More speedball issues... [resolved]

[StivVid]

Something very strange just happened...

I plugged the 6080 back in and fired the amp back up.  LEDs lit up in sequence as they normally do, except that 3 lights now burned very brightly on the big board.  The other lights on the 9 pin socket and small board gradually faded out until I don't think they were emitting any light at all.  All that were lit were 3 lights on the big board burning very brightly.

I rapped my thumb against the side of the base while wondering what was going on, when suddenly all the LEDs lit at their normal brightness.  They're still lit now.

Here are the new voltage readings:

Small board:

OA=  75.7
IA=  186.6
B-A/B=  0
IB=  186.6
OB=77.6

Big board:

OA=  110.7
OB=  109.2
G=  0
B+=  187.4

I'm going to leave it on for awhile to see what happens.  What do you think?

[StivVid]

Hang on...  Just lost another LED on the big board.  Only 3 are lit.  2 on the end nearest the octal sockets, and 1 on the end nearest the 9 pin.

I have no idea what's going on.

[Doc B.]

If the LEDs came on with a tap on the case I would guess that you have a loose connection somewhere, or possibly a short that opened when you tapped the case. Look for loose connections, unsoldered wires, loose bits of cut off wire ends rattling around, terminals touching, excess leads touching, etc.

[StivVid]

I've been all over this thing with a fine toothed comb.  I can't see anything amiss.  Visually, everything looks good.

Could there be a fried component on the board somewhere?

[Paul Birkeland]

Yeah, that basically confirms an issue with the big PC board, which is nearly always a TIP50 that isn't properly installed.  Please double check the shoulder washers and the ordering of all the parts. 

[StivVid]

Ok...  I disassembled the entire big board.  Dismounted both TIP50's from their heat sinks.  (The transistors were mounted properly all along, I just disassembled and reassembled in the same order.)  Replaced one thermal insulator pad that looked iffy.  Reassembled everything including replacing all the hook up wires for the big board.

Just fired it up and everything seems to be working.

Here are the new voltage measurements:

Big Board:

OA=  111.4
OB=  109.0
G=  0
B+=  185.7

I hope this is the end of it.  Gonna keep an eye on it for awhile.  Can't thank you enough for your patience and understanding--not to mention your great wealth of knowledge.  It's a pleasure to be a part of the Bottlehead community.