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

[Paul Birkeland]

You now have a short dragging down the high voltage rail.  Typically we used to see this a lot when the TIP50 transistors weren't properly installed on the heatsinks.

It it probably a good idea at this point to remove the Speedball (you can leave the little wires, just let them poke straight up) and replace it with the original resistors, as it's likely that you have an issue with the original build that needs to be dealt with.

[Beranga]

I removed the Speedball boards and reinstalled the resistors. Resistance check passed and the voltages seem fine:

1  76
2  177
3  0
4  177
5  75

6  0
7  105
8  0
9  105
10  0

I might add that I installed the diodes mod on the IEC for computer noise (getting a 13K ohms reading from chasis ground to IEC ground), don't know if it affects anything.

[Paul Birkeland]

The PN2907 on the A side of the small C4S board looks to be cracked, can you confirm that?

If it's not, you can try reinstalling just that board, then retest the amp.

[Beranga]

The PJ2907 is not cracked, it doesn't look like in the picture. I installed just the small board, and voltages are all right:

OA 68
IA 200
B-A 0
IB 200
OB 68

I imagine that the large board must be the problem. On that board, an alligator clip scratched one of the ground to LED vias, so I soldered a jumper in and checked for 0 ohm resistance from ground to the LED and ground to the next resistor. Pics. attached.

[Paul Birkeland]

Can you measure the DC resistance on the big PC board between the center leg of each TIP50 transistor and one of the solder points for the heatsink?  If these touch, your high voltage rail will get dragged down and you'll end up with the issues you had.

[Beranga]

They don't touch, resistance is out of range between center leg of transistors to solder points of heatsinks..

[Beranga]

Don't know if it would help, but when the big board was installed, one of the LED shined overly bright. Pic. attached.

[Paul Birkeland]

I think your best bet would be to contact replacementparts(at)bottlehead(dot)com to see about obtaining a replacement large Speedball board and parts.

[Beranga]

Can I listen to the Crack with only the small board installed while the new board arrives?

[Paul Birkeland]

Sure, you can use the amp with the small board and the 3K resistors.

[Beranga]

Before ordering a new large board, I measured everything and found the short!!! I was a faulty 2N2222A, once replaced all voltages measurements seem fine:

Small board
OA 70
IA 191
B-A 0
IB 191
OB 70

Large Board
OA 108
OB 108
G 0
B+ 190

Paul, thanks for you patience and all the help!!!