TECH: Williams System 3 Power Supply Humming

[Zap]

I am working on bringing a Williams Lucky Seven back to life. It has no audio board, just chimes. I put in a known work MPU, a known working driver board and an aftermarket power supply. Upon power up, there was a continuous loud hum coming from the power supply board. I powered it down and swapped in the original power supply board, which had no visible damage. It too produced a hum when powered up. What could be causing this? What should I check first?

[Tom Zapiecki]

On Friday, June 18, 2021 at 7:13:26 PM UTC-4, Zap wrote:
> I am working on bringing a Williams Lucky Seven back to life. It has no audio board, just chimes. I put in a known work MPU, a known working driver board and an aftermarket power supply. Upon power up, there was a continuous loud hum coming from the power supply board. I powered it down and swapped in the original power supply board, which had no visible damage. It too produced a hum when powered up. What could be causing this? What should I check first?

I disconnected power supply board from the transformer, and still had the hum. It is the transformer humming, not the power supply board. My old ears deceived me. What can I do about it? Is this a sign the transformer is going bad?

[John Robertson]

Check the fuse on the primary (line) side of the transformer and make
sure it is the factory recommended value.

Check that the 6.3 light wiring isn't over fused and shorted! It may not
be connected through the power supply regulator board.

If above is all good, then see if the transformer windings are loose
(unplug the game, eh?) if you grasp the paper wrapping and try to wiggle
it. It should feel rigid. If loose try putting some hardwood wedges in
space between the windings and the metal core of the transformer. Do not
use metal wedges, or soft wood.

Of course you may have a short in the wiring from the transformer
between leads - this I can't guess where you would find it, just check
the condition of the wires and look for pinches and bare or exposed wire.

If that quiets it down, then great, otherwise leave the transformer
powered up for a little while with the windings disconnected from the
power supply board (pull any fuses). see if the transformer gets warm or
hot to the touch. It shouldn't get more than slightly warm with no load...
If it starts smelling or smoking then it is toasted - has a shorted
winding.


John :-#)#

--
(Please post followups or tech inquiries to the USENET newsgroup)
John's Jukes Ltd.
MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
(604)872-5757 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

[Tom Zapiecki]

I will follow these steps!

Thanks, John!

[Tom Zapiecki]

John,

Fuses good and correct rating. Playfield and backbox lightning do not go through the power supply board, and they are good. Transformer seems solid in yoke. it does not move at all. Wiring is intact, properly insulated, and I did not see any opportunity for a short. After running for 5 minutes with no load, transformer felt barely above ambient temperature. I would not even call it warm.

Has anybody ever run into a transformer that just hums? I might even go so far as to say it buzzes.

Thanks,

Zap

[John Robertson]

Check to see if any metal laminations in the transformer core are loose.
It is good that the transformer is not getting hot!

[Tom Zapiecki]

John,

Everything is tight. I tried an unorthodox method which some people had success using. I whacked it with a hammer. All that did was scuff the top of the plates.

At this point, since it functions more or less properly, I will live with it as is.

I do appreciate the help.

Thanks for taking the time to respond!

Sincerely,

Zap

[John Robertson]

As a last resort, make sure the bolts holding the transformer together
are all tight...