WPC CPU help

[roadshow16]

Dear all, a question my Dad has asked for me to pass on. Any help appreciated.


I have a WPC CPU which is confusing me.
There are 4 switches that are not working - all in Row 4
Everything else on the CPU works exactly as expected.
(I have swopped in another CPU, so I know the error is on the board
and not in the wiring or the plugs etc)

One of the bad switches is on PIN 4 on J209,
which goes to Pin 4 on J208 and then....
D6 1N4004
R58 470 ohm 1/4w
R57 68 ohm 1/4w
U18A LM339
R27 4.7K ohm 1/4w
U13B 74LS240

As far as I can tell by comparing readings with IC's on the good CPU
there is no issue (when testing on component legs) or with any of the resistors etc
so I think (if the components above are indeed the correct ones) then it
perhaps is an issue with the Trace at some point- as there is some historic acid damage, which has been neutralised.

Please can someone confirm that pin 4 goes through the components identified and I presume I should get continuity directly between pin4 and a leg of R27
With my meter on Diode test, Black lead on Pin 4 and Red lead on the left leg of R27 I get a reading of 1.862 - which I would think is pretty encouraging in terms of connectivity - yet still not working.

Although U18 appears to test good, perhaps the pins which deal with Row 4 switches are not passing the correct data.

Any help appreciated. Thank you.

 

[keith b smith]

Hi Craig,dont forget U20 ULN2803 with a bit of luck it should be socketed

 

[roadshow16]

Thanks Keith My Dad also sends his regards to you and Gail.

He also says

Thank you Keith.
The U20 pins compare favourably between the two Cpu's (nothing in that area is Socketed.
I did not see the circuit for the row4 switches passing through U20, so I must be reading it wrong.

 

[Wintonmartin]

Check continuity from r27 to U14. I had a similar issue on a board that was acid damaged and it was the trace between these points

 

[Wintonmartin]

The quality of the imagine is a bit poo. Try this

WPC '89 cpu board info | Tech: DMD era games | Pinside.com

| Forum Topic by zaza

pinside.com

 

[Wintonmartin]

Sorry u13

 

[roadshow16]

Thank you for the suggestion, I can see why that trace would be vulnerable going directly under the battery area.
Mine is good.
My acid damage seems mainly around U18 and U19, all the pins test good on those and continuity gets to R27 and beyond to U13

 

[Wintonmartin]

Ahh. Have you tested u18 with a logic probe?
Someone else may be able to advise better but I don't think you can test continuity through the IC so if you've got contunity before and after it could be the IC itself?

 

[roadshow16]

Thank you, yes that was what I was eluding to, that U18 may not be passing the correct data.
I have a probe, but have never used it.
I was hoping to find failed continuity somewhere, but I guess we are beyond that.