51x Power supply troubleshooting

Hey all!

Just finished my first a bit bigger project - or so I thought. I've spent the last few days putting together a 51x floor box power supply and I just plugged in the power to test it.

First thing: excitement goes to horror when I hear the dreaded "bzzzt" from somewhere on the board. About to pull out the power cable when I notice that everything seems to be powered...except +16V. I measure all pins and everything is where it should except +16V AND +48V is showing 70.2V instead. [EDITED]

After some very basic troubleshooting, I notice that JP2 has no connection to star ground. Remove shrink tube, add solder and now it works. Put everything back together, plug in power: +16V now works but phantom still shows too high.

Tried turning the screw on RV1 - no difference. Just de-soldered RV1 - measuring 970ohms 1 to 3, 221ohm 1 to 2 and 750 2 to 3. Seems about right?

I'm quite sure the buzzing noise I heard when I first fired it up was from the phantom power section on the board. The fuse is still working, didn't break.

Could any kind soul help me troubleshoot this and tell me what I should try next? I'm really a novice at this, sad to say.

Cheers,
Tore

EDIT: I'm measuring 70.2V, not 72 as I wrote at first. Also tried now turning the screw on RV1 several rounds and I can confirm that it does absolutely nothing.

If you can't adjust the 48 volt, check the regulator for that voltage.
You might have put it in backwards and blown it. Replace it and check the diagram on
the P.S.B. and make sure the metal part of regulator lines up with the drawn retangle box.

Aaah yes... That might well be it. When I think about it I might have mixed up the ICs, hmmm... I'll have to try to read on them, as far as I can remember I had two different kinds but I guess I was sloppy after that.

If I have to get a new one, would this do?
https://www.elfa.se/elfa3~se_sv/elfa/init.do?item=73-266-58&toc=19820

Can't find one with the same specs as in the BOM here in Sweden so I'd have to order if it has to be exactly the same. Again, sorry for my very limited knowledge of electronics.

Alright, an update: clearly I'm blind so I mixed up IC1 and IC3 on the board. So IC1 got the LM350T meant for IC3, and IC3 got the TL783 meant for IC1.

Since 24V outputs as expected, maybe the TL783 is still working? I guess I fried the LM350T. Anyway, both are unsoldered and taken off the board for now.

My store in Sweden don't have them but this looks equivalent:
https://www.elfa.se/elfa3~se_sv/elfa/init.do?item=10-364-37&toc=19820

Any thoughts? Something more I might have broken? Winetree, thanks a million for pointing me in the right direction.

EDIT: My bad, just found this:
https://www.elfa.se/elfa3~se_sv/elfa/init.do?item=73-266-92&toc=0&q=lm350

tgs said:

Since 24V outputs as expected, maybe the TL783 is still working?

Click to expand...

should still work well in the 48V PSU

Ended up with a fail: after much work getting the components in the right place (man I wish I had a real desoldering station...) I plugged it in and...SPARKS from the heatsink. All fuses intact, oddly. All LEDs went on. Unplugged, took the board out and checked the new soldering points, etc. Plugged it back in: only -16V and -24V LEDs go on. Don't dare do do much more at this point...

Did you isolate the Regulator from the heatsink?
You should use the slip sheet between the regulator and heatsink
with the nylon washer to isolate the screw.

Volker and Winetree: spot on. I missed the nylon washers - feel like a fool now but for a first DIY project I hope I can be excused. I plugged it in and everything works perfectly now! I'll hopefully be more cautious next time. I think I follow instructions well but I do have a tendency to selectively leave out a few words...

Anyway, you can't imagine how happy I am that I didn't actually break anything (apart from that first LM350T). Thanks so much for your help.

Congrats,
A new rule;
Read it three times,  build it once.