Williams power supply no +5vdc under load - tech note

[Dokert]

Williams power supply no +5vdc under load - tech note

Just rebuilt a Joust/Robotron/Moon Patrol/Bubbles/Sinistar style Williams power supply and the +5vdc kept failing under load.

All new headers
all new fuses
All new caps
new dual wipe sockets to go with the new LM723's
new BR1 (3502W bridge rectifier)

Everything tested good, until you put it under load and +5vdc would drop out. Remove the load and +5vdc came back instantly.

R3 seemed to be getting almost smoking hot, so I tested it and it tested good in circuit. Pulled it out of circuit and it again tested good. Replaced R3 with a different R3 from a donor power supply and the problem went away. New donor R3 doesn't get smoking hot either.

I suspect that the internal winding (for lack of better term) of R3 was getting hot and going open.

Maybe this will help someone in the future, or maybe I am just rambling on again.

 

[YellowDog]

Good tip Dave. I have only seen that once and it was with a bad R3.

A couple of other things to look at if the 5V is dropping out:

- Fuse Clips. They get bent out and don't make solid contact. Or there is corrosion and they don't make a solid contact with the fuse. If this happens, you will get 5V with no load, but when you put it under load, the 5V drops off.

- Solder pads on BR1. I have had a bunch of these. People get the Bob Roberts power supply rebuild kits and they show the +5V LED until it goes under load and then it goes out. Because the solder pads are so big for the BR1 the solder doesn't always flow to the top pad. In this case, the plated through hole copper is just not thick enough to carry the current required or it was damaged in removing the old one or inserting the new one. The solution is to solder the top pads as well as the bottom ones. I have had at least 10 of these in the last year or so.

- LM723. There appear to have been a bad batch of these chips. I have had 5 or 6 of these that all they needed was a new voltage regulator chip.

- Zener diodes. The zener diodes in the overvoltage circuit are starting to fail by conducting at a lower voltage.

ken

 

[Slackmoe]

+1 on the LM723 chips. I had the same problem with my power supply for Star Castle.

Andrew

 

[Dokert]

I have noticed a lot of problems with the single wipe sockets that came with some repair kits. It gives the illusion that the LM723 is bad, but when you install a new dual wipe socket the LM723 works just fine.

Somewhere I have a power supply that I can't get the reg 12vdc to show up, and I never checked the big resistor in the circuit to see if it was the problem. Now I can't find the power supply to check it.

Those BR1 pads are the worst, and sometimes you just have to work both sides to get a good connection.

 

[Battlecat]

Hi: I am new to this but I was wondering if you have any tips on how to learn to repair a ps? I just do not want to throw mine in the dump even after I got a new one.

 

[YellowDog]

What kind of power supply is it? There are quite a few different kinds out there.

ken