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Old 29th November 2019, 05:17 PM   #11
Zen Mod is offline Zen Mod  Serbia
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F5 troubleshooting
easiest is to test them outa circ, if you're having problems in circ

jfet matching jig with 9V battery is good and quick

mosfets you can check either with amp psu or with laptop brick

see Papa's FW Article page , if needed , for simple mosfet matching jig
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Old 29th November 2019, 05:29 PM   #12
boywonder is offline boywonder  United States
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Thanks for the reminder ZM....


I matched the IRFs years ago..but completely forget.....so I'll check out the FW article.
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Old 29th November 2019, 07:45 PM   #13
rayma is offline rayma  United States
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Originally Posted by boywonder View Post
Rayma: did you see my post #7 above? is the Q1 in question fine? from gate to either S or D is open....
fresh ones in the pack read 5.66M from G-S or G-D. Should I unsolder the output MOSFETs and test those?
The gate should be open in either polarity to the D or S. A MOS gate is an electrode that
has no ohmic connection internally, and works by electric field like a capacitor.
The most common fault with the gate is a short to the drain or source due to excessive
voltage from static charge due to handling or a fault in the circuit. Another common fault
is a short from D to S due to excessive current.

For testing any N type fet, the red DVM lead goes to D and the black lead to S, and should be open.
For testing any P type fet, the red DVM lead goes to S and the black lead to D, and should be open.

Last edited by rayma; 29th November 2019 at 07:47 PM.
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Old 30th November 2019, 12:48 AM   #14
boywonder is offline boywonder  United States
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Well....I thought I had figured it out...but it's still has the issue. I tested the 2SK370 and the IRFP9240 and they both checked out normal.


I thought that one side of R1 was contacting ground since the ground connection is right there and the boards are tiny.......I moved the grounds away from R1 and re-stuffed with a new R1 but it smoked too.


Interesting..with nothing connected to the input the bias and DC offset are quite normal, and R1 is happy....when I plug the function generator into that channel R1 smokes....


I haven't checked the ZTX550 yet.......tomorrow.
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Old 30th November 2019, 01:09 AM   #15
rayma is offline rayma  United States
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If you plug an RCA shorting plug into the input does R1 still smoke?
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Old 30th November 2019, 02:13 AM   #16
boywonder is offline boywonder  United States
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I'll try that in the morning....
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Old 30th November 2019, 02:20 AM   #17
rayma is offline rayma  United States
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Originally Posted by boywonder View Post
I'll try that in the morning....
Look very carefully at all the wiring and parts connected to the input RCA connector terminals. If DC conditions
are ok until a connector is inserted, something is wrong related to the socket. Are the two wires reversed?
Maybe the amplifier is oscillating when the input is connected. What happens to the DC voltage at the amplifier's
output when a shorting plug is inserted?

Last edited by rayma; 30th November 2019 at 02:33 AM.
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Old 30th November 2019, 02:46 AM   #18
boywonder is offline boywonder  United States
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The input wires aren't reversed.....I redid both channel's input wires today.....the other channel still works fine....shorting plug in the morning...


The other channel didn't smoke for months while the amp was driving the sub/shaker table from the good channel...nothing was connected to it.
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Old 30th November 2019, 02:50 AM   #19
rayma is offline rayma  United States
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Originally Posted by boywonder View Post
The input wires aren't reversed.....I redid both channel's input wires today.....the other channel still works fine....shorting plug in the morning...The other channel didn't smoke for months while the amp was driving the sub/shaker table from the good channel...nothing was connected to it.
My theory is that this problem is caused by inserting a connector, for some reason that is due to another problem.
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Old 30th November 2019, 04:06 PM   #20
boywonder is offline boywonder  United States
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Shorting the RCA input doesn't generate any R1 smoke...and the bias voltage is stable (just happen to have a meter on there)


I've also noticed that with nothing connected to the input there is a large malformed sine wave on the output ......

Last edited by boywonder; 30th November 2019 at 04:25 PM.
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