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Amplifier Distortion, DC-Offset, and You!

cool... time to find some resistors to blow up!

Should I care about the power rating of the resistors? Am I correct in assuming that smaller is better.. like 1/4 watt??

Update: 7PM
I repeated the measurement using a 1/4 watt 100 Ohm resistor across each pair of speaker terminals.... no exploding resistors!

DC offset fluctuates in a range between +10mV and +15mV on both channels.... I guess that means it is relatively healthy ?

Update: 10PM
Waited a couple of hours to let the amp warm up and stabilize before hooking up speakers.... it works!

Looks like a good cleaning and DeOxit treatment for the switches and controls might be all that is required now. There are only 2 trimmer pots on the circuit board.. presumably for adjusting offset.. but I'm not sure I want to touch them at this point.

Thanks again EW...!!
 
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so what does this mean?

EW...

So now that I know the KA-2002A actually works, and can play music using a tuner as a source... do those DC offset measurements indicate anything peculiar or interesting in this case?

Is there anything wrong with the amp that may require attention, or was the small load provided by the resistors enough to make it "simulate" the load of some speakers attached?

I'm a little confused because I don't recall a similar situation being discussed in this thread... although I confess I didn't go back and re-read ALL the posts again.

If I'm interpreting correctly, the second set of measurements ( taken with the resistors in place ) indicates that the amp is performing more or less within normal tolerances? Is that correct or am I missing something?
 
Since the amp shows pretty normal readings with a resistor connected it appears that the amp is indeed a capacitor-coupled output.

I guess you'd have to consider the way a capacitor works. With a positive voltage on one cap plate, and the other plate (which would normally go to the speakers) disconnected, there is no place for the opposite plate to collect electrons and zero the voltage, therefore both cap plates will show a positive voltage.

Caps need a charge and discharge path...without one measurements get wierd and sometimes make no sense (and knowing this is also a good troubleshooting tool).
 
thanks for the tutorial!

EchoWars..

Thanks for your patient explanation... gradually the pieces of the puzzle are falling into place... I think!

Looks like a service manual is the next step if I decide I want to tinker with this any more than just a routine cleaning.
 
Great thread EW and I do wish I had a bit more knowledge in the electronics field.
I just spent the day cleaning the controls of a SUI G 8000 I scored last week.
Unit is in very nice shape but was filthy in and out. What a job it is to get to all the pots and switches!!
No more scratchy controls and no more drop out on the right ch. so I guess the Deoxit bath did the job.
Now for checking the offset!
My DMM has a scale of 200m.
Left reads 10.1 - 10.4
Right reads -27.8 and keeps going up very slow never quite setteling. It is at -31.5 now.

I have no clue what this means or if there is a problem but it does sound very nice.
Gary
 
Beginner's Luck

Well, that 'New Baby' I've been touting since Friday (Technics 8600) was put to the test on Saturday. I was nervous about what the multimeter would prove for the DC offset, because what I was hearing was pretty dern sweet.
Left: ~8 mV
Right: ~9 mV
:banana: :thmbsp: :banana:
Starting with good health! Woo-hoo!
Hopefully I won't have to crack open the case for a long time. I auditioned many good CD's over the weekend and was very impressed -- downright proud.
Now, as soon as I can get away from work, the main event will commence. Since college, and the subsequent frequent moves, my favorite audio test albums have been: 1. Dark Side of the Moon 2. Dire Straits - Brothers in Arms 3. Tool - AEnima. :yes: I absolutely can't wait. Hopefully my wife and my dogs can take it...
[Edit] Holy... Track #1 of Brothers in Arms just played in my office radio after I posted this. Must be a good omen. :thmbsp:
 
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I tried this with one of my receivers and 2 integrated amps yesterday.

After the 10 minute warmup:

Pioneer SA-7500: about 5 mV on each channel
Realistic SA-1001: L: 30 mV R: 75mV
Yamaha CR-2040: About 30 mV on each channel
Never got around to testing the SX-3600

Well, since the Yamaha is the one I use the most I decided to subject it to further testing. Left it on for about an hour listening to the radio. Put it back on Tpae 2, turned the volume down and

After 1 hour: Both channels about 40 mV

Then I went to dinner, movies, and drinks leaving it playing the radio softly.

After about 8 hours: 80 mV on each channel!

I think this might be a "real" problem because if I leave the receiver off for an extended period, I always have to put the speaker selector switch to the off position before turning it on to keep a "thud" from going to the speakers when I press the power button. No problems like that once it is on.
 
Try setting selector to AUX and choose spkr connections where you will place leads for measurement with spkr switch and Vol @ 0.
I've never measured using Tape position so don't know if this will help or not.
 
Thanks for the advice. I used Tape 2 because there is nothing plugged into it. Aux is my CD player. I did have the volume turned down to 0 and the speaker selector set on "A". My speakers are attached to "B". I will unhook the CD player and see if there is any difference using AUX.
 
Not likely, but what have you got to lose by checking?
That thud is indicative of DC offset, btw.

I suggest you reread EW's first posts in this thread to clarify a few things.
Easy to forget with a thread of this length.
 
Got the new Sansui G-9000DB switches/knobs cleaned up and working well. DC offset is R~ 1.2 mV, L~ -41.0 mV. Need to get hold of a service manual.

Recently picked up a Pioneer SA-7500 at the flea market (free - could not tell if it powered up - the guy said take it - power indicator light was simply out) and it tests at R~ 13.5 mV, L~ 24.8 mV.

Alan
 
Did a yearly (don't I wish! - first the body, then the poor, poor mind) checkup of my HK 770 65 WPC amplifier. On the left, 58.2 mV, brought quickly down to -1.8 mV. But the right… wow. 8.8 V. Yes, 8800 mV. Aren't there supposed to be protection circuits or fire alarms or something?! Well, now it's waiting on the bench for a few new capacitors and resistors, and let's hope all will the soldering goes well in the morning. Time for the aspirin in the whiskey.
 
Hi EW, I am trying this with my $20 Radio Shack doohicky set at 2V.

Sansui 1000X: R) .038 +/-3 L) .023 +/-2
SX-1050: R) -.010 L) -.005

Can you let me know how I did? Did they pass?!
 
2V is the best it can do? No millivolt scale? Hell, even the cheap ones I looked at a few months ago had a 200mV or 300mV scale.

Anyway, your reading on the Sansui is 38mV and 23mV, and 10mV & 5mV on the Pioneer.

See how that decimal works now?? :yes:
 
Help figuring this out?

So, I'm trying to educate myself to this idea as I've acquired a number of older receivers I want to give to friends, or sell, (to make room for more!) but I don't want to pass along anything that isn't working up to spec.

So yesterday I ran down to the local hardware store and picked up a little inexpensive multimeter. It has two possible kinds of settings: "DC V" which runs from 10 to 500; and "DCmA" which runs from 0.5 to 250. Testing it on an old Kenwood, I got effectively no reading on any of the voltage settings, but a small reading when set in the DCmA range.

Am I doing this right? Is the meter I got not sensitive enough to register with speaker leads? Should I be checking voltage or amperage here?

Sorry to sound so ignorant; I've gotten pretty good at fixing mechanical problems with vintage units, it's time for me to start delving into electricity but I don't know anything about it.

Thanks

Roy
 
You volt meter is a very basic unit and does not have the sensitivity to measure Millvolts. the Ma stting is for milliamps, which is current, not voltage.

Harbor freight sells an inexpensive ($3-6? depends on sale of the day...) multimeter which goes have a low enough setting for measureing mv. Millivolts, or thousanths of a volt.

I start at the 20V setting and move down to the lower settings, just in case I get a unit that is way out os spec. (Hasn't happened so far, [knocks on wood]
 
Please don't set your meter for amps and connect to the speaker terminals. You'll let the magic smoke out.

Sorry to be a PITA, but in this very thread I've said that at least a dozen times...never set your meter for amps. Never.
 
I just checked my Carver PM 900 that had been used for five used by a rock band before I got it. It was at 1.9 on both left and right for th first 30 minutes and then setled down to 2.7 -3.0 after that.Interesting that it got higher as it warmed up.
Ed Ruane
 
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