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Old 15th September 2020, 04:29 AM   #31
objext is offline objext  United States
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Q6, Q8, & Q13 drop down to similar temps of Q5, Q7, & Q12 if I lower the supply voltage from 14V to 13V. U202 & U402 continue to be around 116-117. I think U402 may actually be the hotter of the two, but only by a degree or two.
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Old 15th September 2020, 04:51 AM   #32
objext is offline objext  United States
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Q309 and the like (the MPSA56 & MPSA06) are also getting warm/hot, as high as about 123F
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Old 15th September 2020, 09:10 AM   #33
Perry Babin is offline Perry Babin  United States
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Are you saying that the 300 series transistors including Q309 are heating, alone or are all of the Qx09 transistors heating?

Post a photo of the drive signal on the gate legs of one PS FET in each bank when the FETs are heating unevenly and when they are heating equally.

For the hottest op-amp, remove it or at least lift it's power supply terminals and re-check the heating of the adjacent op-amp to see if it's still heating up.
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Old 16th September 2020, 12:57 AM   #34
objext is offline objext  United States
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Q108, Q109
Q208, Q209
Q308, Q309
Q408, Q409

These are all heating, but maybe this is normal. Just trying to find any outliers that might indicate a problem. I don't have a scope, but I guess I need to get one if I'm going to try to figure this out without shipping it off to someone else.

I will try your suggestion of lifting the hottest op-amp. Thanks for your help, Perry
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Old 21st September 2020, 04:30 AM   #35
objext is offline objext  United States
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I lifted PIN 4 and PIN 8 of U402, TL072C. Before lifting those pins I was getting different readings on R43 and R44. I was getting about 40ohm on one and 37ohm on the other. With U402 PIN 4 & PIN 8 lifted they both read correctly at 20ohm.

Regulators are still heating to the same amounts as before. At 13.5V input I get:

LM317
In: 28.6V
Out: 14.2V
Adj: 12.9V

LM337
Out: -12.7V
In: -28.6V
Adj: -11.3V
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Old 21st September 2020, 04:37 AM   #36
Perry Babin is offline Perry Babin  United States
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Is the op-amp next to U402 still heating like it was?

It's possible that the op-amp was defective (leaking internally) but not causing any significant current draw from the regulators.
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Old 21st September 2020, 04:52 AM   #37
objext is offline objext  United States
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Yes. It seems most of the op amps are heating. Some more than others. My IR gun is difficult to aim at such small areas precisely. The LM833 are also heating.
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Old 21st September 2020, 05:06 AM   #38
Perry Babin is offline Perry Babin  United States
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Where is the LM833?

The voltage is well within what the op-amps can withstand. I don't remember TL072s getting hot but they can't all be bad.

Pick one of the hotter remaining ones and post the DC voltage on all of its terminals.


Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
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Old 21st September 2020, 05:18 AM   #39
objext is offline objext  United States
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There are two sets of 4 TL072 and 3 LM833 paralleled together on pins 4 & 8.

DC voltage on U201 TL072:

Pin 1: 0.57
Pin 2: 0.57
Pin 3: 0.56
Pin 4: -12.15
Pin 5: 0.48
Pin 6: 0.50
Pin 7: 0.50
Pin 8: 13.67

I get almost identical readings from U401 on the other side of the amp
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Old 21st September 2020, 05:22 AM   #40
Perry Babin is offline Perry Babin  United States
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That op-amp looks healthy.

What are the circuit board designations for the LM833s?

Do you have the same voltage across each of the resistors in the two pairs of 20 ohm resistors?
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