Marantz 250M: Bias throws DC offset off

Having a weird issue with one channel in a marantz 250M. The bias adjustment throws offset off, and doesn't have as much an effect on bias as it should. Channel sounds distorted.

Changed a bunch of transistors to even up some work previously done. New outputs, drivers, and a couple other transistors. Both channels the same. Pretty sure I got everything installed right.

I checked a few things with the meter. +ive driver base voltage is going down to under 100mV as I turn up bias. -ive driver goes up. Base and emitter voltage on Q507 doesn't change by more than 10-20mV.

Does anyone know what might be causing it? I've never seen this happen before. I've been finding the circuit knowledge here on DIYaudio helpful in some cases recently.

I've attached a schematic but it can be found on hifiengine as well.
 

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Duke58

Member
2009-01-03 8:00 pm
The power amp board is very similar to the Marantz 1200. I own a Marantz 1200 and have had this same problem before.

First thing is to check the differential amplifier pair- looks like Q501 and Q502. I'm going off memory, but that pair of transistors should be matched for Hfe? The output transistors are also supposed to be matched for Hfe. You also need to read the tech bulletins for the amp.

Also, check Q801. This transistor is located on the heat sink by itself.

It's a good idea to not change transistors on this board unless you know they are bad. I spent several days checking transistors on a curve tracer to find one was flaky when it was tested at rail voltage.

A few years ago, I spent more time on my 1200 than it is worth. The payoff was it's still working and I love the sound, almost as much as my tube amps.
 
The power amp board is very similar to the Marantz 1200. I own a Marantz 1200 and have had this same problem before.

First thing is to check the differential amplifier pair- looks like Q501 and Q502. I'm going off memory, but that pair of transistors should be matched for Hfe? The output transistors are also supposed to be matched for Hfe. You also need to read the tech bulletins for the amp.

Also, check Q801. This transistor is located on the heat sink by itself.

It's a good idea to not change transistors on this board unless you know they are bad. I spent several days checking transistors on a curve tracer to find one was flaky when it was tested at rail voltage.

A few years ago, I spent more time on my 1200 than it is worth. The payoff was it's still working and I love the sound, almost as much as my tube amps.
Can a bad diff pair make bias and offset interact like this though?
I changed transistors because some had already been changed and I wanted them to be consistent between the channels. The other channel is working well.
 

Duke58

Member
2009-01-03 8:00 pm
Bad transistors caused the same problem on my Marantz 1200.

Measure the collector and emitter voltages of the transistors on the working channel and compare to your non-working board measurements.

These amps also need the electrolytic caps to be changed. Replacing the transistors using the shotgun method causes a lot of problems with these amps.

Download the manual and the tech bulletins and read through them.
 
Hah, I'm an idiot. Set this aside for long enough to clear my mind of it. Took another look today thinking I must have made a mistake.
The TO-66 original drivers were mounted with two screws. Only one of those screws connected it to the circuit. Guess which screw I connected the replacement TO-220 transistor to :D
Yep, collectors of the drivers were connected to nowhere.
Whoever changed the transistors on the other channel had put a jumper wire on the board to get around this so I did the same.