Accidentally shorted speedball circuit, help!

[ash9r]

I was cleaning my top plate with steel wool and it seems I did not get all of it out from the circuitry underneath. When I plugged in the voltage, I heard a loud pop and one of the channels stopped working. Upon dissection and checking out the voltages, this is what I found.

One of the LED pairs on speedball is not lighting up. Nothing obvious seems burnt/fried out.

The only voltage that is off is that on pin 7. 202 volts vs expected ~84 volts. Note that due to symmetry the expected voltage on this pin should be around 84 volts as it is that on pin 5.

Voltages.
5. 84.1
7. 202 (Incorrect)

I did not do the resistance checks yet, I'll perform the same and update the post.

 

[ash9r]

As the instructions say, you only need to worry about voltages that are +/- 15% out of spec.  Were the resistance checks OK?

So you can edit (modify) the above post with only those that far out of spec (less confusing).  Then give the expected voltage, the measured voltage and all multipliers and units like mV or kV or just V.

Thanks Grainger49, I updated my original post with the voltage that is off. It is just pin 7 which is more than 2x of the expected. I noticed that this goes to the speedball TIP50 transistor.

 

[fullheadofnothing]

Did it work before this happened?

Why are the boards off the standoffs?

What's up with the electrical tape?

Have you done a careful and thorough inspection for loose strands of conductive material?

 

[ash9r]

Yes, absolutely, everything was working fine until i cleaned my top plate with steel wool and failed to clean off some of the spill on the speedball main board. I hooked it up, there was a huge spark causing a short somewhere.

I took the boards off the standoffs to measure voltages, also i added electrical tape on the smaller boards to prevent shorting with the larger board which seats very close to it.

Yes, I've done the inspection and I don't see any more loose strands.

 

[ash9r]

Here are the resistance measurements. They're quite off

I do want to mention i've performed the mod here (http://bottlehead.com/smf/index.php?topic=2946.0) "easy mod to keep startup voltage on the output down" so that's probably affecting some of the readings, not sure.

Pin 6. Expected 2.4K actual 0
Pin 7. Expected 2.9K actual infinity
Pin 9. Expected 2.9K actual infinity
Pin 10. Expected 2.4K actual 0
Pin B3. Expected 2.9K actual infinity 
Pin B6. Expected 2.9K actual infinity

Hopefully this helps. I do want to reiterate that everything was working well, wired up fine before the short, voltages and resistances correct, etc.

 

[Paul Birkeland]

Here are the resistance measurements. They're quite off

The resistance checks aren't in the Speedball manual for a good reason.

You have very likely fried half the big board, and will need to replace all of the components on that side.

-PB

 

[ash9r]

The resistance checks aren't in the Speedball manual for a good reason.

You have very likely fried half the big board, and will need to replace all of the components on that side.

-PB

Thanks, I tested the transistors and found that the 2N222 is blown, just ordered components from digikey, will put those in and update this post

 

[ash9r]

So here is the latest, I used a multimeter to test both TIP50 and 2N222 on the defective section of the bigger board and found that 2N222 was blown. The LEDs were also blown.

I replaced the LEDs and 2N222, now the LED with its anode at the base of TIP50 glows but the LED with it's anode at the base of 2N222 does not.

The voltages check out OK. However when the crack is switched off, the voltage at Terminal 7 does not immediately drop to 0, this means the capacitor does not automatically discharge. It drops to about 30v after switching off the power and then drains slowly (on the multimeter)

Also I noticed that the voltage at the base of TIP50 is around 3.1v (LED glows) and that at the base of 2N222 is about 1.112v (LED does not glow). However on the working channel the voltages are both around 3.1v.

Before I go ahead and replace everything on the big board, just wanted to see If I can debug it.

Any leads?

 

[Paul Birkeland]

I replaced the LEDs and 2N222, now the LED with its anode at the base of TIP50 glows but the LED with it's anode at the base of 2N222 does not.
[...]
Any leads?

You have very likely fried half the big board, and will need to replace all of the components on that side.

 

[ash9r]

Thanks Paul, i did that and my crack works again! thank you so much