Stage Line 500W amp repair project

[Arouse1973]

did notice this when i seperated R172 and R174, thay look to be slightly burned where they were touching, seperated them before i turned it back on and the fan started working!

Maybe the fan should have kicked in normally in a fault condition and this was stopping it. I don't think it's related to the fault LED.
Adam

 

[bikemech]

will do this tomorrow, thanks for your help everyone, feel like am getting closer,
cheers

 

[Arouse1973]

No it's ok. When we say lower and upper end it's looking at the diagram the way it is drawn. The upper end is nearest the top of the drawing, so the supply. The lower end is the other side of the resistor where it connect to the base of the transistor.
Edit: Lower end or bottom end

 

[Arouse1973]

Good night.

 

[bikemech]

OK, R177 is in a different part of the circuit. If Q46 collector is still 0V there is still the same problem with that part of the circuit. Have a close look around R108. There's something strange going on there.

What's the +45VL voltage that Adam asked for?

Where am I mesuring for for this,
Is it The top end of R108 and T4?

 

[Arouse1973]

H Pete
Can we carry on from post #194, it's up to you though.
Adam

 

[bikemech]

Ah I thought you meant R117 also, sorry. Can you when you get time measure the top of R108?
Adam

just did this again and top of R108 reads 66.0vdc eithet side,
black probe to T4 0v
with or withiout Q46 being in circuit

 

[bikemech]

ok adam, thanks,
can you be as clear as possible please about where you want me to test and mesure, am still learning sorry.

 

[bikemech]

Another thing is turn the power off and measure the resistance across the emitter terminal and the base terminal of Q46 with leads both ways around. That's from the label marked +45V (Not +45VL) and the lower end of R108.
Adam

ah think i understand now, the messages are all over the place,
do you want me to test the marked points in the diagram in post #203?
with the transistor installed and the amp powered off?

 

[Arouse1973]

Yes please and do the same with it removed.
Cheers
Adam

 

[bikemech]

what test range would you like me to use adam? Ω or diode test range? or?

 

[Arouse1973]

Ω set on say 2000

 

[bikemech]

ok meter set to 2000 in the Ω range,
red probe on emitter of Q46
black probe on the base of Q46
meter reads 555
black probe on emitter of Q46
red probe on base of Q46
meter reads990,

do you want me to test Q46 the same way out of circuit or test the board where Q46 should be once removed?
also do you want me to test from emitter to lower end of R108, or base to R108

 

[bikemech]

meter set to 2000 in the Ω range
red probe on emitter, black on the bottom of R108
meter reads 554

same test oppersit way round
meter reads 988

both tests with Q46 installed

 

[Arouse1973]

So that was with the transistor fitted. If yes then do the same with it removed.
Adam

 

[bikemech]

So that was with the transistor fitted. If yes then do the same with it removed.
Adam

am i testing the transistor itself or the board with it removed?

 

[Arouse1973]

The PCB. You could meter the base emitter of the transistor also, using the diode feature.
Adam

 

[bikemech]

meter set to 2000 in the Ω range
red on emitter point on the PCB
black on the base point on the PCB
meter reads 989

same test with probed switched
meter reads 989

red probe on emitter of Q46
black probe on bottom of R108
meter reads 988, with probes both ways round

 

[Arouse1973]

Ok so looks like the transistor is not causing any issues here. This is really puzzling now, how can you measure 0 V across R108 when it should be dropping approx. 3 V. Ahhhh! this is doing my head in. For the same voltage to appear across R108 then it must be coming from somewhere.
Adam

 

[bikemech]

took the board out again to have another look!!!