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Old 16th March 2011, 03:50 PM   #1
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Default Driver transistors?

Well once again, i've stumbled over driver transistors.

I have an amp here, with blown mosfets, so i've pulled them all out (12 of them).
Now, the 2 banks of 6 fets are split, there appears to be 1 driver transistor for 6 fets (3 on one side and 3 on another).


Measuring at the pads on the fets I get: 5.4v across 3 of them each bank. (3 each side of the board) On the remaining 6 (2 3's again) I get B+.

So I thought, easy, just a blown driver transistor which is shorting. But here's the tricky part. The transistors are attached to an IC, on both of them at the middle leg, I get milli-volts (very very small). On the leg attached to the IC I get 5.4v (nice). Then on the remaining leg i get 5.4 on the one (nice) and B+ on the other. Now maybe my brain has farted and missed something, or have things really gone mad with this amp? .

Can anyone give me a starter with this? I'll get a pic up and some model numbers in a while.

Edit:
Driver transistors : A1023's
Ic - TL494 (cn) = http://www.datasheetcatalog.org/data...2wr26kdjyy.pdf
Click the image to open in full size.

The row of 4 components are: 2 resistors in the middle and a diode each end.

I have spare driver transistors to hand (not 1023's, but alternates i've used before)

Last edited by agithegreat; 16th March 2011 at 03:59 PM.
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Old 16th March 2011, 04:55 PM   #2
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It sounds like the 1023 is open.
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Old 16th March 2011, 05:18 PM   #3
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Ok, i'll pull the bad one and report pad voltages back .
Thanks PB.
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Old 18th March 2011, 09:07 PM   #4
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Ok, driver transistor removed.
Now I get B+ -2v (B+ =15v now 13v on the gate's of the fets.)
5.6v solid on the end pad and 0v on the middle.

Where this 13v is coming from, i'm stuck. There are a few diodes dotted around, but none seem to be shorted.

Any ideas of how this voltage is getting there? I've followed the traces and there appears to be no shorts?

I'm baffled.
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Old 18th March 2011, 09:16 PM   #5
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Post the DC voltage on these 6 points.
Attached Images
File Type: jpg 2011-03-16155152b.jpg (163.6 KB, 82 views)
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Old 18th March 2011, 09:23 PM   #6
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1. 12.8
2. 15
3. 0
4. 5.6
5. 0
6. 12.8

Thanks PB
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Old 18th March 2011, 09:36 PM   #7
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The transistor with 4,5,6 is defective. The 12v is coming from the driver IC and nothing is pulling it down (the 4,5,6 transistor should be pulling it down).
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Old 18th March 2011, 09:43 PM   #8
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That transistor is now removed.
Should I still see those voltages there?
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Old 18th March 2011, 09:47 PM   #9
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Yes.
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Old 18th March 2011, 09:48 PM   #10
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Great, i'll pop a new 1023 in then and report back

Thanks.
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