Sansui AU 5900 dead channel

Depaj

Member
2015-08-05 5:31 pm
lyon
I started working on a nice Sansui AU 5900. It powers up ok but the relay doesn't engage so I tested for dc offset and got -21v on one channel and an ok but not great 150mV on the other. So far I have replaced those famous vd1212 diodes and R31 and R41 on the dead channel. One of those resistors read 1.3k instead of 220 as it should be. I fired it up again and now I got -33v of dc which seems logical as R31 is on the negative power supply rail for the voltage amplification circuit.

I tested all the transistors one by one disconnected from the board but they all test ok. I've done a quick check of the resistors but they measure the same as on the good channel.

According to the amount of dust and dead insects that were stuck on the circuit I suppose this amp has never been worked on so all original.

Ideas on where to look next would be welcome :)
 

Depaj

Member
2015-08-05 5:31 pm
lyon
The slightly lower voltages are probably due to the bulb tester that I put in series.
Sans titre.jpg
 

Depaj

Member
2015-08-05 5:31 pm
lyon
I will post another one with the voltages on all the transistors.

Hi Sakis!
I have replaced the 4 220R resistors.
I will replace all the electrolytic caps anyway, are you also referring to ceramic caps ?
I tested the 2SA798 of the faulty channel and the reading were ok BC and BE both ways were about 0.65.
By the secondary PSU do you mean the one on top of the amp, with the selectors ?
 

Depaj

Member
2015-08-05 5:31 pm
lyon
I re-soldered the whole board and this seems to have done the trick although I'm not 100% sure. But I still have a DC offset on one channel that is fluctuating between +1.8v and -2.9v. Also the voltages on the bases and emitters of TR11,13,15 and 17 are very unstable as well.
 

Depaj

Member
2015-08-05 5:31 pm
lyon
I changed the electrolytic caps and that didn't change anything so I started de-soldering (1 leg only) all the resistors for testing and they are all within 5% of spec. Now when I was done soldering them all back in the dc stabilised at -76mV so I guess there was another broken joint somewhere. I will replace the two input transistors anyway and maybe I will get even closer to 0v offset. Anyway, I would say, result, thanks for all the help.