Amplifier Distortion, DC-Offset, and You!

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Just tested my Soundcraftsmen A5002 and got the following results

ch A: -108mV
ch B: -26mV

I do have a concerning hum on my ch A speaker, but it goes away with amp gain control at zero. I noticed DC offset is not affected by gain setting.
 
Sx-850 sn: Xi...

Hey all. New here and new to vintage receivers. Just picked up a mint Pioneer sx-850. Absolutely beautiful condition! Bone stock, no replacements or work('bout to change :yes:) that I can see outside of lightbulbs.

Manufactured Sept 1977

R is ~ 3.5mv
L is ~ 5mv
 
Pioneer sx 535 offset

2 questions:

1) The service manual for this receiver says to set the offset to 15mV. But it seems like it can go lower. Should I target 15mV, or try for 0mV?

2) The right channel reads out around 1mV. But using the adjustment pot, it won't go below around .5mV, or higher than around 1.9mV. This seems odd to me, as the left channel pot will adjust to a much larger range. I just recapped the amp board, and replaced the adjustment pots with some bourns 3299 pots.
I guess my question is :
Is it possible that the offset in the right channel is so naturally close to 0 that the adjustment pot isn't going to move the value much? Or do you think that there may be a problem somewhere? The amp sounds good, so I don't think there's a problem.

Thanks,
Jim
 
2 questions:

1) The service manual for this receiver says to set the offset to 15mV. But it seems like it can go lower. Should I target 15mV, or try for 0mV?

2) The right channel reads out around 1mV. But using the adjustment pot, it won't go below around .5mV, or higher than around 1.9mV. This seems odd to me, as the left channel pot will adjust to a much larger range. I just recapped the amp board, and replaced the adjustment pots with some bourns 3299 pots.
I guess my question is :
Is it possible that the offset in the right channel is so naturally close to 0 that the adjustment pot isn't going to move the value much? Or do you think that there may be a problem somewhere? The amp sounds good, so I don't think there's a problem.

Thanks,
Jim

Are you sure that your new pots are OK, and connected correctly?

Did your amp adjust OK before new caps and pots?
 
Are you sure that your new pots are OK, and connected correctly?

Did your amp adjust OK before new caps and pots?
I can't say for sure if the new pots are OK. I'm pretty sure they're installed correctly. I'll need to double check all of that.
Yes, it did adjust OK prior to the changes. But the old pots are hard to adjust, so I decided to replace them.
 
Looks to me that something is up with the right channel. Think I would remove the new pot and check it with a meter. But, don't blame me if something goes wrong. I'm normally pretty careful, but I remember a few years ago a probe or something slipped adjusting a Quad 303. Destroyed the big power supply regulator transistor. Still, that replaced, all OK.
 
2 questions:

1) The service manual for this receiver says to set the offset to 15mV. But it seems like it can go lower. Should I target 15mV, or try for 0mV?

2) The right channel reads out around 1mV. But using the adjustment pot, it won't go below around .5mV, or higher than around 1.9mV. This seems odd to me, as the left channel pot will adjust to a much larger range. I just recapped the amp board, and replaced the adjustment pots with some bourns 3299 pots.
I guess my question is :
Is it possible that the offset in the right channel is so naturally close to 0 that the adjustment pot isn't going to move the value much? Or do you think that there may be a problem somewhere? The amp sounds good, so I don't think there's a problem.

Thanks,
Jim
Actually the manual does NOT say to set the offset to 15mV. It makes no mention of offset. In fact, it makes no mention of what the adjustments are at all. It's obvious to a skilled or trained technician however, that the adjustments are idle/bias current adjustments. They should indeed be adjusted for 15mV's across the TP's. Make sure you have good contact to the TP's with your meter while measuring. If you can't adjust the bias then you have problems and I'd suggest you start by checking your work carefully considering you say you were able to adjust prior to your tinkering. Start a thread in the Pioneer forum.
 
Yamaha CA-710 (CA 610II)

Arrived yesterday: meter lamps out, otherwise very clean.
Yet to listen but:

Left: 6.9 mV
Right: 7.4 mV

I think the manual says 10 +/- 3 mV...

:thmbsp:
 
EW, (or anyone else),,,

After finding this thread very informative, and not realizing how important DC offset was, I decided to check my 7 channel HT amp. I did not read all 260+ pages, so hopefully how I checked mine was appropriate.

I found this thread at the perfect time, because I am swapping interconnects between the amp and pre. When I tested my Adcom 7707, I had nothing connected to it, so there was no "unused input" selected. Is the way I checked the amp acceptable???

If so... the measurements are as follows,,, not measured by specific channel, but from left to right facing the rear of the amp.

CH 1 - 4.2mV

CH 2 - 8.9mV

CH 3 - 5.5mV

CH 4 - 0.4mV

CH 5 - 4.7mV

CH 6 - 1.8mV

CH 7 - 5.5mV

One channel is a touch higher than the rest, but all fall well into the "Damn good!!" range. I don't plan on doing any adjustments, as long as the method I used for testing is OK.

TIA,

---Mike---
 
Wow... 11 y/o thread! Proud to partake...

Picked up (literally) a Onkyo TX-8500 from the "FREE" box at a garage sale last Sunday. The teenager at the sale said: "Look, that dude is taking that old, heavy stereo!! Hahaha!!"

The most fun I've had in 10 years!

Anyway, here's the DC offset:

L: 9.9mV
R: 4.6mV

Most happy!

There some distortion on the left channel at quite high volume, but goes away with rapid actuation of the scratchy balance pot (needs cleaning!!!).

265 page thread?!?
Thanks EchoWars...
 
Hmm...Well I have just finished completely recapping my Marantz 2252B Receiver
And after adjustment I have checked DC off set with reading of
left between -0.0mV and -0.6 mV bouncing
right -0.7mV to 2.0mV again bouncing between

is this a good result?....or am I doing something wrong here?.....
no speakers connected....channel one ON.....measured from channel 1
aux mode no signal connected....
tone controls all neutral...
 
I take that back...have unplugged from Isolation transformer and Variac...retested on mains....now have around 4mV both channels...
 
Soundcraftsmen A2502, after replacing differential transistors both channels with Zetex 694b ( I think) 20/20 mv, initially was around 70mv both channels. Replacing R4 both channels from 47k to 75k brought it down to L +3mv R -3mv (thanks to Ecluser for that advice).The unit sounds great.
 
Good evening all,

I have a Fisher (Sanyo) CA-78 receiver which is part of a rack system that my father purchased new in '83. While I am aware of their B.S. marketing (Studio Standard plastered on everything) and unfavorability of this era amongst enthusiasts, I am particularly nostalgic for this unit as it's what I endlessly listened to music on as a child. I believe this unit isn't fully in BPC territory, although it does use a STK.

I would love to get the best possible sound and then some out of this stereo and I am just the type that has enough patience and lunacy to do it.

Right Channel -10.4 mV

Left Channel -103.2 mV

Where do I go from here? I do have the service manual.

Thanks!
 
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