Poorly Sansui AU-2900

RolyShoes

New Member
One of your kind members directed me to this forum for further guidance. Feel free to copy and paste/send a link if my questions have been answered a dozen times but here goes...

For a few years now, my Sansui amp has been kicking off intermittently. It currently is emitting a ferocious crackling and popping through the left speaker whilst the right one is okay. I assume the speakers are not the problem as when we plug headphones into the socket, we get the same terrible noise. I have tried outputting from both sets of speaker connections but the growling is still there.

I opened it up six months ago and found the capacitors were a bit leaky so I replaced them but it didn't solve the problem. I tried changing the speaker cable but that didn't make any difference.

One of your number has suggested I need to replace the transistors so my first question is: Do I replace ALL of them? It hasn't had an overhaul for at least twenty years so it could be worth doing but if I one fails, how do I identify it? I could do one at a time, but that would be a pain.

Second (maybe third) question: where do I source the correct replacements?

I've had this amplifier for years, it's mostly great and I'd like to keep it rather than starting again. I've not been able to find anyone local confident in fixing it so thought I'd keep trying to repair it myself. I'm usually pretty good at these things but my husband disapproves of dismantled electronics on the dining table so the quicker I sort this the happier we'll all be.
 
A couple questions- Is the character of the noise affected in any way by the volume control or the position of the tone controls? Is the noise constant under all conditions of input selection? The service manual is on hifiengine if you don't have it. Schematic shows a very conventional circuit design and it should be easy to troubleshoot. I don't see any particular "problem child" transistors, but that doesn't mean anything. I also don't see all the parts listed in the manual- can you see if any caps on the boards are tantalum types? Where are you- can you put your general location in your profile?

General rule- it's always best to localize and fix a problem before attempting any overhauls and upgrades. Failure to do that usually leads to more complicated problems that are a lot harder to troubleshoot! I'd never replace transistors that don't have a problem, or at least a history of problems for that type.
 
Conrad,

The noise isn't affected by changing any of the settings on the control panel. The only way it stops is when the offending speaker is disconnected.

I've got the manual and schematic which I'm just trying to get my head around. I'm not sure how to identify tantalum caps, attached is a photo I took this morning of one board.

Glad you said that about localising the problem - I suspected if I went all in and took them all out bad things would happen. I read somewhere it's possible to identify the damaged ones using a (voltage?) meter. Is that the case?
 

Attachments

  • DSC_1003.JPG
    DSC_1003.JPG
    90.3 KB · Views: 67
I can't see any tantalum capacitors there, I see two replaced capacitors the rest looks original.

If the noises are caused by failing transistors which is very likely, it is unlikely you will be able to see the failures with a voltmeter. The transistors seem to fail with age, despite most not being 'over stressed' which can be a reason for early failure. Sometimes the failing transistors can be identified by careful use of 'freeze spray', however most replace the suspected faulty transistors for modern substitutes, as the originals are usually found to be obsolete types that are no longer made.
 
Last edited:
Hey John. Thanks for the input - there is another board which I couldn't photograph but am I right in looking for something like that in the attached photo?

I gather freeze spray is applied directly to the suspect component before testing, is that right? Can I use an inverted can of compressed air or shall I shell out for the good stuff? Is it safe to use when the amp is connected to the mains or will I have to plug it in after application?

Thanks for your patience. I'm used to basic soldering and cable making so circuit boards is a whole exciting new world.
 

Attachments

  • tantalumcapacitors.jpg
    tantalumcapacitors.jpg
    3.2 KB · Views: 6
Freeze spray is great stuff when it works, i.e., if the component is sensitive to it. You can use an inverted Dust-off or similar can too. Sometimes if I want to be a little less aggressive, I use a Q-Tip moistened (not dripping) with alcohol, which cools as it evaporates.

It doesn't look like you have any tantalum caps on the board. They can get noisy when old and defective, but most last near to forever and don't degrade like other caps.

Be sure to do a good visual, looking for suspect solder joints and such. You can also take a plastic rod (pen body or whatever) and gently tap parts while listening to the noise. If it changes it may be hard to localize the problem but it's a clue that it's a connection of some sort.
 
It will most likely be one of the 2SA726 transistors in the pre amp/tone control section, it could also be one of the 2SC1313 in the same section. Both are known to of noisy, in particular the 726's. The other likely culprit is the differential input transistors on the main amp section (5 legged package - two transistors) from memory either a 2SA796 or 2SA798.

Cheers

John
 
Thanks for the tips chap, I'll have a go this weekend. I'll try the alcohol first before forking out for more aerosols.

John - do you have any suggestions for replacement transistors or do I need identical parts? I did find a list of suggested alternatives online but more opinions/leads are welcome.
 
at the risk of stating the bleedin' obvious, the alcohol is not gin. It's IPA, isopropyl alcohol and comes in aerosol cans from places like Maplin etc. Gin may well have the same effect but it's such a waste I think...
 
Hey chaps, the saga continues. I thought I'd identified the unruly transistors so I sourced replacements and switched them out. The crackling seems to have gone but is now replaced by a hissing on the phono input which gets louder when I turn the volume up. Also the phono input seems to be weaker - can barely hear the source I feed through, even with the volume all the way up. Suggestions please?
 
Switches with oxidised contacts, dry/cracked solder joints, tired electrolytic capacitors, more noisy transistors, and so on.

Expect some noise from the phono input with the volume all the way up, it should ideally never be heard at normal volume control position, and I believe this is achievable with your AU-2900 after the suspect components have been replaced.
 
Thanks, I'll keep looking. Just noticed the labels on the board for the PNP legs don't seem to correspond with the diagram on the specs. I turned around one of the transistors I replaced and the sound from the record player is coming through perfectly. Should I trust the diagram or the circuit board map?
 
ThinkI identified the problem - the transistors I'm replacing are the opposite configuration to the new ones so I put them in backwards. Switched them around and so far it all seems to be working. Watch this space.
 
Hi RolyShoes, I seem to have the same problem. How did you identify the unruly transistors and where did you source your replacements?
 
Hey, Arno. Unfortunately the problem has come back with avengeance so I'm back to square one. Going to try to narrow the problem down properly with some freeze spray to identify the culprit as I suspect it's not just one component.

I found some 2SA726 transistors on eBay but I'm struggling to source the 2SC31313 ones. I'll let you know if I have any luck!
 
Back
Top Bottom