Question about Electro-Harmonix 12ay7 Tube Microphone Preamp

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A big hum of 100 c/s in the output,i need the schematic for repair

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You must be on 50Hz power. You don't really need a schematic to repair the unit. Since the hum is twice power line frequency, the likely cause is the electrolytic filter cap for the high voltage has failed. The cap should be obvious in the unit. It will be near rectifier diodes and have a voltage rating above 50 volts. To test, bridge the one in there with another cap observing the correct polarity. If hum changes, replace the original cap.

Posted on May 25, 2011

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I don't have a exact schematic for your amp, but I can give you some hints and suggestions...

USUALLY when the output transistors fuse there is a problem at least one level back and sometimes two levels. Often the driver transistors also get fried and sometimes one stage farther where the bias is regulated for the final amp transistors. A schematic is imperative to diagnose and repair unless one replaces every part.

Next, to prevent damage when restarting the amp, put a 100 Watt incandescent lamp in series with the hot power into the unit. This acts as a current limiter and auto-resetting fuse. The lamp will come on and limit the current while one troubleshoots the amp to find what is causing the current drain.

This schematic MIGHT be pretty close to your unit:

http://elektrotanya.com/swr_swr2000_power_amp_module_sch.pdf/download.html

I would expect to see Q4 through Q12 destroyed and maybe a few more components. If transistor Q4 opens, goodbye all the transistors! The 1N4004 diodes are PROBABLY big enough to take it, but all the small transistors probably are damaged or degraded... The high value resistors will survive, but the low ohm ones that can draw enough current may be damaged.

Try to analyze what each part of the circuit is for... I think Q13 and Q14 are part of a soft start circuit. This amp is NOT well protected in case of component failure and MANY that fail in the final stages cause catastrophic destruction.

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Here is the schematic:

http://elektrotanya.com/fender_vibrolux_6g11_schem.pdf/download.html

Check the quality of the DC bias as being free of ripple.across teh 25 mfd/50v cap filtering the bias.
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This should be an easy fix, but you might need an oscilloscope to diagnose it quickly... or just parallel caps across these to see if it kills the hum.

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I don't have a schematic for this unit, however good design would not demand matched transistors.
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Hi there. The final output IC (big and black with 15 pins? attached to heat sink) is shorted and needs to be replaced. If you can still see color bands on R63, it should still be good.
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