Amplifier Distortion, DC-Offset, and You!

Hello guys.
I'm new to the forum.
I have made minor repairs on my equipments like replacing capacitors and improving some soldering and although I have an engineer training my career has been in configurations of telecommunication equipment ... so the few school knowledge of electronics are almost completely forgotten :(

However, as all of us here, music lovers, like to take and enjoy the best sound that our equipment can deliver.
As such, I have already made substitutions of poor quality potentiometers for the multi-turn type, but I have some doubts that I'm sure you can clarify as well as others.

My actual gear are a almost 20 years old Pimare Pre-30 and Primare A30.2, love them, by the-way :)

For power (A30.2) as the Service Manual is explicit the doubt is in the procedure ... should I short-circuit the inputs? I.e. get the 20mV with the xlr pins 2 and 3 shunted? The service manual called it idle current adjustment, what about dc-offset?

Now for Pre-30, that's a hard-one for me... I really don't know where or what should be measured by adjusting that multi-turn's.

Please note, have attached both service manual. Hop that also helps everyone o cares.

Thank's great forum
 

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  • PRE30 Service Manual.pdf
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I just tested my Denon PMA-860. Got good enough results: 7mV on the left channel, 2mv on the right channel.
 
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Okay, so the negative number is the same as a positive value.
Got it.

Seems just high enough to consider replacing transistors or something?
The receiver has been out of use for some time I believe.
 
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Just checked the DC Offset on my NAD 3140 integrated amp. Here are the photos of my meter with the readouts. Do I have the correct setting on my meter? It's set at "DCA", the selector dial at "200m". If that's correct, the readings are high! Or do I use the "DCV" setting with the selector at "200m"?

Left = 165
Right = 107

mtr%20pc%202.png
mtr%20pc.png
 
Just checked the DC Offset on my NAD 3140 integrated amp. Here are the photos of my meter with the readouts. Do I have the correct setting on my meter? It's set at "DCA", the selector dial at "200m". If that's correct, the readings are high! Or do I use the "DCV" setting with the selector at "200m"?

Left = 165
Right = 107

mtr%20pc%202.png
mtr%20pc.png

Yes you should be using DCV, DCA is DC amps. DCV is DC volts.
 
Thank You, Eastham! :thumbsup: Do I use the same "200m" setting on DCV?
Yes, that would be a 200 millivolt setting. If it's higher than that, go to 2000m (or 2000 millivolts, or 2 Volts). If it's higher than that you have problems. Seriously, you should be okay at 200m.
 
Set on the correct settings now. Here are my readings. Do I need to adjust the DC Offset with these readings? I do have the service manual for the NAD 3140.

L = 70
R = 45

mtr%20pctrs%202.jpg
mtr%20pctrs.jpg
 
Set on the correct settings now. Here are my readings. Do I need to adjust the DC Offset with these readings? I do have the service manual for the NAD 3140.

L = 70
R = 45

mtr%20pctrs%202.jpg
mtr%20pctrs.jpg

Yep, as the service manual says power it on for 5 minutes then adjust VR601 to read 0mV on left channel and VR602 for right. Be careful though, those trimmer resistors are probably pretty dirty and very touchy, very slight adjustments with your screwdriver.
 
Yep, as the service manual says power it on for 5 minutes then adjust VR601 to read 0mV on left channel and VR602 for right. Be careful though, those trimmer resistors are probably pretty dirty and very touchy, very slight adjustments with your screwdriver.

Should I spray a bit of DeoxIT on them first? The amp is very clean inside.

Do I need to use a special screwdriver?
 
Should I spray a bit of DeoxIT on them first? The amp is very clean inside.

Do I need to use a special screwdriver?

I wouldn't, you could damage them doing that. A plastic adjustment tool would be ideal but a small flat blade scredriver works just as well. Like I said, just be very careful. don't poke around at anything that might be live or the consequences might be... Shocking... and make very, very, light adjustments. Just enough to barely turn it and always keep an eye on your meter.
 
I wouldn't, you could damage them doing that. A plastic adjustment tool would be ideal but a small flat blade scredriver works just as well. Like I said, just be very careful. don't poke around at anything that might be live or the consequences might be... Shocking... and make very, very, light adjustments. Just enough to barely turn it and always keep an eye on your meter.

OK, I won't use the DexoIT on them. I don't have a plastic adjustment tool, but I have small screwdrivers that will work. I'm very careful at adjustments, I'll definitely be careful at turning them. I know that a millimeter either way can really change the reading. Correct?
 
OK, I won't use the DexoIT on them. I don't have a plastic adjustment tool, but I have small screwdrivers that will work. I'm very careful at adjustments, I'll definitely be careful at turning them. I know that a millimeter either way can really change the reading. Correct?

Yup, they're pretty sensitive.
 
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