Hi all,
Been trying to repair a CPC 464 with a dead video (black screen?).
(70200 motherboard)
What I have done:
- Replaced dead Z80
- Check the Gate Array in another system (works!)
- Desoldered the ROM and installed a socket
- Burnt a 27256 EPROM with the diagnostics binary
- Tried to read the original Amstrad ROM in my programmer, but just got FFs
...still get a black screen and no beeps from the speaker.
Here are some observations:
- No Hsync or Vsync from CRT controller.
- Address outputs from the CRT controller are not pulsing and all are sitting at about 2.5V !!!
So, my next plan was to order a CRT controller off Aliexpress, but was wondering if there is anything else I should as I wait for the IC to arrive?
Much thanks in advance.
oh...I should add that I seem to have good voltages and clock signals.
On AliExpress you can buy a fake chip. Such is the risk.
Yes...it is a risk for sure
be sure that pin40 of GA has a secure connection to pin21 of CRTC
Do you mean pin4 of the gate array is connected to pin21 of the CRTC?
pin40 40007
pin4 40010
id replace that CRTC... and check the logic/buffers for the memory are working
to me, it seems like its copped some reverse voltage...which can take out a bunch of chips....
Quote from: namaman on 00:59, 30 March 25Hi all,
Been trying to repair a CPC 464 with a dead video (black screen?).
(70200 motherboard)
What I have done:
- Replaced dead Z80
- Check the Gate Array in another system (works!)
- Desoldered the ROM and installed a socket
- Burnt a 27256 EPROM with the diagnostics binary
- Tried to read the original Amstrad ROM in my programmer, but just got FFs
...still get a black screen and no beeps from the speaker.
Here are some observations:
- No Hsync or Vsync from CRT controller.
- Address outputs from the CRT controller are not pulsing and all are sitting at about 2.5V !!!
So, my next plan was to order a CRT controller off Aliexpress, but was wondering if there is anything else I should as I wait for the IC to arrive?
Much thanks in advance.
Hi,
just to be sure... What test equipment are you using?
Bryce.
Quote from: McArti0 on 13:04, 30 March 25pin40 40007
pin4 40010
yes, for me its a 40010, and it's connected to pin 21 of the CRTC. Thanks
Quote from: namaman on 00:59, 30 March 25Quote from: Bryce on 18:55, 30 March 25Hi,
just to be sure... What test equipment are you using?
Bryce.
I have a multimeter, scope and a logic probe...cheers
Quote from: Brocky on 16:17, 30 March 25... and check the logic/buffers for the memory are working
Not quite sure how to go about that...I'll poke around and see if I can see anything.
I'm assuming it's the 74ls244 and the 74ls373 I should look at?
yup.. but concentrate around the CRTC for now....it, along with the GA produce the picture.. replace the CRTC or test it in another system if possible..
from there you can determine if the ram and/or logic is bad (with the diag rom).. but you need to get some sort of picture out first..
might wanna just quickly probe the clock into and out of the GA aswell.. 16mhz in.. about 4mhz out to the z80..
I have previously checked that those line were getting a clock, but I never actually measured the timing. I'll do that.
I just ordered an MC6845P yesterday, only to read that the 'P' variant has some potential incompatibility issues. Oh well, I'm hoping it'll be good enough to get the machine displaying something if it is indeed a dead CRTC that I'm dealing with.
Quote from: Brocky on 01:45, 31 March 25might wanna just quickly probe the clock into and out of the GA aswell.. 16mhz in.. about 4mhz out to the z80..
Yep, 16mhz and 4mhz are both good
Side question: How should the CRTC signals look if it doesn't get initialised?
Quote from: eto on 07:36, 31 March 25Side question: How should the CRTC signals look if it doesn't get initialised?
Big frame ~4Hz V, ~7kHz Hz
https://youtu.be/XfvnLdiuokM
Some progress!!!
Removed the CRTC, installed a socket and put in another (found it in an Apple II 80 column card).
I have a EPROM installed and programmed with the Diagnostics binary.
Mostly it looks like it's trying to work, but has some spurious randomness, and it never displays any useful information. Ends with a series os dashed lines at the top of the screen.
No sound out of the speaker either!
Relays on cassette click! (you can hear it in the video attached)
Putting in the standard ROM now shows a white screen.
Any thoughts?
Here is a video:
After switching on, apply Hi permanently to the ROMDIS line and reset by quick Lo on RESET BUS line.
If the memory is fairly stable you should see vertical bars on the screen.
This TV is PAL or NTSC?
Quote from: McArti0 on 06:15, 02 April 25After switching on, apply Hi permanently to the ROMDIS line and reset by quick Lo on RESET BUS line.
If the memory is fairly stable you should see vertical bars on the screen.
This TV is PAL or NTSC?
This as a PAL machine and I'm in New Zealand.
Should I do that test with the Daignostics ROM or standard ROM installed? I'm assuming it doesn't matter.
Ok, did that.
I get about 50 (I didn't count them all) vertical bars, but there are what seem to be random horizontal bars (1 pixel width) that seem to invert the vertical bars...If that makes sense. It's messy.
Is there anyway to attach images to a post?
@namaman If you have the first 4 Ram chips OK Bits 0-1-2-3 the Diagnostic will get further in the test
? are the Ram chips in sockets ?
If so then remove all of the Ram from the second Bank and just install the first 4 Ram chips (as above)
Good luck
Keep Safe
Ray (Australia)
Unfortunately no socketed RAM.
I was hoping to avoid having to desolder them all, and get the Diag ROM working and just identify the dead ones.
:-(
Quote from: namaman on 06:40, 02 April 25Is there anyway to attach images to a post?
Click preview first.
Ps. Show us your motherboard.
https://www.cpcwiki.eu/index.php/Mainboard_Versions
Thanks...Here is the attachment from McArti0's ROMDIS test.
Screenshot 2025-04-02 at 7.18.32 PM.png
It's this motherboard, but with the 40010 gate array in the upper socket:
MC0002B Motherboard (https://www.cpcwiki.eu/index.php/File:CPC464_PCB_Top_(Z70200_MC0002B).jpg)
It isnt good pattern.
Now show by Scope READY/WAIT signal. 1MHz duty 25%
Now add ground to READY line. This will stop the processor.
Is the image degrading itself?
Without ground to READY.
Show by scope READY & nCAS
READY & nRAS
READY & RAMRD
Trigger to READY.
~500ns/div
This video shows:
- Machine turning on and starts the Diag ROM
- ROMDIS connected high (random noise)
- CPU reset briefly to GND
The vertical bars appear but everything is constantly moving as you can see in the video.
If I then ground the READY line the movement will stop until I remove the jumper. At some point the system freezes anyway and all movement stops.
I'll do those other scope tests asap. (Maybe tomorrow as its getting late here now)
Yellow - CAS
Red - READY
500ns
Yellow - RAS
Red - READY
500ns
Yellow - RAMRD
Red - READY
500ns, 5us, 5ms (three seperate captures)
(please keep in mind that this is a pretty cheap scope)
Quote from: namaman on 01:09, 03 April 25Yellow - CAS
Red - READY
500ns
RAS ;D
Quote from: namaman on 01:10, 03 April 25Yellow - RAS
Red - READY
500ns
CAS :D
Screenshot 2025-04-03 at 1.21.20 PM.png
What is this? :o
Try repeat this and write about setting.
...Thats what I saw on:
Yellow - RAMRD
Red - READY
I can have another look on an analog scope...may be better.
*Oops...I was sure I didn't get RAS and CAS the wrong way around...oh well!!!!
How many channels do you have in your oscilloscopes?
Btw. https://bread80.com/2021/06/03/understanding-the-amstrad-cpc-video-ram-and-gate-array-subsystem/
Two, but I have a logic analyser somewhere....but I only have one IC clip for it.
Quick question, I'm assuming the RAMRD signal is actually the nRAMRD from pin 29 of the 40010 Gate array?
so here is a video...
500ns/div
Tested with the Diagnostic ROM installed.
Red channel is the READY signal of the Z80.
Yellow channel is the nRAMRD signal from Pin 29 of the 40010 Gate array. nRAMRD stops doing anything once the dashed lines are displayed on screen.
Have I done something wrong?
Yes.
https://www.cpcwiki.eu/index.php/File:CPC664_Schematic.png
Look at the diagram. 664 is also on 40010 so it is the most similar.
If you confirm this noise on RAMRD then check if Z80 pin21 /RD has a good connection.
I'm still not sure what I have done wrong. On the 664 schematic you sent, the nRAMRD is on pin 29 as I have tested.
I will check Z80 pin 21
*UPDATE Pin 21 connects to IC112 Pin 5...good
Thanks
Quote from: namaman on 01:17, 03 April 25Yellow - RAMRD
Red - READY
500ns, 5us, 5ms (three seperate captures)
The noise on the second one is very bad.
hummm...yes.
I'm using a switching power supply, and just tried my bench supply...no change.
Yes or no...
Yes I can repeat this, but I'm not sure if it's just some artefact from the cheap digital scope. Weird.
Here is the same thing from an analog scope:
Then we are to assume we didn't see it. ::)
Then permanently Lo on resistor R124. And test pin1 of 74LS373
let me think of a better way to investigate that
Yeah...Possibly we could just ignore that :-/
Quote from: McArti0 on 08:09, 03 April 25Then permanently Lo on resistor R124. And test pin1 of 74LS373
Lo on R124 makes no difference.
Here is pin 1 of the 74ls373
RAMRD is so rare that you have to set the trigger on it itself. Analog scope may have a window that is too small.
Now I would check all the multiplexer control signals.
nCPU, CASADDR. Basically everything will be fine here but maybe GA is not mistaken.
Then multiplexers at the end - the verdict - 8x RAM.
Quote from: McArti0 on 09:35, 03 April 25nCPU, CASADDR. Basically everything will be fine here but maybe GA is not mistaken.
Then multiplexers at the end - the verdict - 8x RAM.
Sorry, I'm not sure what you mean here.
All RAM removed and socketed.
Diag ROM now just shows RED screen and then goes to the dashed lines.
@namaman Ok That's good news.
For a quick test Just fit 4 Ram chips at Bit0 to Bit3 and retest
and you should get as per Photo
You are nearly there
Keep Safe
Ray
I'm not so sure it's going to be that easy...
Anyway, is that IC120 the IC117 for the first four RAM chips?
Cheers
Yep 120 to 117
Bit 0 to Bit3
Kep Safe
Ray
Hey...check it out!!!!
I was able to replace all RAM with know good RAM and it works...well, kinda...the keyboard doesnt work :-(
So thats next on my list of fixes.
So far: Z80 dead, RAM dead, ROM dead.
Thanks to everyone for sticking with me so far, especially McArti0 for all his troubleshooting suggestions.
Stay tuned as I investigate the keyboard...
Is the keyboard not working on both ROMs?
Does DiagROM only hang when detecting CRTC?
yes...keyboard not working at all for both ROMs.
The Diag test stops there...and thats it.
Check pin32 GA(40010) it should be 300Hz interrupts
Pin18,20 AY - Quick Hi,Lo series 50 times per second.
Grounded one or more Pin19,20,21 8255 at start CPC
This changes the brand name in the welcome text.
It proves that the 8255 works at all.
Grounded pin 7-14 AY. Should be serial 10 chars on screen at one pin.
Quote from: namaman on 07:04, 04 April 25Hey...check it out!!!!
I was able to replace all RAM with know good RAM and it works...well, kinda...the keyboard doesnt work :-(
So thats next on my list of fixes.
So far: Z80 dead, RAM dead, ROM dead.
Thanks to everyone for sticking with me so far, especially McArti0 for all his troubleshooting suggestions.
Stay tuned as I investigate the keyboard...
If that much was broken it must have got an over-voltage or reversed voltage. That would have damaged the AY too, so I would just go ahead and swap it.
Bryce.
Quote from: McArti0 on 09:02, 04 April 25Check pin32 GA(40010) it should be 300Hz interrupts
Pin18,20 AY - Quick Hi,Lo series 50 times per second.
Grounded one or more Pin19,20,21 8255 at start CPC
This changes the brand name in the welcome text.
It proves that the 8255 works at all.
Grounded pin 7-14 AY. Should be serial 10 chars on screen at one pin.
Does it matter what ROM I have installed?
I'll give those a try tomorrow. Getting late here now.
I also just tried swapping in another known good CRTC and the Diagnostic test still hangs at the CRTC test.
firmware ROM.
Also I recall seeing this video from Noel:
*see the 19min 50second mark
I wonder if my PIO is dead too????
Quote from: Bryce on 09:23, 04 April 25Bryce.
I think you are probably correct.
I forgot to add to the list of dead parts the CRTC...but surprisingly out of all the RAM that was desoldered only two RAM chips were dead, the rest tested fine.
You have a working GA. That's a lot of luck.
Quote from: namaman on 10:11, 04 April 25I wonder if my PIO is dead too????
First tests....
Quote from: McArti0 on 09:02, 04 April 25Check pin32 GA(40010) it should be 300Hz interrupts
Pin18,20 AY - Quick Hi,Lo series 50 times per second.
Grounded one or more Pin19,20,21 8255 at start CPC
This changes the brand name in the welcome text.
It proves that the 8255 works at all.
Grounded pin 7-14 AY. Should be serial 10 chars on screen at one pin.
yes...
- 300Hz on pin 32 of the GA.
- On the AY, Pin 18 pulses (see photo), pin 20 seems dead.
- Grounded one or more Pin19,20,21 8255 at start changes nothing on the boot screen.
*one addition here, Grounding PIN 19 causes the screen to slowly roll as if Vert Sync is slightly off.
"Grounded pin 7-14 AY. Should be serial 10 chars on screen at one pin."
Not exactly sure what this means.
* Dammit...something just pooped itself on the system as I was testing. Just checked its not the RAM, or the GA, or the Z80, or the CRTC, those all look fine I think...more investigation needed
...Well, not sure what was going on there, but I suspect a marginal 74LS32 @ IC112 (I was getting a complete blue screen, and sometimes just randomness). I actually piggybacked it with another, and the system suddenly booted again, removing the piggyback IC and the system continues to boot as before. Unusual.
Quote from: McArti0 on 09:02, 04 April 25Grounded pin 7-14 AY. Should be serial 10 chars on screen at one pin.
I tried individually grounding those lines, but nothing changes on screen. No characters appear.
I have ordered a 8255 PIO, a YM2194F Sound chip, and some AY-8910 to AY-8912 adaptor boards.
I hope thats all I'll need to get this machine up and running again.
I guess it's time to desolder the 8255 and the AY-8912...
Quote from: namaman on 23:16, 04 April 25yes...
- 300Hz on pin 32 of the GA.
- On the AY, Pin 18 pulses (see photo), pin 20 seems dead.
- Grounded one or more Pin19,20,21 8255 at start changes nothing on the boot screen.
*one addition here, Grounding PIN 19 causes the screen to slowly roll as if Vert Sync is slightly off.
- GA working.
- Pin 18 AY, 8255 working at Write
- pin 20 AY ? ? ? 8255 not working at Write?
- Pin19,20,21 8255 not working at Read !!!
- https://www.cpcwiki.eu/index.php/LK_Links
- PIN19 realy not Pin22? its to 60Hz/50Hz Vsync switch
Quote from: namaman on 05:47, 05 April 25I guess it's time to desolder the 8255 and the AY-8912...
First test IC112 Pin6
Test pulse 74LS145 between +5V and Pin2
If Pin2 pulses then cross on working CPC Pin2 LS145 to READY permanently. This should stop CPC at Open AY. Grounded Pins 7-14 AY should be show Lo on Pins 21-28 AY. (Pin 7-Low -Pin 21 Low etc)
Arrrrgh!!!
I mentioned previously I started having intermittent issues, but always seemed to be able to recover...somehow. I suspected the 74LS32 at IC112.
...Then suddenly I couldn't boot at all. I replace IC112, but still no change.
Two steps forward, one step back!!!!
Here is a video of what I'm getting now. Kind of a pulsing on the screen. Also seems like some signals are also pulsating at the same rate. No change if ROM is removed.
I have checked the GA (in my 6128), Z80 (in my 6128), and CRTC (in the Apple II 80 column card), and they still all seem to work in those systems.
Oh dear...What to do now.
I checked the voltages, and the clocks, and again those seem fine.
Frustrating.
Quote from: namaman on 10:12, 05 April 25Arrrrgh!!!
I mentioned previously I started having intermittent issues, but always seemed to be able to recover...somehow. I suspected the 74LS32 at IC112.
...Then suddenly I couldn't boot at all. I replace IC112, but still no change.
Two steps forward, one step back!!!!
Here is a video of what I'm getting now. Kind of a pulsing on the screen. Also seems like some signals are also pulsating at the same rate. No change if ROM is removed.
I have checked the GA (in my 6128), Z80 (in my 6128), and CRTC (in the Apple II 80 column card), and they still all seem to work in those systems.
Oh dear...What to do now.
I checked the voltages, and the clocks, and again those seem fine.
Frustrating.
That's why bridging and shorting pins should never be used in diagnosis, measurements tell you everything you need to know. It's too easy to accidentally break something else and end up where you are now.
Bryce.
You have loose sockets. When You unplug and plug Chip in socket, You cannot be sure that all pins are connected.
Bridging and shorting pins with brain is nothing wrong with doing something, you just need to use your brain and know how it works.
If you don't know how to do it, you scare others.
Now 74LS32 is used to Write to CRTC. But not work properly
Bryce, yes, that is indeed quite possible. I think I'll stop that for now.
So after some more playing around, this is where I am at.
I am fairly sure the CRTC, Z80, GA and RAM are all ok.
I started looking around IC112 again. I removed the new 74LS32, and put back the original that I desoldered...I saw more signs of live, and on some restarts I even got a white screen and black boarder!!!...I then I looked in my parts bin, mostly I have one kind/maker (which proves not to work at all) and then I found a third variety of 74LS32 and tried that...this time I was able to consistently get a boot screen, but with garbled or inverted colour text (see image).
Soooo....not sure what is going on and why the various 74LS32 have different outcomes, but I feel I'm starting to focus in on the problem a little bit, or at least I'm in the right area.
Anyway, it's real late here now, so I'll pick this up again in the morning.
Any help or insights would be greatly appreciated.
Thanks again
Screenshot 2025-04-05 at 11.54.24 PM.png
IC112 is Main Source IORD and IOWR signal for CRTC and 8255
GA dont use IC112.
NOW you have problem with WR and RD from Z80. check Your socket.
Quote from: McArti0 on 11:56, 05 April 25you just need to use your brain and know how it works.
Most people don't know how it works and telling them what to do (especially without explanation why) has a very high risk that they make a small mistake here and there.
Even a person familiar with electronics should send their damaged Amstrad CPC to one of the many Amstrad authorized service centers to take advantage of the lifetime warranty. :picard:
Just a comment before I get into it.
Fistly I actually do have a basic understanding of what could happen through shorting certain signals in that the IC delivering the original signal could get damaged. So I guess thats my own risk, but I also agree that before any mention of shorting pins, then maybe it would be wise to mention that there is some risk involved.
Anyway, I've checked all the connections from IC112 and they all look fine.
I've just checked the connectivity from all my new sockets and I can't see any issues at all.
I've tried a few other varieties of 74LS32s in IC112, but nothing works! I only have one 74LS32 that seems to do anything. I guess my biggest question at the moment is why? Hopefully the answer to that may lead to the underlying issue.
...I did try swapping in some other Z80s, 2 didn't work (possibly dead), two NEC Z80's were as per the screen image I showed before with text corruption...and then there was this one (see video). At one point it actually displayed clean text with a crazy flashing boarder...not sure if it's useful info, but thought I'd share.
Let's get back to the stripes. ROMDIS Hi.
I suspect a lack of contact on one of the address lines and maybe the GA data line.
a lot of characters appearing after Ready means that the 8255 has started reading 00000000 from the keyboard, i.e. all keys pressed.
Here is the halt ROMDIS hi test:
Lots of...noise?
And add RAMDIS Hi?
Quote from: McArti0 on 08:24, 06 April 25And add RAMDIS Hi?
this is ROMDIS HI and then a quick LO on the RESET line.
...do I just do the same but do RAMDIS Hi instead?
if thats not right, can you explain the sequence?
CPC on, next ROMDIS Hi, next RAMDIS Hi.
No reset?
no
ROMDIS HI (screen glitches), then RAMDIS HI doesn't change anything.
Then doing a RESET the screen is just as the video before...noisy lines.
Ok check on the oscilloscope if this noise at ROMDIS is visible on Lines D0-D7 GA (i.e. Pin14 RAMs)
Quote from: namaman on 12:02, 05 April 25Bryce, yes, that is indeed quite possible. I think I'll stop that for now.
So after some more playing around, this is where I am at.
I am fairly sure the CRTC, Z80, GA and RAM are all ok.
I started looking around IC112 again. I removed the new 74LS32, and put back the original that I desoldered...I saw more signs of live, and on some restarts I even got a white screen and black boarder!!!...I then I looked in my parts bin, mostly I have one kind/maker (which proves not to work at all) and then I found a third variety of 74LS32 and tried that...this time I was able to consistently get a boot screen, but with garbled or inverted colour text (see image).
Soooo....not sure what is going on and why the various 74LS32 have different outcomes, but I feel I'm starting to focus in on the problem a little bit, or at least I'm in the right area.
Anyway, it's real late here now, so I'll pick this up again in the morning.
Any help or insights would be greatly appreciated.
Thanks again
Screenshot 2025-04-05 at 11.54.24 PM.png
That could just be a bad connection on the ROM IC. I'd recheck the solder joints and pins of the ROM.
Bryce.
Quote from: Bryce on 09:50, 06 April 25That could just be a bad connection on the ROM IC. I'd recheck the solder joints and pins of the ROM.
Bryce.
Thanks Bryce. Just rechecked the ROM pins, and all have continuity as expected. This is actually the 3rd time I've checked these...lol
Cheers
Quote from: McArti0 on 09:31, 06 April 25Ok check on the oscilloscope if this noise at ROMDIS is visible on Lines D0-D7 GA (i.e. Pin14 RAMs)
Checked on pin 14 of the RAM.
Looks random, but maybe there is a repeat pattern here.
All pins basically look the same, but with variations in the repeated patterns.
this is Pin 2 of the RAM:
And here is RAM pin 14 from my working 6128:
And here is RAM pin 2 from my working 6128:
I'm not adverse to doing a bit of educated shotgun repairs as I'm pretty sure I can remove any IC with no damage insured...It's just about patience.
As I mentioned before, the GA, Z80, CRTC, EPROM have been tested and I an 99.9% sure are all functional. So what does that leave as culprits?
I have spare Buffers and Mux ICs.
@namaman Hi
Ok Its been along Journey (Still is)My Thoughts :- for a CPC 464After Power up Press the "DELETE" Key several time — Do you Get a Beep From the SpeakerAssuming the Cassette is connected. ??Put back the Eprom with the Diagnostic on it.Carefully look at what happens at Power Up.Photo Included of the Diagnostic Stopping ??Have a look around IC102 AY-2-8912 (Sound Generator) Check Pin 16 to see if it is /LOWAnd IC101 74Ls145 (Decoder)Ok Try ThatKeep Safe (and Patient)Ray
Audronic, thanks for the advice about Sound and Keyboard input.
I'm will get back to looking at that shortly...but now I'm trying to figure out why my screen has become corrupt :-(
See post #72 in this tread
Hi
Before you go an look for the problem ?
Try the Diag and see what is on the screen It may help ?????
Keep safe
Ray
Unfortunately the Diagnostic test now does this...
No colour redrawing of the screen at the beginning. Briefly shows results up to 'CRTC' then hangs with garbage on screen.
Ok — Youch
Could you check Ic 108 6845
Pin 40 Vertical sync (50 Hz)
Pin 39 Horizontal sync (15.625 KHz)
As it looks like the frequencies are Incorrect ?
Keep Safe
Ray
Just checked those...they look to be spot on:
Pin 40 - 50.077 Hz
Pin 39 - 15.624 Khz
There is a lot of sync issues when timing the small CRT with my phone unfortunately. Please try to ignore that.
Hmnnnn
I will have to have a think about that
Ok Last one:-
IC118 Gate Array Pin 11 should be SYNC to the Monitor (15.625 Khz)
Keep Safe
Ray
GA pin 11 - 15.624 Mhz
...so looks good
This is the LAST one today.
I am beginning to question the Monitor (or the lead) for part of the Problem.
Do you ave an other CPC available to plug the Monitor cable into ???????????
Keep Safe
See you later
Ray
Yes...I have tried the monitor with my working CPC6128, and there is no problem.
I don't actually think there is any issue with video sync. I think it's just the filming of the screen makes it look funky.
Quote from: namaman on 23:26, 06 April 25Quote from: McArti0 on 09:31, 06 April 25Ok check on the oscilloscope if this noise at ROMDIS is visible on Lines D0-D7 GA (i.e. Pin14 RAMs)
Checked on pin 14 of the RAM.
Looks random, but maybe there is a repeat pattern here.
All pins basically look the same, but with variations in the repeated patterns.
Add nCAS on other channel and set trigger on nCAS.
Show on analog scope.
Try d0-d7 pin2 pin14.
Next
ROMDIS Hi, bars
Scope on IC112 pin3, pin6. There should be no negative peaks.
CRTC pin 23 There should be no positive peaks.
IO must to be off. I hope so....
Hopefully I've done this correctly.
(nCAS on top and set as trigger)
All the data lines basically look the same:
Pin 2 and 14 of the RAM
Quote from: McArti0 on 06:21, 07 April 25Next
ROMDIS Hi, bars
Scope on IC112 pin3, pin6. There should be no negative peaks.
CRTC pin 23 There should be no positive peaks.
IO must to be off. I hope so....
no reset?
and do I keep the nCAS trigger?
Do I keep the trigger?
Quote from: namaman on 06:42, 07 April 25no reset?
We take advantage of the fact that seeing the bars results in one memory read and two writes.
Anything else that happens is a mistake.
ok...
pin 23 CRTC - 0V
Pin 3 ic112 - 4V
pin 6 ic112 - See photo (video on digital scope in next post)
pin 6 ic112 (red) nCAS trigger (yellow):
(Ignore the stopping of the scope here as the laptop it's running to is a little slow)
Ok What is IORQ pin20 Z80 doing at this moment?
With ROMDIS hi and bars on screen Pin6 IC cannot be Lo and IORQ cannot be Lo NEVER!
Also check if it is really ROMEN is very common Lo.
Some other observations.
Doing the ROMDIS Hi is not always consistent in what I see on the video. Sometimes the "noise" moves like in the video, and sometimes the image has "noise" but is static (frozen), and once or twice I've got this (see image).
Also with the different video outcomes (above), the pulsing on Pin 6 of IC112 looks different.
Quote from: McArti0 on 07:27, 07 April 25Ok What is IORQ pin20 doing at this moment?
not much at about 4V (a little noisy)
also...here are the bars in colour for your reference:
ROMEM:
Quote from: McArti0 on 07:27, 07 April 25Ok What is IORQ pin20 Z80 doing at this moment?
With ROMDIS hi and bars on screen Pin6 IC cannot be Lo and IORQ cannot be Lo NEVER!
Also check if it is really ROMEN is very common Lo.
Well, it looks like ic112 pin 6 is going Lo
IORQ looks basically Hi
If IORQ is always Hi then pin6 and pin3 IC112 must also be always hi. You probably don't have pin4 of IC112 connected to pin20 of z80.
Also remember that the socket pin holes should be soldered to the upper and lower tracks. Originally there is a connecting tube in the hole. If you destroy this connection during unsoldering then you have to solder from the top and bottom.
Quote from: McArti0 on 07:56, 07 April 25If IORQ is always Hi then pin6 and pin3 IC112 must also be always hi. You probably don't have pin4 of IC112 connected to pin20 of z80.
it's definitely connected.
I don't think I destroyed anything. Chip Removal was very clean for every chip removed so far.
* Just checked all the connections from IC112 again...all seems fine
I keep coming back to this question...I've tried about 6 different 74LS32s as replacement for IC112, but why only one produces a video image (but with garbled characters)? The other 74LS32s produce no image and possibly no video output at all.
Confirm IC112 pin4 connections to Z80 pin20
Confirm IC112 pin5 connections to Z80 pin21
5V between Pin14 and Pin7 of IC112
Quote from: namaman on 08:38, 07 April 25I keep coming back to this question...I've tried about 6 different 74LS32s as replacement for IC112, but why only one produces a video image (but with garbled characters)? The other 74LS32s produce no image and possibly no video output at all.
74LS32 has 4 OR-gates. The one that has a working gate 1,2,3 pins(Can write to CRTC, 8255,GA) has a broken 4,5,6 ( reading all IO always)
Quote from: McArti0 on 08:39, 07 April 25Confirm IC112 pin4 connections to Z80 pin20
Confirm IC112 pin5 connections to Z80 pin21
5V between Pin14 and Pin7 of IC112
Yes
Yes
4.8V
Quote from: McArti0 on 08:44, 07 April 2574LS32 has 4 OR-gates. The one that has a working gate 1,2,3 pins(Can write to CRTC, 8255,GA) has a broken 4,5,6 ( reading all IO always)
Buy new IC112 and IC110 I don't believe them.
Do you have a breadboard?
Quote from: McArti0 on 08:51, 07 April 25Quote from: McArti0 on 08:44, 07 April 2574LS32 has 4 OR-gates. The one that has a working gate 1,2,3 pins(Can write to CRTC, 8255,GA) has a broken 4,5,6 ( reading all IO always)
Buy new IC112 and IC110 I don't believe them.
I have tried new 74LS32s...dont work.
I could remove IC110 as I have spare 74LS132.
I do have a breadboard somewhere maybe.
"I have tried new 74LS32s...dont work."
OR gate is simple like wire.
If pin 4 is always Hi then pin6 is always Hi.
Quote from: McArti0 on 09:07, 07 April 25"I have tried new 74LS32s...dont work."
OR gate is simple like wire.
If pin 4 is always Hi then pin6 is always Hi.
I have tried MANY different 74LS132 ICs in the socket...only one 74LS32 works and produces a corrupt image on the monitor. The other 74LS32s don't seem to work at all.
I have removed the 74LS132 at IC110 and socketed it and replaced it...no change :-(
Wait...Wait!!!!
Maybe it's working!!!!!
Quote from: namaman on 07:14, 07 April 25pin 6 ic112 (red)
Quote from: namaman on 07:34, 07 April 25Quote from: McArti0 on 07:27, 07 April 25Ok What is IORQ pin20 doing at this moment?
not much at about 4V (a little noisy)
Show it on an oscilloscope pin6 IC112 and z80 pin20.
Next
Show it on an oscilloscope pin6 IC112 and pin4.
Thank goodness for that!!!!
I thought I was going crazy.
So I replaced IC110 and it didn't fix the problem, but then I thought I should change IC112 again in case it's some weird chip combo thing happening..and it works.
So... the 74LS32 (IC112) I was using was actually bad, but in combination with the 74LS132 (IC110) which was also bad, produced a picture (albeit a corrupted one)!!!!
Where as, if I replaced the 74LS32 with a good working one, in combination with the bad 74LS132, it wouldn't produce a picture...thats a frustrating symptom.
so to put it another way... two broken ICs kind of worked, but one broken IC and one good IC didn't work...jeeez!
Anyway...Huge thanks for all your help and support, and also to Audronic who pitched in some thoughts over the last few days too.
Now I'm waiting for PIO and Sound chip + adaptor board.
Screenshot 2025-04-07 at 8.34.29 PM.png
Quote from: McArti0 on 09:02, 04 April 25Check pin32 GA(40010) it should be 300Hz interrupts
Pin18,20 AY - Quick Hi,Lo series 50 times per second.
Grounded one or more Pin19,20,21 8255 at start CPC
This changes the brand name in the welcome text.
It proves that the 8255 works at all.
And 8255 pin35 (reset) is lo
Pin18,20 This is the same as Pin10,11 8255
Well Done
That was some Journey
I am sure a lot of us learned some thing, I did
Thanks
Keep Safe
Ray
Quote from: Audronic on 10:32, 07 April 25Well Done
That was some Journey
I think the last few days was just a side mission.
The system isn't fully working yet though!
Quote from: McArti0 on 10:19, 07 April 25Quote from: McArti0 on 09:02, 04 April 25Check pin32 GA(40010) it should be 300Hz interrupts
Pin18,20 AY - Quick Hi,Lo series 50 times per second.
Grounded one or more Pin19,20,21 8255 at start CPC
This changes the brand name in the welcome text.
It proves that the 8255 works at all.
And 8255 pin35 (reset) is lo
Pin18,20 This is the same as Pin10,11 8255
Pin32 GA(40010) - 0V.
Pin18 AY - Pulsing (but cant read frequency as pulsing is not consistent like a clock).
Pin 20 AY - 0V
Grounded one or more Pin19,20,21 8255 at start CPC - Nothing changes.
8255 pin35 (reset) - Lo.
Quote from: namaman on 10:43, 07 April 25I think the last few days was just a side mission.
The system isn't fully working yet though!
Ok
What else is not working ?
Keep Safe
Ray
Keyboard and sound
Quote from: namaman on 00:03, 08 April 25Pin18 AY - Pulsing (but cant read frequency as pulsing is not consistent like a clock).
Quote from: namaman on 00:03, 08 April 25Pin32 GA(40010) - 0V.
Pin 18 AY pulsing proves that Pin 32 GA is definitely working. (pulsing)
check 8255 pin 14,15,16,17 its scan rows kayboard.
Quote from: McArti0 on 05:32, 08 April 25Pin 18 AY pulsing proves that Pin 32 GA is definitely working. (pulsing)
??? How does that work Pin 32 (Blue Video) ??
Keep Safe
Ray
Quote from: Audronic on 06:07, 08 April 25Quote from: McArti0 on 05:32, 08 April 25Pin 18 AY pulsing proves that Pin 32 GA is definitely working. (pulsing)
??? How does that work Pin 32 (Blue Video) ??
Keep Safe
Ray
namaman has GA40010. You must look on 664 or 6128 schem.
I just have an IBM 5150 Monitor on the workbench at the moment.
Waiting for parts to arrive for the 464 and then I'll replace the PIA first and see if I can get the Diag test to pass, and then will test the sound chip and if dead will build up and adaptor board and install a YM2149F.
You can also see if the 8255 receives the IORD pin5 signal. If Pins 14, 15, 16, 17 are flashing, it means that the 8255 only works as a data receiver, but is unable to send anything to the processor.
Quote from: namaman on 00:23, 12 April 25I just have an IBM 5150 Monitor on the workbench at the moment.
Waiting for parts to arrive for the 464 and then I'll replace the PIA first and see if I can get the Diag test to pass, and then will test the sound chip and if dead will build up and adaptor board and install a YM2149F.
The AY is 1000 times more likely to fail than the 8255. I don't think I've ever seen an 8255 fail except in very extreme circumstances.
Bryce.
Quote from: Bryce on 13:46, 12 April 25I've ever seen an 8255 fail except in very extreme circumstances.
Why dont read LK1-LK3 and Vsync...
Quote from: McArti0 on 16:55, 12 April 25Quote from: Bryce on 13:46, 12 April 25I've ever seen an 8255 fail except in very extreme circumstances.
Why dont read LK1-LK3 and Vsync...
There could be many reasons, /CS, /RD, /WR or some other required pin not being set correctly.
Bryce.
Hi all,
I've taken the PIA and the AY-3-8912 chips out. Socketed the PIA.
PIA and AY-3-8912 both removed cleanly so can use if not dead.
Waiting for a new PIA, YM2149F, and the adaptor board for the AY-3-8910 (YM2149F) to AY-3-8912. Should all be here in a couple of days I hope.
...then it should be super easy to see what works and what doesn't.
Ooops...because I collect and work on a variety of different systems and it's hard to remember all the chip acronyms.
Quote from: namaman on 07:58, 14 April 25Ooops...because I collect and work on a variety of different systems and it's hard to remember all the chip acronyms.
Outed as a Commodore fan! ? :D
Bryce.
Quote from: Bryce on 17:56, 13 April 25Quote from: McArti0 on 16:55, 12 April 25Quote from: Bryce on 13:46, 12 April 25I've ever seen an 8255 fail except in very extreme circumstances.
Why dont read LK1-LK3 and Vsync...
There could be many reasons, /CS, /RD, /WR or some other required pin not being set correctly.
Bryce.
Ok. This is after replacing IC112 and IC110, i.e. control for /RD /WR is new. With a correctly loading system /CS, i.e. A11 must be functional. What is broken?
What other pins are required to operate the 8255?
The sound chip and the PIO have arrived from china.
(The sound adaptor PCB is still in transit but should be here soon)
Installing the new PIO now allows the Diag test to get past the CRTC test.
So some progress. Just waiting on the PCB to arrive and hopefully it'll all just work out
Quote from: namaman on 03:46, 15 April 25Installing the new PIO now allows the Diag test to get past the CRTC test.
Quote from: Bryce on 17:56, 13 April 25Quote from: McArti0 on 16:55, 12 April 25Quote from: Bryce on 13:46, 12 April 25I've ever seen an 8255 fail except in very extreme circumstances.
Why dont read LK1-LK3 and Vsync...
There could be many reasons, /CS, /RD, /WR or some other required pin not being set correctly.
Bryce.
Or 8255 is damaged, which is clearly shown by the logic of the diagnostics like This time.
So stop posting nonsens Bryce...
Quote from: Bryce on 11:15, 14 April 25Quote from: namaman on 07:58, 14 April 25Ooops...because I collect and work on a variety of different systems and it's hard to remember all the chip acronyms.
Outed as a Commodore fan! ? :D
Bryce.
I've been found out!!!!
Quote from: McArti0 on 06:30, 15 April 25Quote from: namaman on 03:46, 15 April 25Installing the new PIO now allows the Diag test to get past the CRTC test.
Quote from: Bryce on 17:56, 13 April 25Quote from: McArti0 on 16:55, 12 April 25Quote from: Bryce on 13:46, 12 April 25I've ever seen an 8255 fail except in very extreme circumstances.
Why dont read LK1-LK3 and Vsync...
There could be many reasons, /CS, /RD, /WR or some other required pin not being set correctly.
Bryce.
Or 8255 is damaged, which is clearly shown by the logic of the diagnostics like This time.
So stop posting nonsens Bryce...
Since when is suggesting the most likely part to have failed nonsense?
Bryce.
Quote from: Bryce on 13:46, 12 April 25I don't think I've ever seen an 8255 fail except in very extreme circumstances.
Bryce.
What exactly were you suggesting with this sentence namaman, who had a broken 8255?
Quote from: McArti0 on 16:10, 15 April 25Quote from: Bryce on 13:46, 12 April 25I don't think I've ever seen an 8255 fail except in very extreme circumstances.
Bryce.
What exactly were you suggesting with this sentence namaman, who had a broken 8255?
Exactly what I said: I've only seen 8255's fail in situations where there was an external cause for the part to fail: Over-voltage, reverse-voltage, static discharge etc. But I don't think I've ever seen an 8255 that failed in normal working circumstances, something that often happens with RAM IC's, CPU's and the AY.
Bryce.
the PCB sound adaptor board arrived today and I promptly built it up...
IT'S ALIVE
So that was a mission. Thanks for everyone who stuck with me through it all.
I'm guessing it must have been an over voltage, or reverse voltage issue:
The only thing to fix is the DELETE key not working...for now I'm just going to pretend I didn't notice...lol
Quick denture.
Key def 10,1,55,127 Makeing SHIFT+F7 as DEL.
10 is F7 key
55 is 7 char
127 is DEL char
Check 50Hz pulses between +5v and pin11 74LS145.
Quote from: namaman on 09:59, 17 April 25The only thing to fix is the DELETE key not working...for now I'm just going to pretend I didn't notice...lol
The DEL key is on a row and column of its own. I'd try cleaning and reseating the membrane first. Then try shorting the appropriate pins between the two keyboard connectors. If you still don't get a DEL, then it can only be IC101 (74LS145) that's faulty. Every other associated IC has already been swapped. If shorting at the connector gives a DEL signal, then the membrane has a problem. Separating and (carefully) cleaning both sides of the membrane might solve this.
Bryce.
I'd be extra careful when cleaning the membranes. Sometimes the lanes get deleted. Thankfully, the PCB repair pens are handy in such cases.
Quote from: dodogildo on 19:05, 17 April 25I'd be extra careful when cleaning the membranes. Sometimes the lanes get deleted. Thankfully, the PCB repair pens are handy in such cases.
That's why you should only use water, no chemicals and only clean very lightly with a soft cloth.
Bryce.
So the final fix for the non working delete is in!!!!!
Corrosion on the keyboard PCB traces in and around the delete key.
(As you can see I don't have the membrane keyboard.)
I scraped off the old solder mask, and ran some jumper wires to bridge the corroded area... and then sealed it all up with nail polish.
Phew...all done!
Thanks again for all your help!
Philip
Hey, didn't know it was a PCB keyboard. In that case it's always repairable.
Bryce.
quicky: no intention to hijack this thread but since im fighting with a similar problem: got my spare 6128 and it shows no sign of life (apart from pwr led). I only checked for hotspots on the board so far and the floppy controller is the most toasty one by a good margin. That normal? or should i try to desolder it? would the cpc start up with a missing fdc?
FDC is clearly warm. CRTC too. Write what tools you have and what you know about electronics.
Quote from: FruityFrank on 19:28, 22 April 25quicky: no intention to hijack this thread but since im fighting with a similar problem: got my spare 6128 and it shows no sign of life (apart from pwr led). I only checked for hotspots on the board so far and the floppy controller is the most toasty one by a good margin. That normal? or should i try to desolder it? would the cpc start up with a missing fdc?
The FDC does tend to get quite warm, so I wouldn't automatically suspect it. If you have a spare CPU and GA, then I would try swapping them first. The CPC won't boot without the FDC, so don't touch it until you have confirmed that everything else is ok (voltages, clock signal, reset signal etc).
You probably should start a new thread for this repair though.
Bryce.