TL494 stuck in protect

i have this no name amp that seems to be old school hi end but unbranded that is stuck in protect.

the protection circuit goes through via a lm358 opamp that sends the protection signal to the 494. but the 494 still reads 4.7 on leg 3 after i disconnect the lm358 from that leg.

these are the off readings on the 494:

leg 1___ 0.390
leg 3___ 4.7
leg 16__ 13.49
leg 9 __ 0.001
leg 10__ 0.001

i know legs 9 and 10 are the outputs and since they are in protect they shouldnt be putting anything out.

any chance the lm358 is defective and throwing out a high voltage?

Perry Babin

Paid Member
2003-11-20 11:01 am
Pin 3 is higher than pin 2 which should make the output of pin 1 high. Here the op-amp seems to be working properly but the inputs to this half of the op-amp would be causing the amp to go into protect (assuming pins 1 and 7 drive the error amp of the 494).

The second half of the op-amp seems damaged. pin 5 is lower than pin 6 which should make the output on pin 7 low. Sometimes, if the input voltage is too close to the power supply voltage, the output of the op-amp will invert. I don't know if that's happening here. The op-amp could be defective.

It's difficult to see how this op-amp could be driving pin 16 of the 494. All of the outputs are lower than the voltage on pin 16. There could be a buffer transistor between the two ICs. That could explain the difference in voltage.

I think I'd try changing the 358 first.

If possible, post a photo of this area of the supply (or email it to me). Maybe something will look familiar.