Stage Line 500W amp repair project

[BobK]

Yep, blue, gray black brown is 6.8K

You want red violet black red for 27K
Bob

 

[bikemech]

Well I went back to the shop , this time armed with my meter. They had them all in the wrong boxes, after much confusion I got the right ones ,
Replaced R117, refitted Q46 plugged it all back in and guess what, no fault left LED !!!! Now what's the next step as it looks like am getting somewhere now, the fan is still spinning to,

 

[KrisBlueNZ]

LOL! The colour codes on those resistors say 6.8 kΩ. Did Maplin sell you those when you asked for 27 kΩ resistors? They must have got moved into the wrong bin. And I guess the staff don't know what the little coloured stripes mean - perhaps they think they're for decoration, like on beads on a necklace!

Yes, the resistors you linked to are fine - if they can sell you the right ones!

But before you go any further, you need to remove that brown goop from everywhere you can find it. Remove the big electrolytics, and any components in areas covered by the goop, and within a few millimetres of the edges of it, and clean every last trace of that goop off the board, before refitting and replacing components. Look carefully for any other gooped areas and do the same. Clean the goop completely off the electrolytics and reinstall them - they probably haven't been damaged, but smaller components that were touched by the goop, or within a few millimetres of it, may have been; I would replace all of them.

 

[bikemech]

LOL! The colour codes on those resistors say 6.8 kΩ. Did Maplin sell you those when you asked for 27 kΩ resistors? They must have got moved into the wrong bin. And I guess the staff don't know what the little coloured stripes mean - perhaps they think they're for decoration, like on beads on a necklace!

Yes, the resistors you linked to are fine - if they can sell you the right ones!

But before you go any further, you need to remove that brown goop from everywhere you can find it. Remove the big electrolytics, and any components in areas covered by the goop, and within a few millimetres of the edges of it, and clean every last trace of that goop off the board, before refitting and replacing components. Look carefully for any other gooped areas and do the same. Clean the goop completely off the electrolytics and reinstall them - they probably haven't been damaged, but smaller components that were touched by the goop, or within a few millimetres of it, may have been; I would replace all of them.

Sorry kris just got this message after I already refitted everything, the brown goop is hot melt glue that has discoloured,
Do you still want me to remove the caps and clean it all off?

 

[KrisBlueNZ]

Yes, definitely. I know it's used for rigidity but I doubt that it's hot melt glue as such. I've seen that stuff cause problems before when it gets old. It can corrode wires.

That 27 kΩ resistor is not in an unusual circuit position. The reasons why it might have failed are (a) random failure due to age or (b) damage from brown goop, and I'd put my money on the brown goop. Leaving any of it on that board is just tempting fate.

 

[bikemech]

Yes, definitely. I know it's used for rigidity but I doubt that it's hot melt glue as such. I've seen that stuff cause problems before when it gets old. It can corrode wires.

That 27 kΩ resistor is not in an unusual circuit position. The reasons why it might have failed are (a) random failure due to age or (b) damage from brown goop, and I'd put my money on the brown goop. Leaving any of it on that board is just tempting fate.

Ok will do this tomorrow after work, so what is next do I need to take the link from Q2 C and E out now?

 

[KrisBlueNZ]

Sure, put Q46 back into circuit.

 

[bikemech]

already soldered Q46 back in and R108, and I have current at Q2 base now two so I think alls good so far,

 

[KrisBlueNZ]

Good. So you can remove the bridge across Q2 (from post #138). Then when you've removed all the brown goop, we can get back to checking the output stage!

 

[bikemech]

Ok all the goop has gone, what do we do next,

 

[bikemech]

Can anyone tell me what the difference is between +45VL and +45V ? Is the VL AC current?

 

[BobK]

If you look on the schematic, 45V is before the fuses, 45VL and 45VR are after separate fuses for the left and right channel. There should be negligible difference in actual voltages (assuming the fuses are intact).

Bob

 

[KrisBlueNZ]

If you look on the schematic, 45V is before the fuses, 45VL and 45VR are after separate fuses for the left and right channel. There should be negligible difference in actual voltages (assuming the fuses are intact).

Right.


OK, so the fault LEDs are all OK now?

Can you go back to post #112 and make those measurements again, and repeat them for the other channel too.

 

[bikemech]

here is my results,
LEFT CHANEL
Q1 base==2.02VDC
top of D9==67.0VDC
bottom of D9== -03.0m
across R30==185 meter set on 2000m
across R32== -189 meter set on 2000m
Q22 base==-2.03VDC

RIGHT CHANEL
Q23 base==1.49VDC
Top of D21==66.1VDC
Bottom of D21==14.6m
across R62==00.0m
acrossR71==00.0m
Q40 base -1189m

not sure if i had the probes the correct way round on R30, R32,R62 and R71

 

[KrisBlueNZ]

OK. Most of those voltages are kind of correct for the left channel, but there's not enough bias for the output transistors.

Here's the schematic for the left channel.



Each diagonal red line indicates the base-emitter junction of a transistor. When you measure between the base and emitter of each transistor, you should get a voltage of around 0.7V (0.6~0.8V is reasonable). This indicates that the transistor is being forward biased and will conduct some current.

In the left channel, I think all of these voltages are reasonable except the base-emitter voltages of the output transistors, Q18 and Q20, which are about 0.2V. I know that the output transistors aren't installed, but there should still be bias voltage across their connection points.

The right channel seems to be even worse - no base bias for the output transistor positions.


For the left channel, could you please measure the voltage at the output (the point I've marked with "~0V") relative to 0V, and then measure each of the eight base-emitter voltages marked with red lines.

For the NPN transistors (top half of the output stage), the base will be positive relative to the emitter, so put the red probe to the base. For the PNP transistors (bottom half), do the opposite.



Then could you please repeat those measurements for the right channel.

There's no need to mark up the diagrams. Just list, for each channel, the output voltage, and the eight marked transistors along with their base-emitter voltages.

 

[bikemech]

for the LEFT chanel i mesured from T4 to D10 and it reads -01.3 and keeps dropping the longer i hold the probes on,

Q1 = 0.60VDC
Q10=0.58VDC
Q12=0.60VDC
Q18=191m

Q22=0.59VDC
Q11=0.59VDC
Q13=0.61VDC
Q20=197m

for the RIGHT chanel i mesured from T4 to D32 and it reads 023m

Q23=0.60VDC
Q25=0.49VDC
Q26=0.38VDC
Q28=00.0m

Q40=0.59VDC
Q37=0.49VDC
Q34=109m
Q36=0.00m

think i have done this correctly!

 

[bikemech]

Quick question guys, thinking of buying this, is it any good and would I be able to to BGA rework with it? Thanks.

http://pages.ebay.com/link/?nav=item.view&alt=web&id=310770452723

 

[Arouse1973]

Not a chance lol BGA needs special equipment.
Adam

 

[bikemech]

Not a chance lol BGA needs special equipment.
Adam

thought it was to good to be true lol

 

[bikemech]

Big ask here but could anyone do something like this for me so i can see the flow, or how its supposed to go,