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Amplifier Distortion, DC-Offset, and You!

Hi EchoWars,
I really appreciate your advice.

I tried it with speakers connected and there is no significant change. The offset on the left channel really jumps up with the input shorted and the low-cut turned-off. I don't know if a picture of the amp would help at all. I can't take one right now. My girlfriend has my camera and I keep forgetting it when I see her. My mind is on other things.

Do you have any advice for my next step?
 
If you want to go to the next step, you will either have to get the unit to someone who knows this stuff, or get the manual and do some reading (and start a new thread).
 
You'd never believe it, but the offset on my just acquired Marantz 2385 is -.3mv left side a 0mv to -.1mv right side....

Using my B&K autobench 3000.
 
Readings and Questions

I've had time to check most, but not all, of my receivers and amps:
L R
McIntosh MC 2505 2.5 1.0
Onkyo TX-8500 14.2 21.3
Kenwood KA-3500 -18.5 12.8
Luxman R-3045 14.8 -20.2
Pioneer SX-737 6.7 8.7
SAE Two A-14 16.8 12.8
Lafayette LR-9090 41.1 30.0
Pioneer QX-747 front 2.4 4.2
back 4.1 6.7

Some observations and questions:
Although high numbers indicate something is wrong, low numbers are no guarantee of problem-free units. I've got several impulse garage sales purchases that misbehave.
The SAE works OK as long as the parametric equalizer is off. Once on, there is a low level gargle out of the left channel, and touching/turning some of the equalizer knobs sends enough noise into the amp to trip the speaker protection relays. (I had yet to discover this when I took the DC offset readings, so I haven't tested with the equalizer on.) I've just bought enough Deoxit to drown those pots (if I can get to them). If the problems persist, am I in danger of "letting the smoke out" if I run the amp with the equalizer turned off?
The Pioneer QX-747, a dirty knob-missing dinosaur, slowly wakes up. I've never hooked up speakers to the rears. When attempting to get the DC offset readings, one of the speaker terminals produced a continually varying output. I got the stable reading from the "B" terminal. Any ideas on what could cause this?
The Lafayette LR-9090 has yet to be hooked up to speakers. While I was letting it warm up before taking readings, it was periodically clicking under the hood. Haven't had time to look into that, but I certainly won't hook up any decent speakers to that puppy any time soon. Are there any other relays beside speaker protection that might be tripping?
 
Thought I would post this here too.
Just tested my Accuphase E-202. Got 17 on one channel and 72 on the other. Needs a little work.... :)
E
 
wdug,

Wish I could give you more info on these units, but in general I'd say that fluctuations are fairly normal as long as they stay within a +/-10mV range. Instability can be caused by a lot of things; most are easy for a tech to address. On the SAE, it is possible that a leaky cap is letting DC get to the tone controls, and that is what is causing the protection to trip. Certainly try to clean the controls and see if it helps. On the Lafayette, open it up and see who's doing the clicking...
 
I got bias adjustment instructions from Packrat for my CA-1010, as there's no specific offset adjustment. After adjusting bias from a found 3mV in class B (spec is 25mV) and found 30mV in class A (spec is 280mV!) to spec, I find my offset figures dropped from 32.5mV to 31mV in the left channel, and dropped from 28.5mV to just 21mV in the right.
Tom
Edit: should I be hearing much of a difference? I can't tell if the treble's better now or if it's my imagination. I do notice it runs a bit warmer now, but maybe the difference in sound quality could only be truly measured by lab instruments?
 
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Just thought I'd throw my findings in here as well, my newfound Technics SE-9060 reads 15.8 and 12.8, and were sometimes lower, having the DC cut on (2hz) made no difference whatsoever.

My JVC Ax-Z911 read 20.1 and 28.6, and would vary from 24-34, but seemed to average out to around 28 ish, on the right channel.

I have a question, I don't know hardly anything about amplification, but how come when I adjust the volume on my JVC the mvdc goes up to about 70mv, then starts to drop, and then when I get to about the 1 O'clock position, I can get it to 0.0mv, and then it starts climbing back up, up to a voltage reading when it is at max? Is that normal, or bad? My Technics would just climb from min volume (at 12.8) up to a max volume reading of 77mv. I don't know how either amp operates, I have the manual for the technics, but It doesn't mean as much to me as I'd like it to yet. Anyway, thanks for the help, and a great thread to make sure this old gear is operating like it should! Crexrun
 
Dunno why it does that...but the volume should be left intentionally down all the way to avoid this type of interference anyway.
 
Yeah, It was for the actual testing, I just wanted to see what would happen, it the voltage would go through the roof, it didn't, so I guess I'm ok. Do you think you'd bother opening either of these amps for the time being? I think they both sound excellent, nice and full at low volumes. Thanks a lot, Crexrun
 
Unless you already know that the amp has provisions for DC offset adjust, and know where the adjustment is, I don't think I'd sweat it.
 
Echo,
In reference to your original post about buying off of ebay. I just bought a Pioneer SA 8800 in mint condition (80 watt per channel amp). I was told it sounded great. After turning this thing up to about 2 watts of power, terrible distortion. It's now in the shop, I'm patiently waiting to see what the issues are.
 
Yikes!

Best of luck...actually it isn't a terribly complicated amp, and whatever the problem is should be easy to remedy.
 
MC-7270 - 0mV (just adjusted it this week after a total recap, 'lytics that is)
MC-2125- less than 2mV
MC-2205-same
MC-2102- to heavy to moveperhaps this weekend ;-)
 
I'm far from a McIntosh authority, but aren't most (if not all) of these using transformer outputs? If so, offset should be very low.
 
My new (new to me) pioneer sa-8800 that I just got had enough dust in it to knit a sock and a little discoloration on the rear circut board where the relays are (looks like heat damage). After a good cleaning it measured just 2.5mv left and -2.1mv right, pretty amazing for 25 years old. Echo Wars, are the power outputs available for these? I am hoping to grab some if before they are unobtainium, if they aren't already.....
 
I don't know what it uses, but if they are standard TO-3 transistors, then some sort of replacement will likely be available for quite some time. The original Japanese transistors are likely long gone though.
 
are the power outputs available for these? I am hoping to grab some if before they are unobtainium, if they aren't already.....
Click to expand...

You may try on www.mark5.com
It's a UK based firm and they carry lots of obsolete and non-supported semiconductors without a large minimum order requirement.
Don't know if you already knew about the site, but I made a search on AK with "mark5" and it returned nothing.
So I assume that this could be a useful info to share with the fellows AKs :thmbsp:

Warmest regards and season greatings!

Carlo

PS: If you won't find what you're looking for there, you may always look on the cross reference database on digchip
 
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