Carlsbro Colt 45K Audio Amp

[habanerocat]

Got one of these to repair and need some advice please.

Basically it's blowing the two input 2AT fuses after the transformer on power up.
I upped them to 3.15A but they also blew.

The transformer output is ok and the speaker reads 4 ohms in circuit.

As you can see the board is a bit burned at the power transistors (TIP137 & TIP 132). One of them is reading 1ohm across two of its pins. The three serial diodes look ok.

I haven't been able to find a circuit for this amp online. Anybody got one?

Obviously I suspect one of the power transistors but I'm reluctant to take them out as the board is old and burned in that area and once I go at it I'm probably only going to get one chance before the tracks start lifting.

So I'd be thankfull for any advice on which components to lift and check first. And I'd be a bit rusty on checking the power transistors, in or out of circuit, if anybody has a good guide.

Thanks.

[habanerocat]

Re: Carlsbro Colt 45K Audio Amp

Seemingly these Darlington transistors don't test very well in circuit, or out of circuit for that matter. So I took them out and built a simple ciruit up on a breadboard. Sure enough the one that was almost short above was blown.

Amp back working............

[momaka]

Re: Carlsbro Colt 45K Audio Amp

Good to hear you got it working!

But yeah, low-resistance between two terminals on a BJT (regardless if regular type or Darlinton) out of circuit usually indicates bad part.