Williams Power Supply

dahnz

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Hi

I need some help with Robotron-era Williams power supplies (the ones with the OV protection built in). I had one that I did a cap kit on (except for C10 - the 18,000 mfd cap). It was working okay until the 5 volts went out and the -12 volts unregulated went up to about -30 volts. BR1 was new and read okay but I replaced it anyway, but that didn't help. Ken gave me some advice via PM but I never did figure it out and in the meantime I replaced it with another PS I had. That one also was recapped (again, I left out C10 because I didn't have any onhand).

Now that one has gone the same way. Was working fine, then the 5v went out and the -12 is now reading -42. I'm a noob when it comes to these things but apart from BR1 I can't see anything in the circuit diagram that would be affecting the -12v in this way.

We have 230v AC down here but the transformer is wired correctly for that and the voltages coming off the transformer measure out okay (pins 8 and 9 on 4J1 show 25v instead of 20v but all the rest are closer and, if anything, lower than they should be).

Any help would be much appreciated.

Dominic
 
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Dominic,

with the game turned off,

- disconnect the plug 4P2 from 4J2 (ie disconnect power from game pcb)
- disconnect the lead between the transistors on the heatsink (4P4) and the power supply PCB (4J4)

what is the resistance measurement across the 2n6057 tranisistor between pins 1 & 3 of the harness (grn and yellow wires) of the 7 pin plug 4P4 (red lead to green wire, black to the yellow wire)

with the game turned on,

- disconnect the plug 4P2 from 4J2 (ie disconnect power from game pcb)
- re-connect the lead between the transistors on the heatsink (4P4) and the power supply PCB (4J4)

what AC voltage reading do you get when you measure the AC voltage between the terminals of Fuse F5 & Fuse F4 ?

what DC voltage readings do you get between Gnd Test Point TP2 and
- Test Point TP1
- Test Point TP5
- Test Point TP6
- Test Point TP1
- Test Point TP1
- Test Point TP1

what DC voltage readings do you get between Gnd Test Point TP2 and
- IC2 pin 12 ?
- IC2 pin 10 ?
- IC2 pin 3 ?
- IC2 pin 4 ?
- IC2 pin 13 ?

What DC voltage reading do you get between Gnd Test Point TP2 and +ve of capacitor C7 (ie at the gate of Q1) ?

What DC voltage reading do you get across the terminals of ZR1 ?

these readings will help us to understand what may be going on...
 
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dominic, after you have done the above, also, make yourself a load tester for the 5v rail to test the output underload.

using an automotive light bulb

Calcs:
P=IV - if V=12, P=25w, I=?
I=25/12= 2A
If V=12, I=2, then R=6ohms
V=IR=2*6=12v . good.
therefore if V=5v, R=6ohms, then I=5/6 and is < 1A

in short, wire a 12v 25w bulb across the 5V output terminal and the ground terminal (use some crocodile clips and ordinary flex wire about 2mm diameter ) at 4J2 (careful not to short out any other pins) that will make a basic load test with out risking your mpu board. if you want to test with more current, add another bulb same type in parallel to double the current

doing the same tests as previously but with the test load connected to 4J2 between pins for 5v and pins for ground per schematic, and note the results to the same questions.
 
Hi Dominic,

You can get the 18,000 mfd caps from Ed at www.GreatPlainsElectronics.com.

If you are seeing very high voltages on the unregulated lines, here are some things to check.

(1) make sure the legs are in the correct position for BR1. There is usually a plus sign (+) on the case of the bridge. Some variations of those bridges also have the AC leads marked with (~).
(2) make sure that the top solder pads are soldered to the leads on BR1. Because the solder pads are so large it is very common to get cold solder joints or no connection to the top side pads.
(3) Pull one leg on each and test D2 & D3. Because of the capacitors and the bridge, it is not always accurate to test them in-circuit.
(4) Check to make sure C3 (1000MFD 25v) was put in correctly.
(5) Put your meter on AC and test the 12V unreg line. If BR1 is bad and bleeding AC it will give inaccurate DC readings. If the bridge is rotated 90 degrees then it will pump half wave AC directly to the unregulated leads.

ken
 
Thanks Ken and Andy for the help. I was measuring the voltages to answer the questions Andy posed when I noticed that they changed after 4J1 had been moved. So I recrimped some of the transformer connections and replaced the 4J1 header on the power supply board and that seemed to fix it on both boards.

Now with the voltages good, I can move onto the next problem.
 
Sounds good. Glad you got it working.

Rule #1 on any of these games is: If the power ain't happy, ain't nothing else gonna be happy! :)

You always have to fix the power first.

ken
 
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