Stage Line 500W amp repair project

[KrisBlueNZ]

Oh ok, is that quite feasible for a 500W amp to drop 20 odd volts with the output transistors connected?

It does seem like quite a lot, doesn't it. I think it could be normal though.

That's really odd, you measured 0 V across R108. That means either the meter didn't measure it correctly, the resistor is short circuit or it there is no current through it.

Yes. There's 82V across R117 which means there is 3 mA flowing in that circuit. That should cause 3V drop across R108 with Q4 removed.

have inspected the board and found a slither of copper from where the cable tidys push through the board, looked like it was shorting to or around R177,

Do you mean R117?

removed this replaced Q46, (just pushed it into the pin recievers i fitted for test), wired it up and turned it on, now i have the cooling fan on! this has never happened before, fault left is still lit tho

OK, cool! (Literally.) What is the voltage at Q46 collector?

 

[bikemech]

Q46 collector reads 01.7mv
and was R177 kris

 

[KrisBlueNZ]

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?

 

[Arouse1973]

Q46 collector reads 01.7mv
and was R177 kris

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

 

[bikemech]

Do you measure different voltages now from last time. What is the voltage on the top end of R108?
Adam

by the top end do you mean the side the power comes in??
either way the results are
R108 measures 66.1VDC one side and 66.2VDC the other side with the negative probe to T4 0v rail
all powered up and Q46 installed

 

[bikemech]

how come R117 is +65.3vdc one side and -14.3vdc the other,
is it possible it has blown???

 

[Arouse1973]

Yeah top I meant power in side. That's why you measured 0 V across R108 then.
Thanks
Adam

 

[Arouse1973]

how come R117 is +65.3vdc one side and -14.3vdc the other,
is it possible it has blown???

No the zener is doing this.
Adam

 

[bikemech]

No the zener is doing this.
Adam

damn it, thought i was on to something then!!

 

[bikemech]

here is the copper piece i found,
wonder if it has anything to do with the fan working now,
i owned this amp about 5 years ago and dont remember it ever coming on!
sorry for the quality of the picture1

 

[bikemech]

No the zener is doing this.
Adam

would that be D38?

 

[Arouse1973]

Yes that's right. If conducting correctly it should have approx. 50 V difference between it's terminals which it has. I think all the voltages you have measured are correct apart from the R108 voltages. So we know that R108 is ok, you measured it.

Now did you defiantly remove Q46 when you measured 66.2 V in one of the other posts, it just R108 should have 3 V drop across it as Kris mentioned.

For it not to, this voltage must be coming from somewhere else. So my initial thoughts are going to be Q46 is naff, but then you said you removed it. If not then there must be a connection somewhere from the 66.2 V supply to the base of Q46.

Do you have any resistors with you?

Adam

 

[bikemech]

Yes that's right. If conducting correctly it should have approx. 50 V difference between it's terminals which it has. I think all the voltages you have measured are correct apart from the R108 voltages. So we know that R108 is ok, you measured it.

Now did you defiantly remove Q46 when you measured 66.2 V in one of the other posts, it just R108 should have 3 V drop across it as Kris mentioned.

For it not to, this voltage must be coming from somewhere else. So my initial thoughts are going to be Q46 is naff, but then you said you removed it. If not then there must be a connection somewhere from the 66.2 V supply to the base of Q46.

Do you have any resistors with you?

Adam

yes i do have various resistors ( on another small amp i could robb)
would you like me to remove Q46 and retest?

 

[Arouse1973]

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

 

[Arouse1973]

yes i do have various resistors ( on another small amp i could robb)
would you like me to remove Q46 and retest?

Not for the moment.

 

[bikemech]

here is how i currently have Q46 installed

 

[Arouse1973]

See if you can find a 1K resistor or something around that value. Then attach one end to a piece of wire and connect that to 0V. With this new lead with the resistor hanging of it connect the other end to the bottom of R108. If this part of the circuit is ok you should measure half the voltage you had originally.
Adam

 

[Arouse1973]

here is how i currently have Q46 installed

Oh Well done, I thought you had to de-solder it. Ok do the other bits first then remove and retest. Then do the resistor test I just mentioned. With and without the transistor.
Adam

 

[Arouse1973]

Then another test. Remove Q46 and use you resistor lead you just made and connect one end to the +45V terminal and the other end dab it on to the top of R119. Does the LED go on and off? This will prove the rest of the circuit is working. Sorry if you have done this before.
Adam

 

[bikemech]

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!