[mewrei]

So I finally worked up the nerve to finish my CKKIII, and I got everything together, my only problem is kind of odd.

I measured the +/- 15v test pads and they both are within 0.3v of 15vdc.

However, whenever I go to bias the output transistors on R19 and R43, only 1 channel will allow me to set it around 8mV as specified.

The other will only measure up to 2mV on the maximum setting of the trimpot.

Where are some likely locations to start looking for the problem?

Bad resistors, bad transistors, bad trimpots? (I'm using Toshiba 2SC2238/2SA968 transistors that I sourced from MCMinone so they should be good).

Thanks for the help

 

[FallenAngel]

Anything getting hot? Take a photo of both sides of the board, you likely have a cold join somewhere or simply a transistor in the wrong place.

 

[mewrei]

(Apologies for the mess)

The left channel transistors got warmer without the larger heatsinks, but the right channel never really got hot without any thermal management

 

[FallenAngel]

Well, that really doesn't help much.

I'd recommend looking over every single TO92 transistor to make sure you haven't put them in the wrong place. Then double check that you haven't swapped the TO220 ones.

 

[FallenAngel]

Did you remember that the Toshiba 2SC2238/2SA968 have the reverse pinout compared to BD139/BD140?

 

[mewrei]

Ok, I think I found at least A problem if not the problem in question.

It looks like one of the leads on the board got scratched back.

I'll try hooking it back up and see if that fixes the problem.

EDIT: yeah that fixed it. Thanks for the help (now I just feel silly)

 

[mewrei]

Yes, I have the heatsinks mounted towards the board (as opposed to away with the BD139/BD140).