of damage from desoldering / soldering it.)

There are some STK-056 modules currently listed various places online which may be originals. (I'd avoid any from sellers in China.) You're not likely to find anything that's both cheap and real, so it's up to you to determine whether it's worth it to you to proceed with a replacement.

 

[JB8]

Thank you, @50 Green , I appreciate the follow up!

I fear I'm now past the point of no return with this unit. That's right, instead of getting the right channel working properly I decided to pull and clean the brightwork and replace the lights - that was a bad idea. The way SONY built out the faceplate, dial glass, etc. I found to be very intuitive and the look is fantastic. My garage receiver is an old HP-610A found at GW for next to nothing, but I'd never had to pull it apart for any repairs, so this 7035 has now pulled me in and I'm still keen on bringing it back to life.

Have you checked R653 and C654 / C655? is there continuity across CB651? Across the parallel inductor / resistor?

Yessir, I did check these and found no issues. Initially was getting an OL reading across C654 when testing in chassis, but when I pulled it the testing indicated it was fine. I was hoping I'd found the tiny culprit, but sadly I think I'll have to try the STK swap now and will report back once I have an update. Thanks again!

 

[JB8]

Hey all, hopping back in on this guy. Actually purchased a PS from France via that huge auction site, was amazed that the item showed up in one week, faster than some other products I've purchased here in the states, wild.

At any rate, my plan was to just pull one of the STK mods and install swap it into my unit, but now I'm thinking, should I just install the entire unit instead and take my chances? Frankly the new unit looks cleaner than mine, but I'll defer to your experience on the best approach. I figure if between the two units I can end up with a working receiver I can hoard away the other STK if it's working fine. Thanks in advance for your advice!

Here are some pics as well for reference:

OLD


NEW

 

[JB8]

Never mind, I went ahead and installed the new power supply, works great!

Thanks for the assist on this thread, always appreciated!