Stereomour issue - low sound, voltage regulation [solved]

[danox574]

Hello - I have had my Stereomour on the shelf for a couple months after building my BeePre & Paramounts.  Brought it out of retirement and found one channel had lost 50% of it's volume and is very muddy.  It's a 2A3 configured amp.  The problem does not move with the tubes.

All resistance checks out per manual specs.

The voltage checks seem to indicate that the IA/OA side of the voltage board is not regulating.  All four LEDs are lit, but I'm seeing ~380V where I should be seeing ~230V (terminal 2).  This was confirmed with a quick check where IB is 380V and OB is 230V, but on the A side - IA is 380V (actual 394V) and OA is 380V (actual 366V).  Other voltages look good.

I am assuming this means I have a bad Q1 or Q2 on the A side, because looking at the schematic I can't see what else is even there in the C4S circuit.

If so, is there a way to test what's going on in circuit to determine the offending component, or is there something else in the circuit that could be affecting this? 

[Doc B.]

Sounds like the 12AT7 is not conducting try plugging/unplugging from the socket a few times. It could just be that gunk collected on the tube pins from sitting for a while.

[danox574]

Socket has been re-seated several times, and tried another 12AT7 to no avail.  I do notice now that one of the pairs of LEDs on the C4S board is brighter than the other.  Voltage remain the same when I'm wiggling the 12AT7, so I don't feel like I'm getting a connection even intermittently.

Could this be a issue on the socket at pin 1/2/3 or 6/7/8 then, whichever is serving the right channel?  The tube is lit of course.

[Doc B.]

The high voltage reading at OA means the tube is not conducting for some reason. Often this indicates an open connection at the cathode pin - either the tube pin is not making good contact with the socket, or the 431 chip is not conducting because of a bad connection. You might check connections at nine pin socket pin 3 and look over the C4S board around the 431 chips to see if there is a bad solder joint. Might be worth reflowing the solder pads on the board's A side and on the socket too at pins 6,7 and 8.

It's not too likely that a transistor would have blown simply because the amp sat for a while, but a less than perfect connection could  deteriorate with sitting. You can measure resistance between each pair of pins on the transistors and look for shorts to see if they are damaged. Usually they only fail if a circuit is mis-wired or an errant test probe shorts one leg to ground.

[Paul Birkeland]

The MJE5731A also will work hard to not solder well on the center leg, as it connects to the metal tab of the transistor to wick heat away.  It wouldn't be all that surprising if you have a cold solder joint on that leg.

If you want to, you can switch the wires leaving OA and OB to see if the voltages stay the same or swap sides.

-PB

[danox574]

Thank you so much for your suggestions everyone.

Hello - I have removed the C4S board entirely and discovered different measurements on each side of the C4S board.  I went around and reflowed the board first, but I am getting very different results from the MJE5731A on each side.

Putting the black lead of an ohm meter on the tab (collector), on the OB side I get an 'open' reading to the emitter, but I am getting about 600ohms from collector to emitter on the A side.

This strikes me as a failed MJE5731A, looking at the schematic and after a reflow (although I am testing in-circuit) this doesn't seem like any other cold solders on the C4S board could cause it.

Does this sound like a failed MJE5731A?

Thanks, -Dan

[Paul Birkeland]

If you are convinced it's the MJE5731A, then by all means replace it.  The ones that I have seen fail will have a dead short between two pins.

If you flip the board around and put it back in the circuit, you would get another datapoint that would help in assessing the issue at hand.

[danox574]

'Strikes me' and then asking certainly shouldn't imply convinced.  Doc B. said check resistance and since I had found this, I figured I'd just ask if that sounds like a failed transistor to y'all.  Happy to continue troubleshooting of course, but would like to get the transistor ordered tonight if that's a likelihood based on the resistance checks.  Is an opening reading on one and 600ohm on the others in the realm of natural variation?

Hope to get down there to put the board back in and swap the OA/OB tonight.

[danox574]

OA/OB swap moved the issue to the other side of the amp, so I replaced the MJE5731AG and bingo.  All is well.

Why this went bad, I really don't know, but we have a solution.

Thank you for the guidance.