Stereomour - dead channel

[Stalemate]

Greetings,
I've had a Stereomour for a few years now, but after the right channel has been weaker than the left for almost the whole time, it now went dead.

I've checked my soldering, and replaced suspicious reading capacitors and resistors, but I'm not able to figure it out. The resistance-tests are within normal range, but the Voltage check on the right channel shows 4V on the first terminal, and I can not for the life of me figure out what the issue might be. The left channel is just as fine as it ever was.
I'm having a theory that it might be the Output transformer that's gone, since I can't get any good reading on either 1 or 2, towards either groundpoint. Also the PCB is dead on one channel and if I connect terminal 1 and 20 (the coresponding on the left channel) it lights up the right side as well.
The tubes are glowing, however the left one gets much warmer than the right.

Any Ideas on how to troubleshoot this further? The OT is a bit of a black box for me, but are there any other tests than swapping sides? ( I dread the job)

thanks
Vegar

 

[Deke609]

... the Voltage check on the right channel shows 4V on the first terminal ...

You are measuring 4V DC on terminal 1 (near the A/right side 4-pin socket) of a Stereomour II?  What do do you get at terminal 20?

If your measurement of 4VDC at terminal 1 is correct, then you've got no power going to the right-side output tube -- so I think there's something wrong with the power supply or possibly plate choke wiring, not the OT which is later in the chain. 

To help pin down where the power supply problem is, and rule out the plate choke misbehaving, I'd take a DC voltage measurement at terminal 27 -- that's the last terminal of the A/right side B+ power supply before being sent to the plate choke and then on to terminal 1 and the output tube anode.

If the voltage measurement at 27 is really low, then the problem is in the right-side power supply (if you look at the schematic in the manual you will see both a channel A power supply and a channel B power supply -- one feeds the right side and the other feeds the left). Since your left left side works fine, it's clear that the power transformer, UF4007 reciifier diodes and the 220uF caps are all fine, b/c these things supply both B+ channels (this is also indicated on the schematic). So, if terminal 27 is really low, that narrows down the problem to the wiring of 4 components: the two 130 ohm resistors and the two 100 uF caps on the A/right side.

I hope this helps.

cheers, Derek

 

[Paul Birkeland]

I've checked my soldering, and replaced suspicious reading capacitors and resistors, but I'm not able to figure it out. The resistance-tests are within normal range, but the Voltage check on the right channel shows 4V on the first terminal,

What are the voltages on each pin of the 4 pin socket on the dead channel?

You absolutely do not have a dead output transformer.  It could be failed as a dead short or completely open and it wouldn't change the DC voltages in the amp.

 

[Stalemate]

You are measuring 4V DC on terminal 1 (near the A/right side 4-pin socket) of a Stereomour II?  What do do you get at terminal 20?

406 V, that's actually a bit on the high side..? Strange.

To help pin down where the power supply problem is, and rule out the plate choke misbehaving, I'd take a DC voltage measurement at terminal 27 -- that's the last terminal of the A/right side B+ power supply before being sent to the plate choke and then on to terminal 1 and the output tube anode.
If the voltage measurement at 27 is really low, then the problem is in the right-side power supply (if you look at the schematic in the manual you will see both a channel A power supply and a channel B power supply -- one feeds the right side and the other feeds the left). Since your left left side works fine, it's clear that the power transformer, UF4007 reciifier diodes and the 220uF caps are all fine, b/c these things supply both B+ channels (this is also indicated on the schematic). So, if terminal 27 is really low, that narrows down the problem to the wiring of 4 components: the two 130 ohm resistors and the two 100 uF caps on the A/right side.

You're right, the 27 measures 2 V
I tested the capacitors with my multimeter, and see that they charge alright. however, hats off to you, the 130 between 27 and 30 is dead.

You absolutely do not have a dead output transformer.  It could be failed as a dead short or completely open and it wouldn't change the DC voltages in the amp.

Yeap, you're absolutely right, I was not thinking correctly, got a bit too stuck on following wires.

Thank you guys, I'll try and replace the 130 ohm, see if it helps, I'll post results.

humble regards
Vegar

 

[Stalemate]

It's alive!! ;D

however still with a small difference between right and left, I have to go maybe 20% to even it out on the balance. Anyone with any ideas on that one? There's no noises, I was maybe thinking about looking at the pot-meters.

thanks
v

 

[Deke609]

PB's the expert, I'm just a novice taking some shots in the dark here.

If you swap left and right output tubes, does the imbalance also switch sides?

If not, do the voltages of terminals 1 and 20 now match?

And regardless of the above, I think it's kinda strange that the 130 ohm resistor failed. Did it look burnt/cooked?  Since you mentioned having issues with the right channel for a long while before it finally went dead, I wonder whether there's something still wrong that killed the original resistor.

cheers, Derek

 

[Paul Birkeland]

Can you post the DC voltages on both 4 pin sockets?

 

[Stalemate]

Can you post the DC voltages on both 4 pin sockets?

Hi,

Starting at "1 o clock"
R: 65, 390, 0, 65
L: 65, 390, 0, 65

So as far as I can see, they are identical

I tried swapping the tubes, but it stays the same imbalance
The readings are now identical 388 - 419 on port 1-2 and 19-20
Thanks
Vegar

 

[Paul Birkeland]

If you use your phone as a source and use the volume control on the phone, then turn the volume control on the Stereomour all the way up, is the imbalance still present?

 

[Stalemate]

Yes it is, along all 3 inputs.
However, sometimes it crackles a fair bit, but tapping the potmeters, both balance and volume helps, which makes me suspicious. What do you think?

thanks
Vegar

 

[Paul Birkeland]

If tapping on the chassis or the pots makes a difference, you have a loose solder joint that's causing your problem.

 

[Stalemate]

I was afraid you were gonna say that... do you know which one? :-[

Joking aside:I'm trying to check the resistances, and there's something that gave me pause: 30 and 31 are both connected to 44 with identical resistors, but they measure different; 83 and 120 (R/L), is that to be expected?

thanks

Vegar

 

[Paul Birkeland]

If I had your amp here for repair, I would just do them all.

 

[Stalemate]

If I had your amp here for repair, I would just do them all.

I see your point, it's been running for a few years, soI guess it's to be expected. I'll just make a pot of coffee and get cracking.

Thanks for your patience, I thought the 30 and 31 to 44 was a bit of a odd reading, but I take it that's normal then.

cheers
Vegar

 

[Paul Birkeland]

You are checking resistance from ground and you get 130 ohms and 83 ohms?  That would tend to indicate an issue that would blow the mains fuse almost instantly.

 

[Stalemate]

No, I checked from point to point 30 - 44 and 31 -44

I went over it and resoldered where I saw the joints were "skinny" and or grey, and now it works a lot better, I wouldn't say that the balance is perfect, but it's getting closer for sure.

 

[Paul Birkeland]

Well, you could replace those resistors with wire and it wouldn't influence the channel balance.