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Amplifier Distortion, DC-Offset, and You!

2X Trio (Kenwood) L-O5M mono-amps; c. mid-late 70's (forget exactly)

Exact history not known prior to my ownership of the last 4 years, but no obvious, physical evidence of replaced electronic parts:

Designated LEFT channel: 23.3 mV
Designated RIGHT channel: 13.5 mV

... Cheers!
 
Sick SX-1980

Wow.......this sucks. I always thought my SX-1980 should sound better than it does, now I know why. My meter doesn't have a millivolt scale, but on a 0-5 volt scale I'm showing over -0.4 volts per channel. I suppose that equates to over -400 millivolts. After those readings, I have taken that receiver out of service. Now I'm gonna have to run a Yamaha DSP-A2070 instead.....dammit. Echo Wars, one of these days soon, when I get the cash, would you be interested in going through my 1980? I was going to try to find someone local, but I don't know anyone who has a lot of experience with a receiver of this caliber and age. Thanks.
 
Results

Had this posted in the wrong place at first. I've tested all of my amps, and the Pioneer sx-1250 at 8mv per side is the hands down winner.

My Yamaha rx1100u natural sound receiver on the other hand, no longer works at all after the test. Just a faint bit of sound if the volume is cranked.

I pulled the cover to see if I let the smoke out of anything, and It appears as though a couple of larger capacitors leaked some goo on the circuit board and on a row of four diodes. I'll see if I can get a picture and maybe someone can enlighten me a bit.

This was a 1200 dollar receiver in it's day, Hopefully it's still salvagable...

Ed
 
My Yamaha rx1100u natural sound receiver on the other hand, no longer works at all after the test. Just a faint bit of sound if the volume is cranked.
Click to expand...
You mean...it was working, you decided to do an offset test, and now it's not? Did you by chance short the speaker terminals somehow? Or set your meter to read amps and then connect to the terminals (which would short them)?

Sorry to hear it. Whatever is the matter can be fixed.
 
I reread the instructional posts, because I was confused by something EW said in an earlier post. It still sounds like I'm supposed to connect the meter probes to the pos and neg speaker terminals to check dc current. Even though this is, by definition, a dead short. Have I missed something?
 
Have I missed something?
Click to expand...
Yes. We are measuring voltage, not current. I can't remember the last time I actually set my meter to measure current...probably years ago. Almost always, if I want to know the current in a circuit, I'll measure the voltage across a known resistance and calculate the current. Safer, easier. It is a rare occasion when you need to measure current directly. Very rare.

DC offset is a voltage measurement. Set the meter accordingly. Bias is a current, but it is set by measuring the voltage across known resistances (usually the emitter resistors) and calculating the current.

This thread concentrates on offset measurements. Bias is too variable from unit to unit, and the method of measuring and setting varies too much to go into here and is thus beyond the scope of our discussion.;)
 
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New member with DC Offset results

OK, after a mis-attemp (measuring AC volts) here's what I have found:
Sansuis
AU-717 - out for repairs; ina
AU-9900A - most recent purchase, unknown history, looks original; this may be my new favorite because of the sound and features:
1mV - Left
1mV - Right
Haflers
220 - #1 - factory wired, my oldest friend of 20+ years, original:
3mV - Left
1mV - Right
220 - #2 - factory wired, owned for ~ 2+ years, original:
14mV - Left
5mV - Right
500 - 2nd oldest friend of 18 years, original;
20mV - Left
86mV - Right
Mitsubishis
DA-A7DC - owned for ~ 9 mos, original:
10mV - Left
1mV - Right
DA-A10 - owned for ~ 12 mos, repaired shipping damage to heatsinks and upgraded binding posts and RCA input jacks:
1mV - Left
7mV - Right
DA-A15 - owned for ~ 12 mos, repaired shipping damage to heatsinks and upgraded binding posts and RCA input jacks:
1mV - Left
13mV - Right

With the exception of the 500, all are used at least 1x/week and I don't hear any problems; but maybe I don't want to?

Hmmm, 8 amps plus an Advent 300 in the shop and a Tivoli Model 2 in the office - do you think I may need help!
:hdphones:
 
Well, here's the problem, I don't know the difference between voltage and current. All I really need to know is how to do it, I don't need to understand it. Am I correct that the probes go on the speaker outputs? Am I correct to set the meter to measure DC? Am I correct to turn the input selector to an input not in use, like aux or something? Am I correct to turn the volume all the way down? If so, I'm doing it right.
 
Originally posted by dsk
Well, here's the problem, I don't know the difference between voltage and current. All I really need to know is how to do it, I don't need to understand it. Am I correct that the probes go on the speaker outputs? Am I correct to set the meter to measure DC? Am I correct to turn the input selector to an input not in use, like aux or something? Am I correct to turn the volume all the way down? If so, I'm doing it right.
Click to expand...

Think in terms of plumbing. The rate (speed) that the water is flowing can be thought of as voltage. The amount of water could be considered current(amps). the size of the pipe it flows through is resistance. Ohm's law.

You have it right in your above post.
Ed
 
Originally posted by fropiler
Think in terms of plumbing. The rate (speed) that the water is flowing can be thought of as voltage. The amount of water could be considered current(amps). the size of the pipe it flows through is resistance. Ohm's law.

You have it right in your above post.
Ed
Click to expand...


Acually, Voltage is best thought of as the pressure of the water. The speed and amount of water flowing is the current and the size of the opening is the resistance.
 
Thanks. I need things dumbed down when it comes to electricity. Wish I could repeat a couple of high school subjects now!
 
Originally posted by CDFixer
Acually, Voltage is best thought of as the pressure of the water. The speed and amount of water flowing is the current and the size of the opening is the resistance.
Click to expand...

I stand corrected. Pressure is a better way to describe voltage.

Ed
 
Originally posted by EchoWars
You mean...it was working, you decided to do an offset test, and now it's not? Did you by chance short the speaker terminals somehow? Or set your meter to read amps and then connect to the terminals (which would short them)?

Sorry to hear it. Whatever is the matter can be fixed.
Click to expand...

It was working, but it had been removed from service when The Sansui came into play. (Must be jealous). It seemed to have a problem in the phono section, as the volume control was noisy and the output seemed lower than usual. Might have been on it's way anyhow.

The meter I used is a Fluke 77. You have to move the leads on the meter itself in order to test current, so I'm pretty sure I didn't do that - Can't ever remember testing current for that matter.

Ed
 
One guy explained to me volts/amps/resistance this way, and it was very easy to understand.

Imagine a wheelbarrow, the size of which would be the voltage, the larger the more it can carry. If the wheelbarrow was empty, then it had little amps (current), if the wheelbarrow was full it was carrying lots of current. Now if you were pushing the wheelbarrow uphill, then that would be considered resistance. It maybe an oversimplification, but it was easy to remember.
 
Originally posted by dsk
Thanks. I need things dumbed down when it comes to electricity. Wish I could repeat a couple of high school subjects now!
Click to expand...
Read here. There is a link at the top left for 'Elementary Electricity'. Go for it.
 
For recently snagged SX-980: 52 & 70

How hard is it to adjust the DC offset in this unit???
 
10 min job...if you know which pot to adjust (there are four pots, two for offset, two for bias). Tweaking the wrong one will result in smoke.:eek:
 
Hi OMI
I have the service manual if you can wait I can post a pic of the section that deals with the adjustments.But with the server not posting pic I will have to wait.

Ron
 
No hurry... I dont want to SMOKE this find:) I can hold off until the server issue is resolved... I am intering into uncharted waters here as I am electronicaly challanged for the most part. :)
 
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