Voltage problem with new stereomour [solved]

[ffivaz]

Hi,

I finished building my stereomour. All resistance check. I ran into a problem with the voltages around the C4S PCB Board, B side.

IA 387V (380V)
IB 387V (380V)
OA 218V (165-220V)
OB 343V (165-220V). And the problem follows with 343V on terminal 14, the metal plate of MJE 5731 is also 343V.

B Kreg 2.5V
A Kreg 2.5V

-Reg is 0 on both sides

C1 59V (60V)
C2 358V (350V)
C3 0V (0V)
C4 59V (60V)

A side is similar

HV+ is 387V. The rest is close to the specs.

Could you help me ?

[Paul Birkeland]

OB 343V (165-220V).

Try reheating the center leg of the MJE 5731A, as this joint likes to wick away heat.

[ffivaz]

I tried reheating every joint, especially that center leg. Voltages are still the same. Could the MJE be broken ?

[Paul Birkeland]

I tried reheating every joint, especially that center leg. Voltages are still the same. Could the MJE be broken ?

It could be.  To check it, use your meter to test for continuity between each pair of legs on the transistor.  If you don't get 0 Ohms across any pair of legs, the transistor is OK.

The lack of voltage drop is very common from that middle leg not being all the way soldered, from the bA/bB jumper not being solidly connected (though I think it is in your amp), or from a backwards transistor.

You can feel free to pop up a pic of your PC board and we can be your second set of eyes.

-PB

[Paul Joppa]

I see some drop across the B side (the technical term is "compliance") IB=387, OB=343, compliance=44 volts. And the bias is right, 2.5v at kreg both sides. So I suspect the emission of the 12AT7 is not up to snuff. Often this will repair itself as the tube burns in over the first 50-100 hours. If not, ping the spare parts email address and ask for a new tube.

All the other voltages look great.

[ffivaz]

Before seing Paul's RE, I desoldered all connections to the board, reflowed all joints and resoldered the board. The voltage are still the same. I'll use the amp for a couple of days and see if it settles down.

BTW, the amp sounds beautiful without any burn in of the tubes. Thanks for your help and thank for another beautiful BH product!

[Mike B]

If it sounds good, it works.

Is it balanced?  Same SPL out of both channels?

[ffivaz]

I have to test that. I'll report!
Thanks for the tip
Fabien