Author Topic: Quick and dirty RAM test for CPC  (Read 8531 times)

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Offline gerald

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Re: Quick and dirty RAM test for CPC
« Reply #75 on: 23:10, 18 September 18 »
Hi,


I have two problems.
The CPC 6128 with standard RAM.
Original problem:
If the machine is cold (room temperature) then i can load the R-type Reloaded from floppy (3") and HxC and run the program.
... and then not cold the machine (after 3-4 minutes) press reset and can not load the game. (Freeze the program during loading)


Second problem:
The dirty ram test is failed. (attached picture)


I have replaced:
- 8 RAM modules (new rams: TMS4164-12NL)
- 74LS244 and 74LS373
Could you please help me?


Jonni



Did you run the RAM test with a cold or warm CPC.
Also, there are other ICs involved in the RAM access. Mainly the gate array and the address multiplexors (4 74LS153 : IC104 IC105 IC109 and IC113)
The gate array being on socket it may suffer a bad connection when getting warm. Can you try re-seating it ?

Offline jonnixx

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Re: Quick and dirty RAM test for CPC
« Reply #76 on: 23:21, 18 September 18 »
Did you run the RAM test with a cold or warm CPC.Yes. Both.
i am waiting for 74LS153... next week, maybe...
Can you try re-seating it ?Yes. Tomorrow.
Is it exist any test program for GA?
Many thanks.



Offline Audronic

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Re: Quick and dirty RAM test for CPC
« Reply #77 on: 01:47, 19 September 18 »
Please send a Photo of the RAM test when the machine is COLD
Thanks    Ray
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Offline jonnixx

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Re: Quick and dirty RAM test for CPC
« Reply #78 on: 08:43, 19 September 18 »
Hi Ray,
The ram test is same (failed) in both.Thanks.


Offline jonnixx

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Re: Quick and dirty RAM test for CPC
« Reply #79 on: 18:19, 20 September 18 »
Hi,
- GA re-seating is done- replaced 4 x 74LS153- replaced 2 x 74LS32- replaced 765 FDC
Memory test still failed as before.
any idea?  (GA?)

Regards,Jonni

Offline gerald

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Re: Quick and dirty RAM test for CPC
« Reply #80 on: 18:43, 20 September 18 »
Hi,
- GA re-seating is done- replaced 4 x 74LS153- replaced 2 x 74LS32- replaced 765 FDC
Memory test still failed as before.
any idea?  (GA?)

Regards,Jonni
Without looking at the RAM without a logic analyser, your last bet is indeed the GA itself.

Offline jonnixx

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Re: Quick and dirty RAM test for CPC
« Reply #81 on: 19:19, 20 September 18 »
Without looking at the RAM without a logic analyser, your last bet is indeed the GA itself.
I have oscilloscope (Tektronix) and multimeter and any other but i do not have logic analyzer.I am looking for a 8 channels logic analyzer ... now.
I will come back if i have a new logic analyzer.
Regards,
Jonni

« Last Edit: 19:30, 20 September 18 by jonnixx »

Offline LambdaMikel

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Re: Quick and dirty RAM test for CPC
« Reply #82 on: 19:24, 21 September 18 »
A logic analyzer and oscilloscope is unnecessary for 99% of the repairs.
I suggest you get the CPC schematics and start doing some continuity checks first.
Next, get a 10 $ logic probe and check if the chips are getting their select signals, if
their outputs are active, etc. That way you can track down non-functioning chips and replace them. 

I have done a couple of quite challenging repairs, and none of them required an oscilloscope or logic analyzer. Unless you are very good at digital electronics, you will have a hard time understanding what's going on anyway.

The suggested method allowed me, for example, to perform this repair, so I know what I am talking about (that thing had about 20 defects - including broken traces, defect RAM, defect glue support chips, ...): 

https://stardot.org.uk/forums/viewtopic.php?f=3&t=15501&start=30&hilit=repair+problem

In addition, you will need people that can guide you through the checking process. Probably the most important thing.

Good luck! 
« Last Edit: 19:28, 21 September 18 by LambdaMikel »

Offline gerald

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Re: Quick and dirty RAM test for CPC
« Reply #83 on: 10:40, 27 October 18 »
For those interested, I've added a zip file with the source code of the tool to the 1st post.You will need SDCC to compile it.

Offline gotcha

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Re: Quick and dirty RAM test for CPC
« Reply #84 on: 10:07, 02 November 18 »
I just noticed this post  :o This is very interesting, thanks @gerald !

Yesterday, I used one of my 6128 as a test bench for testing the RAM of a old PC (they use the same RAM) https://forum.system-cfg.com/viewtopic.php?f=18&t=9388&start=30#p144477
I used a memchecker program (in German) that I had found some time ago. This solution worked because I believe I've put the potentially non working memory on the second 64K bank only. 
Can anybody confirm that IC119->IC126 is the second bank ?

But in case of a CPC with a failing first memory bank, the solution I was using until now (also given by @gerald ) was to switch banks and/or disable a bank by removing the PAL and re-wiring the PAL socket.
This software solution looks ideal. I'm going to give it a try with the X-MEM  :D

Offline gotcha

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Re: Quick and dirty RAM test for CPC
« Reply #85 on: 10:23, 02 November 18 »
A logic analyzer and oscilloscope is unnecessary for 99% of the repairs.
I would say, yes and no.

Most of the issues on CPCs are indeed simply on-off switch to clean, keyboard to clean etc...

But when it comes to a problem on the motherboard, the scope is IMO more than useful, just to check clocks and some other signals like the video output, activity on busses etc....
For some problems, the logic analyzer is also needed. As an example, I've got a failing CPC and I didn't want to change blindly chips:
  • With the scope, I detected that the synchro signal was not generated by the CRTC. Without a scope, you can't see this
  • But the scope is not enough. This problem can come from many parts : failing CRTC chip ? Not well programmed CRTC due to a failing memory ? bus ? Z80 ?
     I then had to use a logic analyzer to check that the CRTC initialization sequence (at boot) was correctly coming on the data bus on the input of the CRTC and determine that the CRTC was faulty.
Without logic analyzer, i could not have located the actual problem accurately.
« Last Edit: 10:48, 02 November 18 by gotcha »

Offline LambdaMikel

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Re: Quick and dirty RAM test for CPC
« Reply #86 on: 16:19, 02 November 18 »

But when it comes to a problem on the motherboard, the scope is IMO more than useful, just to check clocks and some other signals like the video output, activity on busses etc....
For some problems, the logic analyzer is also needed. As an example, I've got a failing CPC and I didn't want to change blindly chips:
  • With the scope, I detected that the synchro signal was not generated by the CRTC. Without a scope, you can't see this

My logic probe has a "pulse mode". With that you can check if the output / chip is alive and generating a non-steady signal. I used that mode to check RAM chips (chip select, RAS, CAS, inputs, output).

A logic probe is onle 30 €, whereas a scope that can be used for Mhz digital signals is a couple of hundred €,  even the cheap ones (there are little DSO pocket scopes for about 100 $, but they are toys and don't have the bandwith for digital signals, they are only useful for audio / analog really)

I am not saying that a scope and/or logical analyzer is not useful. But I would not advise people to get one if 99 % of the situations (including some of the ones that you have just described) can be analyzed perfectly fine with a logic probe.

And from a time management point of view, before you have figured out how to hook up and read and understand the databus and CRTC signals, you would have already replaced the CRTC with a fresh one. The good thing about the CPC is that it only has a couple of big chips... in principle, I am not a fan of blind repairs, but for the CPC it is possible. Buying a new CRTC is not only more cost efficient than buying an oscilloscope or logic analyzer, but also much more time efficient.

All this is of course only advice for people who are not electronic geeks and don't already have an oscilloscope and logic analyzer. So I would not suggest to people to buy such equipment in order only to repair a CPC (unless they want that equipment anyway of course).


Just my few cents.
« Last Edit: 19:08, 02 November 18 by LambdaMikel »

Offline Audronic

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Re: Quick and dirty RAM test for CPC
« Reply #87 on: 23:35, 02 November 18 »
Hi All


I have just been working on SOME CPC464's and have made a table for the Bits used in GERALD's RamTest


It may be of some use  ?


Thanks.    Ray
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Offline GUNHED

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Re: Quick and dirty RAM test for CPC
« Reply #88 on: 17:14, 03 November 18 »
Great! It would be even greater to know which bit corresponds to which RAM chip.


Just to be clear: It's the case that for every bit of a byte we have one physical RAM chip, right?
So if a bit is 'broken' in one byte it is probably 'broken' in all bytes, right?

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Offline Audronic

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Re: Quick and dirty RAM test for CPC
« Reply #89 on: 22:46, 03 November 18 »
Ok Part 2


Ray
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Offline GUNHED

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Re: Quick and dirty RAM test for CPC
« Reply #90 on: 03:20, 04 November 18 »
Awesome! Thanks!  :)
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