What's up with my Sansui Hifi amplifier?

[Electricblue]

Hi, sorry if this is in the wrong section, feel free to move it!

I picked up a Sansui A-80 amplifier for £35 GBP today. He said it had been repaired before. It sounded fine when it was at the guy's house but when I got it home and turned it up, I started noticing problems.

The left channel starts to fuzz when I turn it up past 3/10, the right channel stays clean throughout. Also, it's got DC coming out of the left channel. It varies with volume, when you turn it up, the woofer sucks inwards enough to slightly crease the surround...

The right channel is 100% fine as far as I know...

I've tried swapping the speakers over, and the L/R inputs from my CD player. I've tried all the RCA inputs, it still remains the same. There's no distortion through the headphones, I don't know if there's any DC.

I've given the amp to my father temporarily, he's going past the guy's town during the week, so he's gonna see what he says about it.

I'm no electronics guru at all, but are there any clues as to where my problem lies? I love the sound of the right channel, if the amp could be fixed, I'd be happy with it. Surely it's rescue-able?

[B-string]

The DC offset in the left side amp indicates trouble. I'm fairly certain it could be repaired, it will depend on the production status of the parts (unobtainable obsolete parts could be a possibility if no current equivalent parts are available).

[Electricblue]

Thanks for the reply! The DC is definitely worrying me, I'm not hooking up my decent speakers until it's gone.

I'm guessing the repair would involve transistor replacements, and
I assume they are a lot harder to find than regular resistors and caps, especially as it's a 1983 amplifier.

If I could get it working, I would be very very happy with it.

I've heard of DC on output, but in the mV range, and I don't think that's enough to bottom out an 8" woofer. Would the distortion be another side effect of the fault, or another problem altogether?

[B-string]

The distortion is caused by the DC offset AT the speaker. Since it varies with master volume I'm not suspecting the output devices.

[Electricblue]

Originally Posted by B-string

The distortion is caused by the DC offset AT the speaker. Since it varies with master volume I'm not suspecting the output devices.

You mean the driver itself? Surely this wouldn't make the distortion come from the tweeters?

[B-string]

Originally Posted by Electricblue

You mean the driver itself? Surely this wouldn't make the distortion come from the tweeters?

The distortion from the speaker is being caused by the DC offset from the amp. Two things are happening: 1) The LF driver is no longer in center alignment. 2) The power supply rail is now limited by the offset so the voltage swing in one direction is limited.

[BassmanPaul]

It's been a looooong time since I worked on a Sansui amplifier. Many of them had both a bias pot and a pot to null out the DC offset. It could be as simple as a needed adjustment.

[B-string]

Its been quite a long time for me as well.

[Electricblue]

I had a quick peek inside before I gave it to my dad. It had 2 trimpots with BIAS written on the PCB next to each, they were opposite ends of the board pretty much.

The silver on the pots looked a bit rusty, maybe that's something to do with it?

I don't want to go poking about and break something, when I could have just taken it back to the guy that sold it to me.

So in theory, getting rid of the DC would fix the distortion too?

[B-string]

Correcting the DC offset should, yes.

[Electricblue]

Great, maybe it can be healed after all What should I expect if I fiddle around with the bias?

Should I put a multimeter on the left output and twiddle until it reads as close to 0V as possible? Bearing in mind no DC happens when there's no input (I don't think so anyway), it's related to the audio volume.

[B-string]

NO! Get the schematic/service info. If you start playing with the bias control without the proper info you may fry the outputs. Bias is used to keep the outputs above cut-off just like a vacuum valve (keeping them slightly conducting at idle).

[MuthaFunk]

Bstring is right.

You should run a search here:

http://audiokarma.org/forums/forumdisplay.php?f=39

There's a buch of great people that frequent that forum that should be able to help you better than perhaps us MI amps guys could. I had a lot of guidance with old Pioneer amps there. Good Luck.

[Electricblue]

Ok, we have a PDF service manual/schematic now.

We're going to measure some voltages/adjust the bias carefully and hopefully solve the problem. My dad's pretty good at schematics and electronics in general, we just needed pointing in the right direction

I'll post the results afterwards if anyones interested in how it turns out

Thanks for all the help guys, keep shredding!

[B-string]

Of course we are interested.

[Mr. Foxen]

I have a Sansui, took bunch of photos when I opened it for cleaning/curiosity: http://ampstack.wordpress.com/2012/0...six-hi-fi-amp/

[Wagz]

I believe the Sansuis need to have a load on the speaker outputs when you set the bias.
Setting bias on an unloaded output (just connecting a multimeter instead of a speaker) will result in an incorrect bias setting. Scratchiness in your left channel can be lots of stuff, start with cleaning/lubing the balance and volume potentiometers with some DeOxIt.
If the amp isn't hummingor has a dead channel, that's generally a good sign that it could be fixable.
Old Sansui gear is sweeeeeeeet amd worth fixing, newer stuff not so much. They sold the name at some point and it became cheap mass-produced stuff like a lot of formerly great audio brands.

[BassmanPaul]

That was not a requirement back in the early Seventies when I worked in one of their service departments.