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Old 6th January 2015, 10:48 PM   #1
Flibble is offline Flibble  United Kingdom
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Default Help with NAD3020 DC Offset Problem - Please!

I recently got a NAD3020 (off another diyaudio member ) which seems to be working fine apart from a 0.5v DC offset on one channel in the power amp section. I've fixed a handful amps in the past but I'm not an electrical engineer and I'm stuck with this so any help would be gratefully appreciated!

If you look in the piccy below you can see the voltages I'm measuring and what they should be, all the other voltages match the other channel. There is 0.5v DC on the output but it plays music and reproduces a sine wave fine on my scope. The green dots show the bits I've replaced or tested out of circuit so far and I've not found anything obviously failed yet testing in-circuit. Given the age of the PCB I've been a bit reluctant to pull components.

I'm hoping this may be an easy diagnosis for one of the awesome diyaudio experts

Thanks and Happy New Year!!
Dan.

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Old 7th January 2015, 11:01 AM   #2
nattawa is offline nattawa  Canada
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Can VR5 not do anything to the DC offset? What voltage readings do you have on C638 and C630 far side from the output supply rails?
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Old 7th January 2015, 11:23 AM   #3
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It's probably also worth checking that your 3020 does actually correspond to the schematic / manual you're using - there are several different versions (3020, 3020A, 3020B, etc. - even at least 2-3 versions for the 3020 itself), and a lot of pcb's that differ slightly to the manuals.

Did you also check / reset the bias after the component replacements you made ? and how do the actual voltages look on the working channel ? do they agree with the schematic you have ?

Replacing / pulling components shouldn't be a problem, the early pcb's are pretty good - just be sure to use a temp. controlled iron, and desoldering braid (flux as well, if needed), and don't 'sit' on a pad too long. I'd suggest finding the problem first though.
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Old 7th January 2015, 09:47 PM   #4
Flibble is offline Flibble  United Kingdom
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Default Fixed!!!!!

Thanks for the replies nattawa and Goldie99 - turned out to be Q604 that had gone open between base and emitter. Pretty hard to spot (for me anyway!) as voltages around it looked ok, ended up just lifting things to test them.
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