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Rotel RA-1412 strange preamp problem

Could try adding some bulk capacitance at the power pins of the tone/pre board, see if there is any change Can you lay your hands on an external power supply that you can hook up to supply the +17/-12 volts to the tone/pre board? Does the amplitude of the spiking change with the setting of the volume control?
 
External DC power supply is one thing that I do not have. My old one died long time ago and I never got a chance to refurbish it :(.

The Rotel power supply is very basic, and it is shared through all boards, just using RC combinations to drop the voltages.
Rotel ps.jpg
 
It's *possible* that the PS starts generating spikes when the load on it increases. That's why I suggested an external supply. Based on listed supply voltages and the series resistors, it looks like about 500 mA on the +15.7 v line and about 20 mA on the -12 volts line. Could possible try a pair of series connected 9 volt batteries for the +15.7.
 
Not sure if you ever resolved the issue but I had this exact same problem on my 1412 so I figured I would post what I did to fix it.

I found on the right channel power supply board Q903 was shorted and burned looking, and Q904 not working either. I replaced Q903 with a MJE15032G and Q904 with a KSA1013YBU. I also went ahead and replaced the same transistor (Q901) on the left channel board even though it was not shorted, it was burned looking as well so I'm sure it was on its way out.

This resolved the issue for me, if you haven't fixed yours yet I would give it a try.
 
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BF PIerce said:
Not sure if you ever resolved the issue but I had this exact same problem on my 1412 so I figured I would post what I did to fix it.

I found on the right channel power supply board Q903 was shorted and burned looking, and Q904 not working either. I replaced Q903 with a MJE15032G and Q904 with a KSA1013YBU. I also went ahead and replaced the same transistor (Q901) on the left channel board even though it was not shorted, it was burned looking as well so I'm sure it was on its way out.

This resolved the issue for me, If you haven't fixed your yet I would give it a try.
Click to expand...

Interesting timing. I just picked it up a couple of hours ago. Apparently cause of the issue was an intermittent ceramic capacitor on a power rail for the preamp board.
 
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