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Um... actually if there is zero volts between the opposing bases, that sounds less like the four diodes. They would all four need to be shorted, and you need not remove them to find that out. If any one was open, the whole string would be open, and there would be way large voltage between bases. At least that is how I see it. And while they are in there, you can measure the voltage across each one directly.
Your readings are highly suspect. First, if you tell me there is 31 volts at R70, that sounds like the voltage across it, otherwise, you need to specify which end of the resistor. I will assume you mean the bottom end on the drawing. And 14.5v at the bottom of R71, which is the base of Q1. Now you show 15v at the (presumably) top end of R72. Now normally that would be impossible - how could the voltage become MORE positive than it was closer to the V+ rail, as we measured at R71? And indeed it goes back up to 30v at R73. I HOPE you meant those last two to be negative voltages. It is crucial to include polarity unless we can assume all voltages are positive.
Considering you mentioned you took all readings with respect to ground, did you ever read the voltage actually from the base of Q1 directly to the base of Q2? Or did you measure each to ground and then add them together? Since you left off the polarity I have to ask that, because then you'd have +14 and -15, which ought to turn them on very hard. And even if they were both positive, 14.5 on one and 15 on the other is still half a volt apart.
TP18 SHOULD center at about zero volts DC. The Q1,Q2 bases should each have roughly a volt or two on each, Q1 positive and Q2 negative.
I tried to calculate the current through the diode string. By the voltages (and assuming negatives where needed) I get about 3ma through the 4.7k resistors and about 7ma through the 2.2k resistors. I would expect the same current through both resistors, they are in series. We have to account for the extra 4ma somehow. So I have to ask, though it seems unlikely, is the TIP142 on the right, next to the pair of main filter caps? And is the TIP147 over near the IC? There should be something like 8040 ohms between B and C on those transistors. Then again that doesn't make sense either. Even if caps C39 and C40 are bad, they still would drag extra current through the 4700 ohmers rather than the 2,2k. I'm baffled. Or maybe I am tired, it is late.
And did we ever measure voltage on R74,75 end to end rather than to ground?
Might as well check CR9-12 for shorts too.
Your readings are highly suspect. First, if you tell me there is 31 volts at R70, that sounds like the voltage across it, otherwise, you need to specify which end of the resistor. I will assume you mean the bottom end on the drawing. And 14.5v at the bottom of R71, which is the base of Q1. Now you show 15v at the (presumably) top end of R72. Now normally that would be impossible - how could the voltage become MORE positive than it was closer to the V+ rail, as we measured at R71? And indeed it goes back up to 30v at R73. I HOPE you meant those last two to be negative voltages. It is crucial to include polarity unless we can assume all voltages are positive.
Considering you mentioned you took all readings with respect to ground, did you ever read the voltage actually from the base of Q1 directly to the base of Q2? Or did you measure each to ground and then add them together? Since you left off the polarity I have to ask that, because then you'd have +14 and -15, which ought to turn them on very hard. And even if they were both positive, 14.5 on one and 15 on the other is still half a volt apart.
TP18 SHOULD center at about zero volts DC. The Q1,Q2 bases should each have roughly a volt or two on each, Q1 positive and Q2 negative.
I tried to calculate the current through the diode string. By the voltages (and assuming negatives where needed) I get about 3ma through the 4.7k resistors and about 7ma through the 2.2k resistors. I would expect the same current through both resistors, they are in series. We have to account for the extra 4ma somehow. So I have to ask, though it seems unlikely, is the TIP142 on the right, next to the pair of main filter caps? And is the TIP147 over near the IC? There should be something like 8040 ohms between B and C on those transistors. Then again that doesn't make sense either. Even if caps C39 and C40 are bad, they still would drag extra current through the 4700 ohmers rather than the 2,2k. I'm baffled. Or maybe I am tired, it is late.
And did we ever measure voltage on R74,75 end to end rather than to ground?
Might as well check CR9-12 for shorts too.
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