Power supply blowing its MOSFETS only when earthed

[Electronic Santa]

I have a guitar amp that beer got spilled on. It was blowing fuses straight away.

It has a seperate power supply board and I was quickly able to determine that the 2 power mosfets BUK456 800A were blown. I cleaned the mess with iso. When I removed the mosfets the short went away.

 I ordered some compatable ones (cant find the BUK456 800A original) and put them in, it fired right up. On my work bench (I used a sencore power nite variac)  I turned it off and on for 3 days and it was fine playing it multiple times.

As soon it was unplugged and transported and plugged back in, boom the power mosfets blew again? (Only difference earth ground my bench uses a isolated or lifted earth)

So I changed them again, this time being very careful to make sure they were heatsinked proper and hooked up correctly and this time I ran it over night, off and on 20 times no issues


Maybe efi is connecting to the chassis? Would this cause this? It's isolated with rubber on inside? Came with no bolts

[CaptDon]

I am going to take a wild guess that your bench or test outlet has an isolated ground and when you plug it in to a normal socket there is a ground path. Sounds like you forgot an insulating washer or mica insulator under one of the mosfets and now there is a power path to ground. BOOM!!

[Electronic Santa]

Yes I  agree. I am on an isolated ground, and this made sense that this was the cause.

The only issue is, where and what? I guess I'm going to have to tear it all apart and look at all the grounds, and see what I missed.

But i also agree it has sometbing to do with the ground issue.

Any ideas on what to look for?

[CaptDon]

Some devices have 'isolated tabs' and don't require insulated shoulder washers or mica / silicone insulator pads underneath. Check the hardware you disturbed in your repair also look for burned carbonized tracks on the circuit board where the beer got spilled. Funny side note, while doing sound at an outdoor pig roast a drunk girl fell down a set of steps and hurled an entire 'slow gin fizz' into my amplifier rack. It went into everything but everything kept working!! I expected the worst!! That night I took every piece of equipment out of the rack, opened everything up and washed the boards and air dried with a hair dryer. All is well!! Worst experience ever was a drunk spilling a large glass of beer into my mixer board! Ended the gig for the night and destroyed the mixer completely!! Far to much work and expense to repair it. Some traces were gone completely off the circuit boards!1

[Electronic Santa]

Very cool!! Too bad about the mixer!!!

Yes this is an odd one. It's a small power supply boards and 2 other daughter boards for the preamp and digital fx etc. The power supply board has a big metal EFI box and I'm thinking the nuts and bolts that bolted it to the metal chassis were isolated and It came to me apart, so they were missing and I put whatever in there.

I attached a pic wouldcthat efi inuslator thingy need to be isolated? It appears to have a rubber piece shieldingit from the face and maybe i used the wrongs screwed and connected it? . But its for sure something to do with the earth grouding.

Sorry I was on holidays took me a while to respond.

Appreciate anyone's input.

FWIW. The 2 mosfets that are blowing when this happens is, the 2 south part of the pic. Works fine on my isolated variac powersupply and as soon as it's earth grounded they blow.

[Audiorepair]

It looks like you have a mains inlet module with built in filter stuff.

Perhaps this is the problem.

Maybe try to remove this or to bypass mains to the unit otherwise to test.


An isolation transformer floats the ground.  A faulty inlet may connect the ground to a live rail.

[fzabkar]

Run the unit on an isolated supply, and then measure the voltage between chassis ground and each of the MOSFET pins. That's assuming that your resistance checks don't find the cause.

[Electronic Santa]

Thanks I'll try. the first post explains it. Works perfect on variac (sencore powernite) and then when plugged into regular wall power it blows.

But interesting I'll check that out

[Electronic Santa]

Will try. I assume you mean with working mosfets installed? Waiting for some new ones now.

[Audiorepair]

Have you still not considered your mains inlet filter may be faulty?

[Kim Christensen]

FWIW. The 2 mosfets that are blowing when this happens is, the 2 south part of the pic. Works fine on my isolated variac powersupply and as soon as it's earth grounded they blow.

What do you mean by "south part of the pic"? If you mean Q509 & Q511 then those need insulating washers under them. The tabs cannot be connected to the chassis.

[Electronic Santa]

I mean the one's to the right in the picture. All of the mosfets (2 on the left and 2 on the right) have Insulating tabs underneath them. I guess it hard to see. YES Q509 AND Q511

Does anyone know if that mains filter EMI metal can, does that need to be isolated from metal case?

[shakalnokturn]

EMI can must be connected to earth and even if defective shouldn't cause the primary MOSFETS to blow.
Although MOSFETs have been insulated have you tried measuring resistance between Drain and heatsink? Same thing between cathodes and heatsink on secondary rectifiers? (Short circuit condition with no protection.)

Any other components been replaced? It could be as simple as a through hole lead cut too long touching the heatsink on the solder side.

[shakalnokturn]

There are also a couple of capacitors located between the mains filter and secondary diodes, they look like RIFAs, I don't know what their role is in the circuit but if the are between primary and secondary they're suspects.

[Electronic Santa]

Interesting. I'll check everything you mentioned.

Please look at these 2 pics. When I got this there were no screws in the EMI to the chassis. I went and put some standard nuts and bolts through here. Did I cause the connection between earth and the negative rail? It's clearly got a rubber pad to Sheild it from the chassis on the inside. And is this why the amp works fine on my isolated bench supply?  (Is this also why it came to me with no nuts and bolts cause the brass ones or non conducive originals were missing and someone fiqured this out before me) this has been worked on by 4 others. I had to repair ALOT to get it working on my bench)

This is a learning experience for me forsure.

[Electronic Santa]

I've considered this. Is there a way to really check this other than bypassing? Waiting for new mosfets.

[TimFox]

I had a similar problem with an -hp- 6209 320 V/0.1 A lab DC power supply.
My first one (from eBay) worked fine.
The second one (another eBay vendor) worked fine out of the box (into a DMM) until I connected the negative terminal to the ground terminal, whereupon it immediately went into current limit.
It turned out that a trace on the two-sided circuit board touched a rubber grommet that spaced the board from the mounting rails (which were grounded).
Somehow, the rubber had failed and turned into charcoal, effecting a short circuit.
I replaced the grommet with a plastic spacer and it has worked since.
If something is shorting to the metal, it may not make a difference until the metal connects to something like PE (safety aside).

[Kim Christensen]

The shell of that EMI filter was designed to be grounded. There should normally be a connection from the AC plug ground to the shell of the EMI filter and a lead/tab to ground the shell to the chassis of the equipment that it's installed in.

[robert.rozee]

check the two "gold coloured" capacitors in the below picture, they may be shorted internally or have liquid trapped underneath. thems be evil, replace even if not found to be the fault.


cheers,
rob   :-)

[Electronic Santa]

Interesting so you think regardless I should change these caps?

[Electronic Santa]

No other components have been replaced but im being told those yellow caps need to be changed.

I'm waiting for new mostfets to make more tests.

[robert.rozee]

they appear to be "RIFA" brand capacitors, which are known to break down and fail catastrophically after 20+ years in service. and it looks like they are wired between the 'hot' side of the PSU and the DC side - most likely chassis. these going short may well be sufficient to let out the smoke at the PSU's front end.

see:
https://www.eevblog.com/forum/testgear/rifa-madness-identifying-rifa-capacitors/


cheers,
rob   :-)

[coppercone2]

once I tightened a screw too hard which caused a board to bend and contact a grounded heatsink in a sorensen power supply that I ended p throwing away because it was delaminating too much to allow for reasonable repair.

[shakalnokturn]

they appear to be "RIFA" brand capacitors, which are known to break down and fail catastrophically after 20+ years in service. and it looks like they are wired between the 'hot' side of the PSU and the DC side - most likely chassis. these going short may well be sufficient to let out the smoke at the PSU's front end.

Well they don't usually go short straight-out, they usually go hygroscopically resistive and in this case should trip the RCD/GFCI before destroying anything else (diode bridge rather than MOS actually...).

Although I mentioned them too I doubt they are the problem here.

We're lacking information on primary to secondary leakage / resistance measurements to be able to help much more.

[Electronic Santa]

Interesting.


I will get the resistance numbers and reply back a tad later today.