Sansui 5000x No Right Channel

[Dahjelle]

Both boards read a steady 82.5 volts on one multimeter and 81.7 on another. Since neither is an expensive multimeter, I assume the readings are reasonable.

It appears from your post that the picture I posted on Sunday at 2:05 PM above has the left and right amp boards labeled backwards. If so, my idea that the capacitor in my original post might be bad is wrong since it is on the right amp board facing the front of the unit. Please confirm that I am understanding your post correctly.

Is there anything else I could easily check?

Thank you.

 

[tom3]

Yes, the driver boards are backwards looking from the front end. Check wiring color from the Right/Left labelled upper fuses to the boards. Might also do a probe to the input point on the driver amp for the dead channel. See picture for the point to check this. (different boards, same input location) Take a small screwdriver, finger on the metal shaft, and touch the points shown in the picture with the receiver turned on. No high voltages here but be careful not to short out anything. Should hear a quiet but distinct hum from either channel when touched. If that left hand board is quiet, then the problem is indeed in the driver amp or output transistors. If you get a hum on both channels equally, the problem is ahead of this.



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[Dahjelle]

Thank you. I apologize for the slow reply. It's a busy time. I checked and double checked the wiring and continuity between the fuses and the amp boards. I am convinced the right amp board is for the right channel facing the front of our receiver. I also touched a small screwdriver to what I believe are the points you indicated in your picture (please see my attached picture). There was a hum in my headphones on the left when I touched the left board and on the right when I touched the right board. What would be the next thing to check? I would be happy to attach more pictures or do anything else that might help diagnose the problem.

 

[archibael]

Hmmm... I'm seeing from the thread that you disconnected the pre-amp jumpers as requested. Did I get that when you drove the amp directly from a source (your phone) the right channel was still dead while the left worked?

Might want to check that again to be sure, but if everything else I'm reading in this thread is right (and you hear a right channel hum when you touch the F6013 input), perhaps there's a connectivity issue between the RCA in on the right channel and that input to the F6013 you just touched.

Do you hear a similar right channel hum when you touch the signal terminal of the RCA jack?

There just aren't that many resources shared between the two driver boards that you'd see one go out when the other had a load (right speaker working when left is unplugged) apart from the DC source, and you have already said that's at ~80VDC on both boards (correct me if I'm wrong on that). I do note that the "late" version of the 5000X service manual shows another set of optional "wired in fuses" in series with the normal set of speaker fuses, and that is sourced from a shared resource to drive TR807/8, but I'm struggling to understand how a problem there would result in the symptoms you're seeing... if one of those fuses is out, you shouldn't be getting any power from the right speaker even if the left is unplugged.

 

[Dahjelle]

I did the test Tom requested using headphones. Now I connected two Sansui SP-30 speakers in case that made a difference.

I tested the phone on another receiver to make sure the podcast was broadcasting on both channels and the cable was good. It was.

I connected the speakers to System A and the phone to Tape 1 Mon. The selector switch to Aux. Tape Monitor Switch 1 pressed. Only the left speaker worked. Pressing Reverse or Mono made no difference. The volume control is much more sensitive than it used to be.

Then I disconnected the left speaker and the right speaker worked.

I was then going to disconnect the pre-amp jumpers, but didn't wait long enough after turning off the receiver. The speakers buzzed when the jumpers were touched. The left much more than the right.

After waiting a few minutes, I disconnected the jumpers and connected the phone to the Main Input. Only the left speaker worked.

When I touched the F-6013 input (blue wire) the right side hummed nicely but the left was really loud. I don't think the difference was that great when I used headphones.

There is a hum coming from the left speaker when nothing is connected and the volume is turned all the way down. The bottom cover is off if that would make a difference.

Both boards read ~80 VDC.

I don't see any wired in fuses, but I don't know what they look like. I believe this unit was manufactured in July 1971.

I don't know what you mean by the "signal terminal of the RCA jack" or which RCA jack you are referring to.

After looking at all of this again it really seems that somehow both the right and left signal is getting routed through the left channel. Is that possible?

Please let me know if I missed anything.

 

[archibael]

I meant the incoming RCA jacks once the jumpers were removed, but it sounds like you did that experiment just now albeit accidentally when you touched the jumpers before the caps were fully discharged.

If there's a difference in the two hums when you touch the driver in, then the driver board or the attached transistors are definitely the culprit, as folks have been saying.

I don't see a signal path on the schematic for both channels to route through the left channel unless someone has been messing around in there and miswired something when they were done, but I believe you said this unit has never been serviced, so that's unlikely.

My 5000X with F6013s is several thousand miles away right now so I can't offer suggestions on specifically which nodes to test for voltage, but that might be one place to start: multimeter base, collector, and emmitter voltages on transistors on both the left and right boards and see where the discrepancy is on the right.

That might be more than you feel up for as you'll have to figure out good probe points from the schematics, but that's where I would go next to troubleshoot further.

 

[Dahjelle]

I just had a thought. Maybe the right side is functioning normally and the left side is putting out several times it’s normal volume. It’s worth a quick test. ... I just discovered that if I set the volume of the phone so the left channel is at a reasonable volume when the receiver is at 1/4 volume and the balance is at full left I can get about the same volume from the right channel with the receiver volume at about 3/4 and the balance at full right. So it’s not that the right channel isn’t working, it’s that the balance is way off.

 

[tom3]

Something you might try, a bit of work though. Switch the output transistor pairs from side to side, see if the problem reverses. They are plug in units but you have to get the big heat sinks off to access them. Could be just a bad output transistor? Do you notice any unusual or uneven heat from those rear heat sinks?

 

[Dahjelle]

I don't know what to expect for the power transistors. I ran the unit for a little over an hour on AM radio with SYS A speakers (only the left side working) at less than 1/8 easy listening volume. I measured the temperatures by placing the temperature probe of my multimeter on each of the 2SC1079 transistors. Facing the front of the unit from left to right I obtained: 92-93, 95-96, 93-94 and 92-93 degrees F. Are these temperature readings reasonable? Is there anything I should have done different for this test?

Out of curiosity I did a quick search for 2SC1079 transistors. The ones in our 5000x have 3 pins but the ones I found have only 2 pins. The service manual says they are 2SC1079 (Y,BL) or 2SC1116 (O,Y). Am I missing something?