So I got a 300/4 in from a friend, 1st Gen. Top channel (as in picture orientation, Q300/Q301 channel) blew, pulled all outputs and checked resistance between gate and source on all FET sockets and all high sides (bottom Fet sockets of each pair, Q301, Q305, Q501, Q405) match at about 5.6mil Ω, yet all low sides (Q300, Q304, Q400, Q404) were off? Between 7mil to almost 18mm. So I powered it up to test gates cuz everything else probes ok/not dead (resistors, BJTs, etc) and I get a wave at each low side gate, yet each of low side waves are higher/lower voltage versus others on my Scope screen, guessing bias? Does bias control the gate voltage on these amps? After testing and confirming to make sure drive is working and is sending signal to each of the low side gates (albeit all at different voltages) now the resistance is much lower on each low side gate to source after draining rail voltage, and the “bad” channel that blew is still the lowest but at 700k Ω now, but the middle 2 low side gate to source channels probes at 8mil Ω and the bottom channel measures at about 10mil Ω. What is giving me these crazy all over the place Gate to Source measurements yet everything seems to be working? I did notice the BJT in front of the top/bad channel’s output card (Q306) is reading 30 Ω between drain and source and the other 3 channels BJT are all reading 20ohm. I’ve just yet to come across anything like it.
I marked on picture what each low side was probing at after testing to ensure drive is working.
I marked on picture what each low side was probing at after testing to ensure drive is working.
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