Low Phantom power from 500 rack?

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The Scum
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Re: Low Phantom power from 500 rack?

Post by The Scum » Wed Feb 03, 2021 11:59 am

Analyzing a little more, the LM317 seems to be configured as a ~42V regulator.

475 Ohm feedback resistor, 15,400 ohm shunt. 1.25*(1+(Rshunt/Rfb)) = 41.77.

(or use this calculator)

It it were standing on a 5 or 6V Zener, then we'd be in the ballpark.

The regulator should develop a 1.25 V offset between the Vout and Vadj pins...if your prior measurements are correct, you've got about 0.4V.
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Re: Low Phantom power from 500 rack?

Post by The Scum » Wed Feb 03, 2021 12:00 pm

Keep following the Adj pin: What's beyond the 15k4 resistor?
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Re: Low Phantom power from 500 rack?

Post by DrummerMan » Wed Feb 03, 2021 12:13 pm

The Scum wrote:
Wed Feb 03, 2021 12:00 pm
Keep following the Adj pin: What's beyond the 15k4 resistor?
It disappears under the PCB. Theres a resistor right next to it that COULD be in its path and that looks like it SHOULD be 10k, but is measuring at 2.17k. That one disappears under the PCB as well.

And I was incorrect about my correction earlier. Those 2 diodes do in fact reconnect with leg 3 by a very thin trace.
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Re: Low Phantom power from 500 rack?

Post by The Scum » Wed Feb 03, 2021 12:17 pm

I'll guess that the 10K is in parallel with the trimpot, combining for the 2.17 K.

Adding 2.17K to the 15400, the calculator yields approx 48V.

Your regulator is the prime suspect. 10 years of a slight overvoltage likely killed it.
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Re: Low Phantom power from 500 rack?

Post by DrummerMan » Wed Feb 03, 2021 12:20 pm

The Scum wrote:
Wed Feb 03, 2021 12:17 pm
I'll guess that the 10K is in parallel with the trimpot, combining for the 2.17 K.

Adding 2.17K to the 15400, the calculator yields approx 48V.

Your regulator is the prime suspect. 10 years of a slight overvoltage likely killed it.
Would you say it's worth just replacing that voltage regulator first and seeing how it goes from there? Looks easy enough to swap out.
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Re: Low Phantom power from 500 rack?

Post by The Scum » Wed Feb 03, 2021 12:34 pm

Based on the remote diagnosis I can do through the tiny window presented, yeah, swap the regulator. Be sure to get the high voltage version.

There's the risk that my analysis is incomplete...but a new regulator will be revealing.
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Re: Low Phantom power from 500 rack?

Post by DrummerMan » Wed Feb 03, 2021 12:41 pm

The Scum wrote:
Wed Feb 03, 2021 12:34 pm
Based on the remote diagnosis I can do through the tiny window presented, yeah, swap the regulator. Be sure to get the high voltage version.

There's the risk that my analysis is incomplete...but a new regulator will be revealing.
Of course, but I appreciate the energy you've put into helping me. I'm also going to recap a Rheem Kee Bass keyboard, so once I figure out all the parts for that I'll put in a digikey order. Hopefully this all shouldn't take too long and I can see if this works!

Thanks again. To both you and Scodiddly! Let me know if either of you need a good drum track or semi-competent vibraphone/mediocre flute/shitty trombone track. I owe you guys one!
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Re: Low Phantom power from 500 rack?

Post by The Scum » Wed Feb 03, 2021 10:25 pm

Looking over Doug Self's Small Signal book, I'm reminded that there's a better regulator for phantom power: the TL783.

I think it's compatible with the LM317, but you might compare datasheets. The big benefit is that it's rated to 125V input. I think the pinout is the same, and the voltage setting resistors equation is too, but please doublecheck my work.
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Re: Low Phantom power from 500 rack?

Post by DrummerMan » Thu Feb 11, 2021 4:01 pm

20210211_155152_copy_514x906.jpg
20210211_155152_copy_514x906.jpg (228.79 KiB) Viewed 429 times
Fuck.

Yeah.


I replaced the lm317 voltage regulator. Not my cleanest solder job but I used a solder sucker for the first time ever (what an invention!). I started with the empty chassis and tested the power supply and the chassis' pinout, then put each preamp in one at a time and checked again, in case one of the pres was doing something funky. All 4 pres in now and phantom turned on across the board and everything's reading a solid 48.2v DC.

So extremely happy.

I'm going to leave the pres on with phantom engaged for a little bit and check back to make sure something isn't slowly overheating. I'll plug in mics later and make sure that's all working as intended too.
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Re: Low Phantom power from 500 rack?

Post by Scodiddly » Thu Feb 11, 2021 6:16 pm

Congratulations! 8) 8) 8)

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Re: Low Phantom power from 500 rack?

Post by Magnetic Services » Thu Feb 11, 2021 6:30 pm

DrummerMan wrote:
Thu Feb 11, 2021 4:01 pm
Not my cleanest solder job but I used a solder sucker for the first time ever (what an invention!).
Congrats on your first suck!

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Re: Low Phantom power from 500 rack?

Post by Scodiddly » Thu Feb 11, 2021 7:10 pm

If you followed my advice and bought more than one replacement, then put the extra into a little baggy and tape it to the side of the power supply.

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Re: Low Phantom power from 500 rack?

Post by DrummerMan » Thu Feb 11, 2021 7:47 pm

Scodiddly wrote:
Thu Feb 11, 2021 7:10 pm
If you followed my advice and bought more than one replacement, then put the extra into a little baggy and tape it to the side of the power supply.
Well, I've got a little container for studio spare parts like fuses, etc., but I did take your advice and get like 4, just in case!
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Re: Low Phantom power from 500 rack?

Post by digitaldrummer » Fri Feb 12, 2021 6:27 am

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