Trying to repair a IIe (my adventurer log)

[AMenard]

Hi,

 

I've bought an "untested, power on" A2e. Of course it doesn't work as expected. I bought it to try to repair it or worst case scenario, use it for spare parts. It does power on but I get this display:

 

 

What are your opinion about what could be wrong with it?

Edited June 6, 2018 by AMenard

[AMenard]

Good thing, all the chips are socketed. I've got some TMS4164-20NL on order. I also have an enhance kit which include new roms, video and CPU. Will take the board out tomorrow and check the trace to see if any defect is visible. Don't seem to be any electrolitic cap on the mobo so there's one less thing to worry about.

[Osgeld]

try reseating the ram

[Keatah]

Ensure open Apple and closed Apple keys are functional. Continuity test the function from the key switches to the motherboard to be certain.

Edited June 6, 2018 by Keatah

[AMenard]

try reseating the ram

Will try that tomorrow. Thanks.

[AMenard]

Ensure open Apple and closed Apple keys are functional. Continuity test the function from the key switches to the motherboard to be certain.

Will try that also.

[Osgeld]

Will try that tomorrow. Thanks.

 

Also, I dont want to insult anyone's intelligence, but there's been a number of cases round here where people take "reseat your ram" to mean yank all the ram chips out at once without looking at them and placing them back in random orientation, which of course causes damage...

[AMenard]

Also, I dont want to insult anyone's intelligence, but there's been a number of cases round here where people take "reseat your ram" to mean yank all the ram chips out at once without looking at them and placing them back in random orientation, which of course causes damage...

Of course not! While I don't have much experience repairing A2e I'm still an IT tech with 20 years experience under my belt. I'm just more used to more modern tech while some of you have already debugged a few and saved them from the scrap pile

.

[AMenard]

I took a small video of the display to better show you the problem

 

https://m.youtube.com/watch?v=32xRlahwJ2M

Edited June 7, 2018 by AMenard

[AMenard]

Taken with my ipad so the dark bar is the ipad not the display.

[Keatah]

Does it look the same after 40 seconds?

[AMenard]

Ithink I may have a lead.... LS166 (SN74166N) which sit between the video rom and the ram gets quite hot while everything else is cold or barely warm after a minute.

[AMenard]

I'll also have to find some plastic stand-off for the motherboard as most of them are broken off. I may have to use some brass one, with fiber washer if I can't find some originals.

[shoestring]

I'm not sure replacing the serial register will fix the problem. That's part of the character generator circuit which is fed by the character rom and all your characters appear to be fine.

 

The LS166 clocks the data from the rom into the video output, it does this serially.

 

Based on your video it looks to me that bad data is crashing the 6502, several things that could be causing it.

Edited June 19, 2018 by shoestring

[AMenard]

Receiverd 12 new ram chips. Will swap the old one tomorrow.

[shoestring]

How do the +5 voltages look at the chips ?

 

If replacing the DRAMs doesn't help you could try

 

The two LS244 address line buffers near the CPU. If you have a logic probe those are easy to troubleshoot.

Verify roms at UE8 and UE10, those are tied directly to the address and data bus of the CPU.

MMU @ UE4

The Output enables and DRAM RW enables.( UB8, UE1 and UE4 )

FInally try swapping the CPU.

[AMenard]

How do the +5 voltages look at the chips ?

 

If replacing the DRAMs doesn't help you could try

 

The two LS244 address line buffers near the CPU. If you have a logic probe those are easy to troubleshoot.

Verify roms at UE8 and UE10, those are tied directly to the address and data bus of the CPU.

MMU @ UE4

The Output enables and DRAM RW enables.( UB8, UE1 and UE4 )[/size]

 

 

FInally try swapping the CPU.

Will try that, thanks!