Amplifier Distortion, DC-Offset, and You!

Lefty, that was my next question, could the bias adjustment impact my high reading. Now, it's not like I know what I'm talking about, it's that "little bit of knowlege" causes problems for me.

EchoWars The guy who repaired this for me was a nice guy trying to do me a favor. I have no problems with what he did, but I agree I need to get the thing doctored. Not much selection for quality repairs around here.

Thanks guys for your guidance.
 
I'm sorry. If someone was doing you a favor, and they got it running, then you are more than one step ahead. I assumed that you paid out the nose (like most do) to have it repaired. My apologies to your friend.

Bias can affect offset, but not usually to the tune of 1/2 a volt. The 9700 could have an adjustment for DC offset, in which case it is a 10 min job to fix, otherwise we are talking about replacing transistors.
 
Holy crap. My Sansui 5000A reads high. Over 300 mv on each channel. Hmmm... Looks like I have some work to do. I'm not looking forward to it, considering this receiver is the most time consuming to work on EVER. It has sideways mounted amp boards with an inch of space on either side, so they almost have to be removed to do any work on them. Hell, just changing the bulbs takes a good hour.

My Onkyo A-7055, on the other hand, read at 11.8 and 8.7, not bad for a 30 year old amp.
 
See if the Sansui has an offset adjustment (at lot of Sansui's do). If not, and nothing else is wrong (measure a lot of voltages and make sure ground potential has not been compromised between the power supply and the amp PC board), replace the input differential pairs with new ones. If they are NPN, use a 2SC1775A or a 2SC2547. If PNP, a 2SA872 or a 2SA1085 (I prefer the C1775A and the A872, but any of them would work well). With no other tweaking, this should get offset to below 50mV, and much lower if you are lucky. ;)
 
EchoWars Ok, With some hesitation, I crept up on the bias adjustment and was able to reduce the reading to 39.7mv on my problem channel. Now I am 39.7mv left and still around 7.0mv on the right.

Result? Power meters show closer matched output channel to channel. Paranoia forces me to still use cheap speakers, but it seems like the bottom end is better.

Thanks for help.
 
EchoWars,Where do you learn this stuff?? Gee Whiz,If my stuff turns on and works I am happy.I am in awe of your knowledge!!
Do you have a tech background?
Your contributions to this form are enormous

Best,
Alan
 
Wideone, Hopefully, you were measuring across the emitter swamping resistors (see bottom of page 2 of this thread) and not measuring the output of the speakers terminals when you adjusted, otherwise bad things can occur. If you adjusted the bias current correctly, and the offset just happened to drop to 30-something mV as it is now, then all is cool. If the amp does not allow for offset adjust, then all you can do is set bias to proper value and cross your fingers. Beyond that it's transistor replacement time (still not a huge deal, BTW).

Opt80, you guys are going to give me a big head. :stupid:

Compared to a lot of guys out there doing this stuff, I'm a hack. However, compared to the average clown at the local TV repair shop, I'd like to think I'm a couple of notches better. As far as my edjmacation, I've a degree in Eng. Technology and a degree in Comp. Science. Most of my career has been spent as a robotics technician in the print industry and in cleanroom fabs (Intel, Moto, AMD, Hitachi, etc, etc..), but I have been fooling around modifying and repairing classic audio gear since the early 80's (before they were classics).

I love this old equipment. I mean, I really love it. Comparable quality today (if anything is even really in the same league) costs upwards of $1200 for even a simple integrated amp. I was just out of HS when this great 70's gear was being sold, and could not possibly afford it. Now all but the most exteme examples of the best 70's gear are within most budgets.

The problem with the old stuff is that it is, well....., old. Drifting transistors, old high-ESR caps, or leaky caps, and general neglect conspire to see to it that a small minority of those with this old great gear will ever really know what it is supposed to sound like. My mission in life is to see that they do. ;)
 
"I love this old equipment. I mean, I really love it"

me too EW what a pleasure;)
I take a look tonight to my oldmarantz 4300 receiver and it s wonderful L 2 mV R 4.5 mV not so bad for a 28 years receiver never adjusted :cool: I just repaired the Power supply.

Dom
 
Sansui 9090db I just picked up from a junk yard for $10, 22mV in left and 4mV in right. Need to get a service manual so I can really tune this thing up.

Pioneer sx-828, another $10 find, awsome condition, under 5mV each channel.

Kenwood ka-9100, bought fom ebay at going rate and had 25mV in left and alost 50mV in the right channel. After adjustment I now have 2mV in left and 5mV in the right channel. I really need to replace those trim pots they are so damn sensitive.
 
EchoWars,

I just tested the Luxman L-450 I got for $10 and left channel was 1.1 mv and the right was -0.5mv
 
Not too shabby. The Lux may be cap coupled with a reading that low...or else it simply may be well balanced.

rhinofly, nothing really wrong with the readings on the Sansui. Not perfect, but still fallling well into the acceptable catagory.
 
EchoWars,

Tonight I checked the Concept 16.5 and it was -4mV on the right channel and 14mV on the left. The strange thing about the left channel is that most of the time it is at 14mV but it will drift up to 100mv and then come back to 14 to 15mV. What would be causing this? The schematic shows two trim pots on the power amp driver board. Could these be dirty causing the readings to drift?
 
Does it drift with no signal input? In other words, are you measuring while listening to music?

If there is only one pot, then it is probably for the bias adjustment. I'd have to see the schematic to tell. Certainly the bias should be set. You should see if the bias also drifts around after setting it. If it does (and I doubt it), probably the pot is whacked out.

1.) Put a new battery in your meter, or try a different meter.

2.) That much drift is whack... :eek: I smell leaky/bad diode junctions in the input pair transistors, or a bad electrolytic capacitor in the feedback loop, or even possibly a dry solder joint somewhere.
 
EchoWars,

I cleaned both left channel pots with some deoxit and retested it. The left channel when warm stayed at about 8-10mV
 
Hopefully that reading is in millivolts. ;) If so, left is a little high, right is fine. Nothing to freak out about, but as I've said, sonics begin to suffer audibly (to me anyway) once the readings hit about 10-15mV.

Anything below 50mV is probably below manufacturers repair specs, but an amp with 50mV of offset is unlikely to meet published distortion specifications.
 
EW.
The reading were done on a beckman industrial 300A digital meter
that goes down to 200m dcv. so it should be somewhere in the ballpark

rgds larry
 
And another cause for thumps...

Originally posted by EchoWars
The MF amp is pretty decent if those are your readings.

As far as the Yammy, a rule of thumb in troubleshooting is...fix the obvious first. In your case, it would be the fuse.

A chunk of DC offset can certainly cause a nasty thump on turn-on. Get a proper fuse and take a look.


Yet another cause for Thumps and DC Offset would be power supply caps that are not charging up at the same rate. Such unequal voltages will be the primary cause of thumps and possible output offset ( maybe not 32 Volts of offset, or possibly enough to blow a fuse ) yet a blown fuse is likely due to some silicon device that is already shorted or blown open...

-Steve
 
Kinda resuscitating an old thread here, but my interest was quite piqued reading it. I grabbed my trusty Rat Shack DMM and measured the DC offset of the Pioneer SX-450 I've had since childhood. I recently put it back into service when my 1992 Denon Rx packed up, driving Polk Monitor 4a's. I live in an apartment, so its 15wpc output is for the nonce plenty.

AAnnnnyway, I set about measuring the DC offset. My readings were:

Left Channel: -02.2 mv
Right Channel: -25.1mv

I was a bit concerned about the right channel, but since it (a) is still within the realm of acceptability and (b) I cannot detect any sonic defects I'm leaving the machine alone, other than replacing the faceplate and dial glass when my replacements I won on eBay arrive. :)

http://www.afterburnerpro.com/pioneer.html has pics of the 450 in its current broken-dial state, along with my RT-707 reel deck.

Thanks EchoWars for the info...I've been into this for years but had never heard of DC offset...you truly do never stop learning in this hobby! :)

TP
 
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