Checking DC offset on Technics SA-500

[redbeard74]

I have read EW DC offset thread but the speaker terminals on this receiver are a strange twist-lock type and I can't seem to get the mm probes in there.
I tried using a short section of speaker wire to read but the mm keeps jumping around wildly.
I have the service manual for this receiver and found a section that seems to describe checking the DC offset. Here is an image of the section.

I powered the receiver on for a few minutes to warm up, set it to AUX, no sources attached, volume all the way down. I read -8.2 on the left and -8.6 on the right. After a few minutes, the left has dropped to -7.3 and the right has risen to -9.7.
Here are some photos of how I got these measurements, could someone let me know if I am reading in the right place? I have made no adjustments.

In the photo below I have circled where I think I would make an adjustment if necessary. A small screwdriver would be used to turn the pot in the center, correct? Although, if these readings are correct, then they would be well within tolerances.

 

[avionic]

Not DC offset Icq is quiescent current ie. BIAS :yes:

 

[petehall347]

looks to me like you are checking the bias there .. i would leave as is it isn't to far out .or maybe warm it up some more and check again .
as for dc offset that is checked at the speaker terminals . ov d.c would be ideal on each side .. there may be 2 other trim pots for the adjustment

 

[avionic]

Your flirtting with disaster. Get yourself some mini-grabbers for your meter probes. Had you have made just a minor slip,you would have kissed your output transistors farewell.Refering to the second photo!

 

[avionic]

No provisions for DC offset adjustment.

 

[larryderouin]

Avionic is correct. OP is doing ok except for not having Mini clip/grabbers. The one I just sold after 4-5 years would tend to drift slightly from 30 to 60 minutes. Once it stabilized it wouldn't drift until the next cooldown/heatup cycle. Warm it up until it settles down and adjust to as close as you can to spec.

It's a pretty good receiver, I'd say fairly equivalent to a SX-780/SX750 range.

Larry

 

[terra1]

Minigrabbers are highly recommended. If they won't grab on the solder side, try the white block side. If the probes are hard to attach without popping off, try taping electrical tape across the fins as support for the probes to lean on. Sometimes I will use electrical tape to mask off nearby metal posts or contacts when working in narrow spaces just in case.

 

[redbeard74]

Thanks I didn't realize Icq = bias.

I do like this little receiver. It sounds very nice, and is very compact, measuring only about 13" in depth, so it can fit in small spaces.

I don't like the pressboard bottom, or the fake wooden sides/top. I am working on replacing those.

So far I have cleaned the dust out and sprayed contact cleaner into the pots and switches, which was difficult to access.

OK I am ordering some minigrabbers and will check the DC offset at speaker terminal when they arrive.

Avionic how did you conclude there is no provision for DC offset adjustment?

 

[avionic]

Thanks I didn't realize Icq = bias.

I do like this little receiver. It sounds very nice, and is very compact, measuring only about 13" in depth, so it can fit in small spaces.

I don't like the pressboard bottom, or the fake wooden sides/top. I am working on replacing those.

So far I have cleaned the dust out and sprayed contact cleaner into the pots and switches, which was difficult to access.

OK I am ordering some minigrabbers and will check the DC offset at speaker terminal when they arrive.

Avionic how did you conclude there is no provision for DC offset adjustment?

Looked at the service manual for the 700 . It has the same adjustment procedure without any documented procedure for DC offset.

 

[redbeard74]

Ok I have my mini grabbers in use now.
Problem is, the DMM readings won't settle down...
They keep jumping around erratically. I have had the power on for about half an hour now.
The readings jmp between 0 and 10mv but sometimes hit in the 20s or 30s for a split second.
Others seem to be getting an exact reading...
Here is how I am checking.

 

[avionic]

Measured with "no signal" applied.Tone controls defeated,volume control at min? Fluctuating offset of such a low amount is nothing to loose sleep over and not really worth the effort to try and minimize.

 

[redbeard74]

No signal applied, tone controls off, volume at minimum.
Guess I am just concerned that I can't get a "stable" reading like others have posted. But at least the numbers are low. Looks like it will be safe for my JBLs.
Thanks for your input avionic.

 

[avionic]

The offset may also settle down after the amplifier has been on for a while and thermally stabilized.I your case it would require replacing amplifier front end components,matching components..yada,yada,yada..

 

[larryderouin]

The SA-500 I had also had a "jumpy" offset. It was 10-15mv and centered on 0mv. -5mv to +5mv or so. Agree with Avionic as long as it's not extreme (+/-50mv or more) and the bias is stable you'll be ok.

Just make sure when you take readings, you have no signal (selector in AUX with nothing plugged in to it), tone controls off(Loudness) or centered(Bass/Treble), and volume to ZERO. Otherwise the readings WILL jump around. The readings are based on matched or balanced driver transistors at the front end of the amplifier section rather than an adjustable pot. As they age they get unbalanced slightly and will give a fluctuating reading. Generally, as long as it's under 100mv you should be ok. Some guys are more demanding of the numbers and insist on as close to 0mv as possible. It can be done, but it can be expensive ordering 20-30 transistors and matching them for 1 unit. I'm of the camp that if it's working don't (****) with it (as long as it sounds good. Others will tell you different and we're all allowed our opinions. It's your ultimate discision as to how far you take it.

Right now, with the numbers you have, I'd call it good and close it up and enjoy it.

Larry

 

[redbeard74]

Thanks Larry. Yes I had the same "jumpy" readings as you on the dc offset. After reading the DC offset sticky i thought when I finally figured out how to check it, I would be rewarded with a firm number--no such luck! At least it stays mostly in the lower range of milivolts

So I put it back together and am listening with headphones. I will probably hook it to speakers this weekend.

And check out the fancy hifi table