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Old 1st December 2017, 03:53 PM   #11
Jeff Mills is offline Jeff Mills
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The absolute saddest part of this whole thing is I earned a electronics engineering degree back in the mid 80's. that I never put to use and now that I'm old I have basically forgot a whole lot about it So please forgive me in advance if I ask stupid questions about this repair job.
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Old 1st December 2017, 05:49 PM   #12
Perry Babin is offline Perry Babin  United States
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It appears to have 47 ohm gate resistors (check all of those) so you could use something like the IRF3205.

The driver transistors may be damaged. They appear to be QP3/4.

Check the solder connections on D23/24.
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Old 1st December 2017, 06:20 PM   #13
Jeff Mills is offline Jeff Mills
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Quote:
Originally Posted by Perry Babin View Post
It appears to have 47 ohm gate resistors (check all of those) so you could use something like the IRF3205.

The driver transistors may be damaged. They appear to be QP3/4.

Check the solder connections on D23/24.
Thank you!

The other transistors are TIP41&TIP42, KB770&KD998 (complementary pairs). I got the data sheets and replacements on order for those. I figured as long as I got my hands in there - I'll put in all fresh.

The symptom:
Channel 1 went full power briefly and then into protect and won't come out. I'm thinking I have a bank with shorted gates??? But I haven't got that far into it yet.

I could just go buy a new amp easy enough... but honestly it's kind of fun to do the repair. Beside God knows I could use the practice and re-learn some of what I've forgotten.
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Old 1st December 2017, 06:37 PM   #14
Perry Babin is offline Perry Babin  United States
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Where did you find the output transistors?
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Old 1st December 2017, 06:55 PM   #15
Jeff Mills is offline Jeff Mills
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Quote:
Originally Posted by Perry Babin View Post
Where did you find the output transistors?
I ordered substitutes FJA4213 FJA4313 as the data sheets were good matches.
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Old 1st December 2017, 07:02 PM   #16
Jeff Mills is offline Jeff Mills
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Sorry but I did want to add channel 1 tests shorted across the speaker outputs with a MM (diode setting).
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Old 1st December 2017, 07:57 PM   #17
Perry Babin is offline Perry Babin  United States
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A short across the speaker terminals generally means that there is a strand of wire across them.
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Old 1st December 2017, 08:15 PM   #18
Jeff Mills is offline Jeff Mills
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Quote:
Originally Posted by Perry Babin View Post
A short across the speaker terminals generally means that there is a strand of wire across them.
or... I got some output circuitry work to do
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Old 1st December 2017, 08:22 PM   #19
Perry Babin is offline Perry Babin  United States
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Output transistors don't connect to ground so shorted outputs cannot short speaker terminals.
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Old 1st December 2017, 11:23 PM   #20
Jeff Mills is offline Jeff Mills
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Quote:
Originally Posted by Perry Babin View Post
Output transistors don't connect to ground so shorted outputs cannot short speaker terminals.
Very good point... I just rechecked.

MM set on diode/continuity test. Measuring speaker terminals.

Channel 1 beep, numbers flash and quickly end at 474 ohms
Channel 2 no beep, numbers flash and eventually ends at 1367 ohms

Last edited by Jeff Mills; 1st December 2017 at 11:25 PM.
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