Stereomour II - Issue with Shunt Regulator?

[lskiii]

Hello

So, I'm having an issue that is isolated to just the left channel of my Stereomour II, and I'm wondering if it might be my shunt regulator...

To start - I have a Steremour II w/ all upgrades (Shunt, DC Filament and attenuator) and it normally sounds great.  Today however I noticed that the left channel seemed very faint.

Things I tried:

Swapped tubes (no difference)
Directly connected one speaker to the left, then unplugged and plugged it into the right (no difference)
Directly connected my DAC to the amp and tried different RCA cables in Left/Right (no difference)

After none of that worked, I felt I'd narrowed it down to the amp so I did a resistance check, voltage check and looked over everything - it all seemed normal.

Then, I did the tests for the DC Filament (checked out) but the Shunt Regulator tests came back off (and asymetrical).

My readings:

IA 301
OA 227
KREG 1.15

IB 392
OB 338
KREGB 4

So, my hunch is that's the problem, but I'm not sure where to go from here.  Any suggestions?

One other clue - I've noticed that when I shut down the amp while the left channel is hooked up, there's a sudden burst of volume right as it's shutting down.  It's almost like it momentarily pops up to the proper volume as the power is being cut.

Thank you!
Louis

[Paul Birkeland]

Can you give IB and OB on the small PC board as well?

[lskiii]

IB = 391

OB = 339

and, in case it helps:

IA = 301

OA = 209

Thanks!

[Paul Birkeland]

So IA on the big board is way out of whack.  It could be the SR board, or it could be something else.  On the bigger PC board, heat up the red wire that feeds IA, then slip it out and leave it poking straight up, then carefully measure the DC voltage on that wire with the amp running. 

If it pops right up to 390-ish volts, we will need to debug half of your SR board.

If it stays at 301V, there is something else wrong with the rest of your amp.

Can you also measure and let me know what the R2 resistor value is on the large PC board?

[lskiii]

So, the red wire, once disconnected, measured 400, so I guess that helps to isolate things.

I'm going to have to re-connect it right now and will test R2...

Thanks!

[lskiii]

OK - I reassembled everything and tested the 2 resistors at R2 on the big board.  The one on the "A" side tested 250.  However, the one on the B side showed no resistance to ground.    Also, the LEDs lit up when I touched the other side of the resistor.

[lskiii]

One other thing - w/ power off, tested resistance across both of the R2s and they both registered 222. 

[Paul Birkeland]

Yes, when measuring resistance you need to have the amp powered off.

If the LEDs are reacting to you poking around, often that means there's a flaky connection or a broken wire. 

[lskiii]

Got it.

So, powered off, the R2 measurement was OK from what I can tell.  Anything else that I can do to troubleshoot?

[Paul Birkeland]

What I would do next is to put the wire you removed back, then remove the wires going to IA and IB on the smaller PC board.  You can just leave them poking up next to the board.  Then fire up the amp and see if your OA voltage pops back up. 

That would be the final test in my mind to narrow down which piece of the amp is giving you problems.

[lskiii]

OK - So, I've disconnected the 2 red wires that go to IA and IB on the small board.

 Now, here's what I'm reading:

Small Board
OB 852
OA 530
IB 700
IA 700

Large Board
OB 300
OA 372
IB. 398
IA. 1.3

[Paul Birkeland]

Large Board
OB 300
OA 372
IB. 398
IA. 1.3

Can you check again?  IA cannot be lower than OA.  How about the Kreg voltages?

[lskiii]

Sorry about that.

I just redid the test - here are the correct #s:

Large Board
OB 300
OA 402
IB. 401
IA. 404

KREG A 1.2
KREG B 2.5

Thanks!

[Paul Birkeland]

OK, so what needs to be worked out now is why the shunt regulator isn't working properly.  1.2V of bias and the 400V on OA would mean about 30mA of current being sucked down by the 12AT7, but that isn't happening because your IA voltage would drop like a stone in the process. 

In the stock build there's a 220 ohm resistor that goes from the center post of each 9 pin socket to pin 7 or 2 depending on which socket you're looking at.  I would be checking those to ensure that this part isn't loose.  I would also make extra sure that both halves of the 12AT7 on the problematic side are glowing.

It would also be a decent idea to post some photos of your build. 

[lskiii]

Hi,

I've tried a couple of times to upload photos, but I get a website error.  Perhaps the files are too large?

Anyway, I have tested the resistor that goes from the center post of each 12AT7 socket.  Both register 217 ohm from the center post to pin 7 or 2. 

In case it helps, my hunch has been that the issue lies somewhere in the 9 pin socket for the left channel pre-tube.  The reason I think that is that it's the only part of the amp that I've messed with.  The shunt has been installed for over a year now and I've never had any problems.  The left channel, however, has developed a hum and in trying to trouble shoot it I did try to create more space between the various connections that go to that socket.  Perhaps that did something?

I will try again to post the photos.

Thanks!