I just received a nice Sansui AU-719 for restoration

[richiem46]

Another follow-up. I removed TR612 and tested it and tested ok. I then re-installed it and powered the AU-719 up on the DBT with no issues. The voltage on terminal 17 now reads normal! I then powered up the AU-719 on the mains and checked the bias and DC offset which were fine. I then attached speakers and an audio source on the AUX and played some music. I immediately noticed a difference, the Bass was much more authoritative (it wasn't bad before). I guess the main filter caps were in need of a refresh.

I losened the F3101 board and removed TR613 with it's heatsink to gain clearer access to TR612 so I did not have to remoce the board which greatly pleased me.

I will work on the driver boards next........

 

[Robisme]

Nice job.

Rob

 

[richiem46]

I restored the first driver board in the AU-719 over the last 2 days. What a pleasure removing it compared to the Power/protector board, just some screws and three connectors to unplug!

I replaced all of the electrolytic caps on the board plus C03 the Black Flag capacitor. The 1UF electrolytics were replaced with 1UF Wima caps. Fuse resistors R04 and R05 were replaced. TR02-TR05 were replaced with KSC1845 transistors and TR06-TR09 were replaced with KSA992's. Zener diodes ZD01 a 27V Zener diode and ZD02 a 13V Zener diode were replaced and ZD03 was replaced with two 1N4148 diodes in series. I also replaced resistors R45 and R46 based on the recommendation od another AK'r. I replaced Trimmers VR01 with a 100 ohm Bourns multi-turn and Vr02 with a 500 ohm Bournes. I replaced the mica/silicone grease on the driver and output transistors with silpads. I noticed that theinput connector socket had a suspect connection on one of the pins and so I re-flowed it.

I cleaned up the rosin on the back of the board and re-installed it in the AU-719, powered up on the DBT and everything looked good. I set the bias and DC offset (DC offset still very touchy) and then plugged the AU-719 into the mains and re-touched the bias and DC offset. I hooked up some speakers and played some music for several hours last evening while re-organizing the garage a bit. The AU-719 played perfectly for the entire time. I will restore the other board over the next day or so.

Here ia an "after" picture of the Driver board:



Here are the output transistors with the silpads installed:

 

[richiem46]

Yesterday I restored the second driver board from the AU-719. The process went smoothly and more quickly than the first board since I was familiar with the board logistics after restoring the first board. The list of components replaced is the same as on the first board so I will not detail them here.

Here is a before picture of the board after I removed it:




Fortunately there was not too much of the Sansui Glue on this board:




I used the Acetone 10 minute soak and scrape technique to remove the glue:



Here is a picture of the board after cleaning using a flat bamboo skewer section to scrape off the glue. The bamboo does not scratch the board.




I was amazed at how dirty the board connectors on both driver boards were. I cleaned them up before re-installing them.



After re-installing the second driver board I powered the AU-719 up on the DBT and everything looked okay. I set the bias and DC offset reasonably close to their specifed values. The DC offset on this board was initially over 1 volt before I dialed it down. I was kind of surprised the board came out of protection with the offset that high. The offset setting was as touchy as ever.

I then powered the AU-719 up on the mains and re-touched the bias and DC offset.

Hooked up some speakers and a music source to the AUX input and listened to music for about an hour. With both driver boards restored the music seemed a bit more smooth for lack of a better word. I didn't expect much of an impact after restoring the driver boards but the AU-719 just sounded more "pleasant" afterwards, subtle but noticeable if that makes any sense.........

I am taking the rest of the week off to prepare for company on Saturday to celebrate my 75th Birthday. I will restore the remaining two boards starting on Monday.