IIc strange image

[WillJac]

Can I see the schematic? You need to trace pin 4, the WE pin, it's got some issues in the write enable circuitry

This for the 16 chip ram logic board A2S4000. I have the A2S4100 that schematic is VERY bad quality and is on page 407-411 of this attached manual. I decided send the entire manual as it could help you.

 

[WillJac]

The signal starts at the TMG chip as 01 pin 14 good solid signal nice form. Goes to R59 then to the 74LS32 pin 4 comes out on pin 6 and is garbage. See the pics. Going in, out and last is at pin 4 on the ram. Seems everything coming out of this 74LS32 is bad garbage.

 

[Lycus]

The signal starts at the TMG chip as 01 pin 14 good solid signal nice form. Goes to R59 then to the 74LS32 pin 4 comes out on pin 6 and is garbage. See the pics. Going in, out and last is at pin 4 on the ram. Seems everything coming out of this 74LS32 is bad garbage.

Awesome, that's exactly what I was looking for, I'd replace that 74LS32.

 

[WillJac]

Awesome, that's exactly what I was looking for, I'd replace that 74LS32.

I hope it’s this but I made a error. This pics and results are with ROM and CPU removed. Does it make different? I can redo with them in socket.

 

[Lycus]

I hope it’s this but I made a error. This pics and results are with ROM and CPU removed. Does it make different? I can redo with them in socket.

You need all chips in their sockets. This would be potentially caused by the CPU R/W line not being there so it's just floating.

 

[Lycus]

I hope it’s this but I made a error. This pics and results are with ROM and CPU removed. Does it make different? I can redo with them in socket.

If those readings are from with all chips installed, then I'd say for sure that 74LS needs to be replaced.

 

[WillJac]

The first set of pics are with cpu and rom the last set I send were with them out. I will do with them in.

 

[WillJac]

Ok here is a full set pin 14 on TMG, one side of resistor, other side resistor, pin 4 in on 74LS32, exit on pin 6, pin 4 ram 1, pin 4 ram 2, pin 4 ram 3, pin 4 ram 4.

 

[Lycus]

The last five are kinda funky to very funky. I'd probably replace that 74ls

 

[WillJac]

The last five are kinda funky to very funky. I'd probably replace that 74ls

Those 5 from pin 6 on 74LS which is the out pin goes to 4 and ram 1 and 2. Ram 3 and 4 I need check as that comes from another source. That is the Aux memory.

 

[Lycus]

Those 5 from pin 6 on 74LS which is the out pin goes to 4 and ram 1 and 2. Ram 3 and 4 I need check as that comes from another source. That is the Aux memory.

Well no matter what I'd say it needs a replacement.

 

[WillJac]

i will see where I can borrow a 74LS from to test it. Don’t have any new right now.

 

[Lycus]

i will see where I can borrow a 74LS from to test it. Don’t have any new right now.

Crossing my fingers!

 

[Lycus]

https://www.mouser.com/ProductDetail/Texas-Instruments/SN74LS32N?qs=q2XTDbzbm6DA9Mnew5GiLA%3D%3D

If you order new ones, this is the exact same chip, just new.

https://www.mouser.com/ProductDetail/Texas-Instruments/SN74F32N?qs=sGAEpiMZZMuyBeSSR239IW7xIH5N0hSKEOF3zh3ZcDo%3D

There's also these which are even faster, which won't really do much for you, I don't think it'll make it not work, but it's slightly cheaper.

 

[WillJac]

I will need to order I don’t have any laying around. I going to see my other machines maybe borrow from them.

 

[WillJac]

Thanks. I just order some but had to use Amazon as I need it delivered tomorrow as I have a friend coming to visit me Tuesday from Miami. I live overseas in the Cayman Islands so it’s hard for me to get parts. I know is not best quality but it’s what i can do in a pinch.

 

[Lycus]

Hey, whatever works! As long as it's a 74LS32 it'll work. Let me know if that fixes it, I think that might be it.

 

[WillJac]

Hey, whatever works! As long as it's a 74LS32 it'll work. Let me know if that fixes it, I think that might be it.

Well I open up my other IIc to probe the chips and I get same results. It seems the keyboard needs to be connected to get the proper signal on pin 4. I will send you some pics shortly. Outside with the kids a bit before they disown me.

 

[Lycus]

Strange, reprobe with the keyboard connected then

 

[WillJac]

Here are the pics for current machine with issues. In order pic 1 is pin 6 on 74ls32. Pic 2 is pin 4 RW on chip 1, pic 3 is chip 2.
Pic 4,5,6 same pins different machine that is working.