Speedball Troubleshooting - High OA/OB values

[Justin]

Hi, forum. I'm currently putting a Speedball together and ran across a snag I'm hoping to get some help with. I previously built an tested the Crack which worked well, was in spec and sounded impressive. I installed the small board for the Speedball and after some troubleshooting I realized I installed the MJE350s the wrong way. Before fixing this I was getting readings of:
OA - 0
1A - 202
B-A/B - 0
1B - 203
OB - 0

After correcting the orientation of the MJEs I remeasured and am currently at:

OA - 144
1A - 162
B-A/B - 0
1B - 162
OB - 137

All LEDs light up. When I follow the diagnostic flow chart I'm hitting a dead end where it tells me to check voltages at A2 and A7 - both values in my kit are below 1 volt. I've re-soldered anything on the board that looked remotely sketchy. Anything obvious I'm missing that would explain the high OA/OB numbers?

Justin

[Paul Birkeland]

Would it be possible for you to post some photos of your board?  If you can unscrew the screws that hold it down, you should be able to take a photo of the top and bottom without pulling it out of the circuit.

[Justin]

Sure, see attached.

FYI the middle lead of one of the MJEs broke when I was removing them to fix the orientation but I feel like I was still able to get a decent connection when I re-soldered it to the board. I don't think that's causing my problem but who knows.

[Paul Birkeland]

I think I see some of the board trace that has come off in this photo.  This can happen without the proper tools if you try to pull the transistors.  To be honest, I would contact replacement parts about just getting a new small board plus parts, as this one is likely beyond being salvaged.

[Justin]

10-4, hadn't noticed that. Thanks for the quick replies.

[Justin]

Hi Paul. So I ordered a replacement board (got it lightning quick btw, like 3 days quick!) and after I reassembled and installed both boards I'm getting similar readings on the little board along with high readings at OA and OB on the bigger board.

Little Board:
OA - 155
1A - 184
B-A/B - 0
1B - 184
OB - 140

Big Board:
OA - 150
OB - 160
G - 0
B+ - 184

All LEDs light up although the ones on the little board are dim. The amp works and sounds incredible. These numbers are clearly out of range - any ideas what might be causing them? Just let me know if you need any more photos.

-Justin

[Paul Birkeland]

Please don't listen to an amp that doesn't pass its voltage checks, it can damage your headphones and potentially cause bodily harm.

Can you pull the 6080 tube and measure just the small board with the 12AU7 installed?

[Justin]

Okay, thanks for the advicel.

The values on the small board with the 6080 pulled are:

OA – 150
1A – 228
B-A/B – 0
1B – 229
OB - 135

[Paul Birkeland]

Do the LEDs on the 9 pin socket light up when you perform this test?

[Justin]

Yes, all LEDs light up on the 9 pin socket and the small board with the 6080 removed.

[Paul Birkeland]

Can you very carefully get DC voltage measurements for A2, A3, A7, and A8?

-PB

[Justin]

Sure.

A2 - 0
A3 - 1.58
A7 - 0
A8 - 1.57

[Paul Birkeland]

And you have a 12AU7 plugged into the 9 pin socket, not some other type of tube?

[Justin]

Yeah, its the one that came with it. I just substituted a different 12au7 from another amp and got 111V OA and 115V OB on the big board. You think that was it?

[Paul Birkeland]

No, there's a bigger underlying issue here.

Can you post some new photos of the top and bottom of the current small PC board? 


 Do you still have the 22K resistors that you removed when installing the small PC board?