Speedball, checks pass, but smoke + L channel pop after 5 mins

[Paul Birkeland]

You can measure each pair of legs on each of the transistors to look for very low resistance; that indicates a short.

Measuring the resistance of an LED on its own isn't typically that helpful, though some meters will light them up with the diode check function.

When you buy a TIP50 and 2N2222, just make sure they are from a reputable electronics supplier.

-PB

[cabcabaret]

I tried testing the transistors and the 'north' 2N2222 does show a 100 Ohm resistance between two of the legs where the 'south' 2N2222 does not.

That's without removing them from the PCB.

I've ordered replacement TIP50s and 2N22222s so fingers crossed! I'll rejig the output caps in the meantime to make things neatly seperated.

Thanks for all of your help, it's much appreciated.

[cabcabaret]

Hello again, it seems you were spot on.

Replacing only the 'top' 2N2222A resulted in all of the LEDs lighting up minus one on the large PCB and led to very different voltages between A & B side connections. Before attempting to replace the TIP50, I thought I may as well replace that LED and that seems to have done it. I think that LED is the one in parallel to R1 in the circuit diagram, I guess that blew along with the 2N2222A.

Now all LEDs light up evenly, and voltage checks pass. I've run it for an hour now the 270 Ohm resistor is pretty hot but not overly (I turn it off and touch it carefully). In any case it's not smoking!

The only thing is that the upgrade seems to have exposed a usb/ground loop noise from my DAC and PC, but I can fix that.

I have to say while i enjoyed making and listening to the solo-crack I was a slightly underwhelmed by it, so I was hesitant to invest more in it with the upgrade, but to my ears the speedball + crack sounds brilliant. Instruments are much more defined and present without losing any musicality. Vocals  especially sound so much better. 

I'm really pleased with it, thanks again for assistance.

[Paul Birkeland]

Now all LEDs light up evenly, and voltage checks pass. I've run it for an hour now the 270 Ohm resistor is pretty hot but not overly (I turn it off and touch it carefully). In any case it's not smoking!

This is not a great idea.  The 270 Ohm resistor is in the power supply, and the power supply voltage takes some time to bleed down, so you're potentially putting your fingers right near high voltage.

Also, we tend to limit the temperature of such a part to 100C in our designs (usually a bit lower), and I believe that is plenty hot to burn your finger.