Stage Line 500W amp repair project

[bikemech]

Hello,

The MPSA42 is the part number, the E112 is likely a production code.
View attachment 50441

Bertus

Ah ok, so as long as there MPSA42, they should be good, also think Q10 is dead, but again, going to have to remove and test out of circuit, i am trying to test as much as possible before i stick an order in ,
Cheers

 

[bikemech]

Will these do as a replacement ?

https://www.ebay.co.uk/itm/10-x-MPS...-/232562261199?_trksid=p2349624.m46890.l49292

 

[WHONOES]

The supply voltages seem a bit high but I guess as they are off load they are not unreasonable but it does suggest that transformer regulation is not very good.

If you haven't removed Q10 yet, you could try soldering a100Ω resistor between the bases of Q1 and Q22. Then connect a DMM switched to Volts from output to 0V, remove any output load you have added. Put small vale fuses into F1 and F3. Disconnect the 55V supplies from the amplifier front end circuit. Whilst monitoring the DMM switch on and observe whether the O/P voltage is close to 0V or has swung towards either the + or - PSU. If it has then it points to a problem in the output stage. If not then, the problem in in the front end. If the problem is in the output stage then the DMM will tell you which half the problem is in. If both the fuses blow then the entire O/P stage is at fault.
BUT, if you cannot disconnect to 55V supplies then ignore all of the above.

 

[bikemech]

The supply voltages seem a bit high but I guess as they are off load they are not unreasonable but it does suggest that transformer regulation is not very good.

If you haven't removed Q10 yet, you could try soldering a100Ω resistor between the bases of Q1 and Q22. Then connect a DMM switched to Volts from output to 0V, remove any output load you have added. Put small vale fuses into F1 and F3. Disconnect the 55V supplies from the amplifier front end circuit. Whilst monitoring the DMM switch on and observe whether the O/P voltage is close to 0V or has swung towards either the + or - PSU. If it has then it points to a problem in the output stage. If not then, the problem in in the front end. If the problem is in the output stage then the DMM will tell you which half the problem is in. If both the fuses blow then the entire O/P stage is at fault.
BUT, if you cannot disconnect to 55V supplies then ignore all of the above.

Wow, lots to take in there, i have ordered some new mpsA42 transistors and some more kit, ie new temp controled iron with different tips, solder wick and flux, i am determined this amp will live again,
Thank you to everyone who is interested and willing to help with this project,
Pete

 

[bikemech]

The supply voltages seem a bit high but I guess as they are off load they are not unreasonable but it does suggest that transformer regulation is not very good.

If you haven't removed Q10 yet, you could try soldering a100Ω resistor between the bases of Q1 and Q22. Then connect a DMM switched to Volts from output to 0V, remove any output load you have added. Put small vale fuses into F1 and F3. Disconnect the 55V supplies from the amplifier front end circuit. Whilst monitoring the DMM switch on and observe whether the O/P voltage is close to 0V or has swung towards either the + or - PSU. If it has then it points to a problem in the output stage. If not then, the problem in in the front end. If the problem is in the output stage then the DMM will tell you which half the problem is in. If both the fuses blow then the entire O/P stage is at fault.
BUT, if you cannot disconnect to 55V supplies then ignore all of the above.

If am honest, i dont see where the 55v input is, apart from to the psu as both sides has an ovpl and + and -, am i right in thinking the OVPL is over volt protection ?, but seeing as the fuses all have good voltage to the left and right chanel amps am not sure why i would need to do this, i think the only problem i now have is somewhere in the left chanel amp circuitry somewhere

 

[bikemech]

Ok, so Q10 and Q1 replaced, all looks good, all power cables attached and no fault lights anywhere , tempted to fit the output drivers ?

 

[bikemech]

 

[bikemech]

View attachment 50472

Tried to clean the board up a little

 

[bikemech]

Tried to clean the board up a little

Also, npt sure if its that important but all the output transistors now have 64v to each base terminal

 

[WHONOES]

Which transistors are you referring to in post #409?

 

[WHONOES]

If you are referring to Q1 and Q10 then NO they should not have 64V on them, they should be close to 0V. They will be turned hard on and so will the rest of that half of the O/P stage. What voltage do you have on the bases of Q11 and Q22?
Before you make any further checks, I would suggest removing Q6 and Q9 from the previous stage and as mentioned previously, connect a 100Ω resistor between the bases of Q1 and Q22 then check the above voltages. If all is well then the problem is in the front end.

 

[bikemech]

If you are referring to Q1 and Q10 then NO they should not have 64V on them, they should be close to 0V. They will be turned hard on and so will the rest of that half of the O/P stage. What voltage do you have on the bases of Q11 and Q22?
Before you make any further checks, I would suggest removing Q6 and Q9 from the previous stage and as mentioned previously, connect a 100Ω resistor between the bases of Q1 and Q22 then check the above voltages. If all is well then the problem is in the front end.

Its the 4 clipped to the heat sync and the main 4 output transistors

 

[WHONOES]

Have you checked voltages on Q1 and Q22 as suggested. If so, what are the results.

 

[WHONOES]

Just noticed that there is a part missing from the board at PR1. It's used to adjust the quiescent current in the O/P transistors and is quite important. Did you remove it?

 

[bikemech]

Just noticed that there is a part missing from the board at PR1. It's used to adjust the quiescent current in the O/P transistors and is quite important. Did you remove it?

No, if you look on the schematic it says NF, (NOT FITTED )

 

[bikemech]

However i may fit these later, and also the missing output transistors

 

[bikemech]

Have you checked voltages on Q1 and Q22 as suggested. If so, what are the results.

I will check this tonight after work and post my results

 

[bikemech]

If you are referring to Q1 and Q10 then NO they should not have 64V on them, they should be close to 0V. They will be turned hard on and so will the rest of that half of the O/P stage. What voltage do you have on the bases of Q11 and Q22?
Before you make any further checks, I would suggest removing Q6 and Q9 from the previous stage and as mentioned previously, connect a 100Ω resistor between the bases of Q1 and Q22 then check the above voltages. If all is well then the problem is in the front end.

Ok Q1= 0.370VDC and Q22= --0.782VDC

 

[WHONOES]

Were those voltages taken with Q6 and Q9 removed. Have you replaced the output transistors as well? The figures you quote are quite reasonable. I assume they are referred to 0V.

 

[bikemech]

Were those voltages taken with Q6 and Q9 removed. Have you replaced the output transistors as well? The figures you quote are quite reasonable. I assume they are referred to 0V.

No nothing was removed, the 4 main outputs are now in, used 2 of the old ones to test first as 3 of them still tested ok, now i have amplification on the right side but not the left, the left chanel has the old main outputs, Q10 and Q20, and yes negative probe on t20