Another Offset problem...

[rtate]

Hi all , I have a Marantz 2385 that has an offset problem on the right channel , 109mV.
The 2k pot used to adjust the offset has absolutly no effect on the offset reading. I have checked the pot and it seems to work fine.
I had thought that the problem had to be in the matching of the input stage transistors(2SC177AE), bu tafter doing some reading I'm not so sure anymore!!
Is it more likely to be a capacitor or resistor problem????
What should I be looking at first?

I am finding it difficult to source the transistors and I am worried about getting a lesser quality for replacements, nevermind trying to match them better than Marantz did...

 

[Mooly]

Do you have a circuit ? 100mv is small for a "fault condition" to be causing this. Does the pot for the other channel work as you would expect. Measure at the speaker terminals with meter on D.C. volts, no input signal and no speakers connected. Really need to see circuit. If you can identify the input transistors (I am assuming a Long Tailed Pair here) just the heat from you're finger on one or other of these transistors should give a noticeable change in offset. Does it sound O.K. ?
Regards Karl

 

[rtate]

I do have the service manual and schematic.

It is not an actual"fault" where the speaker protection kicks in, I'm just refurbishing the receiver and would like to get the specs as good as possible.

The pot for the other channel does control the offset as it should.

I do measure at the speaker posts with no speakers attached and no input with my meter set to DC millivolts. The oter channel has an offset of <1 mV.

It sounds OK on my test speakers, I have not hooked it up to anything good yet!!

 

[Mooly]

Hi, If you could post the part of manual with the pot that is not working, may be able to advise a bit better. Usual arrangement is to try to balance the input pair by altering the balance of the tail current. Has pot no effect whatsoever ?
Regards Karl

 

[EchoWars]

I would add that a lot of Marantz receivers of this vintage seem to have a lot of problems with differential pairs, but it is quite unusual that the offset pot seems to do nothing. I'd be tempted to remove the pot and check its functionality.

 

[rtate]

Mooly , the reading changes from 108- 109 mV

Echo. I have not removed the pot but I measured from the back of the circuit board and the value changed from 0-2k ohm.

If I wanted to replace the pot with a bourns multi-turn, is there a current rating for the original pots??

 

[Mooly]

If pots O.K. all ways, end to end and wiper to each end (if not sure swap with other channel) something is wrong. Still need to see circuit to advise further. Does the offset change if you warm either of the input pair of transistors.
Karl