Sansui au 555a amplifier problems

tomthar

New Member
Hey all, I'm new to the forum, I recently got an old sansui au555a that was in pretty sad shape, I cleaned it up and tried it out, and the left channel has some problems. On both a and b systems when playing through the speakers it is silent with little bursts of distorted audio, once you turn up the volume it plays distorted audio constantly but still distorted and weaker than the left channel. This all happens in both mono and stereo. On the headphone jack it is also distorted but not weaker.

So my question is, how would I go about fixing this? I'm good with solder and have experience with electronics, but nothing as complicated as an amplifier.

Thanks in advance

Also I am not opposed to getting this thing fully restored, It just seems like a very daunting task even with my (albeit limited) experience with electronics. So if anyone has any resources for something like this ( or knows of a good place in the Burlington Vermont area ) that would be huge.
 
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I would check the fuses first. From hifiengine.com you can download a service manual and see their location.
 
By 'cleaned it up' do you mean you cleaned all switches and pots with contact cleaner like De-Oxit?
 
First check everything carefully... especially with a new old amp.

- verify the amp's voltage selector (on the back) is expecting the appropriate mains voltage for your location
- make sure you aren't connecting a CD or other non-phono source to the phono inputs
- set all front controls to "nominal normal neutral" positions
and as mentioned already
- check fuses (pull them and test continuity), clean their end contacts and receiving clip holders
- clean switches and rotary controls with De-Oxit

Then, you might be able to eliminate some things by doing a few simple tests.

If you are using a turntable as your source into the phono input, try using a CD player into the AUX input. If that sounds clear then the phono stage may be the problem.

If a CD through AUX input is not clear, try it in the TUN (FM tuner input) and TAPE inputs to see if the AUX might be cruddy.

Pull out the couplers between the preamp and the power amp (those two metal loop things on the back) and put them back in - those might be cruddy.

Pull them out again and attach the CD output directly to the power amp inputs (don't do this unless the CD has a volume control because you will be bypassing the amp's volume control which is in the preamp). If this sounds clear then the problem may be in the preamp, maybe just one channel. Don't forget to put the coupler loops back in the right way.

It will help if you can narrow down the problem area to something like a channel in a gain stage.
 
Thanks for the help guys the problem area (thanks to paulns tips) is definitely in the right channel and after the preamp. I'm going to check the fuses right now. And yes all the pots have been cleaned with deoxit. If the fuses are clear what would be my next step?
 
It was a fuse, it was near destroyed but right at one of the ends so when I did my first inspection i stupidly missed it
 
Thumbs up ! Of course there is always the question why it blew in the first place. Does it play well now, equal sound in mono L/R ? Are the outputs at even (lukewarm) temperature ?
 
both a and b and left and right seem to play consistently as well as in mono vs stereo. I'll take a multimeter to it soon to see if there is any big difference in the current going through each channel to see if there is something that could have caused a problem. I figure the fuse might well have just blown from being so old. (the contacts were pretty oxidized when I first opened it up) so I went ahead and replaced both of them to be safe.
 
I have one of these and had it restored by fellow AKer Blaine Hagstrom.He did a fine job at a reasonable price. Fine little amp.
 
Okay last question just to make sure everything is alright with it. With these amps is it normal for the power switch to not immediately cut power to the speakers? ie; it fades out slowly rather than just turning off all together.
 
yes that's fine ..its a good sign as it happens .it proves the power supply capacitor can hold a charge .
 
From your description of its behavior I suspect a bad capacitor or transistor.
Which one or ones is impossible to tell without detailed voltage analysis.

IIRC those have known problem child small transisters that are on the always replace hit list.

I have one here from an AKer that I need to check out soon.
 
Many fuses have lived hard lives in perfectly working amps because of their owners mistreatment - from gross overuse to coarse screw drivers held in shaky hands. Guilty as charged. Sometimes changing the fuse is really just all. Having said that, this amp is about as easy as they come - if you are good with a solder iron, you could begin board by board (only 3 !) and replace caps first, then as skills and confidence grows, get new trim pots in there and signal transistors if it is noisy or pops at times.

This amp is very special, in a good way :music:
 
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