old Sessionette 75 Mk 2 - no output

[Bushbaby]

I've just noticed that the fuse in the power amp circuit has blown as a matter of interest! It could have been like this to start with too.

[Bushbaby]

.......and as a matter of interest(?) I have taken some passive tests (if any good) of the mosfets. The green mosfet values appear as 2 diodes by looks of it, does this suggest its faulty??

Best Regards

Rob

       

[Murphyv310]

Are they in or out of circuit?
Well Rob. Ask yourself a question if you have removed them, why is one reporting its a MOSFET and the other two diodes?
The simple answer is one is good and the other faulty, which adds up to a blown fuse that thank God you didn't already check as that would have been curtains to the speaker. You know the saying....... A little knowledge... Etc!!

[Bushbaby]

Hi Trevor
The are still in circuit as I thought I would make a quick look first (yes if connected to other components it will alter the readings). Of course, I can remove (carefully) both of them and retest. It would not surprise me if though the green 2SJ82 HAS failed and therefore did blow the fuse. Logic suggests that this was the reason, as was indicated, why the original speaker was blown. I wish now that I had done more research (you are correct of course in indicating 'a little knowledge') about this amp BEFORE my friend bought it, as the vast majority of this age up for sale are indicated to have major hums, severe crackles OR are not working which all point at this power amp.
I feel terrible about this now and feel completely wrong in my approach. I even thought about looking for equivalent Mosfets but as we have discussed, if there is anything else that is wrong then I will be back to square one!

Learning curve for me yet again.
Best Regards

Rob

[Murphyv310]

Hi Rob.
You cannot successfully test MOSFETS in circuit, in fact many components can not be tested successfully that way. If in doubt always check out of circuit.

[Bushbaby]

Ok, out the will come and I shall test.
Best regards.

[Murphyv310]

Best way Rob
If it still reads the same its definitely duff, then of course everything will need to be checked
If it was here I'd be quoting for both MOSFETS and probably other components too. Definitely BER considering the price of the upgraded full board.

[Refugee]

MOSFETS can do some very funny things when you test them out of circuit with a meter.
The gates hold charge leaving them switched hard on so you have to remember to discharge the gate before each test.
If all you have is a car brake light bulb put your MOSFET on a heat sink and connect the bulb from B+ and drain and B- to source. A dashboard indicator bulb will not need the heat sink.
Connect the gate to a pin next to pins with B+ and B- on them and rub your finger over them and you will find that you have got a completely analogue light dimmer. You will also get a feel for how long the gate stores charge for.

[Murphyv310]

Hi
Simon, I'm pretty sure the tester Rob has will discharge the gate, mine is the same and it does.

[Mike Watterson]

.......and as a matter of interest(?) I have taken some passive tests (if any good) of the mosfets. The green mosfet values appear as 2 diodes by looks of it, does this suggest its faulty??

Almost always.

There is no need to pre-discharge gate with those testers. All capacitors should be discharged, not just Electrolytics. Very high voltage ceramic have a piezo effect so a sudden momentary short stores mechanical energy (if it was charged to 1000+ volts) and there can be a few hundred volts a little while later, so they should be left shorted longer if the discharge is a spark.