No adjusting the idle current

V

Vintagecasey

Active Member
I had a chance to get back looking at the marantz 2240 today. I have another question, this one is regarding adjusting the idle current between pins 708 and 710 and adjusting trimming resistor 719. One side I can get 8 mv like specified, the other side I can only get 1 mv on the high end. I removed that trim pot and checked it with my meter and it shows smooth operation from about .1 ohms of resistance up to about 279 ohms of resistance which seems to be good (it's a 300 ohm pot) ? Is there somewhere else I should check to find out why I can't get 8 millivolts on that side? Or should I just change the trim pot even though it seems good? I did recap the power supply and both amp boards because other issues. All with quality Nichicon capacitors. (Again, these questions may be basic knowledge for most people, but this is a new area for me now that I've been put out to pasture.)...
 
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Did you change any semiconductors on that channel? Which way is the pot end stopped, betting it's wide open 280 ohms? Is there sound on that channel at the speaker?
 
rBuckner said:
Did you change any semiconductors on that channel? Which way is the pot end stopped, betting it's wide open 280 ohms? Is there sound on that channel at the speaker?
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No semiconductors, just capacitors. I have the pot set right in the middle now because cranking all the way either direction makes no difference while it's in the circuit.
 
The dropping may be a switch, usually the tape monitor ones being the worst. But it could be anywhere in the chain. Is this the plain 2240 or the 2240B? I'll look at the schematic and suggest some things to look at.
 
Less than +/-10mV is excellent. Doesn't look to me like you have a DC offset problem.

Amplifier Distortion, DC-Offset, and You!

Time for one of those long, boring semi-technical posts that no one here reads... As a few of you know, I bought a rare Kenwood 700M amplifier a few weeks ago on eBay. It arrived with a weak channel which was taken care of by replacing a bypass cap. Since then, I have gone through the entire...
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I'm talking about "idle current" which I thought was quite different than DC offset? I thought I understood that idle current and bias are the same thing?
 
Vintagecasey said:
Thanks, under normal operation, could someone sketch the normal path on this amp board? ThanksView attachment 3274024
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I thought you had a bias problem? Why do want to know the signal path? You need to focus on the bias circuit.

The signal flows through C701 to base of H702. H702 collector to bases of H707, H714, H715, H003 and H004.

You need a multimeter. Signal tracing is not the way to troubleshoot a biasing problem.
 
Start by testing variable resistor R719. There are a number of resistors in the bias circuit. Start by checking the voltages. Base of H714 should be +1.1V. Base of H715 should be -0.6V.

If R719 is bad, replace it. if it is good , get BCE voltages on H714 and H715 and go from there.
 
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Vintagecasey said:
Thanks, I'll check those... Sorry if I'm jumping around, I don't know any better:) trying to make sense of the spaghetti.
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The bias is at 8mv where it should be? Adjusting VR719 has no effect? Odd that bias is set correctly, yet variable resistor R719 has no effect. Did you test R719? Are you sure you had probes on 708 and 710.

Left channel dropout might be dirty controls.
 
Sorry, to clarify, The bias is 8mv on one channel, 1mv on the other. The channel with 8 mv can be adjusted up and down, the one with 1mv has no effect. Thanks
 
Yep, did that, it has smooth operation from .1 ohms up to 270ohms (it is a 300ohm pot) I assumed it's good or at least should show some change? I pulled the 100ohm resistor just below it and it tested 99 ohms...
 
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