Low Voltage on OA (Resolved)

jimjitsu · 1328

0 Members and 1 Guest are viewing this topic.

Offline jimjitsu

  • Newbie
  • *
    • Posts: 5
on: February 14, 2018, 08:45:08 PM
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.
« Last Edit: February 16, 2018, 04:07:00 AM by jimjitsu »



Offline Paul Birkeland

  • Global Moderator
  • Hero Member
  • *****
    • Posts: 19694
Reply #1 on: February 15, 2018, 03:59:34 AM
I see solder wicked up onto the top side of the board on the PN2907, is that solder connecting two legs of the PN2907?

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline jimjitsu

  • Newbie
  • *
    • Posts: 5
Reply #2 on: February 15, 2018, 12:10:55 PM
That may have just been a bad angle for the photo. Here are a couple other photos that might give a better view.



Offline Paul Birkeland

  • Global Moderator
  • Hero Member
  • *****
    • Posts: 19694
Reply #3 on: February 15, 2018, 07:04:04 PM
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.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline jimjitsu

  • Newbie
  • *
    • Posts: 5
Reply #4 on: February 16, 2018, 12:31:46 AM
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.
« Last Edit: February 16, 2018, 01:37:33 AM by jimjitsu »



Offline jimjitsu

  • Newbie
  • *
    • Posts: 5
Reply #5 on: February 16, 2018, 03:15:41 AM
I just noticed one of the LEDs on my 12AU7 socket is not lit (center to A8). What would that indicate?
« Last Edit: February 16, 2018, 03:37:30 AM by jimjitsu »



Offline jimjitsu

  • Newbie
  • *
    • Posts: 5
Reply #6 on: February 16, 2018, 04:06:25 AM
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!