REALISTIC STA-78 Mono/Stereo Puzzle!

[JB8]

Hey there, thanks in advance for any advice on this issue, it's been a real head scratcher!

Unit originally had a separate thread due to blowing fuses, previous owner had bumped up a ton of cap values and removed the phono pre-amp for some reason, so I restored most of the cap values to close to original, but, there's a persisting issue with the mono/stereo functions. Here's where we're at:

• When listening via any function, with none of the switches engaged, the sound is mono - engaging the Mono button only changes the sound very slightly, but still mono
• When listening via any function, if I engage the loudness button I get a full, more expected spread along with stereo signal. I've used test tracks via the aux port to confirm separation between R/L yet it still feels like the stereo sound field is not as it has been on the ~8 other 78's I've had on the bench
  
• If, when Loudness button is engaged, I engage the mono button, the signal goes to mono
• With only the left AUX channel being fed with a test tone from my phone, switch in the stereo position, the signal split the uprights, right down the middle. If I pan left and right the signal stays the same, centered, engaging the Mono switch didn't change anything spatially but the signal got louder. If I engaged the Loudness switch the signal moved to the left channel only with a little bleed into the right
• The wiring on the switch PCB appears to be original, nothing altered
• I've cleaned the pots multiple times with D5, cleaned the pushbuttons as well plus using a bit of F5, it's pulling in stations nicely, stereo beacon lights appropriately, you can hear the signal get fuller an wider when the light comes on and you've landed on a strong stereo signal, yet the sound is just a wider, fuller version of mono
• With the phono preamp removed the previous owner just wired the leads coming from Ss-1 and Ss-2 directly to the outs, so it's essentially a second line-level input now. With the 33V at pin 13 no longer heading to the phono pre-amp I swapped in a 2K ohm resistor at R706 to protect D705 based on advice from ecluser who's been a huge help on this unit thus far, much appreciated!

At this stage, since the Mono switch seems to work when the Loudness is engaged can we assume it's not the Mono switch? Is there a way to bypass the Loudness switch to test the stereo separation before it enters the switch matrix? Any advice would be great, happy to test, probe, clean, jumper, remove, photograph, etc.

 

[Semanon]

My guess is that the previous owner has tampered with the wires going to or from the switch PCB. My first move would be to verify that the wires still match the schematic.

 

[Lavane]

The wire wraps look intact at the board itself. But I agree with the above. Trace every wire connected to that switch PCB and make sure they terminate at the right point at the other end. A PITA, but worth the trouble.

 

[JB8]

Thanks for hopping in, Semanon.

I've checked continuity and visually inspected them (all connections at the switch PCB), all looks as it should against the schematic. That said, is there a specific wiring manipulation that could lead to this condition? Does the absence of the phono board play into the diagnosis? Feels like that was the biggest disruption, would be great to eliminate it as a possible culprit.

I'll pop back in and double-check every wire, good times, ha!

 

[Lavane]

I wouldn't think there would be a problem with the phono board being missing or wired wrong unless it was switched to phono. Unless there was something weird done with the wiring connecting to the selector switch itself. I guess that's another area to check. Its a lot more wired to check unfortunately.

 

[JB8]

Traced all of them, no luck. All the pigtails look good throughout the board and on the other ends of each lead.

 

[Lavane]

I'm thinking something between the output of the input selector and and where it connects to the preamp stage. Its an odd one for sure.

 

[Semanon]

Never mind. Wrong interpretation of the schematic.

What happens using the tape in switch. That bypasses the selector.

 

[JB8]

Totally stumped, what a shame.

I've traced every wire I can get my hands on, I've had the schematic out for a week now, pouring over each scenario and I've come up empty. No proper stereo, no bueno.

If anyone comes up with any additional troubleshooting idea's please send them my way, would love to fix this guy.

 

[Lavane]

Never mind. Wrong interpretation of the schematic.

What happens using the tape in switch. That bypasses the selector.

Thats a thought.

 

[JB8]

Very interesting! Here are my test results:

• Feed a tone into right side of Tape In, engage Tape Mon switch, I get a center signal that can be panned L/R - if I then engage the Loudness button the signal moves to the right, but not hard right
• If I feed a L/R audio test from youtube into both channels of Tape In, with Tape Mon switch engaged, I get a very center signal. If I engage Loudness I get that wider spread but it doesn't sound like true stereo, BUT, if I then engage the Mono switch the signal is then distorted, which is a new discovery
• Another observation is that with the test tone (using 500 Hz) fed into one channel, when I engage the Loudness switch, the signal morphs a bit, sounds like it's being phased, changes frequency some, feels like a more complex frequency signal and not just the pure 500 Hz signal moving in the field
• Lastly, when listening to a strong FM station with no buttons engaged the sound is clear but mono of course, when engaging Loudness the signal spreads to fill the left/right field, low end is fuller, but it also adds a tone of high frequency content as well, noise

Again, great suggestion, feels like progress!

 

[Lavane]

Either one or more of the switches is acting up maybe. Or there is some wire connections that just aren't right. You said the wiring looks all untouched though. Time to test the switches. Maybe use an ohm meter and see if your getting anything bleeding from one side of the switch to the other?

 

[JB8]

It's fixed!

Seems my wire tracing skills are not great, missed a very clear issue in spite of all the testing. After feeling stumped I got out my "donor" unit so I could compare the units side by side, and boom, there it was, plain as day and something all of you had me chasing from the get go - the former owner had crossed the left/right leads at the volume pot... Pics show the donor unit in pic #1, current unit in pics 2 and 3. Such a dumb oversight but I'm learning to be much more diligent, apologies for the extra time this took, totally an oversight on my part.

Switched the wires back and it's as it should be - great stereo separation, mono and loudness switches working perfectly. Thanks again to Lavane, Eclucer and Semanon, really appreciate the support!

 

[Lavane]

Awesome! It can be a real PITA to have to repair all the "repairs" others have done. Now you can relax and ennoy it.
A Marantz 1060 I bought cheap that came to me in chunks and pieces with wrong parts installed, cut wiring, pulled boards, etc., was fun. I spent several years off and on repairing a Sony power amplifier. I recently pulled it back out and got it straightened out once and for all. Same thing, someone replaced resistors with random values, wrong signal transistors wrecked traces, etc. The new owner likes it.

 

[JB8]

Thanks, Lavane! Yes, quite a relief after feeling like the issue was nearly unsolvable, happy to be learning along the way as well. I'd had that SX-440 that Markthefixer was helping me on but we were struggling, so to get this one out of the way is great, very happy.

I've always been impressed with these STA-78's, and they're dead quiet at idle and have a fantastic sound after a recap, so listening for a few hours last night was pretty cathartic.

Taking a couple days off but there's a long line of units behind this one, I'm sure I'll be back, ha! Love this stuff!

 

[Lavane]

It's a good looking receiver. It is never ending though. I just finished building a simple single stage tube preamp, and just before that built a Fender F51 tube head guitar head amp for my son. Next up is my 300B amplifier. I want to build a different wood base and change the color of the plate chassis for it. I finely finished all my SS stuff though.