Quiet and muddy sound [resolved]

[charleskeough]

The sound coming from both channels of my 2A3 Stereomour is very quiet and muddy. Like a quiet distortion. When I power down the amp it gets loud for less than a second before going silent. During the voltage tests there were some terminals way off and I think this must be where the problem lies but I can't figure out what to do from here. Those terminals were:
 
2          363
14        358

OA        361
OB        355

Any help would be greatly appreciated. Thank you!

 

[Paul Birkeland]

The OA and OB voltages on the C4S board are so far off that the amp would not have passed its voltage checks and shouldn't be used. 

This kind of problem can commonly happen when the center lead of each Q2 transistor is not heated adequately.  Also if the 12AT7 tubes aren't visibly glowing, then there could be an issue with the heater wiring that needs to be tracked down.

 

[charleskeough]

Thanks Paul, I should clarify that this is the original Stereomour. It was working very well since I built it almost 10 years ago! I think over the summer is when it stopped working. I'm just getting back now from being away since late summer and getting this up and running is now a priority.

Should I try and reheat or resolder the center lead of each Q2 transistor? The 12AT7 tube is glowing when the power is on.

 

[Paul Birkeland]

Ah, OK, and old S2!  An exhausted 12AT7 could produce these symptoms, but that would take a whole lot of use.  Using something other than a 12AT7 can flub up those voltages too.

The Kreg voltages would be of interest.

 

[charleskeough]

I do use it a lot daily for records and television, so it might be exhausted. I did buy the 12AT7 thats in there now in 2019. It's a Genalex gold lion 12AT7. I could order a new 12AT7 and see how that goes if you do recommend that.

The Kreg voltages are both at 2.5

 

[Paul Birkeland]

I would give a new 12AT7 a try.  If that isn't it, I would be suspicious of a shorted Q1 or Q2 transistor on the C4S board, or far less likely a not quite 100% connection going to pins 4/5 on the 9 pin socket.

 

[charleskeough]

With a new 12AT7 installed I'm still getting these high voltages. How do I go about diagnosing a shorted Q1 or Q2 transistor? Do I order some new parts and go from there? Thanks again for your help.

 

[Paul Birkeland]

You can set your meter to beep when you touch the probes together, then check each pair of leads coming off each transistor to see if any pair of leads reads as a short.

 

[charleskeough]

My meter doesn't seem to be able to beep. Should I just set it to read ohms and be looking out for a 0 reading?

Do I need to desolder and remove the Q1 and Q2 resisters to do this check or can I check it while it's still installed into the CS4 board?

Lastly when I am doing the check with my meter is the black negative lead supposed to be attached to the ground bus while the red lead makes contact with each of the leads coming off the resisters?

 

[Paul Birkeland]

You can use the resistance setting and look for very low resistance.  You can leave the transistors in the PC board.  Your black probe and red probe are used to check each pair of transistor legs (there are three tests per transistor); the black probe will not be connected to ground. 

https://www.youtube.com/watch?v=YeHGVsk0jKw
Here's a video of someone doing what you need to do.  You don't have to use the diode test though, just the continuity beep or a test for low resistance.

 

[charleskeough]

Thanks Paul, I just ran the test and I was unable to identify a short in any of the transistors. I was wondering if I should order parts for a new CS4 board to build/install and see if that fixes the issue.

 

[Paul Birkeland]

Can you post some build photos of your C4S board?  I would not recommend shooting the parts cannon at this just yet, especially since you have the exact same issue on two completely independent halves of the same PC board.

 

[charleskeough]

Pictures are attached. Let me know if you need other camera angles. I desoldered a couple of wires to be able to get under the board to run the check on the Q1 transistors and the board is still in that state. Let me know if anything looks like it could be the culprit. Thank you for the help

 

[Paul Birkeland]

That looks a lot like either lead free solder or just not enough heat on those joints (maybe both).  This is especially problematic for the center legs of the MJE5731A transistors which connect to the metal tab and sink a lot of heat.  I would reheat all the joints on that board other than the LEDs and add some leaded solder if you've been using lead free solder.  When you heat up the joint, let the iron stay on the joint for 3-5 seconds after the solder flows out.

 

[charleskeough]

Thank you, I just reheated all of the joints on the CS4 board with Leaded solder but unfortunately I'm still getting the same high voltage readings of:
OA 354
OB 350

 

[Paul Birkeland]

You can give a replacement C4S board a shot.  It is unusual that both sides are having the same issue, so maybe hang onto the old board in case you end up with a spare.

 

[Doc B]

Shoot the parts cannon. Love it. This is a great example of the best approach to troubleshooting.

 

[charleskeough]

Im shooting the parts cannon now. I'll report back when Ive got it all installed. Thanks again for the help.

 

[charleskeough]

With the new CS4 board installed everything seems to be in good working order. The sound is loud and clear. Thank you very much Paul for your help troubleshooting. I'm very grateful for this forum!