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Amplifier Distortion, DC-Offset, and You!

And everything is good in the world again! After replacing a few resistors which at first surprisingly and then stinkily caught fire a number of times before I figured out that my soldering was indeed OK, I came to the realization that some of the semiconductors were not in a good way. A few days and a few bucks later, and with a substantially renewed right channel main amp p.c. board, DC offset is now circa 3.8 mV in the left, -1.8 mV in the right.

EchoWars, this unit is not a capacitor-coupled unit, per se. There was an electrolytic on this particular board, though, which had leaked its icky goo all over the place and was no longer, you know, capacitating. Thank you so much for this thread! My Messiaen recordings have never sounded better!
 
Nice job!!

Nothing like learning on the fly. Your ears, and especially your speakers will thank you profusely. :thmbsp:
 
It appears to only go down to a 20V setting. (hard to tell)

You old meter also lacks settings low enough for the job.

Harbor freight has very inexpensive DVOM's that will do the job. Almost pocket sized.
 
"Harbor freight has very inexpensive DVOM's that will do the job. Almost pocket sized."

True, but once I factor in the price of shipping, and paying to toll to cross the bridge to my US post office box it probably ends up costing me the same as the Radio Shack one.

By the way, has anyone noticed that the Canadian version of radioshack (now called The Source by Circuit City) has almost no specs on their website for products such as the multimeters? They've really become unfriendly to us hobbyists.
 
rulerboyz said:
"Harbor freight has very inexpensive DVOM's that will do the job. Almost pocket sized."

True, but once I factor in the price of shipping, and paying to toll to cross the bridge to my US post office box it probably ends up costing me the same as the Radio Shack one.

By the way, has anyone noticed that the Canadian version of radioshack (now called The Source by Circuit City) has almost no specs on their website for products such as the multimeters? They've really become unfriendly to us hobbyists.
Click to expand...

I looked at there site. I didn't see any thing I would spend that kind of $$ on.

The HF DVOM I'm talking about is like $6 reg price. less on sale. Are you sure its not worth it?
 
The red, 7 FUNCTION, 19 RANGE DIGITAL MULTIMETER appears to be closest to the ones I have been getting. They moved the on/off switch separate from th eselector switch, which I consider to be a plus.
 
That looks to be identical to the ones HF had been selling at their local brick and mortor near my house.

Perfectly fine for homeowner/occasional use.
 
This is a SA 7500II that had been sitting for 7 years in a garage befor I got it.
I refinished the case, pollished the face and blasted the dirt out of the inside
with air. But then I got in a hurry to use it at work and used Radio Shack
brand contact cleaner on the switches and pots. The only kind they had.
It left an oil film but I had never used it before so I assumed it was OK.
After about 5 hours it started tripping the protection circuit and making a lot
of noise through the speakers. :scratch2:
I checked the offset with an analog meter ( all I had at work ) and it was fanning the needle on the rt channel.
Then I cleaned the oil out with CRC cleaner and then Deoxit. :thmbsp:
Now the rt is higher than the left but both are very stable and its been on
for about 10 hours and sounds great.
I don't think I'll use Radio Shack cleaner any more.
Thought I would share this, maybe save some one my grief :cry:
Terry
 
EchoWars said:
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.
Click to expand...


Well, live and learn. I did it to a Kenwood KR-4600 that sounded lousy anyway and doubtless needs some internal stuff replaced. What possible additional damage might I have done?

Sorry I didn't read more of the earlier posts, I'd just had some good luck changing cassette belts and got cocky I guess!

Thanks

Roy
 
If it's still working, then probably none. If DC offset was high enough, then basically you are shorting the output to ground and rather large currents can flow, depending on the size of the fuse in the meter (one of mine has a 15A fuse, so obviously the amp would self-immolate before it blew).

Let's be careful out there. :yes:
 
hmm, picked up an old bell and howell FET stereo am/fm reciever today, hitting 60-62 mV on three terminals and 68 on the fourth, inside there are four pots, and four FET's, but how do i figger out which one will help me set the offset, is there any quick way? or should i do trial and error? it still sounds good, but bass controls are wacky and cause a kind of motorboating at full tilt, scratchy pots, a whole wack of small problems, we'll see how this turns out!
 
Then it sounds like offset is the least of your problems.

If this is a early/mid/late 60's to early 70's job, it very possibly uses cap-coupled outputs, and therefore there is no adjustment (but there may be a 'symmetrical clipping' adjustment which is similar but is set under load, with a generator and a scope).
 
lucky me, it was an easy fix, a bad cap near the bass pot, some deoxit, and fiddling with the controls has me down to 12 mv/channel and at first i freaked myself out till i discovered i was messing with the bias rather than the offset, but its all good now, at least it sounds good.
 
Rotel RB-870BX Power Amplifier

I tested the following set-up:

Rotel RTC-850 preamp connected to RB-870BX power amplifier.

I just got my Cen-Tech multimeter that I ordered on ebay, and set it to 200m.

Left: -11.4 Right: -5.2

I also tested my Kenwood KR 4140 which is basically dead in the water:

Left 0.4 Right 0.2

The Karman Kardon hk 330i :

Left 6.2 Right -11.0
 
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Yamaha RX500: Sometimes a circuit protection switch kicks on with this one, not sure if its a faulty switch or indication of another problem:

Tested with Multimeter set to read <200mv.

RIGHT: 10.00mv LEFT: 32.9mv

YAMAHA A-450 RIGHT: 4.0mv LEFT: 5.1mv

Pioneer SA-550 RIGHT: 20.5mv LEFT 20.2mv

Awesome Sticky Thread!
 
Picked up a Pioneer SX-3700 at a garage sale for $5.00. Took it home, blew out the dust and plugged it in.
Right channel: 17.9
Left channel: 16.8

It was very "scratchy" sounding so I cleaned the volume pot and the sound is a lot better than I expected. :thmbsp:
 
Pioneer SX-1050:
Right: 27mv
Left: -1.5mv

Fisher RS-1060
Right: 11.9mv
Left: -4.5mv

Fisher RS-1050
Right: 50mv
Left: N/A

I guess my readings are pretty good for the most part. I'd like to get the right channel a little lower on the SX-1050. On the RS-1050, the right channel looks a little off, and I couldn't get a measurment for the left channel. That probably has to do with the fact that on that reciever the left channel doesn't output any sound until you turn the volume up just a bit.

Interestingly, I use both of the fishers together due to the fact that the pre-amp on the RS-1060 is broken. I use the RS-1050 as a pre-amp figuring that they are similar enough. Kinda funny that both the sections that I don't really use on both amps are the sections that have problems.
 
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