Replacing CPC 464 CPU

[brazoai1]

Hi everybody, nice to see the CPC has so much fans still today. I found a CPC 464 in a flea market just a couple of weeks ago. It comes without power/video cable but I've arranged to make one by myself using the guide on settorezero.com (unfortunately there is no translation from italian...). The most important thing is missing, however, is the CPU... You know is really hard to run the CPC 464 without it  . I have no equivalent Z80 so here my question: I've also a broken VG 8010 (an MSX from Philips) that have a Z80A compatible CPU (same pinout), the only difference is the Clock frequence (3.58 against 4.0 of CPC). In your opinion can it be used to run a test on the CPC? Thanks in advance and... I apologize for the bad english.

Brazo

[gerald]

You can use it without any problem.
In Z80A, A means it can run up to 4MHz.
for reference :

   
• Z80   = 2MHz
• Z80A = 4MHz
• Z80B = 6MHz
• Z80H = 8MHz

Welcome on board 

[TFM]

The Z80 is used in a lot of CP/M computers at 2.5 MHz


For the CPC (4 MHz) a Z80A is needed though.


So yes, the CPU from the Phillips can be used, or get a cheap one (couple Euros) from ebay.

[brazoai1]

Ah thank you Gerald and TFM, I was sure I would get an answer. Well, so I tested the CPC with the Z80A, but no luck... The Computer turn on (the led is on and the tape is working at least the FWD and REW), but no signal at the TV. I could be the custom cable, but the strange thing is that I can hear noise from the internal speaker... It seems like a buzz. I tried to get an error beep (DEL keystroke) but no way. Again I ask for your help. As many other italians I'm a commodorian and I have no idea where to start investigation on this fascinating computer (yes I buried the war axe long time ago  ). My guess is there should be a short circuit somewhere but I'm not sure where I should test for it. I'm studying the service guide, but there aren't many helps on it... Thanks again in advance.

[Bryce]

If the LED is on then there's not a short circuit. Most likely a RAM IC has failed. Check whether any ICs are getting hot.

Bryce.

[arnoldemu]

Welcome brazoai1 and good luck with fixing the 464.

[brazoai1]

Thanks Bryce, I'll give it a try later. I'll post you back any findings. It couldn't be a "well known" failed resistor or capacitor? Just wondering... (consider that my electronic knowledge needs a refresh...)

Welcome brazoai1 and good luck with fixing the 464.

Thanks arnldemu, I'm start to thinking I'll need it... 

[Bryce]

No, resistors and capacitors rarely fail in the CPC. No.1 culprit is RAM, after that comes the CPU and Gate Array.

Bryce.

[gerald]

First thing to do is to check the video output.

A CPC with failing ram is usually still able to configure the CRTC properly, so you should have a proper video signal (ie 50Hz vertical sync / 15KHz horizontal sync) with a visible border.
A quick and dirty way is to use the monochrome output to a TV with composite input so you explicitly switch your TV to this input. The monochrome output is on pin 6 of the DIN connector (the central one).

[brazoai1]

No, resistors and capacitors rarely fail in the CPC. No.1 culprit is RAM, after that comes the CPU and Gate Array.

Since it's a very easy test I'll surely check the RAM temperature. By the way, is there a "normal" temperature? Can I assume 40 to 50 celsius degrees or less?

First thing to do is to check the video output.

A CPC with failing ram is usually still able to configure the CRTC properly, so you should have a proper video signal (ie 50Hz vertical sync / 15KHz horizontal sync) with a visible border.
A quick and dirty way is to use the monochrome output to a TV with composite input so you explicitly switch your TV to this input. The monochrome output is on pin 6 of the DIN connector (the central one).

Ok I guess that to test video signal I need an oscilloscope... I have one, but as I told you I need some refresh about usage  . Long time is passed...

To test the monocrome I guess I can use crocodile clips to a standard RCA connector, isn't it? So I plug PIN6 and ground to the corresponding on RCA, should it works? I will use the old VS-0040, time to fire up that monitor again.

I'll keep you informed...

[gerald]

Since it's a very easy test I'll surely check the RAM temperature. By the way, is there a "normal" temperature? Can I assume 40 to 50 celsius degrees or less?

The usual test is the 'finger' test  , but carrefully ! If one of the memory IC is significantly hotter that other, you have your winner.

To test the monocrome I guess I can use crocodile clips to a standard RCA connector, isn't it? So I plug PIN6 and ground to the corresponding on RCA, should it works? I will use the old VS-0040, time to fire up that monitor again.

Yes, this should work. You can either grab your signal on the DIN itself (pin6 = LUM, Pin5=GND) or directly on R136, which is connected between LUM and GND.

An oscilloscope will be useful if you do not get anything on the monitor.

BTW, what is your motherboard type (Mainboard Versions - CPCWiki)? R136 reference is not correct for pre-asic version.

[Bryce]

If the DEL key didn't give a beep, then it's highly unlikely that it's producing a video signal.

Bryce.

[Sykobee (Briggsy)]

Yeah, could type in SOUND 1,478,40,15 and see if anything is generated.


But I guess that if the RAM is bad then the firmware won't have initialised, hence no audio, no beeps, etc. The CRTC output check is probably the best check at this stage.


Also, maybe check all the chips are seated properly - an uncared for CPC might have been bashed about a bit.

[brazoai1]

If one of the memory IC is significantly hotter that other, you have your winner.

And a burned finger... Nice! 

Yes, this should work. You can either grab your signal on the DIN itself (pin6 = LUM, Pin5=GND) or directly on R136, which is connected between LUM and GND.

An oscilloscope will be useful if you do not get anything on the monitor.

BTW, what is your motherboard type (Mainboard Versions - CPCWiki)? R136 reference is not correct for pre-asic version.

Yeah I completely forgot to tell you that is a REV 3 motherboard. It comes from Spain since the printed quotes on power, volume and other controls are in spanish (I wonder how much travel before the Flea market in Italy...).

If the DEL key didn't give a beep, then it's highly unlikely that it's producing a video signal.

Actually I'm no sure there are no signal from DEL key... I heard no signal but probably I should try it with the CPC completely disconnected from video. I've only unplugged the audio cable and raised volume to MAX at that point I heard the noise coming from internal speaker and then... My wife called me for dinner...  So EOL... 

Yeah, could type in SOUND 1,478,40,15 and see if anything is generated.

But I guess that if the RAM is bad then the firmware won't have initialised, hence no audio, no beeps, etc. The CRTC output check is probably the best check at this stage.

Also, maybe check all the chips are seated properly - an uncared for CPC might have been bashed about a bit.

Chips seem to be properly seated, I will push them again. I'll try also SOUND command, hoping for no typo without screen feedback.

Men I'm really impressed for all the support, thanks again. No CPC should be left alone or forget! 

[MacDeath]

I guess you can cannibalize one from a ZX Speccy... aren't they done for that to begin with ?

[brazoai1]

Guys time to celebrate and have a beer. It works again.



No burned fingers on faulty RAM IC (fortunately...), the monochrome connection did the trick, so it is definitively a cable problem. I will investigate. Now the easiest part, tape belt substitution (it's really loosen), tape counter reallign (is a little bit pushed) and an internal and external clean. The fun can start! Any appreciable title to start with to suggest? 

I guess you can cannibalize one from a ZX Speccy... aren't they done for that to begin with ?

Yes Sir Sinclair did them only to be spare parts for other computers...  Poor speccy... Such a great machine to learn assembler and such a worst machine for any other works. I've a Spectrum+ in the queue waiting fixing, for this time it's safe.

Cheers men, thanks to your suggestion a CPC was saved from dust and doom. Thanks again!

[Bryce]

Congratulations. Now get a colour connection, RGB, SCART or whatever and load few old games. My test game for repaired CPCs is Jet Set Willy, just because I liked the title music I was shit at playing it.

Bryce.

[MacDeath]

the black and white reminds of ZX81... with like 4x time the resolution of course..   and 4x to 16x times the RAM ?


seriously i would love the ZX spectrum if it had other video mdoes compairable to CPC...
Still the Speccy+2/+3/128 are great for AY chiptune or learning assembler.


But you can learn Assembler as well on a CPC6128... and the 80 large characters in Mode2 are quite better for all text applications.


640x200 is always better than 256x192, even though you have attributes coloured texts...
320x200x4 is indeed also better and those Amstrad pixels are quite fine and sharp, if this is just for "text mode".

[Gryzor]

Oh wow, I don't think I've ever seen a B&W CPC running on a TV before. Looks cool!!!

[Munchausen]

I tried a bunch of scart wirings before I found one that worked. The one that finally worked for me was the "Alternative RGB wiring" (from Bryce I think) here: TV SCART cable - CPCWiki


You can always add a 9V source to pin 8 if you want it to automatically switch to the CPC when turned on (that's what I did too).

[brazoai1]

My test game for repaired CPCs is Jet Set Willy, just because I liked the title music I was shit at playing it.

It could be age? I found I'm not so good at certain title on my C64... But you know. 40 is coming...  I'll surely try the title as first one after years of dust.

the black and white reminds of ZX81... with like 4x time the resolution of course..   [CUT]

A little bit OT but it worth an answer. My favourite 8 bit computer is surely the C128 a true masterpiece. Double CPU, double text mode, 128k of RAM and the mighty SID. Definitively out of time when released to the market, but it is a great computer. I know am I risk a permanent ban saying this, but you know the chickenhead is tatooed on my soul... 

Oh wow, I don't think I've ever seen a B&W CPC running on a TV before. Looks cool!!!

Nice huh? Thanks to you all even in 2014 I took this picture.

I tried a bunch of scart wirings before I found one that worked. The one that finally worked for me was the "Alternative RGB wiring" (from Bryce I think) here: TV SCART cable - CPCWiki
You can always add a 9V source to pin 8 if you want it to automatically switch to the CPC when turned on (that's what I did too).

Yeah first of all I will check connections. The guide on settorezero uses the same power source of the computer to feed scart. I've only added a dyode to avoid accidental voltage swap, but the scheme on CPC Wiki seems a little different from one I used, so as first step I'll recheck the pinout.

[KaosOverride]

Strange thing to see a (v1) 464 on spanish case... should be (s1), or maybe is one of the first ones here at Spain, prior to the "ñ" obligation...


The keyboard is English or Spanish? Maybe only the case is from an spanish CPC?

Anyway, enjoy your 464!!!!

[gerald]

Strange thing to see a (v1) 464 on spanish case... should be (s1), or maybe is one of the first ones here at Spain, prior to the "ñ" obligation...


The keyboard is English or Spanish? Maybe only the case is from an spanish CPC?

Anyway, enjoy your 464!!!!

Looks like an English one
Cost down and pre-asic 464 case have port marking in both English and Spanish. At least this is the case on both french model I have.