Speedball upgrade - voltage problems & explosive LED

[SharkFist]

Hi, first-time builder and first-time poster here... Hoping for some more experienced to help..

So I built my Crack successfully - everything checked out (all voltage/resistance checks were within tolerance levels) and I spent the evening listening happily - sounded great ...

Next day I installed the speedball kit and not so much luck...

Several of the voltage readings were off. Terminals 1, 5, 7 and 9 were all reading way more than they were supposed to (but all the others pretty much checked out). I double checked them and noted the levels (pasted below) and looked everything else over. The one other thing I noticed was that the LED's on the smaller PCBs were faint compared to all the rest (D2 was glowing but dull, D1 was almost not glowing at all - same on both).

Then, just as I was doing a triple-check on the voltages to make sure I wasn't crazy I noticed that the LED's all lit up brightly, and then a few seconds later - boom - one of the LED's exploded - sparks and some smoke and electrical burning smell... so I quickly pulled the power plug. (OK explosion might be a little exaggeration, but it made a fizzling noise and definitely a pop as it sparked).

In hindsight, it's possible this explosion event coincided with me pressing on terminal 1 with my Multimeter tip and which I guess could have connected a bad wire or something (although that might have been a coincidence) although, of course, correlation doesn't equal causation.

Now, basically I don't know what to do and I'm hoping I haven't killed it! Please help what I can do to troubleshoot and hopefully get it back up and running... Thanks!

Here are the voltage and resistance readings I took before the explosion:

TERMINAL     CORRECT     ACTUAL     RESISTANCE

1                   75-90         188          18.75(M)
2                   170            190          175(K) Slowly Climbing
3                   0                0              0
4                   170            190          7(M) Slowly falling
5                   75-90         188          18.75(M)
6                   0                (60mV)          2.475(K)
7                   100            187           0
8                   0                0               0
9                   100            187           0
10                 0                (60mV)          2.475(K)
11                 0                0              0
12                 0                0              0
13                 170            170          320(K) Slowly falling
14                 0                0              0
15                 185            205          9.5(M) Slowly falling
20                 0                0              0
21                 206            222          320(K) Slowly falling

[Paul Birkeland]

Can you measure the R1 resistors on each PC board to double check that you have the parts correctly placed?

As always, it's also easy enough to mix up the 2N2907 and 2N2222 transistors as well. 

[Grainger49]

Also, triple check that the MJE transistors are oriented properly.  I put them in backward about 2 months ago.

[SharkFist]

Thanks for the thoughts so far. I've gone through the things you both suggested to check and can't see any problem - all is as-per the manual AFAIK.

I checked the resistance across the R1 resistors and labeled on this image - http://imgur.com/nf5nDtf [nofollow]

Also labeled the transistors and their orientations - so maybe if I've misinterpreted the manual somehow maybe you could spot an issue?

I notice in this test that the left R1 transistor looks a bit "fried" (in addition to the left LED) and it's reading 1K Ohm as opposed to the right one which is 32 Ohm. By "fried" I mean it looks a bit more dull than the other one, and maybe a little burned spot on the side.

Guess I should replace that? But if so, it seems like it was a result, not the cause of the issue I had..?

[Paul Birkeland]

Yes, you have the R1's swapped between the boards...

Also, you installed the LED's in such a way that we can't see if they are backwards. 

[SharkFist]

Aha! Wow, just goes to show you can't triple check things enough times! Thanks so much for spotting that.

So I need to switch the resistors around from the boards...  how might I find a replacement for the burnt resistor and LED? Can I just use any 31.6K resistor, like from RadioShack?

[Paul Birkeland]

31.6K is not the correct value.

I would e-mail replacement parts and we can send you some replacements. 

I would recommend putting the 22K resistors back, then putting the 31.6 Ohm resistors into the big board and running the amp, just to be sure the big board is OK.

Both small boards need to be replaced.

-PB