Speedball Upgrade help

[KaminKevCrew]

Hi there,
I assembled a Crack the other day (stock) and it sounded great! Today, I decided that I'd like to upgrade it to the Crack + Speedball, as I have the Speedball upgrade lying around. However, I can't get it to function properly. The only thing that has changed has been the installation of the speedball kit.

The issue that I'm running in to is that the 270 ohm resistor that runs under the transformer is overheating, to the point that it starts smoking within 30 seconds of use. All of my solder joints are good joints (except for the resistors on the boards, which I moved around a few times so they look ugly, but they are solidly connected.

I got through terminal 1 voltage checking, and that wasn't correct - instead of getting 75-90V, when I turn on the amp, I see about 200V, then over about 20 seconds it drops down to about 2V.

Does anyone have any ideas what I might have done wrong?

[Paul Birkeland]

Could you post some photos of your build? 

If this is the current Speedball kit, then the instructions call for installing the small PC board first and testing the amp, did you make it through this section successfully?

[KaminKevCrew]

I'll post some photos of the circuit boards in a bit - I have pulled them out of my Crack at this point and returned it to stock, as I was wanting to listen to some tubey goodness. Fortunately, everything in the stock Crack seems to be working just fine still, so I suppose that narrows it down to my Speedball upgrade, which is nice to know.

I bought my kit secondhand, rather than new so I'm not entirely sure what version I ended up getting. I found some instructions online, which I've been using. I don't think I have the new instructions though, as there wasn't a section in my set that talked about testing the small boards first.

Is there any way I can test the smaller boards? I have a lab power supply that can do up to 30V, and has to separate outputs. I've also got an oscilloscope and a DMM, but I'm not sure what voltage I would need to be sending here. I could probably figure it out based on the recommended voltages at the terminals that the PCBs connect to, but if you happen to know that would be nice!

Edit: Also, the TIP50 transistors on the bigger circuit board didn't seem to come in my kit with a thermal pad - there was what looked like a mica insulator in my kit, but mica is generally an insulator of heat, so I decided to not use them and instead use some thermal paste I have from building computers. The thermal paste is completely non-conductive, but if the heatplate on the transistor has direct contact with the aluminum heatsink, could that cause issues? I can put an insulator through them if I need to.

[Paul Birkeland]

You need to use the parts that came with the kit and your problem will likely be resolved.

Mica is unique because it is both a very good electrical insulator and a very good thermal conductor. 

[Doc B.]

Thermal paste is technically speaking an electrically insulating material (most often, at least, unless it is of the type that has electrically conductive particles in it). But the spots where it doesn't cover because it got squished are definitely not insulated. You need a mica or sil-pad insulator. IIRC mica is a little better than sil-pads in terms of thermal conductivity.