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Marantz 140 Amp has 3.4 vdc at Speaker jacks

TubeJunkie

TubeJunkie

Master of None
Hello. I have a Marantz 140 power amp that I'm looking at for an old friend. Upon power up, it has -3.4 VDC on the speaker terminals. I'm afraid to hook up speakers to it. Both meters are in the middle when powered on. The two peak indicators are lit too. I replaced all caps in the protection circuit. The relay clicks like it should. I've also marked and cleaned all DC, bias, and meter adjust pots. as well as volume pots. No change. I also removed all output transistors and they check all the same. I have a link to the SM and multimeter. Any suggestions are greatly appreciated.
 
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EchoWars said:
Adjusting DC offset does nothing? Both channels read the same offset?

Sounds like a broken ground wire somewhere.
Click to expand...

DC offset adjustment does change when moving each pot. But the negative DC just gets higher. ( more negative DC) the -3.4vdc is the best I can get it. ???
 
Well, I've been all over this thing looking for a ground problem. I've cleaned all connectors thoroughly. They were heavily corroded (see pics). Removed all boards and touched up any suspicious solder joints. Removed ground from main 2 section filter cap and resoldered all connections. No change. I did notice the output transistors are different from one side to the other. Any other suggestions?
 

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Update: I tried using a different multi meter and was able to set the DC at speaker terminals to .01 vdc. I guess the first multimeter was faulty. Not sure how to set the bias. I just left those pots in their original positions. The manual says to use an oscilloscope. Still have the peak indicators lit. Was able to adjust the meters a little. I have an osillascope, but I've never used it.

Would it be safe to hook speakers to this amp at this point?
 
I hooked up a set of speakers to this amp and I got a loud mid range humming/buzzing noise, so I immediately unlplugged it. Do you still think it's a ground issue?
 
Might be time to turn this one over to a pro, as I think the output needs to be scoped to see what's up.
 
Thanks EchoWars. I really appreciate your time. Hopefully someone will chime in. I really need to start learning to use my o-scope. I know it's a valuable tool.
 
The first thing I would do is clean the thing, looks filthy inside.

Theres no scope needed for bias adjustment, it's needed for the DC balance adjustment but if you've got a hum, there's an underlying issue which needs to be adressed.

I would take it to a pro or leave it off, do a few months of research on how to operate your scope, buy or make dummy loads and then report back.
 
Hum in both channels would have me looking at the power supply. I dont recall having to use a scope for bias or dc offset on the two 140's I rebuilt.
 
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