Amplifier Distortion, DC-Offset, and You!

Pioneer SX-850 has four small pots , two blue and two white. Does anyone know which one is for calibrating offset? Blue or white? And also about bias, what is the manufacturer recommended value for current?
Thanks
 
My 780's got trimmers like that too...the white ones are next to each other near the regulators and the blue ones are situated in each channel's input/driver stage circuit.

I want to be sure which ones adjust offset before I try to zero them.

TP
 
One will be for bias, and the other for offset. If you aren't sure which is which, leave them alone. These old pots are super-touchy (which is why I usually junk them on units I repair) and cranking on them even the tiniest bit can change the setting a huge amount.

I still recommend that..if you have a 'keeper' amp/receiver/whatever, to track down a service manual. Cheap investment, and if you really want to learn a little bit about the inner workings of your gear then it's the logical place to start, even if you have no intention of getting in there and doing it yourself.
 
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BeatleFred kindly scanned the section of the SX-780 manual dealing with DC balance and posted it in a another thread. Two of the SX-780's pots are for DC balance (the blue ones) and the two white ones are for power output meter calibration. The balance trimmers are still good; I got the offset to pretty much zero in both channels, never going beyond +/- 2mv during fluctutations.

I am going to get the manual for it though, and for some other models as well.

As for that SX-850, those are going to be bias and balance for sure as the xx50-series lacked power output meters.

TP
 
Nice tip echowars, just checked my KA-9150 on Speaker A;

LEFT: 5.0mV
RIGHT: 5.3mV

According to your post that must be "Damn good!"
Thanks!!!
 
B3Nut...I saw the post from Beatlefred. Was a little tough to read for these weary eyes. I'd want to look at the manual before I started tweaking.

bolly...the 9150 has an adjustment, so with a bit of patience (the pot is touchy) you can set it to zero...
 
Hey Ew,
I just picked up a minty little Nikko 40 watter and it measures around 2 mv on each side, but i still have distortion on the left side. Where else should I look for distortion to be coming from? I know I could sift through the archives, but I am feeling a bit lazy :rolleyes: Is it very likely that cleaning pots will clear up the distortion, it isn't nasty, just a bit of ...well........distortion on the voices for the most part.
 
Never hurts to clean up the controls. I've seem wierd problems disappear once controls were cleaned up. And it is also possible that the bias is off...

Much deeper than that, and it may be necessary to start tracing signals through the amp with a scope and generator. If I had it, I'd put it on the distortion analyzer to see if what you think you hear is indeed what is going on.
 
My Results

Hi Echo!! Been lurking on the board for a while and decided to join.That was a cool tech note on the DC offset.I turned off my Sx-1080 and grabbed the meter.Darn thing was jumpin all over the scale.Sooo..I powered down the bose eq and it settled down.After about an hour of tweeking the pots this is what I have now.

Before

Left channel=75mv
Right Channel=17.5mv

After

Left Channel=4.5mv
Right Channel=4.5mv

Thanks for the tip!I think I`ll tackle a ground loop problem in my TT next
 
Excellent post. I am happy to reply with my success story. Before readings were:

91 right
66 left

After adjustment

.5 right
.5 left

Only took about 2 minutes to adjust them.
 
Thrift store Sansui 881 down from around -87 mV's and 4 mV's to 2.1 mV's and 4 mV's. Fairly simple stuff now.

I remember the days when I had to use chat just so EW could help me recognize a trimpot.

Brad
 
Great thread

I just completely read this entire thread. . .Great thread. .
 
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Post Edit

I did noit know untill just now that somehow I had put a "Puke" emoticon in my last post. . .This was inadvertant and a total mistake. I thought this entire thread was great reading and if I offended someone by mistake I am truly sorry. . .
 
Sansui AU-999 Ser # 222010985

Amp has been running for 3 hours.

Condition: barely warm.
Balance adjustments to both channels done yesterday.

Readings:

Left: 41 mv
Right: 12 mv

Paul
 
Success!

So, I opened up the AU-999 and adjusted the Balance once again and came up with these settings:

Left: - 3.3 mv
Right: - 4.0 mv

I'm now a happy, WELL BALANCED, listener.

BTW, the adjustment pots on the daughter boards are very hard to adjust. A small change in the pot can results in readings jumping more than + or - 100 mv. I used a non-inductive alignment tool that I've had for 35 years. Tricky work but worth the effort.

Thanks, Echo!

I think these are pretty good readings for a 1970 amp that has had no parts replaced.

Paul
 
Since DC balance is dependant of the base current of the input transistor pairs (see pg. 2 I believe) and base current changes as the transistor heats and cools, I leave an amp on for several hours before making adjustments, just to ensure that the transistors have reached some sort of thermal plateau.

Some amps will actually get to this point fairly quickly...others take an hour or more. I just turn 'em on, walk away, and come back to check 'em later.
 
EchoWars,
Quick question for you. On my "Monkey Wards" amp, it uses Darlington Power Packs STK-0060's. While adjusting my DC Offset, I was able to go from it's start readings down to below 0. For a second, because of the sensitivity of the switches, it was around -100.0 until I quickly turned it the other way to as close to 0 but staying positive, finally leaving them both at +0.5. So, does this mean I have a capacitor output type amplifier? And is it consistant with the later 80 series Pioneers using Darlington Power Packs?
 
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