[HW problem] CPC464 with "poked" screen

[Joss]

Hi. I was getting a CPC464 from ebay with the problem that by switching it comes a screen which seems poked. I was changing the ULA (4007) with other, which I think it works, but it comes the same problem, but with an update of the screen, as if it pokes it again line after line ...... Funny is that in both cases green is the most color on screen .....


Pressing DEL a long time does not bring any sound.


Could it be a corrupted ROM the problem?


Thanks in advance.

[gerald]

Have you also tried resettling the Z80 in its sockect ?

If you get a visible screen, the z80 has properly booted from rom after reset.
You may have :
  - bad ROM(unlikely)
  - bad RAM, you could check if one of the chips is heating more than others with your finger. Carefully, you can burn your finger if really hot, or burn the chip in case of electrostatic discharge (ground yourself). Running a memory test will need to at least get basic prompt.
  - bad Z80 (ie weak signals on addr/crtl/data lines). I got that once but only when an extension was plugged.
  - bad GA. You changed it, but changes in behaviour may indicate a socket issue : either CPU or GA which affect signals.

[Badstarr]

I second Gerald's suggestions, my money would be on the RAM, my new 464 plus was doing something similar, but only every now and then. The last time it did it I happened to have the case open, and I touch checked the first RAM IC and it was getting very hot. I switched the machine off left it for 20 minutes and switched it back on all seemed ok for a while until I powers cycled the machine and got a corrupted basic screen, and the RAM was hot to the touch again.


I removed the offending IC and replaced its capacitor too and now with a new RAM IC installed the chips don't overheat at all.

[Joss]

Thanks to both for your replies.
I was doing another check. I put the GA and the CPU, not both at the same time, first GA and then CPU in other 464 which worked and both of them work fine.


The problem has to be in RAM or in the ROM, I think, but with the finger I cannot find any IC which overheats. I touch first the RAMs and then all of them (PPI, AY, and so on), but I didn't sense any burning effect. After a couple of switchs I can see how the Video RAM is being writing, with random value, but the same all over the screen. If RAM is damage it should be the lower one.


I will be doing more checks, but not the next days, perhaps next weekend. In case of RAM failure, can it be replaced by actual ICs, which can be buy from normal electronic shop? Or it has to be used the RAM ICs from another CPC? The same question is for the ROM.


Thanks again.

[Bryce]

Can you take a picture of the screen and post it. I'd like to see the effect.

Regarding the RAM ICs, you won't find the 4164 IC in most electronics shop, they haven't been manufactured for years, but they are often on ebay, and I might have a few of them somewhere, if you can't find them anywhere else.

Bryce.

[Joss]

Hi Bryce ... sure .... but ..... big mess ....  .... as I said, I put the GA and CPU in a working 464 and the "not working" was without ICs. After mounting them again from the other one, it shows another screen. You can see it here, but it is not the original "fail" screen:

I noticed, the GA in the "working" 464 had a pin not very "straigh". You can see it here:

it was making contact and working well in the "working" 464. I tested it by loading a game from tape. I think by inserting it in the not working 464, it does not make contact. I will try to solve it next weekend, or when I have a little more time, I promise. I want to reproduce the error.


About the RAMS, the not working 464 should help others to work again, but I don't need it now. I would like to know which part is not working, to avoid taking it to other CPC, when needed. Thanks for the offer.
After your reference I saw this online shop in Germany sell the ICs:
4164-120 - RAM 64K X 1 bei reichelt elektronik


P.S.: I don't know which is the best way to insert images, sorry. Here the links:
fail but working pin: DSC_0021 | Flickr - Photo Sharing!
fail screen: DSC_0025 | Flickr - Photo Sharing!

[Bryce]

Hi Joss,
      I didn't realise Reichelt still sold them, cool. Reichelt is my main supplier of parts for my projects, but I never went looking for 4164s, I usually have some here.
The broken pin still looks long enough to make contact. If not, you can just solder a small piece of wire (The wire from a standard resistor or similar) onto the pin and that will fix it. Don't leave the iron too long on the pin though, the GA is quite a sensitive IC.

Bryce.

[Joss]

(I could not wait ... i had to "fix" it) I didn't want to touch the pin just in case it were broken. I have done it carefully and now it is in his old place and works fine. The "not working" CPC does the poked screen once again:
Video by swiching on: SDC10940 | Flickr - Photo Sharing!
Another video ( I had to reduce the quality, sorry): SDC10953 | Flickr - Photo Sharing!
Fotos:
SDC10952 | Flickr - Photo Sharing!
SDC10942 | Flickr - Photo Sharing!
If I get this CPC working I am going to call him "Green Latern" 


I will check it again later this week. I was thinking, that perhaps the problem has to be with condensators. If CPC works a little time, the "poked" screen is different ... I think. I forgot to say I was checking the consumption and the working CPC and this one show similar W values (around 55 W, I think, I did it yesterday ....). If RAM is broken, is it a shortcut? or the line is cut?


About Reichelt, I think they are selling once again parts that they think they could be well sold: "old timer" parts. I saw too this connector, which is very difficult to find and they are going to have them in november:
STECKER 50-254 - SLot-Stecker, RM 2,54, 2x25-polig bei reichelt elektronik
I think it is the right one for the expansion port .....

[Bryce]

Yes, that's the edge connector, but it's for soldering directly to a PCB. I bought a few while they were available. The edge connector for cable is even more difficult to find and unfortunately Reichelt doesn't offer them.

Regarding the CPC, I'm not 100% sure that's a RAM problem. What equipment do you have available for testing? A Multimeter? Oscilloscope?

There's a simple multimeter test for the RAM ICs, even if none are getting hot.

Bryce.

[Joss]

I have a multimeter.

[Devilmarkus]

It remembers me when I had a faulty Z80.
Did you change it already?

[Bryce]

Ok, with a multimeter you can do a quick test to check the RAMs without having to remove them:

With the power turned off, measure the resistance across oposite pins of each IC. ie: Resistance between pins 1 and 16, resistance between pins 2 and 15, 3 and 14, and so on. Write the value down for each IC in a table. If an IC is damaged, it will be obvious from the table, because the value of one of the measurements will be a lot different from the others.

If no RAM IC seems to be damaged, then I am guessing that it's one of the RAM related logic ICs that's broken, but this could be any of the following: IC104, IC105, IC109, IC113, IC114 or IC115 and this could be difficult to diagnose without further equipment (unless you spot one that's getting hot.

Bryce.

[Joss]

Hi Bryce. I will try it and give you feedback here.


@Markus: Yes, I was changing the CPU with a working 464 and it works well in the "good" CPC.

[Joss]

Hi. I was testing the RAMs today, but I don't find any differences between them. My multimeter shows in all the cases "out of range" (40MOhm is the limit). Only between 8-9 shows ~30MOhm, but in all of them. It is very difficult to get a stable signal. I was getting the half of this value in two of them which are near the GA but only for short time, and now all of them shows the same value (out of range and one with this 30MOhm).


I don't think I will spend more time with it. It will go to a corner. Perhaps it will be needed in the future (CPU and GA work anyway). I have another one which works, and I want to check the project with the PS/2 keyboard.


Thanks for the support. If I reach any success some time in the future with it, I will report here 

[Bryce]

It doesn't sound like any of the RAMs are damaged, it's more likely that a logic IC has failed.

Bryce.