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Marantz SR2000 DC offset and bias adjust

Orcinus

Orcinus

Active Member
Been a while since I posted, but I've got a bit of a problem! My Marantz SR2000 has some pretty bad DC offset. -254mv on the left channel, +270mv on the right, after warming up. I've had the amp open but I can't find any trim pots on the amp board to get this down to more reasonable numbers. Unfortunately, this amp has those stupid STK080G power amp chips, instead of mostly discrete components, and I'm not sure it's even possible to adjust the DC offset and bias. Does anyone have a service manual, or know how to adjust this model of amp?

PLEASE HELP!!! :tears:
 
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I would feel pretty goddamn stupid paying $21 just to find out I can't even accomplish what I need to, especially considering that's more than what the entire amp is worth.
 
The STK 080G is a complete amplifier stage, with built-in differential stage.

There is no provision for DC Offset adjustment in this receiver.

Follow the traces from pin 1 of STK080G, there is a small capacitor (~1uF). Remove this capacitor. This will disconnect the input from the pre-amp section. If the offset changes, this is a sign that this input cap is leaking and some DC voltage is present in the preceding stage, replace it.

Follow the traces from pin 3, you will see a 1200 Ohm resistor and a capacitor (~50uF) on the other side of this resistor. Remove this capacitor. If the offset changes, it means this capacitor is leaking. Replace it.

If there is no change when you remove the capacitors, this means the transistors in the differential input stage are mismatched, or the tail current source within the power IC has drifted, or other internal drifting. There is no way to change the currents inside this IC...

In this situation, your only option is to change the input resistance connected between pin 1 and 2. Actually, it is a 27KOhm resistor. In the channel with the positive offset, you must reduce the input resistance in order to reduce the offset. I think it can be safe to reduce this resistor as low as 10k or 5kOhm. I imagine the pre-amp stage is buffered. But you may need to replace the input cap in order to keep the same low frequency cut-off point.

In the channel with the negative offset, you must raise the input resistance in order to reduce the offset voltage. this is not a problem, and you can replace the 27K by anything as large as 500KOhm.

If you try this, start by a factor 2 change in the resistance values.

Keep in touch with us... I am curious to see what you will find.
 
I know this is an old thread, but I’m puzzled as to what happened with my SR-2000. I found it by the side of the road in pretty bad shape (wood case is pretty much toast). Since I’ve gotten better at soldering, I figured this would be a good first unit to recap. I cleaned all the dust and debris out (no deoxit used yet) and turned it on. I then tested the dc offset, which ranged from 480 mv to just over 500 mv on both channels. I worked the controls, no effect. From page 10 of the service manual, I figured out the 4 caps mentioned above were C703 and C704 for the two 1 uF caps and C705 and C706 for the two 47 uF. I removed C704 and checked the dc offset, both channels now steady at 22 mv. Again, I worked the controls, no effect. I have limited knowledge in this area, but I don’t see how removing one cap could affect both channels?
 
bzb882 said:
From page 10 of the service manual, I figured out the 4 caps mentioned above were C703 and C704 for the two 1 uF caps and C705 and C706 for the two 47 uF. I removed C704 and checked the dc offset, both channels now steady at 22 mv. Again, I worked the controls, no effect. I have limited knowledge in this area, but I don’t see how removing one cap could affect both channels?
Click to expand...

I don't see neither.
 
Popped the C704 back in today, no effect. So that's not what caused the dc offset to drop, which is what I expected. I'll start removing caps tomorrow so I can attemt my first full recap,
 
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