Hi Silvervest,

Great Job 😀

I have got the excatly the same problem on my model 80-111 !!
It is the same problem as you have described in your first post.

I have been looking at the schematics for the model 80 , and found the Hex inverter ( U185C ) and it is connected to pin 18 on the SRAM.
I do nat have a scope, so I can not do the same probing as you did
My questions is now, could it be an idea to try and ground the output from the Hex inverter ?
I´am not an electronics engineer 😀 , but I can use a soldering iron and a multimeter 😀

Happy New Year -

Best Regards
Carsten

Hey Carsten! Bummer you're having this issue too, it was a wiley one!

If you've got a multimeter, you can prove the same issue I was having - I only used the scope because I was already using it while checking the address lines.
Just need to see if that pin stays HIGH (should be around 5v) or gets pulled LOW (goes down to 0v) when the computer is switched on, so probe pin 18 on the SRAM IC with the multimeter in volt mode and see what happens. If it stays with a positive voltage, that's the same problem I was having and you could try to pull it to ground.

Just an addendum, I'm still showing a few issues which I think are related to constantly having that pin tied to ground, so will be waiting to replace the hex inverter fully before calling the all-clear on this particular issue. Mostly fixed though!

Good luck! Cheers, Nick

HI,
I will try it after new year 😀 , and I will give you an update

Cheers, Carsten

Hi,

I have done some probling on the board, and this is what I found out.

On pin 18 on the SRAM IC it get 3,8 volt, when the machine is turned off. And when it is turned on I get 0,012 volt.
The VCC voltage on the SRAM IC is only 3,66 volt - that seems to be to low, it should be 5 volt
I have pulled pin 18 to ground, but it did not do the trick -

On pin 6 of U185C I get 3,8 volt when it is turned off, and 0,012 volt, when it is turned on - same as on SRAM IC.

On pin 5 of U185C I get 4,1 volt

On U77 and U133 I get 4,7 volt on VCC

Regards
Carsten

Carstenl wrote on 2022-01-13, 12:50:

On pin 18 on the SRAM IC it get 3,8 volt, when the machine is turned off. And when it is turned on I get 0,012 volt.
The VCC voltage on the SRAM IC is only 3,66 volt - that seems to be to low, it should be 5 volt
I have pulled pin 18 to ground, but it did not do the trick -

If pin 18 is ~0v when powered on, it's working fine. When it's powered off, the 3.8v is sourced from the CMOS battery and is a little low but still acceptable. VCC on the SRAM should come up to ~5v when the system is powered on.

The fault in my case was pin 18 was always HIGH so the /CS line on the SRAM was never pulled down and the chip was thus always deactivated. Since you've got a ~0V during power on, it means the /CS is pulled LOW correctly, and the chip is selected as normal. Must be some other problem in your case.

Hi,
You are right, I have to do some more testing.... 😀