Well, I am quoting myself here coz I know assumptions are like stinky A$$es. When I tested 2SJ73 for the third time, I am reasonably confident that it is working fine. The reason 2SJ73 was tested bad in my previous test is due to the gate is triggered by +V in the P-Channel JFET.
Since 2SJ73 is a very high Gm JFET, it's very sensitive to the way it's being tested. The metal can surroundong J73 will pickup residual noise and any voltage from your bare hand and the gate nearby picks up those noises and activating the gate. So, S-D is shorted when the gate picks up these noises.
I found that not touching the metal can will avoid these false readings so I lay down the JFET on the anti-static mat to give me accurate S-D reading. One other thing is that JFET's stray capacitance can give inaccurate S-D result so hooking up a bleed-resistor between S/D-G before test yield better result.
Since the K146/J73 is good and the later K147/J72 is bad (gate busted), my previous stinky assumption of the preamp failure causing the damage in the JFETs is INCORRECT. Well, did I say assumptions STINK and without the proper testing procedure, they stinks even more! 😀
So why is the JFET's gate in the later stage busted?
John,
Did you get my email with the Schema attached? No doubt Lighting would destroy the amp but when I first looked at the PCBs, I thought it's probably lighting from inside that damage the amp.
1. Very Dirty PCBs with all connectors between PCBs are heavily oxidized and blackened. In fact, that's what I thought is the main reason why the JFET is over loaded. It's was over biased for mid (neutral) point.
2. One of the Electrolytic caps in parallel with these JFETs are shorting.
3. Flux - yeah lots and lots of old solder flux. Looks like Denon never bothered to wash and rinse the PCBs at the 70s-80s time. I have 70s Denon with same Flux problem!
4. Heat - Push-Pull Class A with poor thermal ventilation for the critical parts.
5. ?
6. ?
7..... to be continued....
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