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The little diodes near the input JFET are for input voltage protection. Sunn specifies 1N458. Any signal diode like 1N914 or 1N4152 should work fine. The input stage is an N channel JFET driving a PNP 2N4250 transistor with feedback from the collector of the PNP back to the source of the JFET. The collector of the PNP should be about +1 to +2 volts if everything is working. Since it seems like almost no current is being drawn, I would guess that no collector current is flowing and the collector voltage is -15V.
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WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
REMEMBER: Everybody knows that smokin' ain't allowed in school ! -
Hey good news! I replaced the 2 small signal diodes and resolder the fet back in and the signal is going thru thanks to your help.... Now some where on the preamp board the lower half of the sign wave is being cut off when i turn up the volume. The circuit board is not marked at all. So I'll have to slowly track down the circuit. The volume control is dual which is different from what I would think. This is great. I'll be posting my problems/progress every day or so.... thx again!Comment
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It's done! A bad cap, the usual pre-out/power amp in jack had dirty contacts, and that was it. It lives! Thx to everyone for the help.Comment
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