Sansui AU 3900 Static LC

jim249

Active Member
Hi All; I just picked up an AU 3900. The left channel had really bad static at times that the speaker relay would kick on. I reset the bias on both channels and now the static/hiss is reduced but is still there. The bias voltage left channel jumps and gets erratic when ever the static starts. It is very intermittent with periods of very quiet. Any ideas for me on where to look. Thanks for any help!
 
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Sounds like you have a noisy transistor/s in the left channel.

I would start by separating the preamp/tone section from the power amp section by lifting one end of R07 (2.2K resistor). Power up and listen for the noise only (no music- the pre/power are separated)

If the static is still there, it is a power amp issue and likely the front end (2SA798 diff pair).
If the static noise is not there, it will likely be TR07/09- most likely TR09 (2SC1313).

Either of those transistors are known to get noisy.

It could be other issues, but I reckon there's a good chance it will be one of those two.

3900.JPG
 
Thanks for the replies. All switches and pots have been cleaned. Thanks for those points for me to look at. It has been running for an hour now with no static. I wonder if resetting the bias and burning it back in might have corrected it. I will keep this thread informed.
 
Thanks John, you are so right. It is very random now but still doing it. I would assume that I could remove the brown jumper wire between boards 2556 and 2554. That wire comes of the R07 trace. Much easier to get at.
 
Could it also possibly be the relay contacts?

I would say no. The static/noise was associated with bias voltage jumping and being erratic according to the OP. The relay contacts are on the end and can't contribute to erratic bias voltages and/or random noise.

Bias will jump if there's transient/random noise to amplify from further back and it can also jump or not stabilise if there's a dc balance/noise problem in the front end of the power amp stage. By separating the pre/power we can zoom in on the problem. It's a simple design so it should be an easy fix...
 
LOL. Unhooked the brown wire and it is still doing it. I am going to have to change out the 2SA798. Have to order those so this thread will be quiet for awhile. Thanks again for all the input so far.
 
The 2SA726 in the pre amp and phono amp are also known noisemakers.

If you have a can of freezer spray you cna usually isolate the noisy transistor. Give each transistor a short spray - less than a second - and wait 5-10 seconds between each transistor. In any case, it is worthwhile replacing all of the transistors in the preamp and phono amp (2SA726 and 2SC1313)

Edit - looks like you have isolated the problem to the differential pair - these cn be hard to source depending on your location. You can use separate gain matched transistors in place of the single package pair.
 
Many thanks to all who posted info to this thread, especially to John. I replaced the 2SA798 diff pair and the amp has been running now for several hours with no distortion at all. A quick and easy fix! Thanks again!
 
Please post the serial number for the database. Click on the link in my signature block.

- Pete
 
Yep - just did the same thing in my Pioneer M22 - the 2SA798 failing at the front end can cause a number of sumptoms including noise, hiss, crackling and also fluctuating DC offset - so that was a learning expereince for me thanks to great help from here.
 
Hi everybody. I did a recap of my AU3900 and have set the bias to around 13mV. I noticed that the bias goes up as high as 60mV when playing with music. Is this normal?
I'm using TIP41a and 42a as the replacement for the original output transistors.
The right channel blew once when I turned the balance pot to the max right channel.
Thanks in advance for any advice and suggestions.
 
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Hi pmchoong, welcome to AudioKarma Exclusively Sansui. Please start a new thread so we can help you with your issue and not get it confused with the original issue in this thread. If you are measuring bias current across a power fuse, current draw will increase with volume. Does it come back to 13mV at zero volume? Also, moving any of the controls should cause output transistors to blow. Something else is going on there. Do you have more than one pair of speakers connected? If so, what is the impedance of each pair?

Last, please post the serial number for the database. Click on the link in my signature block.

- Pete
 
Hi. Thanks Pete for the reply. I've changed the bass, treble and the balance pots. It seems to be working at the moment. I do not know which is the power fuse. I measured the resistors across 37,39 and 38,40. And yes, they settled back to around 13mV when volume is turn all the way to 0. Is this normal? If it's not, then I'll start a new thread. I've also post the SN number to the database.
Thanks again.
 
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