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Rotel RB990BX - Loud Hum (with no source)

J

johnspd22

Member
Was rearranging my equipment and installing an RCA switcher and cables to Amp/Receivers (for my DAC input), and somewhere along the line, my main amp is no longer functioning/blew up. Regardless of input, the unit only outputs a loud hum.

I checked the fuses on the left/right amplifier boards and these are ok. I did not test any further as this is beyond my troubleshoot skills.

Any ideas what this could be? I had a few thumps while connecting other receivers. I am concerned I blew some transistors etc.

Thanks.
 
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Did some checking online and came up with the following test values:
a. Unit on - Negligible AC voltage on speaker outputs (0.02v, 0.03v)
b. Unit on - Small DC volage - 7mv, 15mv
c. Tested resistance in between speaker terminals - 1.5M ohms. i wasnt gettgin readings on my digitl multipmeter in ohm, kilaohms only Megaohms.

I think the Zobel network (ground reference bridge across speaker terminals) might be toasted. Thoughts?
 
I also disconnected the main inputs in earlier troubleshooting and got the hum so the issue is not an upstream ground loop etc.

thanks.
 
Start by checking all grounds and connections around the power supply like transformer, filter caps and bridge rectifiers. Then check power supply voltages.
 
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