Lux amplifier transistor replacement

[petervk2mlg]

I just bought a faulty Lux SQ77T/II
On investigation there was a blown 2 amp fuse on the secondary side of the transformer.
I traced the fault to one of the output transistors - an SD 119 which had a base to collector short.
Each channel has a pair of 2SD119s as the final transistors.
I replaced the faulty transistor with a 2N3055. Will this be OK or should I replace all the transistors with the same type?
peter

[SaSi_SiDi]

I think that replacing all 4 would be more prudent. It all depends how close a match the 2N3055 is to the 2SD119.

[studiot]

Your main problem is likely to be the higher frequency response of modern transistors introducing unwanted instability.
This amp is not compensated for these.

The 2SD119 has a modest Ft of a few hundred kilohertz, as do older 3055s, but more modern 3055 go higher.

I should avoid MJXXX TO3 transistors are they are good for several Mhz.
BDX XX eg BDX20 will probably do as they are limited to .8 Mhz
Nearer 2N equivalents are 2N3442 and 2N3773.

I should certainly substitute in pairs at least.

[Mike-repairman]

If just replacing the output transistors does not cure the fault I would suggest replacing all the 5 transistors after the audio coupling capacitor, 10mfd 25V.

As the circuit is DC coupled the driver transistors could be leaky of have been hammered when the output ones failed.

Failure to do this may result in damaging the new output ones again.

Mike.

[petervk2mlg]

Thanks guys.
I have some more work ahead of me, then.
Peter