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Amplifier Distortion, DC-Offset, and You!

IT'S ALIVE!!!

Just finished bringing my Pioneer SX-780 back to life. Installed two new Darlingtons, resoldered a bunch of stuff and last but not least- checked the DC on the outputs.

Initial: Right -112.6mv
Left -87.9mv

After adjustment: Right: -001.2mv
Left: -002.4mv

After replacing the Darlington STK0050's and resoldering some of the joints that looked like they had gotten hot or just didn't look nice and shiny. I am very pleased I was able to adjust easily and it is very stable, within +/- 0.3mv ! My new RatShack meter worked fine in place of my old Simpson 260.

THANKS EchoWars, your repair tips and this thread have been priceless. My wife said, "so you mean you could have fixed the old stereo?". I said "yes, if I had known what I know now". She said "so you mean we spent $$$$ on that new receiver and now you want to hook up the old one...."... I said "HELL YES"....

I'll be ordering my upgrade caps and reading more about that kick butt 780 modification you did. Gonna need something fun to work on next winter.

Thanks again!
griz780
Kalispell, MT
 
Nice going!

Long winters up there in Kali, a 780 might be a nice project. Spent most of a winter in Helena back in the 80's. Lotsa snow, very beautiful, very cold. Went back to Boise where it usually just rains in the winter.

Glad the info helped. Best of luck...
 
O.K. I gotta try this¡K But I am ashamed to admit that I am¡K I am¡K I am¡K ELECTRONICALY CHALLANGED.. O.K. I said it¡K

Down to Rat Shack to buy my FIRST meter¡K get the el cheapo $20.00 one that is on sale at http://home.earthlink.net/~kaito/m830bf.htm for $8.95.

Just as I was ready to motor home I stopped at the door¡K..

¡§Does this need a battery¡¨ I asked¡K Sure did¡K Found the one it called for so lets put it in and see if all works¡K Screws¡K Take 2 out and put battery in¡K. And nothing¡K Hmmmmm.. Dead battery or meter???? Wait¡K Now is that +- and +- or ++ and --¡K Take battery out and turn around¡K WHOOAH it works¡K Remember ELECTRONICALY CHALLANGED¡K. Now back home¡K With my $25.00 meter and battery¡K

Fire up my Kenwood KM-205 AMP¡K Turn volume down¡K. Hit speaker set ¡§B¡¨¡K. Stick those pointy things in where the speaker wire goes¡K. Turn meter on to DCV setting and¡K. Hmmmmmm nothing again¡K Let¡¦s try the DCA setting¡K We have lift off¡K.

R= -1.3
L= +2.3

As I read the other post this is GOODļ

My FIRST journey into the magical mystical world of ELECTRONICS¡K.

OMI
 
Not only am I ELECTRONICALY CHALLENGED, but I am BUTTON CHALLENGED also. Went to do one last check and had both the "A" and "B" speaker buttons on… DUH… After just keeping on the "B" speakers I had

R= -2.6
L= +5.0
 
Great thread EW,

I tested my Sansui AU999, I got

L 1.8 mV
R 9.9 mV

I was surprised that these were so low considering the amp has never had anything adjusted or replaced and it has been in heavy use for 30+ years
 
Re: IT'S ALIVE!!!

Can you tell me where you purchase those Darlingtons IC's? I would like to do the same thing to my SX-780, thanks.
newcomer Stoner :dunno:


Originally posted by griz780
Just finished bringing my Pioneer SX-780 back to life. Installed two new Darlingtons, resoldered a bunch of stuff and last but not least- checked the DC on the outputs.
<snip>
Click to expand...
 
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Just wondering. Did anyone go back and reread the very first line of the post from echowars?
" Time for one of those long, boring semi-technical posts that no one here reads..."
I think Echowars was a little off on that statement.
:D
Looks like one of the longest lived post so far, and absolutly one of the most informative. 14 pages and counting!!!
Thanks again Echowars, and of course everyone else.
Ron
 
Yes...had kinda suprised me that this thread is responsible for widening the 'Views' column to 5 digits.
 
Such an outstanding technical review and post, first, that you have the knowledge to begin with, and second, that it can be written in understandable English.
All your posts make great reading, you should be writing for the major magazines. . . . . . .
Thanks and Kudos to EchoWars!!
Charley Baird.
 
Being elctronicaly challanged let me make sure I got this right and the correct meter. One cheap rat shack 15 range digital multimeter.

dial goes from DCV, 550V to 2v, then OHM, 2M to 200, then DCA from 200mA to 2mA, then ACV 500V to 200V.

Now is it the DCA section that I read when I check the offset????????
 
You are looking for voltage here. A meter set to read amps will short whatever points you connect the meter to...not good, so don't do it.

Set to DCV at the highest resolution possible...for you that appears to be 2V range. Measure at the speaker output terminals.
 
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O.K. Assuming that I have not already fried some internal parts here are the numbers when I set the meter to DCV at 2V

For the SX-9

Speaker Set A 0.013 and 0.001

Speaker Set B 0.016 and 0.002

How do the numbers look.

Have not hooked any speakers up yet. Still have the monster in the garage. Weighs a ton... Needs a good cleaning... Not sure how to go about that yet... One step at a time for me as I am electronically challanged as you can get.... In fact I have to keep my fingers away from plug-in sockets so I dont electocute myself:)
 
Good Evening EW!

Measured my Kenwood KA 7100 and have the following

A channel: Left side is 22.9 mV, right side is 8.0 mV
B channel: Left side is 23.4 mV, right side is 8.4 mV

Are these cool or simply trouble in the making?

Thanks
 
OK you guys....what makes you think that the 'B' speaker terminals are going to be different from the 'A' terminals?

These are stereo amps/receivers. Two channels. Same amplifier for 'A' speaker terminal as for the 'B'. :D

Sorry...

I don't think the 7100 has a offset adjust, and I don't know about the SX-9. Both amps are well below spec (generally considered to be 'good' at 50mV or less). Best to be below 10mV for minimum distortion.

On the Kenwood, only a replacement of the differential pair with a closely matched set will lower it (or add a DC servo to hold at at 0.00V no matter what.)

On the SX-9, if it has an offset adjustment, it could be zeroed out. But being below 20mV means that you're doing pretty decently, and I don't think I'd sweat it.
 
EchoWars,
Thanks...

Like I said "I'm ELECTRONICALLY CHALLANGED"... but I see your point on the one amp thing:) See I learned a little more today. I thought those values were pretty darn close to each other:)
 
Optonica 5606

I checked the offset on this Optonica and it was 21mV on the right channel and 55mV on the left channel. I thought it sounded nice but I thought I was hearing some distortion especially on top so since I have the service manual which BTW is almost completly dedicated to its tuner but has about one page where it shows where to read and adjust Idle Current and Output DC voltage. . . I assume Idle current is "BIAS" and DC voltage is "OFFSET" So armed with the manual I started working. . .

I did learn that both these adjustments go hand in hand, I would set the Idle current to 10mV where it was about 7 on one and 14 on the other channel. Anyway now it is dead on 10mV for idle current and offset (+/- 0-30mV factory) is not hoovering around 0. . . It swings to about -3mV to +3mV but it is staying there after one hour of sitting there on with power connected.

Now the good part. . . Hooked everything back up and I was blown away by how good it sounds now Crystal clear highs , much much cleaner, better bass by far no mud anymore. What I thought sounded good now sounds great, very easy to listen to, a very sweet clean crisp sound. I was hearing some distortion. . . Much better now.

This old girl sounds just fine, lots of life in her. BTW the pots were also pretty stable, although you dont turn it much it is not near as touchy as most I have fooled with (which is not many anyway)
 
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