Marantz 1152DC Offset

[goldswimmerb]

Hey all, looking for some assistance here, I have a Marantz 1152DC on the bench with a rather "drifty" DC offset, it tends to start around 200mv when the relay closes and as things warm-up drops down to where I have the adjustment point set, once there it seems to drift around slightly (eg I will set it and come back 30 minutes later and it will have drifted by 20mv or so).

So far I have replaced capacitors, as well as transistors, the diff pair are two matched ksa992s. Drivers are KSC2073 and KSA940, Predrivers are KSC2690/KSA1220. MV-1 Diodes replaced with a single 1N4148 diode. Outputs are original but were pulled and tested to verify performance. The main heatsink runs warm to the touch but not concerningly so.

The two transistors in the current mirror (after the input FET) were replaced with KSC1845, do these need to be matched?

The reso of the 2SA872/2SC1775s were replaced with KSA992/KSC1845. Outputs are original. I am suspicious that this is just a very temperature-sensitive design as the diff pair is mounted above the driver/predriver section and exposed to the significant heat this section creates, however, I wouldn't think Marantz engineers would design something to be "passable".

 

[ocdecio]

This is the one with varistors and thermistors on the power amp board, right? I wonder what would happen if one of them got bad.

 

[goldswimmerb]

This is the one with varistors and thermistors on the power amp board, right? I wonder what would happen if one of them got bad.

I wonder too, though I believe they're for thermal tracking on the bias, not sure how they integrate with DC offset. What's weird is the DC is high in both channels at the start and then drops, as opposed to just one

 

[hofferwood]

Watch your input voltage to see if it follows idle drift.

 

[catrafter]

Check the +/- 45VDC power supply for drift.

Tom

 

[goldswimmerb]

Check the +/- 45VDC power supply for drift.

Tom

The regulated Supply? Or the unregulated B+ and B-?

 

[goldswimmerb]

So I went and made sure that Q709/Q711 were matched and that the current mirror also was. Heating/cooling Q709/711 causes a pretty big shift in DC, the diff pair is matched as well. Makes me wonder if perhaps the input gets are okay...

 

[cbusuttil]

So I went and made sure that Q709/Q711 were matched and that the current mirror also was. Heating/cooling Q709/711 causes a pretty big shift in DC, the diff pair is matched as well. Makes me wonder if perhaps the input gets are okay...

Hey there! i seem to have this exact same problem on my 1180DC. did you manage to sort it out? im restoring mine and even after the restoration this problem still persists!! this thing is making me really nervous now as i did make sure the differential pairs are matched etc!! i dont know what else i should be looking at! i even checked all the resistors and all seem to be well within spec!!

 

[goldswimmerb]

Hey there! i seem to have this exact same problem on my 1180DC. did you manage to sort it out? im restoring mine and even after the restoration this problem still persists!! this thing is making me really nervous now as i did make sure the differential pairs are matched etc!! i dont know what else i should be looking at! i even checked all the resistors and all seem to be well within spec!!

I think I just wound up increasing the relay delay. Not really optimal but it got the job done.

 

[cbusuttil]

I think I just wound up increasing the relay delay. Not really optimal but it got the job done.

i seem to have narrowed it down to the little filter board… still trying to understand what could possibly wrong as i did renew the semiconductors and electrolytics! good idea re. increasing the delay

 

[goldswimmerb]

i seem to have narrowed it down to the little filter board… still trying to understand what could possibly wrong as i did renew the semiconductors and electrolytics! good idea re. increasing the delay

I have seen replacement transistors cause a change in circuit behavior, in some pioneer units they'll go as far as shifting the polarity of the DC across the output coupling caps. Something similar may be happening here.

 

[cbusuttil]

I have seen replacement transistors cause a change in circuit behavior, in some pioneer units they'll go as far as shifting the polarity of the DC across the output coupling caps. Something similar may be happening here.

That might be the case as well. I tried to match the hfe as close to the originals as well. .... i mainly ised ksa992 and ksc1845 instead of the 2sc1775 and 2sa872 ... Differential pairs matched as well.... i am really lost! or maybe i am being too perfectionist? lol