No need to take them out.
With your multimeter and with the 4 big caps fully discharged, you should be able to find the guilty one comparing their resistances.
Handel
To resume a few facts:
1) Both lamps have more then 70V on them, proving that both NPN and PNP TO3 output transistors are conducting.
Question is: how much current is this, it would be helpful to know as an indication whether all TO3's are O.K.
2) There is a 0.52 Voltage drop over Q38, a prove that Q38 is supplying current to what's following. But on R45 there is no voltage drop, so this current is not flowing into R45 and must go into Q39 and Q39. (or could it be that R45 is +1.28/-1.28V instead of +1.28 /+1.28V ?)
2) On R50 you have a voltage differential of 1 Volt, meaning 25mA. This 1Volt being enough to moderately let the TO3's conduct.
4) Point is however that this 1V is generated because of the current flowing through R50 which should not be the case with R45 having zero volt differential.
5) So this current through R50 has either to flow through a defective PNP TO3, which seems not to be the case, so the only logical possibility is that either Q41 or Q42 is malfunctioning.
So first check whether the 0V voltage difference on R45 is correct. If true you will have to check the above point 5), Q41 and Q42.
Hans
To resume a few facts:
1) Both lamps have more then 70V on them, proving that both NPN and PNP TO3 output transistors are conducting.
Question is: how much current is this, it would be helpful to know as an indication whether all TO3's are O.K.
2) There is a 0.52 Voltage drop over Q38, a prove that Q38 is supplying current to what's following. But on R45 there is no voltage drop, so this current is not flowing into R45 and must go into Q39 and Q39. (or could it be that R45 is +1.28/-1.28V instead of +1.28 /+1.28V ?)
2) On R50 you have a voltage differential of 1 Volt, meaning 25mA. This 1Volt being enough to moderately let the TO3's conduct.
4) Point is however that this 1V is generated because of the current flowing through R50 which should not be the case with R45 having zero volt differential.
5) So this current through R50 has either to flow through a defective PNP TO3, which seems not to be the case, so the only logical possibility is that either Q41 or Q42 is malfunctioning.
So first check whether the 0V voltage difference on R45 is correct. If true you will have to check the above point 5), Q41 and Q42.
Hans
So you will have to find where the 25mA flowing through R50 is going to.
To recheck the PNP TO3's, you can measure the voltage drop on the six 10R base resistors, showing cumulated no more then a few mA.
If confirmed that the TO3's are O.K. you can measure the voltage drop on resp. R1 and R4, the emitter resistors of Q41 and Q42, because one of these should carry the remaining current.
Hans
To recheck the PNP TO3's, you can measure the voltage drop on the six 10R base resistors, showing cumulated no more then a few mA.
If confirmed that the TO3's are O.K. you can measure the voltage drop on resp. R1 and R4, the emitter resistors of Q41 and Q42, because one of these should carry the remaining current.
Hans
So that’s where the 25mA is going to.
What happens to the voltage drop on the 100R collector loads and on R50 when you disconnect one side of this 10R resistor with 0.3V voltage drop from the PCB.
Hans
So voltage drop over R50 is now 0V, that’s what it should be.
With 71volt on the 100R resistors, there is still 710mA flowing, right ?
Now take out the TO3 with the disconnected 10R resistor.
Hans
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As long as you don’t plan to put 1000Watt into a 2R Speaker, you can very well leave the sixth TO3 out and continue with 5 PNP’s.
Although everything seems OK now, you could remove the short between the two WH’s and turn the pot until you have 10V on the 100R resistors.
Then measure the voltage on all 10R base resistors to check if they are roughly the same between the 6 NPN’s and between the 5 PNP’s.
When that’s o.k. remove the 100R and set the Bias to is final value and you are ready to play music.
Hans
A friend purchased a No 23 which suffered a total breakdown at the previous owner for no apparent reason with lots of smoke and stench. Inside a lot of shorted power transistors in the output buffer stage so as various burned resistors.
I'm still unsure if I should get this power amp on the table after looking at the circuit in post #2 because I didn't understand the theory of operation of the Vbe stage. But following the hints in post #56-59 under
https://www.diyaudio.com/community/threads/optimizing-the-vbe-multiplier.216385/page-3#post-6947104
and my conclusion for performing the troubleshooting procedure in post #60 I might start this repair project.
It would also be interesting to know which deficiencies exist in the circuit design from the circuit in post #2, that should be eliminated in general, regardless of individual bugs and how many different circuit designs of model No 23 are there.
I hope I don't get nasty surprises with burned PCB areas like with the No 23.5 - see photo 4-6 in post #15 on page 1 here.
I'm still unsure if I should get this power amp on the table after looking at the circuit in post #2 because I didn't understand the theory of operation of the Vbe stage. But following the hints in post #56-59 under
https://www.diyaudio.com/community/threads/optimizing-the-vbe-multiplier.216385/page-3#post-6947104
and my conclusion for performing the troubleshooting procedure in post #60 I might start this repair project.
It would also be interesting to know which deficiencies exist in the circuit design from the circuit in post #2, that should be eliminated in general, regardless of individual bugs and how many different circuit designs of model No 23 are there.
I hope I don't get nasty surprises with burned PCB areas like with the No 23.5 - see photo 4-6 in post #15 on page 1 here.
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