Ql problem

[cybervet]

Hi I received a QL board issue 5 that was missing the zx8301 and the 68008 which I replaced (Thanks to Rich for providing it.).
The problem is that when powered on and after the initial colored memory pattern I am faced with a totally white screen, no pattern no lines , just white.
From what I found out that might be a RAM problem , but I don't know how to troubleshoot it ( I don't really want to desolder all the memory if It's not necessary ).
It seems like the IC4 IC12 IC21 IC20 and IC19 have been replaced and placed on chip stands .
I will be thankful for any directions.
Tassos

[Dave]

Well, if you get the stipple pattern, then it goes right, you know a few things:

The CPU and ZX8302 are working, and so is enough of the memory for you to know the address decoder logic is working correctly. That's a bunch of logic you don't need to check.

[cybervet]

Any suggestions as to where I should start checking?
Thanks
Tassos

[tofro]

The simplest thing, probably, would be, for starters: Get yourself a Minerva ROM and plug it in - First, because you'll want it anyways, and second, it has a much more sophisticated RAM check routine that not only shows a white screen if it finds a fault, but also locates the fault and gives you some information about fault address and value written on the screen. Also, it will restart the RAM test after a fault and try to live with the RAM below the faulty address, so you could see whether the QL is at least partially working.

Regards,
Tobias

[cybervet]

I am waiting for an eprom programmer in order to burn the minerva rom, I have already made the PCB mentioned in another post.
I also found this in the service manual "If RESET is pressed and a 'tweed' pattern is seen on the screen for a
certain time and it then reverts to white, this suggests that some
failure has been found in the second bank of RAM."
What it means second bank of RAM?
Thanks Tassos

[tofro]

I'd guess
the "first bank" is IC1 - IC8
the "second bank" is IC9 - IC16

Simple, isn't it?

Also, "white screen" means data could not be written and immediately read back. (That is, most probably, a faulty RAM chip).
"green screen" means RAM refresh doesn't work (content is lost after some ms)

I keep asking - Have you got access to a scope? (Even a simple one, liek a USB scope) Makes fault finding muuuuuch easier.

Tobias

[cybervet]

I have one coming is a simple one DSO Nano, will it help?
Tassos

[tofro]

Sure will, if least you can see whether a lineis Live or stuck somewhere
Tobias

[cybervet]

Tobias can you give me a link where I can see how I can check the memory with the scope?
Thanks Tassos

[Dave]

Code: Select all

  n/c 1      16  Vss
   D  2      15  /CAS
  /W  3      14  Q
 /RAS 4      13  A6
  A0  5      12  A3
  A2  6      11  A4
  A1  7      10  A5
 Vcc  8       9  A7

The IC is organised as 64k x 1bit, so 8 are ganged together to make up each byte, and two sets of 8 make up the internal memory space.

A good place to start is to check Vcc to Vss to make sure every pin has power, and to check you get inputs on A0-A7 and i/o on D. Check /RAS, /CAS and /W are all high and pulse low.