Cool. It's listed as one of the acceptable parts (3299W-1-202LF). I think I ordered it for an F6 build (which I had to wait on due to availability of the input transformers). I actually built the Aleph J as a second choice.
Yep, that will work in either orientation. While you're at it, you might as well put Q3 back in so you don't have to do it later. If you have solder-wicking braid, you can remove the bits of the pins still in the board from where you clipped it. You might just be able to use your iron to poke out the old bits before soldering it back in.
Same issue with the replacement pot. Perhaps the condition melted the wiper or windings in the old one?
Measure the old pot now that it's out of the circuit - does the resistance change between the middle and outside pins?
Do you get the same resistance of the pot in situ using the both polarities of the multimeter leads?
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If your lower mosfets (Q7 & Q8) have failed with the gate shorted to the source (i.e., the 221R is shorted to the 0.47R) then you will have similar symptoms. Is there continuity there?
If the lower mosfets shorted the drain to the gate *and stopped conducting*, then you'd have 20 V on the output and 1 W + dissipated by the trimpot - that might melt a wiper.
If the lower mosfets shorted the drain to the gate *and stopped conducting*, then you'd have 20 V on the output and 1 W + dissipated by the trimpot - that might melt a wiper.
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That's interesting... I'm assuming the source and gate are the outer pins if I'm reading the data sheet correctly. Q5 and Q6 measure 16k (but rise as I test), while Q7 and Q8 measure about 470R and stay steady.
When the mosfets are in the circuit, you're measuring a lot more than the mosfet - you're also measuring every other path between the two points. There are also capacitances which may be charged just by the act of measuring, which is why it may change.
I'm not sure how to interpret the resistance measurements, other than that it doesn't seem to be a pure short.
With the gates at the negative rail (-22 V, I presume), the mosfet should show a very high resistance between gate and source, so the 470R should be just R7, since R7 is in parallel with the mosfets.
Does the 470R change if you alter R7?
Does the 470R change if you alter R7?
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I don't see a 470R on the schematic. Maybe I'm missing it? Which resistor is it? And, do you want me to just measure across it while I turn R7, no power?
Just as a matter of curiosity, did you run the amp without the Keratherm pads at some point? I don't see them in your first picture.
The gate-to-source resistance of a mosfet (when off) should be very very high. If you're measuring 470R, that's either the resistance of R7 - which is in parallel with the mosfet - or it's a blown mosfet gate. If you turn your trimpot and the measurement varies between 0 and 2k, then you're probably just measuring the trimpot. If it doesn't vary as much, then the gate is probably blown.
Even though anodized coatings are somewhat insulative, running without Keratherm could have shorted the mosfet drain to earth ground - or to the positive rail, where the other mosfet drains are. This could have damaged the mosfet.
Even though anodized coatings are somewhat insulative, running without Keratherm could have shorted the mosfet drain to earth ground - or to the positive rail, where the other mosfet drains are. This could have damaged the mosfet.
Wowbagger: Thanks for your patience - and the explanation. The resistance between the source and gate doesn't match R7, but it does vary when R7 is adjusted (about 10 ohms a turn on a 25 turn pot).
