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Amplifier Distortion, DC-Offset, and You!

Good thread!
I havent read all 46 pages of it yet but I have a 636 with some pretty high offset. upon turning it on the right channel is 30mV and the left bounces around from .3V to just over 2V.
After it warms up for an hour the right channel is the same and the left has "calmed down" to .1V to .3V.
I have just ordered a service manual but its not in yet, is there anything obvious for me to check before it arrives?
Thanks!
 
Hey! :)

I just bought my first real amplifier and tuner. Yamaha CA-1010 and CT-1010.
Don´t have to even say how good they sound :)

I bought those from my friend and he had already changed every electrolytic cap (except 18.000µF and bipolar). So he had done much work, but didn´t repair the problem what it had. It repeatly dropped the speaker relay. So I get those quite cheap and spend ~ 5h to get it work! The problem was TR402, the 8.9V line regulator. This seems to be usual problem with CA-series. Altought I think that the powersupply stage is done with too bad components. Some transistors runs at really high temperatures! (So i improved a little of those heat sinks)

But today I measured my DC offsets. I had no glue what they could be.. because the caps were changed and it´s so old beast!

I WAS REALLY SUPRISED!! :banana:

Right Channel DC offset Class AB 7.0mV Class A 6.8mV

Left Channel DC offset Class AB 4.7mV Class A 5.2mV

Both A and AB class looks really good to amp at age of that! (and sounds also :naughty: )

Today I also repaired my friend old CR-840 (beautiful one also)

It was in the position which it left from the factory (26 YEARS AGO!) . So I thought it would be necessary to check it´s DC offset also. Gladly I found two trimmers for that on the board. Left Channel was -100mV and Right 150mV. I trimmed those to +-3mV and was satisfied! :) Maybe it was ruined more than mine because it has hybrid stage at the output and predrivers are those also :cry: (CA-1010 has TO-3 transistors) and it had no new caps. And It have had never any problems, but time has made it´s work to potentiometers and lamps (every of them were burned).

So I have to check the bias current from my CA-1010 and maybe trim it because I can see the other channel is more powerful than the other. The right values should be 100mA in class AB and 550mA in class A. Or am I wrong?

You guys have a really great forum here! I just found this! :lmao:

And thanks to EchoWars! You´re doing great work when teaching us who don´t know yet too much of this amplifier stuff :thmbsp:
 
So I have to check the bias current from my CA-1010 and maybe trim it because I can see the other channel is more powerful than the other. The right values should be 100mA in class AB and 550mA in class A. Or am I wrong?
Click to expand...
Dunno...maybe one of the Yammie guru's will chime in...
 
AF6130, Don't hold your breath waiting for your needed users guide to arrive at your email address.
I highly suggest you sit and do some reading thru the many threads here at AK.
We are a group of Friends who do infact help other Friends with their needs.
To join one day and immediately post your needs and desires shows us anything but friendship. To place your personal email address in a post is idiotic and opens you to all sorts of BAD KARMA.
To be a respected member here, you have to sit around and do some reading and join in the discussions ADDING when you have some INFO and earn the respect you so desire.
If you want to be a friend then stick around and show us that you are worthy of our friendship and save the requests until you have been here for a while and added to our discussions.
I seriously doubt any AK member will be helping you at this point so you might want to do some reading and search this site for a few evenings and figure out how to be a REAL MEMBER.
Gary
 
Hello and thanks to all for your past assistance.

This is my first post, although I have been reading and learning from your fine website for some time.

I have a Tandberg TR-1055 that shows 4.1 Volts - that's right - Volts, on one channel, while the other reads 1.5 mV or so. As a result, I did not connect speakers but could not resist the temptation to listen through headphones held at a distance from my ears. A loud buzz was audible over the audio signal.

Obviously, a serious problem exists. I suppose the immediate question is whether to spend the money it will take to diagnose and repair the amplifier or just use the unit as a tuner/preamp.

Thanks for any insights you may be able to share.
 
measurement question...

EchoWars... another newbie question.

I happen to have access to a couple of digital muti-meters....

I was imagining that the easiest way to dial in the bias and offset would be to connect one meter to each of the channel's test points to measure bias current, and observe variations in both channels as the pots are trimmed, and also to allow me to observe how both channels stabilize ofer a period of time.

Is there anything wrong with leaving the DMMs connected to the test points for up to an hour as the amp warms up and the bias current stabilizes? I am concerned whether doing this may affect the behaviour of the amp and introduce unforseen measurement errors.... I can't imagine it can cause any damage to the amp, but I would be concerned about that as well if it is a possibility.

Thanks in advance!
 
Leave them connected as long as you like...the impedence of the meters is so high the amp does not see them.

Heifetz, I know nothing about your amp, and certainly you are the only one who can decide if it's worth the time, effort, and $$ to get it fixed. I was under the impression that the Tandberg's are nice units, so if you consider that a vote to fix, so be it.
 
My offset is now .6volt! I guess .5volt was as low as the tech could get it. .6 is the lowest I can get it,now I have a bit of noise in that channel. While I was doing this I noticed heat damage on both amp boards by a resistor.
 
Great Post! (I would have never thought to give this a try). I'm gonna break out the Fluke and see what I get for measurements on the various pieces of equipment later this week... :D
 
The Marantz 2500 I worked on last month was 2.5mv and 0.1mv. I didn't see an obvious DC offset pot so I left it alone - no sense messing with a near perfect reading. Those readings are better than anything I've seen on a vintage receiver.
 
gonzothegreat said:
The Marantz 2500 I worked on last month was 2.5mv and 0.1mv. I didn't see an obvious DC offset pot so I left it alone - no sense messing with a near perfect reading. Those readings are better than anything I've seen on a vintage receiver.
Click to expand...

If you're desperate, I have a 2600 service manual and can probably point you to it from there (if someone with a 2500 service manual doesn't chime in). But I agree with you that it's not worth fiddling with. If it ain't broke, don't fix it.
 
A really informative and helpful thread!

Pioneer SA-9500
Before 45mv/65mv
After 3 mv/5 mv

Sansui 5000x
-both left and right swinging from -40 mV to +30 mV
-I read somewhere that this is capacitor coupled. The directions in the manual requires the use of an oscilloscope. I guess I need to bring this to a tech who has one.

Pioneer SX-1080
504 mv/187 mvolts !
-Can't adjust. I don't know which pot to turn. Could someone direct me towards the correct pot, please?
 
I don't have the manual, or I'd help. Go shopping for one, or start a new thread and see if someone has one and will help out.
 
Check your Private Messages Rusty, I can help you :) Need your email addy
 
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EW, I tried measuring the DC offest on a Sansui 5000. It almost acts like a tube amp when I tried measuring for DC once upon a time...

Is this a design that doesnt do DC offest? :scratch2:
 
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