insanely-high voltages on green PSU

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kato

Well-known member
Joined
Aug 16, 2004
Messages
1,597
Location
Indianapolis, USA
I took 20 steps back and fired up my Green PSU, no smoke, no flames, no exploding caps.

After a few minutes I dared to approach with a DMM to see what it was doing.

32+v where there should be 15.

And phantom is giving off 170v :sad:

I quickly switched it off.
This seems dangerous as my caps are not rated for such high voltages.
Where should I start? Any ideas what would cause this doubling of voltage?

Kato
 
did you check your DMM setting and batteries? if there's 170V on any place after your rectifiers you will have noticed regs and caps bursting immediatly if they are not rated that high and not after minutes.
 
I had that same thought just before bed last night.
32v doesn't freak me out - my first thought was that I somehow connected the transformer secondary in series (double volts.)

But that 170 just doesn't sound possible from a 15v transformer.
My DMM has showed low-battery icon for weeks. Duh, why didn't I think of it sooner.

So this morning, I put in a recently recharged 9v.
Curiously, it's labeled 8.4v? Is that enough difference to throw off readings?

Low battery light is gone.
I thought my problem would be solved.
But my readings were these:

positive rail: 34v
negative rail: 0v
phantom: 170v

Clearly something has blown. And now I'm afraid of molten, exploding electrolyte blowing my face off. Could the 8.4v battery be the cause of the 170v reading? Either way, there must be a problem.

I guess I'll start disassembling and reassembling one component at a time... Thanks guys,

Kato
 
Well, neg rail is 0v, so you either have a short from neg rail to ground, or you are accidentally connecting the ground of your DMM to neg rail instead of ground.
 
[quote author="Mbira"]Well, neg rail is 0v, so you either have a short from neg rail to ground, or you are accidentally connecting the ground of your DMM to neg rail instead of ground.[/quote]

that wouldn't give 170V eighter.
Measure a known good voltage first (VAC source from your wall outlet and a known operating DC-voltage), to assure your DMM is reading in plausible range. Case it is, separate the transformer from your board and measure the raw unloaded AC secondaries voltages. Check polarity of your rectifying diodes. Rectangular bridge-rectifiers come in different pin layout, so there might be something mixed up.
 
[quote author="Harpo"]that wouldn't give 170V eighter.[/quote]

I know, I just wanted to forget about that part!
 
[quote author="Mbira"]I know, I just wanted to forget about that part![/quote]
happens to me frightenly often when my clicking finger gets faster than my buggy brain :grin:
 
170V ? :roll:

Sounds like you took the Dutch approach and skimped on a power-TX :wink:


But serious, what are the AC-voltages on the secondaries of the TX ?
 
Check the diode orientation for the 34v.

What is the rectified DC voltage after the bridge?

On the Phantom supply, how many stages have got in there? Should only be 3 in your case, I think.

Peter
 
[quote author="clintrubber"]170V ? :roll:

Sounds like you took the Dutch approach and skimped on a power-TX :wink:
[/quote]

maybe my primary and secondary have shorted :!:

[quote author="clintrubber"]
But serious, what are the AC-voltages on the secondaries of the TX ?[/quote]

I will check that this evening.
Never again will I hook up a power supply board without getting a reading on the secondary first. I will use PeterC's ultra cautious (smart) method. :cool:

Thanks guys for your help - I will report back when I have made sense of it. Kato
 
I made a voltage-quadrupler supply once and got a cap polarity wrong. I was fortunate to be turned away as the thing exploded (so much for the safety vent), but I was cleaning up debris for months afterward. I wish I could have recorded the sound it made before and as it exploded.
 
how far can you go on one of those doubler-trippler, etc circuits? As far as you can still get the current needs met?
 
[quote author="peterc"]Check the diode orientation for the 34v.
What is the rectified DC voltage after the bridge?
[/quote]

I decided to disassemble and restart. The big caps are only rated for 16v so I'm afraid it's dangerous to poke around with it live.

I still don't understand why nothing exploded.

[quote author="peterc"]On the Phantom supply, how many stages have got in there? Should only be 3 in your case, I think.
[/quote]

Stages? I need to learn what this means.
I used a board (nicely-)etched by ptownkid.
Caps seem to be correct (all facing same dir.) Diodes look to be in correctly. No visible shorts:

full.jpg


[quote author="clintrubber"]But serious, what are the AC-voltages on the secondaries of the TX ?[/quote]

22.3v with no load. Is this normal?
(it's a 15v Amveco/Talema.)
 
22.3v with no load. Is this normal?

I would have thought about 18-19v with no load.

The caps before the regs should be at least rated for 35v. 22v AC will rectify to 31.5v which the 16v caps will probably not like.

Dunno which version of the PSU PCB Ptownkid is selling but in the latest version the 2 diodes between the regs should be facing the same way.

Ptownkid, which one is it?

Peter
 
You mesured 170 millivolts, not 170 volts, only you did not discearn the difference due to the fact that you were pre disposed into volts. the vehicle to your subsequent mis representation being the preceding measurements of the rails, , and by the fact that most meters have a crappy mv LCD display.
The caps would have either blown, or tripped the fuse if you had 170.which I do, by the way, on a daily basis, so suck it up and deal with it pal.
:roll:
 
[quote author="peterc"]
22.3v with no load. Is this normal?

I would have thought about 18-19v with no load.[/quote]

It's an extra-small (VA) transformer. A compensating technique? No that makes no sense. Poor quality control. Yes, that seems more likely.

Note to self: measure first.

[quote author="peterc"]
The caps before the regs should be at least rated for 35v. 22v AC will rectify to 31.5v which the 16v caps will probably not like.
[/quote]

Parts kit was a little underspec-ed. Before going live I thought, "yeah, I'm going to be replacing those..."

[quote author="peterc"]
Dunno which version of the PSU PCB Ptownkid is selling but in the latest version the 2 diodes between the regs should be facing the same way.
[/quote]

Uh oh. :!:
underside.jpg

Is that 21-9-4 a date code? Sept 21st, 2004?

Thanks for helping Peter. Kato
 
[quote author="CJ"]You mesured 170 millivolts, not 170 volts[/quote]

Thanks. For pointing me in the general direction of reality.
The sheer improbability of it made my brain throb.

A sigh of relief as my sense of relative normalcy in the universe is restored.
abduct.gif
 
[quote author="peterc"]
Dunno which version of the PSU PCB Ptownkid is selling but in the latest version the 2 diodes between the regs should be facing the same way.

Ptownkid, which one is it?
[/quote]

Peter, is there a more recent version than the one at your site?

New PSU unit for Mic Pre's shows the same (date?) code: 21-9-4 but your overlay shows the diodes between the regulators in opposite directions.

Thanks, Kato

[edit]
problem sorted out! (for e's and whizz)

swapped out power supply for a new one - same result. 0 on the negative rail, super high on the positive.

i was using the AC side of my DMM, instead of DC. :oops:
switched over to DC and all voltages are normal...


[end edit]
 
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