Sansui AU-777 Bias problem

[LBPete]

Thank,
Yes i know the 777 have multi voltage selector.
I have a rack of Sansui gears full of 100v amp, i don't intend to have an odd ball that run on 240acv local voltage because I interconnected them by the ac point @ all the back of each amp. Sorry for about confusion.

Be careful using the auxiliary outlets on the back of the amps. The "switched" outlets have very low current carrying capacity usually around 100W. If you draw more than that, you will overload the unit's power switch and it will fail. The "unswitched" outlets don't have this issue but are still somewhat limited by the gauge of the power cord.

- Pete

 

[stereofun]

That could absolutely be your bias culprit. Try to swap and see if fault follows. I havent had a 777, only a 777a, but I recall something about the outputs being soldered in ? I also think there were some other oddities like different outputs for the neg / pos side...or something around that.... stuff you dont see on other amps.

 

[Peter lye]

measurement taken without DBT from TR-8LB and TR-8RB

left R831 =70.5V right R832 =69.5V
R833 =32.5V R834 =38.7V
R835 =31.7V R836 =37.9V
R843 =31.8V R844 =37.9V
R837 =1.16V R838 =1.01V
R839 =1.11V R840 =0.97V
R841 =0.45V R842 =89mV
R845 =0.75v +^ R846 =0.45V

R845 =0.75v can climb up to 1v, because of the TR811 & Tr813 heating up, had shut down the amp. R845 is a 100ohm resistor, pull out the resistor and test ,still measure 100ohm+-1.

Peter

 

[RobinTW]

measurement taken without DBT from TR-8LB and TR-8RB

left R831 =70.5V right R832 =69.5V
R833 =32.5V R834 =38.7V
R835 =31.7V R836 =37.9V
R843 =31.8V R844 =37.9V
R837 =1.16V R838 =1.01V
R839 =1.11V R840 =0.97V
R841 =0.45V R842 =89mV
R845 =0.75v +^ R846 =0.45V

R845 =0.75v can climb up to 1v, because of the TR811 & Tr813 heating up, had shut down the amp. R845 is a 100ohm resistor, pull out the resistor and test ,still measure 100ohm+-1.

Peter

I don't know what these readings are. These are resistors and their measurements should be in Ohms. What are you trying to get from these readings?
Since the amp sings without problem, I think all transistors are working. You need to find what's causing the high bias on the Left channel.
I would start measuring R841, R839, and then R837. Do not measure them in circuit. Be sure to lift one leg before you make measurements.

 

[Peter lye]

Here the culprit: damaged mica on the TR805



Thanks All
Peter

 

[GrisPato]

Here the culprit: damaged mica on the TR805
View attachment 2302423
View attachment 2302424

Thanks All
Peter

Good find. So we can add
"leaking current from collector through mica isolation pad" to the list of causes for high bias current.
Interesting!!
This is why this forum is so useful. To engage all this faulty possibilities on your own will take you 20-30 years.
Thanks for sharing and lean back and enjoy!!

 

[Peter lye]

VR802 i can adjust to 39V, But VR801 can only go 27to 28V


Thank
peter

Further Progress

After the bias was stable and further testing. still have the problem with the left channel "main amp section output adjustment" ,swap out the pair of 2SD143 (TR807/9) for a pair central 2N3054 and that solve the voltage problem.
After another round of testing , the right channel were distorted while the volume were crankup and the transistors (TR812/814) was 10+degree warmer than the Left. Swap out the final pair of 2SD143 with 2N3054, no more distortion. Guess the 2SD143 does not age well.





Thanks For Reading
cheer
Peter

P.S >Yes,will change the VR801/2 pot later