Low Phantom power from 500 rack?

[DrummerMan]

And here's an overview of both sides.

20210202_163131_copy_567x1008.jpg (240.89 KiB) Viewed 2092 times

[The Scum]

OK, that LM317 is the one we're interested in. The 7915/7915 are +/- 15V voltage regulators, the LM317 is an adjustable regulator, the HVT is a version that can take a 60V max input.

So it looks like they've bypassed a couple of the flakier design options with the HVT part. But the 63V you get at the PRE test point still exceeds it's stated max input, which may have led to mortality.

What voltage do you measure between it's center leg and ground?

[The Scum]

The circuit in the closeup of the 317 appears to look a bit like Fig 15 in the datasheet.

https://www.ti.com/lit/ds/symlink/lm117 ... 2289239523

[Scodiddly]

I *think* you can run the HVT rather higher, really the max voltage is relative to the other pins.

Good news is that with that nice high voltage going in there's no dead short and no transformer failure. So it's only going to be a couple bucks for whatever part failed, likely that LM317 HVT regulator. Even without a schematic any good tech should be able to figure it out and do the repair in under an hour.

First thing I would do at this point would be to measure voltage (referenced to ground) on each of the pins on that LM317. None of them should be zero or close to zero volts DC. Left pin is voltage in (60v), middle pin is voltage out (supposed to be 48v), right pin is a reference that determines output voltage. I'd also ponder for a moment about whether the tab at the top has gotten a short to ground, but since it ties to the middle pin which still shows *some* voltage then unlikely to be the problem. You could actually buy a couple extra of those LM317's, they're cheap, and just replace it to see what happens. Reasonably good chance it'll all just work, or else it'll burn out the replacement and you have to keep looking. I usually buy extras just in case I botch the replacement or get a dud.

As always big capacitors are suspect, and if you're feeling adventurous you could think about just replacing them anyways. Digi-Key is good for having all the sizes and values. With the circuit powered off and the capacitors discharged you could measure resistance across any resistors; the surrounding circuit may affect the resistance you read but only by making it lower. So any resistors that measure higher than marked are bad.

[Scodiddly]

D1 and D2 give me 0.018
D3 and D4 give me 0.47

0.47 is a good value for a diode. 0.018 is very suspect, though it's possible the surrounding circuitry is causing that.

[DrummerMan]

So, here's what i got on that LM317:

Left Leg 15.78
Middle leg 16.2
Right leg 71.6

Wouldn't that be the opposite of how it should be?

[DrummerMan]

lm317-audio-amplifier.gif (11.26 KiB) Viewed 2069 times

(Edit) Ok this says that the right leg is input

[The Scum]

The question in my mind is whether the regulator is standing on top of a zener diode, to boost it's operating range. If the zener failed short, the regulator would still regulate, just without the boost.

[DrummerMan]

The question in my mind is whether the regulator is standing on top of a zener diode, to boost it's operating range. If the zener failed short, the regulator would still regulate, just without the boost.

I don't THINK I see any zener diodes. Do they ever look black like generic/schottkey diodes? There's a couple that look they're coming off of and bridging the outer legs on that voltage regulator, but I can't read the numbers to see what they are.

[FNM]

triple post!

[FNM]

double post

[FNM]

I don't THINK I see any zener diodes. Do they ever look black like generic/schottkey diodes? There's a couple that look they're coming off of and bridging the outer legs on that voltage regulator, but I can't read the numbers to see what they are.

They sometimes do look like that. Double check with your diode function on your meter. If D1/D2 read short, I'd probably pull 'em, or take it to a tech. De-soldering on these type of boards can go south real quick.

[DrummerMan]

I don't THINK I see any zener diodes. Do they ever look black like generic/schottkey diodes? There's a couple that look they're coming off of and bridging the outer legs on that voltage regulator, but I can't read the numbers to see what they are.

They sometimes do look like that. Double check with your diode function on your meter. If D1/D2 read short, I'd probably pull 'em, or take it to a tech. De-soldering on these type of boards can go south real quick.

Just tested those 2 diodes. They both come off leg 1 (adj) in series. Not connected to leg 3 (V in) like I had thought.
They both start with 1N. Can't read the rest but I think that makes them general purpose, no? Either way, they gave me .55 & .48.

[The Scum]

Those are likely 1N400x, just basic power diodes, as shown in the datasheet. If the transformer gets disconnected, they discharge the output caps...as shown in Fig 15 of the datasheet.

(No guarantee about what 1Nxxxx device is. They come in plain silicon, Schottky and Zener.)

Can you trace the Adj pin? How much does it go through on it's way to ground? A resistor and that trimpot, or something more?

[DrummerMan]

Those are likely 1N400x, just basic power diodes, as shown in the datasheet. If the transformer gets disconnected, they discharge the output caps...as shown in Fig 15 of the datasheet.

(No guarantee about what 1Nxxxx device is. They come in plain silicon, Schottky and Zener.)

Can you trace the Adj pin? How much does it go through on it's way to ground? A resistor and that trimpot, or something more?

That adj leg splits 4 ways, follows the paths below, then disappears onto the bottom of the PCB where I lose them.

- 470 ohm resistor

- Positive side of those 2 diodes mentioned before in series

- 15.4k ohm resistor

- Positive side of 10uF 63v electrolytic cap

I tested the resistors and those at least are resisting as expected.