Speedball voltage problem [solved]

[menbom]

So, I have built the standard Crack and have been enjoying that for a couple of weeks. Voltages measured fine, worked perfectly, sounded great, the usual. 

Next up, the Speedball. Assembled the boards, attached the wiring, mounted and started measuring. Most voltages check out fine, tubes light up, but there are a few anomalies that I'm wrestling with:

1 - 170V instead of the expected 75-80V
B1 - 170V instead of the expected 75-80V
A8 - 15V instead of the expected 1.56V(the diode here does not light up)

This seems to affect only the small "A" board of the Speedball, the voltages from the "B" board(terminal 5 for example comes in at 82V) all check out fine.
At first, the diode at D2 on the A board did not light up, so I replaced the 2N2907 and then it lit up. After that I tried replacing the MJE350 on the A board, but still the same.

Any troubleshooting suggestions?

Thanks,
/Magnus

[Grainger49]

If you have one good board and one bad one you can compare the voltages into and out of them.  The most often problems with C⁴S boards are:

1) Reversed LEDs
2) Swapped Transistors
3) Transistors oriented the wrong way
4) A bad solder joint on a transistor
5) A wrong or badly soldered wire lead to or from the amp/circuit board

Give all these a close look, use a magnifying glass.  Report if you find something.

[Paul Birkeland]

Is T2/4 about 170V also?

Do both the diodes on the 9 pin socket light up? 

I'd pop off the board that feeds T1 and reheat all of the solder joints, as they are the likely culprits.

[menbom]

T2/4 are 176V.

Only one of the diodes on the 9 pin socket light up, the one on A8 is dark.

Measuring a bit on the boards themselves, it seems that the mid pin of the MJE350 is producing the 82V on board B, but 172V on board A. Tried reheat the whole of board A, taking extra care with the MJE350, as that seems to be the one that should be producing the 82V, but still the same 172V coming out to T1 unfortunately.

[Paul Birkeland]

Can you verify that the red wires leaving the "I" and "O" pads are going to the proper terminals? ("I" is fed by either 2 or 4, "O" goes to 1 or 5)

Do both halves of the 12AU7 glow? (A break in the junction of A4/A5 can cause one half to go dim)

If all else fails, you can swap the little green PC boards to see if the issue follows the board.

-PB

[menbom]

Thanks for all the help Paul!

Red wires correctly installed, both halves of the 12AU7 glows(double checked A4/A5).
Swapped the boards around and interestingly enough the problem does not follow the board. I.e. Board A connected to T4/5 measures correctly 82/176V and board B connected to T1/2 measures 172/176V, just like board A in that position did. A8 is still dark.

So I guess I need to trace back further to somewhere before the boards?

[Mike B]

The LED on A8 creates the volt and a half bias.  It should be lit.

It's either blown or in backwards.

[Paul Birkeland]

Mike is correct, the LED is loose or broken. (When they blow you'll hear it from across the room!)

[menbom]

Tada! I had turned the LED the wrong way around when replacing it, thanks for your help troubleshooting.
All works fine now, and sounds great. 

Now it's time to try this guy: http://www.ludd.ltu.se/~ragge/ecc82.jpg

[w0lfd0g]

Silver plate?  Very nice!

[menbom]

Yep, unfortunately the right channel is a bit noisy. When I first plugged it in, both channels were noisy, but the left one got better after a few minutes. Hopefully the right one will get better after a while.