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Marantz 1040

It is one for sell in my country and is not cheap, arround 200$. 1040 is the last untouched amp in my collection
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so guys busy with adjustments now. Dc voltage currently at 9mv. Do I have to wait a bit for to see if any change before adjusting trimming resistors or do I go ahead and adjust straight away
 
Hi guys. A lil confused now. Managed to set all trimming resistors as per spec except R761. The DVM just won't stay still. It is all over the place fluctuating mostly on negative readings but does give some positive readings some times. Tried powering off and trying again but still same. Moving R761 you see the range change but still fluctuating all over the place.

Any advice on what's happening here?
 
Hi Pete. How can I test this pot and is there a way I ca clean it. Maybe dirt is causing the pot to react like this
 
JRM said:
How can I test this pot and is there a way I can clean it.
Click to expand...
It is probably dirty and cleaning with de-oxit will probably fix it. Dc offset is very sensitive when adjusting and will fluctuate between positive and negative as you adjust the trim pot. Small changes are necessary, sometime I just bump the screwdriver the smallest amount when you get close. Try the de-oxit and be patient, it sounds like you have brought this unit back! Rich
 
Thanks Rich. If it is back it is only thanks to you kind and helpful guys here. I did not have a clue of where to start or what to even look for.

Is there an opening of sort to look for on the pot to get the de-oxit into. And I dnt think I can even get de-oxit here so will normal alcohol based electronic spray work.

Thanks in advance
 
Hi Guys. Would appreciate any suggestions with this R761. Can't get de-oxit here so I sprayed some electronic contact cleaner I had gotten locally a short while back. Also bought spanjaard lubricating switch cleaner and applied that. Did this with pot in place and unfortunately no difference. Do I need to take it off the board and try cleaning as I am not looking forward to changing it as I would like to keep original as far as possible. If i do have to change the service manual says 5k +- 20% so can I use any pot that spec that would fit?

Thank you in advance guys :bowdown:
 
Will do but does that mean you recommend changing it out. I just don't know if my cleaning attempt was effective. Can I not remove from board and soak in something like WD40for a while?

Sorry in advance if its a stupid suggestion
 
you can sub in a new one and see if it behaves then ..if not we need to look further . cleaning is never guaranteed .. even new can be bad but no need to think like that right now .
 
Thanks Pete. I think for now I'm going to remove it from the board and test what it measures on DVM. Will come back with those measurements and maybe you can offer some insight from that.
 
Wildly fluctuating DC offset can be caused by a poorly matched diff pair of transistors. I would replace the bad side (either H701/H703 or H702/H704) with gain matched KSA992FBU. Check out the trimmer first, and if no love, I think next step would be the diff pair.
 
Hi Steve. Firstly thank you for the advice. I decided to check out your suggestion before responding and just got done.

I checked out the pot and it tested fine. Measures exactly like the good side even off the board. Tested transistors you suggested and they test good as well. Can they test good but still pose such a fault? :dunno:Strangely I don't know why I did this but when I dropped the bass level it seemed to improve. Reading on DVT dropped and hovered around 1 to 3 mv. Thought I'd mention that just incase in meant something. Also tested a few other components randomly whilst in there and all measured good. Compared alot to the right channel which is setting fine at the moment.

Was this caused by me when I (in Avionics words) put in the cap Ass backwards?
 
Hi
JRM said:
Thanks Pete. I think for now I'm going to remove it from the board and test what it measures on DVM. Will come back with those measurements and maybe you can offer some insight from that.
Click to expand...
HI Pete. Pot measured 4,7k either way and reading kept stable on DVT for 5 minutes.
 
By the way. I am a new learner to all this and am curious to know what the effect of this erratic DC offset will be?:crazy:
 
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