Hello everyone. I know this relates to solid state gear, but the Sansui 800 dates to about 1969, so I guess it counts as vintage. My Sansui 800 was faulty in one channel. I have traced the fault. hopefully, to a transistor on the board which immediately precedes the two output transistors. The Base-Emitter junction tests as open. In the schematic below, the faulty one is marked TR808. It's a 2SA561. There are quite a few equivalents listed on the internet, such as the KSA1015, or KSA1015GR. What I don't know is whether I should also replace the transistor TR806, which is a 2SC734. I'm new to this, but it seems the two transistors are part of a push-pull pair, as are the output transistors, and I'm wondering how closely they should be matched. Also, after replacing one or more transistors, are there any other adjustments I need to make? Anyone got some pointers on this situation? I don't want to start desoldering anything until I'm pretty certain what I'm doing. Cheers...thanks in advance.

A BC640 will replace a 2SA561. The '640 has higher current and voltage handling capacity as well.
It is a drop-in replacement.

Here it is at Jaycar:

https://www.jaycar.com.au/bc640-pnp-transistor/p/ZT2180

If you wanted to replace its mate (not necessary IMO), you'll need a BC639.

You should not need to touch bias settings, the Vbe is the same.

It's not really necessary to match the more modern transistors, they are all close enough to "perfect" anyway and the negative feedback around the amp will correct any deficiencies.

Thanks Ian. As always, I appreciate the help I get on this forum. Cheers.

Diagram uploaded.

Whilst it predominates in "Solid State" car radios, if there is a considered lack of volume, & you have the facilities to test them? The biggest cause of loss of volume in that sort of stuff, often relates to electrolytic capacitors "drying out". That causes them to lose capacity and is significant in that they used them to couple one stage to the next: Loss of capacity equals loss of coupling.

If C810 dries out you will get low gain. Low capacitance, the low frequencies will be rolled off.

But Japanese electros of this vintage are generally OK. Not so Philips.

With this type of circuit, if you want to test the amplifier without the risk of damaging the output transistors, you can disconnect them and the amp will work at low power on the drivers.