Trace Elliot Bass amp repair

[SLEEAudio]

3 of the transistors have shorts.

With the transistors out, B+ 77v, and there is still 50vdc on the output socket.....

[SLEEAudio]

Also, would the ECF10N16 be a good replacement for both the J50 and K135? There are 2 of each for output transistors

Cheers

[Steve Conner]

No. They are two different devices of opposite polarities. The K135 is N-channel, so you replace it with the ECF10N16. The J50 is P-channel and you replace it with the ECF10P16.

Get it wrong and the amp will blow up immediately on powering up.

There may still be voltage on the output socket with no power transistors, but that voltage should have very little current behind it.

Then again, maybe one of the driver transistors is shorted too. Check TR3, TR4 and TR5.

[SLEEAudio]

Driver transistors are fine. Ill replace the output transistors and possibly the PS caps too and come back to you.

Thanks

Charlie

[Enzo]

And do not connect a speaker or other load until we are sure the amp has no DC at the output.

[SLEEAudio]

OK, replaced the transistors and there is now no DC at the output at all. The is however also no signal at the output. Unfortunately lent my oscilloscope to a friend so wont be able to fault find for a while.

But thought Id let you know I didnt blow the amp up! Thanks for the help

Charlie

[SLEEAudio]

Very basic fault finding shows that there is a problem in the preamp stage. Is it usual with SS amps that if output transistors short that smaller signal components will short too?

[SLEEAudio]

Hope you guys can help me again,

Ive replaced one of the ps caps (-70v) because it wasnt properly discharging. But now the new cap isnt discharging at all (taken 10 mins for it to go below 50v).

Does this sound familiar at all? Am I doig something wrong?

Cheers

Charlie

[SLEEAudio]

Question questions questions... sorry.

Ive traced the signal all the way up to TR1, and then it stops. Is it possible for a transistor to not work even if it isnt shorted between pins?

Cheers

[Steve Conner]

Yes. They usually fail shorted, but if the circuit can deliver a lot of current through the short, the guts then vaporise and they go open again. But if this happens the plastic case often cracks or blows apart.

[SLEEAudio]

Thanks.

The case seems fine to me, is there another obvious reason why the signal is not passing through this transistor?

Cheers

[Jazz P Bass]

Tr1 & TR2 comprise the differential pair input to the power amp circuit.
The preamp enters TR1 base.
The output of TR1 is the collector lead.
That lead then drives TR4 base.
What Vdc reading do you get on the collector leads of TR3 & TR4?

[SLEEAudio]

Collector leads read

TR3 -2v
TR4 -0.3v

[SLEEAudio]

Sorry, they are both reading -2.8v

[km6xz]

Why did you replace a capacitor because it is not discharging quickly. That is a good sign. It also suggests that the -rail has no load on it at all. Do you have -70 on the lower side output transistors?

As far as TR1 is concerned, yes a transistor can be open. Don't replace it unless you prove it is a problem. Measure the voltages on all 3 pins and report your findings. That will tell you if it is working or not.