Speedball OA voltages low on SB and high on LB [resolved]

[Beranga]

I have a problem with OA voltages: it is too low on small board and too high on large board. Before installing Speedball, I checked the amp resistances and voltages, all were in range. Then I installed the small board and checked its voltages and LED's, everything fine. Finally I installed the large board and got the following voltages:

Big Board:
OA 208
OB 94
G   0
B+ 215

Seeing the OA was too high, I rechecked voltages on the Small Board, which now had OA too low:

Small Board:
OA 34
1A  213
B-A/B 0
1B  213
OB 76

Then I checked the Crack voltages and found terminal 1's too low:

1 33
2 215
3 0
4 215
5 76
6 0
7 94
8 0
9 211
10 0

A pair of LED's on the large board remains unlit, and on the small board the led close to 1A is off now (before the big board it was fine).

I've gone throgh the boards reheating and re soldering some of the joints, but the problem remains the same. Please help!!!

[Paul Birkeland]

If you remove the 6080 and run the amp with just the 12AU7, do the small board voltages check out?

What does the bottom of the small board look like?

[Beranga]

Hello Paul,

Thank you for answering on a Sunday! No, without the 6080 the voltages are still out of range:

OA 34
1A 238
B-AB 0
1B 238
OB 76

I attach a photo of the SB bottom.

[Paul Birkeland]

Are you using lead free solder by chance?  How hot of an iron?

[Beranga]

No, rosin core 63/37 solder. The iron goes up to 800°, I use it at max temperature.

[Paul Birkeland]

I would turn it up to max temp.  Your solder isn't getting hot enough.  You probably don't need to reflow the joints for the LEDs, but I would reflow the resistor and transistor solder joints at max temp.

[Beranga]

Hi, Paul. I went through every joint on both boards, reflowing them at max temperature. Still get the same voltages on OA (34 on SB and 210 on LB). The strange thing is that when the small board was first installed, all voltages checked out. It was only after I connected the large board that the problem appeared.

[Paul Birkeland]

With the small SB board removed and tilted up, what's the DC voltage at A8?  Also if you have reflowed the LED solder joints several times, the LEDs themselves will melt and stop working.

[Beranga]

With the small board removed (I'm not sure what do you mean by titled up) voltage at A9 is 0.

I checked the LEDs with the diode setting on the multimeter. Some of them show a small voltage and bip, others show no voltage but light up when tested. Infinite resistance when tested backwards. I attach a picture of them on the powered circuit.

[Paul Birkeland]

The DC voltage at A9 will be zero, but what's the DC voltage at A8?

[Beranga]

Sorry, at A8 is 800mV

[Paul Birkeland]

Can you rotate the small Speedball board 180 degrees so that OA/OB/IA/IB are flipped to the other side? 

0.8V of bias and 30V on the plate suggests the C4S on that side is not allowing sufficient current to pass.  Flipping the board will either move the problem to the other side of the amp or it will stay put if there's an issue in the original build.

[Beranga]

I flipped the small board and voltages are now worse! But the OA voltage did not change to the other side.

OA 27
1A 105
B-A 0
1B 105
OB 73

[Paul Birkeland]

I would e-mail replacementparts(at)bottlehead(dot)com and get yourself a fresh small board.  It will help if you have your iron turned up as high as possible so you can get the solder to flow out in a shorter amount of time. 

[Beranga]

Hello Paul,

As you suggested, I installed a new small board, soldered with the iron at max temperature (pics. attached). I'm now getting low voltages on IA and IB:

OA 67
IA 97
B-A 0
IB 97
OB 63

The large board voltages are:

OA 61
OB 75
G 0
B+ 95

Thanks for your help!