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-   -   Stumped - Hum / Bias Problem in Marantz 510M (http://www.diyaudio.com/forums/solid-state/19327-stumped-hum-bias-problem-marantz-510m.html)

iso_510M 23rd August 2003 08:59 PM

Stumped - Hum / Bias Problem in Marantz 510M
 
Well I finally found my amplifier!

I have not been able to figure out a problem with one channel. It has an annoyng 60Hz hum (just audiable enough to bother me), which goes away if the bias is reduced to almost no idle current or if the heatsink of that channel becomes electrically isolated from the case. With the bias current set according to the service manual, the hum is there. If I back it off to where the hum is reduced that channel distorts (naturally). The hum / noise troubleshooting tips in the service manual have not helped either. Both channels have a very slight no-load / shorted input oscilaton in the several MHz range. The troubled channel shows about .5V and the other is in the 10mV scale.

There's a 510M schematic as a PDF here:
510M


I have checked for changed resistor values and bad electrolytics to the best of my ability without just replacing all the caps. There is NO electrical connection to the heatsink unless I missed a ground other than the chassis. The reservoir caps tested OK and the other channel is A-OK. I checked and cleaned all electrical connections, including all grounds. Also, aside from the hum, both channels sound excellent. The hum is about the same weather the ground pin on the AC plug is actually grounded or not. There is no DC offset and I believe I'm the first person to have opened the amp since it left the factory - All components and soldering appear original.

Any ideas?

djmiddelkoop 23rd August 2003 09:48 PM

I noticed that a Fan is used.
I have seen noise and hum problems due to fans in audio and video equipment.
Try disconnecting it. If that helps try reversing the fan leads.

Hope this helps.
Dick.

Rudy 23rd August 2003 10:29 PM

seems like both channels have there one transformer and supply caps, perhaps measure the ripple voltage of the supplies ( - and + ) on the working channel with the normal bias current, and compare this with the ripple of the not so good working channel. Offcourse with the same bias current set on the bad channel.

If the ripple voltage isn't allmost the same, perhaps replace al power supply caps in the bad channel, ore both channels, your chose offcourse.

Greetz Rudy, good luck

iso_510M 25th August 2003 05:12 AM

Got It
 
Thanks for the pointers Dick and Rudy.

I replaced the reservoir caps anyway. They tested allright but looked like they had slightly vented. 27 years old anyway....

What the culprit was: C330 in the output network had failed and let the amp oscillate. Even though the oscillation (3+ MHz) was <.5V at the output terminals, there must have been a large current consumption. I replaced that capacitor for each channel and now output is a flat line into the 1mV range. :D

P.S. I still don't understand why isolating the heatsink for that channel made the noise disappear :confused:

wintermute 25th August 2003 09:37 AM

Re: Got It
 
Quote:

Originally posted by iso_510M
Thanks for the pointers Dick and Rudy.

I replaced the reservoir caps anyway. They tested allright but looked like they had slightly vented. 27 years old anyway....

What the culprit was: C330 in the output network had failed and let the amp oscillate. Even though the oscillation (3+ MHz) was <.5V at the output terminals, there must have been a large current consumption. I replaced that capacitor for each channel and now output is a flat line into the 1mV range. :D

P.S. I still don't understand why isolating the heatsink for that channel made the noise disappear :confused:


I've been trying to track down hum in my kit amp for years and never got it. I have a simmilar RLC network in the output so will be replacing the caps to see if that is the culprit! Was your hum audible even with the volume at minimum? Mine is detectable in all drivers woofer mid and tweeter (but only if you get up close). Also I think I did a less than stella job when I wound the coils, maybe should consider doing them again too.

Regards,

Tony.

iso_510M 25th August 2003 04:20 PM

Tony,
I'm glad I have possibly helped you. Yes it always hummed from the left channel. Even with the input terminals on the circuit board shorted. The big hint was the oscillation (which I planned to deal with after the hum problem). The noisy channel had 1/2V p-p 3MHz sine wave at all times, which carried over to the quiet channel. Both channels behaved normally as soon as I replaced that cap (C330) on the noisy board. Have you observed your amplifier output with an oscilloscope? Check for oscillation.

As far as isolating the heatsink from the case, I suppose there could have been a ground loop or eddy currents in the chassis at that frequency ?

wintermute 25th August 2003 09:32 PM

Quote:

Originally posted by iso_510M
Tony,
I'm glad I have possibly helped you. Yes it always hummed from the left channel. Even with the input terminals on the circuit board shorted. The big hint was the oscillation (which I planned to deal with after the hum problem). The noisy channel had 1/2V p-p 3MHz sine wave at all times, which carried over to the quiet channel. Both channels behaved normally as soon as I replaced that cap (C330) on the noisy board. Have you observed your amplifier output with an oscilloscope? Check for oscillation.

As far as isolating the heatsink from the case, I suppose there could have been a ground loop or eddy currents in the chassis at that frequency ?

Thanks. I haven't got access to an ossilloscpe, but I think I saw a kit the other day that lets you use your pc as one, will have to check that out. I used to get (I think) oscillation if I advanced the volume knob all the way up (with no signal), added some components in the pre amp stage (can't remember what, it was a long time ago) and that went away (at least in as much as you couldn't see the speaker cones fluctuating wildly!

I also notice that the hum isn't just a fixed freq, tends to warble up and down a bit, which could also be a sign of oscillation I guess.

I think a scope would be good, then I can check for ripple in the PS too.

Regards,

Tony.

mikek 26th August 2003 10:15 PM

interesting parallels to be drawn by comparing the marantz and the harman kardon here:

http://www.hilberink.nl/amps/amps8.htm

odiepsock 29th January 2008 09:35 PM

SCHEMATIC
 
Do you still have that schematic? That link seems to not be working. Thanks, Ryan


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