Marantz 140 Power Amp – Intermittent Fluctuating Right Channel

Heat sink compound aka-- Thermo Compound is fine.
You are on the right path, good luck.
 
Hey raidermc,

Thanks for the encouragement :)

I realized I didn't replaced the H701/H702 transistors............ what idiot I am !! I was sure they was replaced.

BIAS adjusted, DC Adjusted.

I did some test yesterday for around 5 hours and no more issue for now !!! Both channel are silence (no hum, rumble, static issue).

I will do a follow up later.

Thank you !!

Guy
 
Hello!

Hummm..... the issue is still not resolved and i'm still hunting.....

I checked if it's my pre-amp : Revert L and R RCA cables on the amp.

Result : The rumble is still on left channel (VU meter goes crazy) and the volune knob has not impact.

H704 and H703 are still original.

H709 is still original.

Driver and pre-driver transistor (H711, H712, H713 and H714) are still original.

I will try H703 and H704 then.

If someone have some ideas. go ahead!

Thanks,

Guy
 
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It is the right diodes ?
From my tech: The diodes, 2 per channel, are designated H720, H721. If they are a small black diode, they should be replaced with 1N4148 diodes. Be very careful of the polarity. Also, look for any glue on the top or bottom of the boards. If there is, it has probably turned black and is possibly conductive. Carefully scrape it off. After the repair, set the D.C. offset adjustment R741 for 0 volts D.C. at the speaker outputs. The other adjustment is for bias and is a trickier adjustment. You should not need to adjust this unless the amp tends to run hot with no signal after being on for 10 to 15 mins. The 1N4148 diodes should be available at any electronics shop. Rich
 
From my tech: The diodes, 2 per channel, are designated H720, H721. If they are a small black diode, they should be replaced with 1N4148 diodes. Be very careful of the polarity. Also, look for any glue on the top or bottom of the boards. If there is, it has probably turned black and is possibly conductive. Carefully scrape it off. After the repair, set the D.C. offset adjustment R741 for 0 volts D.C. at the speaker outputs. The other adjustment is for bias and is a trickier adjustment. You should not need to adjust this unless the amp tends to run hot with no signal after being on for 10 to 15 mins. The 1N4148 diodes should be available at any electronics shop. Rich
Hi Rich,

Thank you for this great information.

Last month, I replaced both diodes (H720, H721) on the left channel amp and THERE WE GO, no more rumble/static sound. I use my Marantz 140 almost everyday and all seems fine since the replacement of these diodes.

Victory!

Thanks a lot!!

Guy
 
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