Technics su 3500 help needed

[rrmitko]

Some progress from last night.I found the guilty transistor causing the distortion It was TR407. It's 2SC1567 and was measuring good but in the circuit all voltages are big mess. Hfe of that particular transistor is 97 and 133 on the good one from the other board. Now the question is which is a good replacement of the bad transistor.Do I need to replace any other transistor like TR404 or TR408(2SA794)?Is any of them complementary to TR407? Thanks!

 

[rrmitko]

Are these suitable replacements ?
TR407 2SC1567 - sub KSC3503
TR408 2SA794 - sub KSA1381

 

[rrmitko]

BD139/140 seem ok

Thanks a lot mate These I can find even local (uk farnell/uk rs) Do I need to much them by hfe?

 

[rrmitko]

the higher gain the better, but there should be loads available and cheap i am guessing, Hfe ! doesnt need to be matched

So I'll look for BD140-16/BD139-16 ,these are the versions with the higher gain.

 

[rrmitko]

Hi all,
Transistors arrived and are on place now. Amplifier is working as it should again,and the idle current is stable at 15mV. Thank you everyone for the help and the advice! Is jt worth replacing the power transistors on the other board as well,so far I just replaced the damaged pair?

 

[rrmitko]

i would leave them

Thank you! I will just replace TR401,402 with matched pair as I did on the other board.