Transistor variants - interchangeable?

[AndrewT]

Originally posted by tigger01
If the differential pair are the ones I think they are (Q702 and Q704 from the schematic) they are quite cheap (2SA970GR and 2SC2240GR) although I can't find the GR flavour. Maybe I'll have a go at matching the transistors.

Are the transistors usually well matched originally or can they be improved upon?

What other transistors in the schematic would benefit from matching. I plan to replace Q702, Q704, Q752, Q754, Q756, Q758, Q760, Q762, Q764, QN02 & QN03.

all the discrete transistors are either singles or complementary pairs. No matching is needed. Just replace like for like where they are damaged, but do try to be gentle with the PCB, particularly around the IC.

[tigger01]

Chris,

Brief synopsis,

The amp stopped working and although the red led on the front came on when it was turned on, the relay never clicked and it made no sound.

I tried to diagnose the problem with a multimeter with the amp powered on and the power amp board loose. Yes.. very very stupid. I fried Q764, it actually burst into flame!

I sought advice here and have eventualy bought replacements for all the transistors in the left channel, after Q705. I did this because the advice I got was that there was no easy way to know if any of them were damaged, and if they were they could cause the replcement transistors down stream to blow again.

Please let me know if you have any thoughts on what may be damaged and need looking at. Incidentaly I found that R802 was blown and having replaced that, the relay clicks in and I get sound from the left channel. Q764, Q762 and Q752 are currently removed completely. When I turn on however, R764, Q760 and Q756 get very hot very quickly.

Neil

[anatech]

Hi Neil,
You need to replace Q752, 754, 756, 758, 760, 762, 764 and check QN02 or just replace it. Also you need to test R758, 760, 762, 764, 768, 766, 764, RN02, 04.

Have a good look at the traces and other parts in the area. Sounds like you had fun there! Replace the bias control with the same style part if you found Q752 damaged in any way. Test it if Q752 was okay. You also need some heat sink compound and the mica insulators for those transistors mounted on the heat sink.

Good luck with your repair!

-Chris