• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Sansui AU-20000 No preamp signal...

S

shapeshifter

New Member
So I have a friends AU-20000 on the bench right now. (He bought it on Ebay and it worked for a while.)

I have some knowledge of HIFI (my expertise is in tube guitar amps though) and the necessary equipment (scope, tone generator, multimeter). There is a problem somewhere in the preamp, the signal does not get trough, but the power amp sections works fine when I feed it a signal from the "Main in" on the backside.

Also, I get a signal at the tape outs, the input meter is working, but no signal at the pre out/main in.

To be honest I am a little overwhelmed right now by the size, the many boards and the small print schematic of the amp.

Any 20000 experts here, the might have an idea or shortcut to find the problem? Any help is greatly appreciated.

Thanks!
 

Attachments

  • IMG_4118.jpg
    IMG_4118.jpg
    132.8 KB · Views: 27
Register to hide this ad
shapeshifter said:
Any 20000 experts here
Click to expand...
Not an expert, never seen one or worked on one, but you should check that S02a & b are OK and not oxidised - this is the 'pre-main switch', (on F-2500 board) which you should be familiar with by now. ;) - it worked when you switched it to 'main in' - so it's probably OK.

Check all power supply voltages first, make sure you have the +/- 35V to the pre-amp stages, the regulator for these rails is located on the F-2508 board. The +/-65V and +/-68V are obviously OK as the power amp works. ;)

By saying the meters work OK, you have eliminated a lot of things, so I am wondering if you have a faulty relay RL01 (HA-224N) located on the F-2504 board, and driven by the protection circuit on F-2511 board. The coil may have power, and the relay may sound like it is actuating, but perhaps the contacts are faulty?

Let us know how you get on.
 
Thanks, John!
I will check the pre voltages tomorrow and take it from there...
I recapped my own AU-505 this spring, but compared to this 'monster' it is a cute litte amp. (But still sounding big for its size.)
 
OK, so I get about 33/-33V on the stages of F-2505 (Flat Amp board) which should be close enough I guess.
I also checked the phono preamps, both are crackling and the left channel of Phono2 is not working at all.


Bildschirmfoto 2020-11-10 um 09.07.24.png
 
Last edited:
Are you sure it's not a switch problem? Cleaning the switches might help, especially the muting, tape play and input selector switches on F-2502.

This photo might help you locate the relay that John mentioned in post #2. It's definitely worth checking, too.

IMG_4629.jpg

The input wires attaching at F-2503 are fragile and easily broken which might explain missing left channel on phono 2

IMG_4624.jpg
 
So I started to service the thing, somebody has done some work before.... I cleaned the input board (F-2500) and the Main in - Pre out switch, although it was pretty clean to begin with. Next up is a checkup of the front switches...

IMG_4162.jpg IMG_4156.jpg IMG_4161.jpg IMG_4172.jpg IMG_4173.jpg
IMG_4175.jpg
IMG_4176.jpg
 

Attachments

  • IMG_4174.jpg
    IMG_4174.jpg
    161.2 KB · Views: 11
I can see you are drawing our attention to what looks like some damaged track in the picture above. I think that needs investigating and tidying up, it looks 'suspect' and unpleasant to my eyes. There is also a pair of joints above it in the view, the right one of which looks suspect as well. All of these lifted tracks, dry joints, joints with 'blow holes' and so on should be checked and receive attention. ;)

AU-20000 experts here may be able to tell you if wire jumpers (like the blue one on the above board), are 'factory' or not, don't remove them.

Note also that when checking the signal path you can use your multimeter (with the unit turned off - and unplugged), to check continuity from one board to another - if you suspect oxidised switches interrupting the signal.
 
Last edited:
Someone definitely has done some work on the filter board. The traces have come off the PCB before, maybe when he took out the board?

Also, one of the yellow wires (33.35) was detached and the black ground connection wire (34) was almost broken before I resoldered it...

IMG_4207.jpg IMG_4208.jpg IMG_4211.jpg IMG_4212.jpg IMG_4217.jpg IMG_4219.jpg IMG_4223.jpg
 
Somebody clearly was a bit over his head here. The wires from the phono inputs have been desoldered (bad job) and one them even was soldered back to the wrong pad, another one was pretty much broke off when I looked at it...


IMG_4224.jpg

Both channels are working again, but the crackle an noise is still there, so the problem must be somewhere on the EQ board or the phono switches. Also one cap has been replaced, who knows why...
Any ideas? I detached the connectors from the PHONO1/2 to the board, so most definitely the problem is on the board. Should I replace the caps? I do not have replacement transistors at hand, but I have the right valure Nichicon caps...

IMG_4234.jpg IMG_4235.jpg
 
Last edited:
shapeshifter said:
Should I replace the caps?
Click to expand...
I wouldn't, I'd replace the transistors first - providing.... they are on the 'Noisy Transistors' list**. Once again the old mantra - "only change components to rectify the faults first" (and this CAN BE be some capacitors), but no blanket replacement, until it works tolerably well. ;)

Update:
Oh yes** - the unit is riddled with them - 2SA726 & 2SC1313 but 2SA917 probably ok
 
You‘re right. Shouldn‘t rush things. Just would love to get the heavyweight off my bench...
I am thinking about replacing the non original cap, checked the caps with an ESR meter, all measure pretty much within spec.
Any suggestions for checking the transistors?
 
You must log in or register to reply here.
Back
Top Bottom