Stage Line 500W amp repair project

[Arouse1973]

When you measured 0.00V across R108 was the transistor defiantly out of circuit? I just don't understand this!
Adam

 

[bikemech]

When you measured 0.00V across R108 was the transistor defiantly out of circuit? I just don't understand this!
Adam

i will do this again now, i will leave Q46 in and test across R108 with the amp turned on, one probe either side,

 

[Arouse1973]

Yes when you did this.

 

[Arouse1973]

Also can you measure across R117.
Adam

 

[Arouse1973]

i will do this again now, i will leave Q46 in and test across R108 with the amp turned on, one probe either side,

And take it out and do the same.

 

[bikemech]

100% 00.0mv either way round with Q46 in or out, seems i have a live running through it but no negatige on the other side?

 

[Arouse1973]

100% 00.0mv either way round with Q46 in or out, seems i have a live running through it but no negatige on the other side?

Well I am gob-smacked. I am going to bed now, I'll think about this tomorrow.
Adam

 

[bikemech]

ok adam thanks for your help m8, night,
on another inspection!!! think the top of R108 has a bad/dry joint!!!

 

[bikemech]

ok R108 did indeed have a dry/bad joint at the top!,
fixed that now if i mesure from T4 to D38 i get -16.5 on the cathode and -66.2 on the anode, hope this help as i am sure it was + one side before i did the dry joint

 

[bikemech]

is it possible D51 could be shot???

sorry my diagrams are not verry good yet

 

[Arouse1973]

is it possible D51 could be shot???

sorry my diagrams are not verry good yet

No that looks about right for D51.
Adam

 

[Arouse1973]

ok R108 did indeed have a dry/bad joint at the top!,
fixed that now if i mesure from T4 to D38 i get -16.5 on the cathode and -66.2 on the anode, hope this help as i am sure it was + one side before i did the dry joint

Now measure across R108, what do you have?
Adam

 

[BobK]

Did the fault light go off?

Bob

 

[bikemech]

No change across 108 and no the fault light did not go off!

 

[Arouse1973]

Ok so it's possible R108 is faulty then, and it's resistance is changing when power is applied. I have not seen this before but who knows. So the only way to do this is to lift the bottom leg of R108 out of the PCB and put a current meter in series with the open connection. Are you happy to do this?
Adam

 

[Arouse1973]

If R108 is going low resistance then you should see over. 3.00 mA. This is where the fun begins, if it's ok you could see something like 2.95 mA. A small difference but it makes all the difference.
Adam

 

[bikemech]

Ok so it's possible R108 is faulty then, and it's resistance is changing when power is applied. I have not seen this before but who knows. So the only way to do this is to lift the bottom leg of R108 out of the PCB and put a current meter in series with the open connection. Are you happy to do this?
Adam

Do you mean between the removed leg of R108 and the PCB where the bottom leg should go?

 

[Arouse1973]

Do you mean between the removed leg of R108 and the PCB where the bottom leg should go?

Yes Pete.

 

[bikemech]

just before i do this , i may have an answer, there is a red link wire on the top of the board from near Q46,comes from + of BR3 then from there to the right chanel amp Q68 collector, then to the left chanel amp Q64 collector!!

 

[BobK]

Are those the power transistors that you have removed?

Bob