Right channel noisy. Help troubleshooting

[denti alligator]

I’ve had my Stereomour II (w/ shunt regulator) for about 6 years now. Almost no issues. Lately I’ve been so swamped at work that I haven’t had the chance to listen to much music at all. It’s been 2 or maybe even 3 weeks since I turned it on. Today I turned it on and got a scratchiness (slight pops, too) in the right channel and no sound at all in the left channel. I have swapped tubes. No change. Tomorrow I’ll put it on the bench. What should I look for as a possible cause?

 

[Paul Joppa]

The first thing to check is that the input and output cables are connected and the source(s) and speakers are working - it happens more often that you would think! (Just last week I lost a channel; it took me a good hour to determine that one of the banana plugs to the crossover had fallen out of its connector  :^\ -  it should have taken 30 seconds!)

The most common cause is (of course) an unreliable connection. Somewhere. But don't just go looking for one, there are too many possibilities, and it might be something else anyhow. Check the resistances and voltages, which will point to many fewer probable causes.

Post some pictures too. High resolution and good focus helps, since one can then zoom in on individual connections, solor codes, etc. The more information, the faster the diagnosis. It's difficult, but try not to make any premature changes, since they often invalidate the list of symptoms and confuse the thread.

 

[denti alligator]

I checked the sources and speaker connections already. Forgot to mention that. I’ll double check again this morning to be sure. I find it odd that everything was working fine for over 6 years. I didn’t move it, either. Can this really be from a solder joint that went bad after this much time?

 

[denti alligator]

What's the passcode for the manual archive? I have misplaced my digital copy and need to re-download.

 

[denti alligator]

resistance:

1. 1.1m (and rising)
2. 1.1m (and rising)
4. 248.7k
5. 1
6. 1284
7. 3.26m
8. 00.9
9. .303m
10. 8.3k
11. 7.71k
12. .303m
13. 00.8
14. 3.28m
15. 2907
16. .008
17. 248.5k
18. .009
19. 1.1m (and rising)
20. 1.1m (and rising)
21. 1494
22. 1298
24. 1298
25. 1284
26. 1.0
27. 1.03m (and rising)
28. 0.8
29. 0.8
30. 1m (and rising)
31. 1m (and rising)
32. 0.9
33. 0.8
34. 1m (and rising)
35. 0.8
36. 2908
37. 2921
39. 2921
40. 3117

These already show some issues. What other terminals should I check? Should I move on to voltages?

 

[Paul Birkeland]

15. 2907

There's a 2K and a 3K resistor in parallel in this region.  Your 2K resistor is likely loose.

When you post a list of resistances, it's a good idea just to post the ones that don't align with the manual.

What are your voltages?

 

[denti alligator]

When you post a list of resistances, it's a good idea just to post the ones that don't align with the manual.

What are your voltages?

Sorry, I can't locate my manual 

I'll post voltages soon.

 

[Paul Birkeland]

I would deal with the 2K resistor issue before messing with voltages.

 

[denti alligator]

1. 370
2. 389

6. 52.6
7. 241.8

9. 305.3
12. 303.8

14. 301.4
15. 64

19. 394
20. 380

21. 53.2
22. 53.8
24. 53.9
25. 53.6

27. 388
30. 395

31. 397
34. 391
36. 63.7

37. 63.8
39. 64
40. 63.6   

 

[denti alligator]

I would deal with the 2K resistor issue before messing with voltages.

I flowed the terminals on either side of that resistor. Same readings after. Above are the voltages. I can try flowing and adding some solder to 16/36. I nicked the cap on 20/30 in do this, unfortunately. I hope it isn't damaged.

 

[denti alligator]

15 is a bit "dirty." Not sure what this black stuff is...

... cleaned it up with the rubbing alcohol, swipes, and a toothpick.

 

[Paul Birkeland]

Again, terminal 15 (and the others that should read 1.2K) get that resistance from a 2K and 3K resistor that are wired in parallel.  You are seeing a 3K resistance here meaning the 2K resistor isn't connected.  This resistance issue would be the most important thing to resolve at the moment. 

 

[denti alligator]

How would you recommend going about it? Should I de- and re-solder both joints? I have reflowed and added solder twice now to both 16 and 36, top and bottom (where that resistor is connected). Should I try reflowing 15?

 

[Paul Birkeland]

Can you use your meter probes to verify that the 2K resistor is reading 2K?

If the 2K resistor is reading 2K but those terminals read 3K, the 2K resistor is not properly connected.

Looking at your solder joints, more heat and a clean tip would be helpful.

 

[denti alligator]

Can you use your meter probes to verify that the 2K resistor is reading 2K?

If the 2K resistor is reading 2K but those terminals read 3K, the 2K resistor is not properly connected.

Looking at your solder joints, more heat and a clean tip would be helpful.

That resistor is reading 2909. Does this mean it’s damaged?

 

[Paul Birkeland]

Why don't you heat up the solder joint that holds one end in and pop one lead out, then measure the resistor that way.

 

[denti alligator]

I’m not getting a reading at all now (having detached one end).

 

[Paul Birkeland]

OK, I would replace that part then.  One possible reason this could happen is inserting the 2A3 tube on that side without the fat pins on the tube aligned with the fat pins on the socket, which could allow a direct connection between B+ and the cathode resistors, which would also possibly cause some swelling of the top of the 220uF cap mounted in that area.  Otherwise it would be very abnormal for a part like that to just randomly open up.

 

[denti alligator]

I haven’t switched out (let alone taken out) the tubes in over a year. And it was working fine just a few weeks ago. Strange. Does it matter what brand I use? I can get some TOUHIA ones on amazon by Tuesday, which would be nice.
https://www.amazon.com/TOUHIA-Wirewound-Ceramic-Resistor-Inductionless/dp/B07VRBRTVX/ref=sr_1_3?crid=2ZG6I62U7OBAR&keywords=Resistor+2k+5w&qid=1639934974&sprefix=resistor+2k+5w%2Caps%2C84&sr=8-3

 

[Paul Birkeland]

Sure, those will work.  If the leads don't reach the terminal strips you can just wrap them around the remaining 3K resistor.

The brand isn't so important because the AC signal current is almost completely handled by the cathode bypass cap.