Problems with Stereomour II Shunt Regulator

[Scott AA6U]

I am having problem with the Stereomour II Shunt Regulator board that I assembled.  Here are results of the voltage tests:

IA 391vdc (should be 365-410)
OA 173vdc (should be 300-305)
KregA 12.2vdc (should be 2.5-5)
IB 392vdc (should be 365-410)
OB 390vdc (should be 300-305)
KregB 1.9vdc (should be 2.5-5)

The two LEDs nearer the "A" on the board light, but two LEDs nearer the "B" on the board do NOT light.

The first time I tested it, I got the above voltages. Then I removed the board, reflowed all solder connections so solder could be seen on the top side of the board, checked that all wiring is correct (it was) and carefully reconnected it. The second test gave me the same incorrect voltages.

Note that when the amp was first assembled, the resistance, voltage and operational tests were done on the amp were done, and all were correct, so the problem shouldn't be on the amp.

I need troubleshooting advice because I have no idea what to do next. I'm a tube guy, not a solid state guy. Thanks!

[Paul Birkeland]

I'd be interested in seeing the bottom of the PC board and the socket wiring in general. 

Also double check that the PN2907 and 431 regulators aren't swapped.  (same for the wires that go to Kreg and the red wires leaving that PC board feeding the driver stage)

[Scott AA6U]

I checked the Kreg and the red wires coming from the board and verified that they are connected correctly.  I also verified that the PN2907As and LM431As are in the correct places and oriented correctly. Attached are photos of the back side of the board and the sockets. Let me know if you need any more info and what you think the problem is. Thanks so much.

[Paul Birkeland]

If you have an adjustable soldering iron/station, I'd turn the heat all the way up.  If your solder doesn't have lead in it, you'll want to get some that does.  For any solder joint other than the LEDs, I would recommend reheating them and allowing them to flow out.


You can also set your meter to beep when you touch the probes together, then check each pair of legs on each transistor on that board to be sure none of them are shorted out. 

[Scott AA6U]

Per your input, here is what I did:

1. Reflowed solder on every connection on the board. I always use lead solder with rosin.
2. Checked all transistor/regulator leads with an HP34401A multimeter and nothing is shorted.
3. Checked and confirmed that all wires going to the shunt board are connected to the correct places on the amp.

I reinstalled the shunt board and checked the voltages. No change from before and the LEDs on the B side still do not light.

I have no idea where to go from here. Please advise. Thanks.

[Paul Birkeland]

Can you confirm that the amp was operational without this upgrade installed?  There is definitely a way this could happen if the rest of the amp had a fault.

[Scott AA6U]

It was fully operational before I installed the Shunt Regulator. All resistance checks and voltage checks were correct. I played it for an hour to test it and it worked great.

[Paul Birkeland]

What voltage do you see on the X pads on the regulator board?

[Scott AA6U]

Voltages measured on the X pads of the shunt regulator board:

A = 2.58vdc
B = 1.41vdc

[Paul Birkeland]

We will focus on making the A side work first.  The conditions I see on that side are sufficient voltage available for the 431 regulator to work, but no operating current.  This is what I would expect from solder joint issues, an R1 on the regulator board that's way off on its value, or a broken jumper wire. 

The B side has (likely) a shorted 431 regulator and that needs to come out and be replaced as the next step of debugging on that side. 

[Scott AA6U]

Both R1 resistors measure 65 ohms in-circuit.

I double-checked all jumper wires on the board to be sure they are correctly connected and they are.

I re-flowed solder on all jumper wires although the connections seemed to be good.

I checked carefully for solder bridges throughout the board and found none.

I powered it up and all voltages remain the same as before.

[Scott AA6U]

I talked to the guy I'm building the amp for and he said to forget proceeding with troubleshooting the shunt board because it's taking too long. I will uninstall it and reverse the other wiring. The amp sounds great without it.