Sony TA-3200F , VAS is heating too much

[Naveed nasir]

Hello friends
I'm upgrading my sony TA-3200F i changed differential pair from original 2SA705 to 2SA992 and also changed current source from 2SA705 to 2SA992 and current constant fron 2SC634 to 2SC1845 these all are almost identical but with high hFe about 400 while the original have hFE 70 , now the problems is VAS SPS885 is heating too much in few seconds , I tried with a different trans like 2SD669 adding small heat sink but the problem is same , before this modification everything was cool and working fine , sound is OK now and refined but extra heat is main problem now .
A little heat decreased when I increased base to collector capacitance frm 22pf to 55pf.
All type of help Advises suggestions 'll be welcomed.
Schematic is attached

 

[roger2]

When one makes changes that have a negative effect, reversing those changes might be worth considering. This is an example of why I am not a fan of wholesale re-transistoring in the same way that it has become common practice to recap. Transistors, and their interactions within circuits, are much more complex than capacitors. Even relatively simple, basic recapping often goes horribly wrong. There are some people here with the ability to do re-transistoring (myself not included), but that doesn't mean that everyone should be doing it as a standard maintenance procedure. IMHO only.

Which of the original transistors had hFE of 70? And was that their original value or had hFE decreased in those transistors since they were new?

 

[Tom B]

When one makes changes that have a negative effect, reversing those changes might be worth considering. This is an example of why I am not a fan of wholesale re-transistoring in the same way that it has become common practice to recap. Transistors, and their interactions within circuits, are much more complex than capacitors. Even relatively simple, basic recapping often goes horribly wrong. There are some people here with the ability to do re-transistoring (myself not included), but that doesn't mean that everyone should be doing it as a standard maintenance procedure. IMHO only.

Which of the original transistors had hFE of 70? And was that their original value or had hFE decreased in those transistors since they were new?

very good advice

 

[staubinr]

I know it is an old thread, but something very nice happened to me when I changed this fragile broken VAS SPS885 transistor and I want to share my experience with you. The base leg of this SPS885 was broken and I had a DC voltage of 3 volts at the speaker post. I measure the voltage of R118 and had mothing, meaning the SPS885 was cut off. When looking carefully, I was able to see a broken base leg. I give up to repair the transistor. I spend a lot of time trying to find a good replacement for it. Looking at my stash of transistors, I decided to use a 2SC3503, because it has high voltage, high frequency response, >1Watts ... The SPS885 is a EBC and the new one a ECB, then I have to carefully twist if legs. I replaced it, and power it up and adjust DC bias and balance on both channel. Using my computer, Arta software and a little board with a 8 ohms load, I measure its THD. Comparing both channel, I was surprise to see a lower noise floor of the new VAS transistor. You can see the graphics here. Left channel had a 2SC3503 transistor and the right the old one. I know it is too low to hear it, but it is good to know.