Au3900 recap

C

carstereoguy

New Member
Hello all

First post here. Always had an interest in vintage audio, but just finally picked some up. I just picked up an AU3900 for cheap, and am considering a recap. For now will be paired with a set of Sansui SP-50 speakers I happened to have until I find something more suitable for the amp.

I wanted to get the forums advice on good distributors to buy from (anyone in Canada that has found a supplier? Seems the shipping is often more than the cost of the caps)

I found this place which sells a full set of caps it seems, and they look to be good quality ones. Any thoughts? Have any forum members used this place? About $65cad shipped for the set, my guess is that's a bit steep, but saves me time.
https://www.audio-high-store.com/product/sansui-au-3900-upgrade-kit-audio-capacitors/

Looking forward to gaining some vintage audio knowledge from the forum!
 
www.digikey.ca is a great place to get parts. Shockingly fast delivery times for only $8.

AU-3900 is a very good starter unit that delivers a lot for only 22 wpc. Easy to work on too. That cap kit would probably be fine but a little more expensive than sourcing your own caps. It does solve the problem of figuring out which cap type to install where though. Considering what Sansui put in there 45 years ago, I'm not sure you need to sweat it too much.

There's other things I replaced in mine such as diodes, transistors, and a relay. If your amp is working fine now, you can address all that after you get your feet wet.
 
Alright, the amp arrived! Plugged it in, and was expecting that there would be and issue, but it worked flawlessly. Listened to music for an hour or so and enjoyed the sound. Then started noticing crackling from the right channel. Found this thread and ran the check suggested by restorer John.

Crackling still there, so 2SA798 diff pair my most likely suspect. Now I've been reading all weekend to try and educate myself, I am a tinkerer, but my electronics knowledge doesn't cover transiaters, diodes, caps etc. But I'm eager to learn!

(2x) BC560B or (2x) KSA992 are the listed replacement options. I'm having trouble finding info on what brands are considered high quality/low noise. Any suggestions? Keep finding "ON semiconductor" which sounds questionable.
 
carstereoguy said:
(2x) BC560B or (2x) KSA992 are the listed replacement options. I'm having trouble finding info on what brands are considered high quality/low noise. Any suggestions? Keep finding "ON semiconductor" which sounds questionable.
Click to expand...
Yup - a KSA992 pair from ON Semi is the correct replacement.

I think the 798 has a common collector. You may want to confirm this. Join the collectors of both new transistors together (if this is the case) and face the 992's together with a dab of heatsink compound between them, slip a small piece of heatshrink tubing over both of them to thermally bond them together.
 
Thanks all. Just ordered a set of 10 KSA992, as well as a set of the other transistors that commonly fail.

In the meantime I need to get a multimeter that can test them when they come. Is 10 typically enough to find a matched pair? I had no idea until joining this site that this was even a thing, any helpful advice on testing them appreciated for this novice!
 
Ok, got the transistors, and replaced the 2SA798's with 2*KSA992, same effect. I also have replacements for the 2sc1313 which are common failures apparently.

Any suggestions on how to test which transistor is likely the culprit as opposed to me just replacing a component at a time? Happy to learn, but this is my first for at into vintage electronic diagnosis and repair.

I'm using the manual here to look at the circuit layout
https://www.hifiengine.com/manual_library/sansui/au-3900.shtml
 
A great and easy way to diagnose is to use freezespray. Small bursts at a time.

Let the unit warm up until the noise starts, hit each suspect tranny. The problem one will cause to change - it will either get better or worse, or the sound signal will get louder or quieter
 
So interesting developments..

I looked into getting freeze spray, couldn't find it anywhere locally! Online order says delivery in 2-3 weeks.

So, what do you do when your amp doesn't work, and it the weekend and you want to tinker. I went with repairer bob's advice from another thread, to separate the tone control board from the driver board (last time I only disconnected one of the 2 wires...I'm still learning). With both wires disconnected the amp is silent, so I'm now suspecting it it the tone control board. Repairer bob also suggested in another thread that the 2sc1313 pair (TR09 and TR10) would be his first suspects, so figured I may as well replace those transistors. Sure enough I replaced them, and the amp was dead quiet. Over the weekend I easily listened to 10 hours of music on my newly acquired and restored yamaha NS-670's and it just sounded great.

Sunday afternoon I had had it on for a few hours and then suddenly there was crackling in the left channel, louder than ever before. I thought I had blown my speaker! (thankfully it turned out not). Tested the voltage on the outputs at the right channel was steady, but the left would jump all over, periodically showing really high voltage for a second (I cant recall how high).

So the saga continues. Looks like this will be a good project for me, I wanted to learn, and I certainly am! I went on to replace all the transistors on the tone control board since I had them on hand. Now the crackling is gone again, but I have a new symptom. I get music playing with the volume at 0, and it's not clear sound, not crackling like before, but it just sounds terrible on both channels, and adjusting the volume does make it louder, but as you move the dial you get cracking in the channel, which settles down when you stop touching the pot. I put some cleaner through the volume and all other pots to no avail. Any suggestions where to go from here?

I'm thinking the problem is in the tone control board. From my limited knowledge at this point it doesn't sound like a capacitor, all the transistors have been replaced on that board (except TR601& 602 which appear to control the headphones). Are there commonly failing diodes/resistors that should be checked? I read about the common issues with fuse resistors, and checked the 4 on the driver board, all within a few % of spec. Not sure if there are fuse resistors on the tone control board, is there an easy way to identify them?

I know I need to focus on diagnosing better before I move ahead any further, but at this point my skills are limited so any suggestions are appreciated. It sounds like transistors are a common failure on these, as well as fuse resistors. What other parts commonly fail so I can check them?
 
Hi, do yourself a favour and replace ALL of those 2SA725/726 and 2SC1312/1313 transistors on the tone/pre/EQ board. Are you sure the leg orientation of the transistors you have replaced are all correct? There are no fusible resistors on the board in question, only the amp board as you previously mentioned.
Also make sure no wires have broken off the board when you manoeuvred it about to gain access to underside.
 
Thanks for the reply! All transistors in the signal path (excluding the 2 that seem to go to the headphone jack) have been replaced (I'm now out of the appropriate transistors for now, but it looks to me on the diagram that the Tr601/Tr602 aren't involved in the signal path (could be wrong).

I did put one transistor in backwards before everything was working perfectly, and had no sound on one channel. Realized my mistake and flipped the transistor the other way, and all was well for about 8 hours of listening. I went back through last night and double checked all my solder joints and transistor orientations, and it looks to be correct.

Is there a way I can input signal directly into the driver board safely? Say an rca jack to a pigtail to check that the driver board is operating properly?
 
Good clarification, yes slimecity, the crackling returned after about 8 hours.

Another update, I went through the tone control board and checked all resistors. Found 2 that were out of spec and replaced them. Went to test the amp after and now the relay isn't activating (never an issue before). This doesn't seem to me like a symptom that would be triggered by something on the tone control board, perhaps a separate problem. I went in an checked for DC before the relay and I'm getting 9mv on the right and 11-14 on the left (the voltage varies). Not ideal, but this doesn't seem like enough to trigger the protection relay to me. Thoughts?

Edit: When i measured the DC at the output before replacing the transistors on the tone control board I was getting spikes to 30-40+ mV on the left channel, but it wasn't triggering the protection circuit.
 
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Ok, figured it out, I had put in an incorrect transistor pair when I replaced the faulty transistor on the tone control board. Had a ksc1845 place of a ksa992! My mistake.
So now the output voltage is stable at 9 and 11 mV with no fluctuation.

The relay is still not turning on however. I tried hooking a speaker up to the contact before the relay and both channels play normally now, so it doesn't seem like the relay not activating isn't because it should be in protect mode.

I went through and tested all resistors transistors andd diodes in the protection circuit, and they seem fine. Looking at the wiring diagram, I don't fully understand the circuit enough to diagnose further than checking all components. Any suggestions where to start?
 
Thought I would recap for any that find this thread in future. I had 2 noisy transistors on the tone control board, first one of the TR9/10 pair, and then a while after another went and I replaced them all. I used the incorrect transistor in one spot which created a new symptom, and resolved once that was fixed

At this point the protection relay isn't activating, but the DC out the output is solidly less than 15mv on both channels so I don't think it's actually triggering the protection circuit. I went through and tested all the diodes, transistors and resistors in the circuit to no avail. For now I've bypassed the relay to test, and the amp has been sounding good all week. I'll need to go back in at some point and investigate further on the protection circuit issue .
 
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Yea I think you are on the right track - that you need to look at the actual protect circuit.

Have you replaced C602 & 604 in that circuit? I would also focus on replacing TR 602 - the relay driver transistor - at this stage.

I note the schematic doesnt include test voltages - so that doesnt help much.

Interestingly - the schematic lists there are several 1N34A diodes in the protect circuit - germanium types. If yours is a later version, these may be silicon - but if they are the original germanium types you may want to revisit these in case some have failed.

upload_2019-4-18_9-34-20.png
 
I had the same thought about the caps in the protect circuit being suspect. I had already purchased a full set of new caps anyways, but had to wait on a second order as it isn't booted in the parts list that 2 of them are bipolar (but it says bp on the schematic).

Anyways, just got the parts and put them in, no change. Also got the transistors changed in the circuit, no effect. All the diodes test fine (well a couple appear faulty in circuit, but with one leg removed they are fine). Do the relays fail often? I seem to have continuity across the trigger though....
 
Does your AU 3900 have fuses near the transformer? I think one of them goes into protection circuit. Check the fuse holder continuity and the fuse itself, if your unit is later revision.
I have AU 4900 (nearly identical) and my unit does have fuses, but there are different revisions it appears. Some units do not have fuses at all.
Just checked the schema and it does not show those fuses are present there, so must be early model version?

For example:
https://audiokarma.org/forums/index.php?threads/sansui-au-4900.789566/
vs
https://www.audiovintage.fr/leforum/viewtopic.php?t=37244&start=10

The service manual also states if your relay does not operate, the problem lies within power supply circuit or speaker selector switch.
I would also check the headphones socket, if it's not been used for a long time.
 
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