TL494 stuck in protect - diyAudio
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Old 22nd February 2008, 01:41 AM   #1
Clipped is offline Clipped  Thailand
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Default 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?
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Old 22nd February 2008, 02:16 AM   #2
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Post the voltage on all of the pins:

Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11:
Pin 12:
Pin 13:
Pin 14:
Pin 15:
Pin 16:
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Old 22nd February 2008, 02:42 AM   #3
Clipped is offline Clipped  Thailand
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Pin 1:_____ 0.390
Pin 2:_____ 5
Pin 3:_____ 4.7
Pin 4:_____ 0.001
Pin 5:_____ 1.4
Pin 6:_____ 3.34
Pin 7:_____ 0.001
Pin 8:_____ 15.79
Pin 9:_____ 0.001
Pin 10:____ 0.001
Pin 11:____ 15.79
Pin 12:____ 13.61
Pin 13:____ 4.9
Pin 14:____ 4.9
Pin 15:____ 4.9
Pin 16:____ 13.49
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Old 22nd February 2008, 02:47 AM   #4
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To answer a previous question... The output on the 494 is off. If you look at the ground pin of the IC, you can see that it has the same voltage which means the outputs are at ground.


Pin 16 is the problem. What's driving pin 16 high?


If it's the LM358, post the voltages on that IC.


Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
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Old 26th February 2008, 01:10 AM   #5
Clipped is offline Clipped  Thailand
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Pin 1:__12.4
Pin 2:__4.9
Pin 3:__5.7
Pin 4:__0.1
Pin 5:__7
Pin 6:__12.5
Pin 7:__12.25
Pin 8:__14.2

it looks like pin 16 is getting signal from pin 8 of the LM358.
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Old 26th February 2008, 01:29 AM   #6
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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.
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Old 27th February 2008, 11:44 AM   #7
Clipped is offline Clipped  Thailand
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youre spot on, i changed the LM358 and it came back to life.

verdict LM358 was dead... i think this is the first time ive came across a dead opamp...

thanks!

ever seen this board before? i almost thought it was a carver because of the heatsink, but carver is slightly different.

Click the image to open in full size.
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Old 27th February 2008, 06:00 PM   #8
jol50 is offline jol50  United States
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I have no idea what it is, but some things look similar to a PPI made amp I have.
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Old 29th February 2008, 08:10 PM   #9
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Also looks a bit like the early Rodek amps, but IIRC they usually had an EQ control (Autotek BTS).


Tim
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Old 2nd March 2008, 07:18 PM   #10
Clipped is offline Clipped  Thailand
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i was thinking the same thing...it also has the tell tale square zed traces.
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