Speedball Upgrade - Fail Voltage Check

[raps1514]

Hello - back again. After a successful Small Board assembly and install, I attempted the Large Board and started having issues with the voltage check. The amp starting emitting a smell and got quite hot around transformer.

Large Board:

OA: 3.4V
OB: 41V
G: 0V (but kinda fluctuates)
B+: 45V

Thanks for any help!

[Paul Birkeland]

If you remove the 6080, what are the voltages on the front board?

[raps1514]

Is the front board the small board?


6080 Removed — Small Board:

OA: 76V
IA: 211V
B-A/B: 0V
OB: 74V

[Paul Birkeland]

The problem you're having most often happens when the hardware is not properly installed on the TIP50 transistors.  If the insulating pads and shoulder washers are not properly placed, the metal tap of the TIP50 will touch the heatsink and cause this problem.  Please double check that.

[raps1514]

Double checked the metal tab of the TIP50 and it is not touching the heatsinks; the insulating pads is buffering. (I'm pretty sure.)

FWIW - Big Board is reading with 6080 still removed:

OA: 2.14V
OB: 1.536V
G: 0.3V

[kGlerup]

To me it looks like the screw touches the metal on TIP50. There should be an insulating washer between.
But maybe it's just the picture.

[Paul Birkeland]

I agree, I cannot see the insulating washer.  Review the warning on the bottom half of page 22 and the instructions that follow...

[raps1514]

Still not quite seeing what y'all are seeing. Here are more photos.

[Doc B.]

The black shoulder washer should be under the screw head on both heat sinks. You have it on the wrong side - under the nut - on Q2B rather than under the screw head. So the screw on that side is touching the tab on the transistor, which it must not do. You need to take the assembly apart and reinsert the black washer from the other side of the heat sink, so the shoulder of the black washer is on the screw head side.

[Paul Birkeland]

On the bottom of page 29 is a resistance check, what values do you get for that resistance check?

[raps1514]

I've gone ahead and attempted this again. Sending two pics showing the placement of the parts, unscrewed in. And another two pics with them screwed in. Note - they look very similar to previous pics because...I'm still not sure what I'm doing wrong.

Also - the LEDs on the small board do not light, nor the ones on the Q2B side. The ones are very well lit, however.

[raps1514]

On the bottom of page 29 is a resistance check, what values do you get for that resistance check?

OA: .285Mohms
OB: 2.15MOhms

[Doc B.]

Looks right now, as I can see the shoulder washer is under the screw head on Q2B. Couldn't see it for sure before.

[raps1514]

Still getting the same resistance and failing voltage checks. Slight chemical-type smell coming from top of chassis. LEDs not lighting up on Q2B side.

[Paul Birkeland]

Next I would suggest removing the large Speedball board and taking pictures of the soldering on the bottom, as well as the build of the stock kit that's under the big Speedball board.  The chemical smell that you are smelling is your amp responding to the near dead short that your large Speedball board is presenting, so don't run it too long and let it cool between tests.