Amplifier Distortion, DC-Offset, and You!

[B3Nut]

Chiming in here a sec...

I dont' think you'd see DC if it was a capacitor-coupled output, and at any rate I'd highly doubt that it is. Most of these were direct-coupled output. And yes, that's the way the 680 and 780 are, and I'd hazard the 580 and 880 (and about everything else that used STK modules) is the same way, since the suggested circuits Sanyo published with their STK datasheets show direct-coupled outputs (through a Zobel network, of course).

The touchiness of the adjustment is due to a dirty trimmer. You can clean these with contact cleaner/lube, though Echo prefers to just replace them with really nice Bourns trimpots. I adjusted the trimmers on my SX-780 until I got them to around 0, with +/- 2mV fluctuation either way from temperature variations.

TP

 

[DanTana]

I did spray those trimpots with a very lightly with Radio Shack circuit cleaner, I don't like the tuner cleaner as it has a lubricant in it. Could be that I didn't move them very much after spraying and not all the crud got removed, but I was able to get them both consistantly at a very low reading with no fluctuation. Thanks for the reply B3Nut about the capacitor output, and direct DC-coupling I'm glad they are the same as Pioneer's then at least someone has knowledge about the basic Sanyo circuitry.

 

[EchoWars]

Even if you get those trimpots cleam, by their very nature they are touchy. Replace the pots, or live with it.

That RatShack cleaner is nasty stuff. Personally I wouldn't let it near my gear. YMMV.

If there is an offset adjust, then you're DC coupled. No need to have one if you were cap coupled. DC coupling is not only better, it's cheaper, since big output caps are expensive. Besides, all STK modules that I'm aware of are designed for direct coupling, running off of split rails.

I'll reiterate this once more...it doesn't mean Jack what sign (+/-) you measure at the output when adjusting. There's nothing dangerous about measuring '-20mV' as opposed to '+20mV'. It's the magnitude of the number that is important, not its sign. After all, you've got to be able to swing both positive and negative at the output anyway, otherwise how are we going to convince a speaker to reproduce music?

 

[RobV]

EW,

What an improvement! :thmbsp:
I finally got a multimeter (RS 29 range), and set the DC offset and bias current on my Sansui AU-919 amp, which sounded a little 'off'. The before / after values (mv):

DC Offset:
L +4.3 / -0.1
R +25.5 / -0.1
Bias Current:
L +20.7 / +20.7
R +14.0 / +20.4

The bass is tighter, and percussion is more crisp, too! I want to do my (other) BA-F1, but I just sent you the service manual!

Thanks,
RobV

 

[RobV]

Really... BIG IMPROVEMENT! My 10 yr old daughter just told me to "turn it down" (Jeff Golub - 'Wanna Funk').
:guitar:

Wow!
RobV

 

[Mr.Toad]

Quad 405 Late '70s

Just measured my Quad 405:

4.1 mv Right channel
2.6 mv Left channel

I switched to tube amps a little while ago, but I thought I'd see how the old Quad 405 measured.

 

[RobV]

Another one!

Today I replaced the connected/separated switch on my Sansui AU-819 integrated amp, which I bought new 24 years ago. Other than the occasional pot & switch cleaning, this is the first service it has required. While it was open, why not check the DCO & BC?

Before/after (mv)
DC Offset:
L +47.4 / +0.6
R +6.2 / +6.2
Bias Current:
L +14.8 / +20.8
R +14.6 / +20.2

Thanks Echowars! The old amp never sounded better!


RobV

 

[Morden2004]

And another classic tuned up to A+++

Yesterday, after burning in my "new" Sansui G-8000 for 3 days, I finally got around to popping the hood to do the adjustments. What did I find? (picture to follow).

Grabbed my DMM, and set about to adjust the balance and final current.

.............. New Value (Original Value)
Balance L: - 0.15 (+4.55)
Balance R: +0.15 (+3.50)

Current (adjusted to spec of DC 16 mV +/- 1 mV)
L: 16.45
R: 16.55

This unit sounds as good as it looks - and it looks awesome.

 

[Morden2004]

G-8000

It was so clean, I wondering where they kept it all these years.

Paul

 

[Elbowgeek]

My SX 636

This is an amazing thread, and perhaps the most edumacational thing I've read in years

My SX-636, bought at a thrift for $40 along with a contemporary Pioneer cassette deck (obviously bought as part of a package deal), is showing >200mV on both channels. No wonder it didn't sound as good as I've seen raved about here.

The wonderful thing about this receiver is that the output board looks very accessible, so I'll have a go at self repair. I'll replace both transistors and major caps and then see what we get.

If it works out, it will become my office system, along with a set of AR 4ax's I'm going to pick up this weekend.

Also, along with the AR's is an RT 701 which I'm totally itching to get me grubby paws on. I'll use it to record vocals in my "studio" in analog before transferring to the digital domain.

Cheers,

Dennis

 

[fliphandywork]

Echowars,

Do you do repairs for people on the forum?

Thanks!

 

[Haoleb]

Yes yes yes.! SS repairs by Echowars

Maybe i should have been more specific. it was the second post down... guess i figured you would read the topic.

 

[fliphandywork]

I did read the topic... but it is so long it slipped my mind. I was also hoping to illicit some type of response... such as how to get it in for servicing.

Thanks.

 

[EchoWars]

Email me. Look in my profile and email from there. I'd rather not post here as I get too much junk mail as it is.

 

[70'sVint.audio]

Hey EW,
My Pioneer SX-636 reads like this;
L 5.5mV
R 6.5mV
Is this good or bad?
Vince

 

[EchoWars]

Thats a fine reading Vince.

 

[70'sVint.audio]

Thanks EW, I check out my SX-880 and it's a total different story, here's how it reads;
L 12.2mV
R 34.6mV
Do I need to make some adjustments here?
Thanks.
Vince

 

[zhagerott]

HK 730 and 930

Checked my vintage HK's

HK 730 did 3.2mv on the left channel and 8.6 on the right.

HK 930 didn't fair so well at 26mv on the left and 38 on the right. The 930 is 31 years old however.............

 

[EchoWars]

Originally posted by 70'sVint.audio
Thanks EW, I check out my SX-880 and it's a total different story, here's how it reads;
L 12.2mV
R 34.6mV
Do I need to make some adjustments here?
Thanks.
Vince

If the 880 has an adjustment (and I think it does) then you can set it to 0. As it is, it may not be perfoming quite as it is capable, but there certainly no danger of anything bad happening.

 

[Tromatic]

Pioneer SA-9100

L: 4.5 R: 2.5