Sansui Au 4900 problem

Hi there,

i am starting to approach electronics, i have more experience in diy loudspeakers, so my knowledge is limited. Maybe you guys could help me.
I have this old Sansui Au 4900 which has some (big?) issues.
So, probably i won't be able to repair it, but maybe with your help i have a chance.
I made a video to give you an idea of what's going on:

http://www.youtube.com/watch?v=0wEJf...ature=youtu.be


As you see (and hear) the relay goes nuts from the very beginning, no music is passing and actually i can't tell what kind of noise is this. Never heard anything like this.
I checked the output DC, but it's not easy, if even possible in this condition, the Voltages on the white resistors are ok i think, between 8 and 10 mV. I've read on some topic to check them.

If i use the phono input, i actually hear some music in the background with the noise, with all other inputs only the noise.

i found the scheme on Hifi engine, but as i said earlier i am not at the level of knowing exactly where to look at. There is a lot of stuff before the relay, so it's not easy. An AK user gived me the service manual.

Could anyone help me or you think it makes no sense at all to try to fix myself this nice amp?

btw, if someone could give me some hint for books i should study, dedicated to amplifiers and audio electronics, i would be grateful. I suppose Douglas Self's book are kind of a reference, uh?

nice day to you.

Hi,

Looks and sounds like you have intermittent DC or noise causing DC offset spikes which is in turn, causing the protection relay to chatter on and off. (there is actually a spark in the relay a one point in your video).

What are your skill levels with using a multi meter, and a soldering iron (removing/replacing components)

I (and others on the Sansui forum) can offer some basic fault finding advice once you let us know what your skill level is)

In any case, first thing is not to have any speakers connected until this current problem is rectified.

Cheers

John

Hi Skippy,

I think i wouldn't find big problems to measure and remove-replace components.
Have a nice day,

Andy

Hi Andy

First question - when the amp was set up as per the video you posted, was the noise coming out of one or both speakers?

Hi,
In the video i wired the amp to a single very cheap loudspeaker, one of those speakz i wouldn't mind to destroy. But i checked all amp out's, the noise is always there, doesn't matter if A or B, left or right...

Thanks for the help, mate.

Nobody out there?

does the relay chatter with no speaker(s) attached?

The video link is broken….

Chi… do you have a meter, can you read a schematic, do you have tools to solder?

Mike

Mikstick:

Look.. you won't believe that.. i just tried what you asked me, and the amp worked fine. I haven't touched it! And i am pretty afraid that this is a bad news, intermittent problems are a nightmare and i think there is little to do if the amp works just fine; so i have to wait that the problem will show again, right?
If i remember well, the relay was switching even without any speakers whatsoever wired.


vigman:

i changed the youtube link in the first post, and you'll find it here as well:

http://youtu.be/0wEJfoN5FwQ

don't know why YT changed it.
i do have a meter, a basic oscilloscope, i can read a schematic, maybe with little more effort than some of you (until some months ago i just worked on passive loudspeaker networks) and i do have equipment basic solder.

thanks for the interest, guys. i appreciate.

i want to stress the concept that it wouldn't bother me to bring this piece to an expert technician.. i would love to repair by myself, though.

have a nice day,

Chiu.

I am going to make a stab in the dark and suggest its a small transistor on the mainboard. I had an au4400 fault with a similar scratchy popping sound, only the au 4400 hasn't got a relay and has different small transistors.

Whether its a dry solder or a transistor on its way out, or something else, I can't know.

With the amp unplugged and some bright light (very important) and magnification, you could look all over for dry solder joints. Use a marker and just put a dot on the board where you think there may be need for solder reflow. But don't do it just yet unless its very obvious that you have found a problem. Wait until everything else is checked.

If it was my amp, I might try bringing a hot soldering iron close to the small transistors to heat them up, not too much, I wouldn't be trying to melt them or heat damage them, but I would be trying to make an intermittent transistor start faulting. I would also be spraying with freezer spray each small transistor on the main board. This would only be useful with the amp running with a source input and speakers connected to output.

I don't suggest anyone work on live equipment, its potentially life threatening
Do not poke around inside an amp that is plugged in!!!
If you do, you need to realise the potential danger in doing so
If I said put your hand in the fire, would you? etc etc etc

hi Mikstick,
thanks for the answer.
i'll give a try, actually i already checked for dry joints, but not with much accuracy.
considering that the amp is now working fine (as described in my reply above), i should detect some issues fo this kind just by touching and moving the "culprit transistor", right?

thanks, i'll let you know… nice day to all of you.

touching and moving the potential culprit transistor would/should show the problem if it was a dry joint

I think heating the transistor(s) may bring the issue up, if it is a transistor

If you don't like to try that, then when the problem arises again, try freeze spraying each small transistor on the main board (only a little) and when the noise goes, you know you're on the right track.

It is worth trying to find the problem while you still have the amp and tools out.

keep us informed

thanks
mike

does the relay chatter immediately at power on ? or does it start a few seconds after ?

mikstick said:

touching and moving the potential culprit transistor would/should show the problem if it was a dry joint

I think heating the transistor(s) may bring the issue up, if it is a transistor

If you don't like to try that, then when the problem arises again, try freeze spraying each small transistor on the main board (only a little) and when the noise goes, you know you're on the right track.

It is worth trying to find the problem while you still have the amp and tools out.

keep us informed

thanks
mike

Click to expand...

Hi Mik,
Actually i couldn't find any dry joint, so i am going to give it a try to the technique you told me. Hopefully i won't damage anything, i never tried yet.
I've finished my freeze spray, so maybe it's a better idea to wait tomorrow and buy it, to have it available in case of necessity.
Thanks a lot, man.


Pete:
When the noise was there, it showed up from the real beginning, but the relay started to switch after some (maybe 5) seconds..


Nice day to all of you.

there was an amp with a chattering relay a good while back ..it was a capacitor in the protection circuit causing it .

Hi guys,
Sorry if i dissapeared for a while, but the amp worked fine until a couple of days ago.. It drives me nuts!
Now i still have the crackling, but only from one channel (the right one).
Now.... Lets these are the charachteristics:
The crackling (and popping) comes out from the right channel always, doesn't matter which input i select, doesn't matter if i balance the pot to "all left", or with volume all down.
I checked all the transitors in the right power amp stage, they are all fine.
It shouldn't be something in the preamp section, right? The noise would disappear shunting the volume down, right?
Obviously the problem starts when the relay closes the contacts, i might try to clean the inner metal plates.. But i am going blind.
Another idea i had yesterday night was that maybe there are some grounding problems.. I remember having similar issues with a tape cassette deck with a bad ground connection somewhere...

I just wanted to keep you informed..

Have a nice day,

Andy

Power off your fridge first

Ehm.... Why?

It's very common that malfunctioning fridges or such appliance mess up with your home power. I chased a buzz for months, was more a frfrffrffrfr actually, it's the bloody heat pump, clean music or warm house?