Lamp Driver issues associated with U3

[kamalackarie]

Evening all,

I am working backwards trying to bring this game back to life and am getting there slowly. I put new I.C decoders in U1-U4.n
All lights associated with U3 do not work. I swapped the decoder from U4 into U3. Issue persists, so not the IC.
Ran a logic probe over U3 and compared with others - all good with one exception.
Leg 15 on U1-U2 -U4 are all in a solid HIgh state. this is not the case on U3
Leg 15 is not clear as to its function.

In attract mode the Resistors associated with U3 are all stable - so nothing is coming from the chip

all help appreciated.

 

[AlanJ]

Sorry. Edited Check pin23 the chip enable line. It comes from j4 and is controlled by the mpu.

 

[AlanJ]

Plus : Have you redone the headers on j4 ?

 

[AlanJ]

Sorry pin 23 not 1

 

[kamalackarie]

Alan - thank you so much for putting me on the right path. i had continuity in all the right places, While testing continuity between pin 23 an R64 i noticed it looked different. The 4 resistors that serve pin 23 are rated at 2.2M ohms
the resistor on U3 is rated at 2K ohms - so need to swap it out and should sort the issue. I have a lot to learn in this area and appreciate your help

 

[AlanJ]

A logic probe helps with these so you can check the signal is coming from the mpu to enable u1,2,3,4 in turn

 

[kamalackarie]

Hey Alan
i have a logic probe and am slowly learning how to use it

old dog....new tricks

 

[AlanJ]

Hey Alan
i have a logic probe and am slowly learning how to use it

old dog....new tricks

Great. So you should see that pin 23 on each ic u1-u4 is pulsing. If u3 isn’t pulsing Trace the signal back and ensure it’s being generated by the mpu.