Low Voltage on OA (Resolved)

[jimjitsu]

Hello,

I recently upgraded my potentiometer to a DACT CT2-2. My amp sounds fine but after the upgrade I noticed that OA on my Speedball board gets a low reading. It's possible this issue existed before the DACT install.

Small PCB
OA: 50V
OB: 78V
1A: 176V
1B: 176V

Large PCB
OA: 70V
OB: 90V
G: 0
B+: 176V

Where should I start looking for problems? I've tried to spot any shorts on the PC boards and reflown a few suspect solders joints but no luck.

[Paul Birkeland]

I see solder wicked up onto the top side of the board on the PN2907, is that solder connecting two legs of the PN2907?

[jimjitsu]

That may have just been a bad angle for the photo. Here are a couple other photos that might give a better view.

[Paul Birkeland]

I'd remove the 6080 and then recheck the small board voltages (large board voltages will be off).

I would also reheat the solder joints on the center legs of both MJE350 transistors.

[jimjitsu]

Okay, I reheated the two middle joints and retested without the 6080. New measurements are:

OA: 50V
OB: 70V
1A: 249V
1B: 249V

I noticed that right after powering on, OA gives a reading of around 70V but it quickly drops to 48-50V within around 15 seconds. I tried replacing my 12AU7 tube but that made no difference in the reading.

[jimjitsu]

I just noticed one of the LEDs on my 12AU7 socket is not lit (center to A8). What would that indicate?

[jimjitsu]

Got it! Removed the LED between the center pin and A8
 Connected a cable from A3 to A8. Now it passes all voltage checks perfectly. Thank you for the help!