BK 3011 repair

[manicdoc]

Got a BK 3011 off Ebay for a song. Turns out the output driver circuit had two dead transistors pretending to be diodes across the +/- 22v lines via two 47 ohm 1w resistors, things had got so toasty one resistor literally unsoldered itself...

Checked everything in that part of the circuit, and all the other semis measure good, I'll go back one more level and check the transistors doing feedback control. I reckon either someone spiked the output or something went wrong further back...

I've already recaped it for good measure, and nothing nasty was found. Got the replacement transistors on order, should be with me within a week.

[Vincent]

Indeed quite possible that other components failed beyond the output transistors.

There seems to be ICs as well? I would also check those and see if they work as expected.

[manicdoc]

Indeed quite possible that other components failed beyond the output transistors.

There seems to be ICs as well? I would also check those and see if they work as expected.

A lot of it is analog, I'll double-check the signal paths.

[manicdoc]

Indeed quite possible that other components failed beyond the output transistors.

There seems to be ICs as well? I would also check those and see if they work as expected.

A lot of it is analog, I'll double-check the signal paths.

Replacement transistors finally arrived. I checked back the signal path 3 levels deep on transistors, nothing else bad...

Decided to power up via an inline lamp and measure the voltage drops across the 46-ohm resistors that were overheating before with the bad transistors. Max measured drop seen was 0.4v which works out to be a small fraction of a watt, so well within limits.

I'll power it for 10 minutes and monitor - if nothing runs away, I'll move on to confirming output behavior and then check calibration.

To me, it looks like the output got zapped and the two driving transistors died as a result or might be old age...

[manicdoc]

Further update, I think there is a fault in the middle of the output stage, not the final drivers. Can't get rid of a negative offset on the output and it's not amping right.

I'll remove all the transistors (7) and then check each circuit fragment left and then swap out the old transistors. They are jellybean and cheap.

In prep for diving in deeper, I've cleaned off all the flux, it was literally soaked in it.

Slow going atm, other jobs to do.