You could duplicate the ROM in both halves of the larger EPROM. It doesn’t matter if A15 floats then because it will read the same byte value regardless.
OMG... I've been trying to fix a BBC Micro B+128 for the last few days that's showing almost exactly the same symptoms on its 6845 as the CPC 464 in this video. The timing is unbelievable! Thanks for giving me some idea of what to check out next.
Hi Noel, perfect video 👍I have just bought a CPC 464 from eBay and it had exactly this fault. I used your video as a step by step guide checking the voltages, clock and timing signals. And it was exactly the same as yours with interrupt low all the time. As the Gate Array was always cold so I got a modern replacement and it starts up perfectly. I couldn't have repaired it so quickly without watching this (several times!!) first. All the Best B 😎
I'm glad you got it working. I wondered why there was a 64K EPROM in and A15 was not strapped low or high but you worked it out in the end! I hope somebody does make gate arrays available in the future. I would suggest you cover the erase window.
Welcome aboard! And yes, the t-shirt collection is something I've gathered over the years, and now I go out of my way to print a t-shirt if I'm working on a new system 😃
This is not really a good idea: leaving the high address bit floating can cause the voltage at the pin to enter the intermediate voltage range between high and low, which can lead to the internal switching logic inside the device to rapidly oscillating. This causes high power consumption and can even damage the device (especially after some time). e.g. See the Texas Instruments application note entitled "Implications of Slow or Floating CMOS Inputs".
I’ve never seen one of these in person, but after watching several of your repair videos I feel pretty confident that I could take a good crack at one. Excellent video.
while working through your videos and listening to your exsplaintions while reading "an introduction to microcomputers" and trying to crame some learnings , perfect afternoon
Buenas! Aquí un seguidor Español, me encantan tus vídeos son una pasada, con cada video aprendo electrónica y retro informática! Gracias por cada uno de tus vídeos, son oro! Te escribo en castellano porque en uno de tus vídeos dices que estás en el norte de España y porque muchos de tus cacharros están en Español, espero que estés siendo feliz en este humilde país. Un abrazo
Thanks! Yeah, I've done a few CPC repairs since I wrote it and it's come in really handy already (even if I say so myself 😃). Let me know what you think when you try it. I'm also happy to incorporate any feedback.
What impresses me is that many RU-vidrs show their mistakes on camera. Like looking at the wrong schematic or the bent pin on the RAM. I don't know that I would have the confidence to show the world my errors. That is not meant negatively on the author as it shows what happens sometimes in the real world. I think my self criticism comes from watching years of repair shows on TV that rarely made mistakes while replacing a door frame or some such.
Thanks for sharing the mistakes you made along the way. That helps us all profit from those mistakes, and learn more. You always keep it real, and that's a great thing, man.
That was an excellent diagnosis and repair video. It really is a pleasure to watch someone who is both methodical and meticulous working through a problem. 🙂 I need to try out an Amstrad CPC, as I never actually used one back in the day.
top stuff Noel...this makes me wanna get my board back out and probe around a bit..(i need a diagnostic cart, and some ram chips!) ...im really keen to see the gate array recreated..im thinking it shouldnt need an fpga...it should be able to be done with microcontroller and the right code...i was gonna look into it if i could get my board working!
That soldering looks a lot like mine. It's messy, but consistently results in a working device, that's the most important thing. I should work on it though, which will help with more tightly packed PCBs.
Smart brother. Excellent vid… love my old cpc464 I picked up from a rubbish pile including green screen monitor. All I had to fix was power input capacitor and it’s all good
Great work. You pealed the problem back layer by layer like an onion. Not only that you are teaching me board level trouble shooting skills. You are awesome! I would love to see you make the FPGA for the Gate Array. You could make it a series and run these mixed in with all your other content.
Noel making a FPGA based gate array? I say "Go for it!" I would attempt it, but I'm having trouble just making a 16-bit ISA riser card with only 2 slots...
Clear, very detailed and informative as always. Well done, Noel. One trick to use 512 instead of 256 is to duplicate contents from 0-7fff onto 8000-fffff, thus A15 is not relevant. However, grounding A15 as Noel does is electrically safer that let it floating.
Grounding the A15 pin is the only real solution, as if it is left floating can confuse the the heck of the address decoding logic, and can result in total garbage data being read. Just never leave any relevant signals in "X" or floating input state. Leaving something unexpetdly floating can result also a current spike, especially in older CMOS chips that have essentially linear input to the CMOS pair.
Thank you! Yes, that's something to always watch out for. I was kind of kicking myself for not seeing it earlier... but if I had it wouldn't have been quite so interesting, so that worked out OK 😃
Good troubleshooting. Not many technicians still can do that. Just one thing, I would have checked the supply voltage on the pins with the scope as well. With a digital meter, what you see is not necessarily what it is. Or at least you don't know what you don't see. I still prefer an analog meter.
Me encanta la didáctica, cómo lo explicas, me ha encantado todo el recorrido buscando el problema y descartando posibles fallos. Muy interesante la diferencia del pin A15. Gracias!
For sure. It's just more of a hassle and you don't always have a good board with the right sockets. And, there's always the possibility of putting an IC so destroyed that it damages something else (but that's not likely, especially if you know the current draw is reasonable).
I think when you burn a smaller image into a larger capacity EPROM, it would be prudent to just replicate it as many times as it takes to fill up the EPROM. Then the specific value of unused high lines doesn't matter. But then leaving them floating still isn't ideal anyway, because no specific behaviour is guaranteed.
Wow to both your troubleshooting skills, as well as to the computer itself.. Amstrads were rather rare in here (now Slovakia / formerly Czechoslovakia), and mostly sold with the 'Amstrad-Schneider' logo. Only one local shop (that I can recall) had a few of these for sale, but unfortunately, without any accessories and programs for it - on top of complete lack of any documentation. Quite a pity, as later on in my life I found out how great these machines actually were. However.. Throughout the whole video I felt like I was still missing something essential, even fundamental in there, if I may say so - but finally, I figured out what it was.. . . . The cat :-)
Back in the days ... oh boy, that's long ago and was a good time. I had a CPC664 myself. I remember the German computer magazine c't had a project with the CPC6128. I think they added even more RAM to this. The important part: they had to modify the Gate Array and IIRC they also published the code for that. I do not have this in my archives, but I'm sure they have. A simple ask for help could meet some support I guess. FWIW and HTH
Thank you. I'm actually just talking to someone working on a Gate Array replacement and I'm hoping to be able to test it. So that will be happening soonish I hope 😃
If the power supply is reading low on the MB you would assume the leads have resistance, ie, losses. Or maybe the external bricks have a higher voltage, eg, 5.2V.
Excellent attention to detail, that floating pin would have driven me crazy for a long time. Great video! I see that at a certain point you switched from Windows to Mac and I imagine it had to do with running the programmer software. Even though it's a Windows program, I run XGPro for my TL866 II Plus on linux under wine . Wine runs well on a Mac and XGPro might work as well. Again, Excellent video!
Hi, some people burn the image twice on the EPROM to handle the floating pin, but I am not sure it is a stable solution (even if it probably works, at least most of the time). Do you have an advice on that?
Excellent as always: Questions because I really don't know and I'm somewhat curious, not stupid... Is the logic known or yet to be reverse engineered? How complex is the Amstrad gate array? In other words does a replacement need to be implemented in a CPLD or will discrete components do as in the gate array found in later Spectrums? Indeed could an EPROM implementation work such as found with the 'less good' C=64 PLA replacements?
About lower voltage with that power suply, maybe increase wire gauges that go from PSU to mainboard you are probably experiencing loss from high resistance wires
This is a great video, I love it’s detail-oriented yet understandible approach. I have a faulty CPC464 and did the debugging steps saw in the video. In my case, the CRTC generates VSYNC / HSYNC signals, but the gate array does not generate interrupt signals out of them. What does exactly the interrupt code do, does it play a role in the initialization? I mean if I pull up the interrupt pin in the Z80, will it display the welcome screen, even if the interactive things will not work? Where do you get a replacement gate array? I have a 40008 to replace, but found nothing in ebay. As a last resort solution, can I install a binary counter between the sync and the interrupt signal, or it’s a more complicated logic than that? The rest of the gate array seems working. Thanks your answers in advance!
Well done (again)! I really appreciate the detail put in explaining every possible cause and further analysis. What about if he had duplicated the same ROM binary content on the 27C512 despite having A15 floating? Un saludo :)
Thanks. Duplicating data is something that's often done and it works. I don't know about the long-term effects of having a floating input though. It could be that some transistors could have a reduced life because of that? Not sure.
@@NoelsRetroLab So far, I've never had any kind of problem with that setup. I mainly used that trick in Apple IIc (converted from rom 255 to rom 4X). Can't say it's 100% safe but as far as my experience goes, it is working without any side effect.
Something that I've had success with in the past when using the wrong ROM capacity in a system is burning the ROM with the data repeated across it. My BBC Micro expects 128kbit ROMs in the sideways slots but when I haven't had any to use I've used 256 or 512 kbit chips and burned the data in either 2 or 4 pages so that no matter how the floating address bits are interpreted by the ROM the data will always be the same. It's not something I'd rely on long term, but it hasn't let me down yet.
Love all the 8bit fun, can't wait to see what you pull out for SepTandy! I have a Model 1+Expansion Int. I want to get working with a floppy emulator like a Gotek. :D
Thanks! September is going to be pretty busy personally, but I'll try to make at least one #septandy video. Possibly two, although it might become #octandy by then 😃
You mentioned in a previous video that you reside in northern Spain. I spent some time in Galicia. A very pretty and green area and I learned that the claim that "The rain in Spain falls mainly in the plain" from the musical My Fair Lady is most definitely not true after visiting the north and then the central plains of Spain.
@@NoelsRetroLab Once I was dinning near Cangas de Onis and it starts rainning, the waitress, out of the blue, commented with us "és así que nosotros regamos los campos"... don't know why, but never forget the way he said it
@@NoelsRetroLab That entire area is beautiful and green and the people are very friendly. I really enjoyed my time there and didn't want to leave nor did my coworkers.
Is the nodge of the ROM socket on the right side instead of on the left side. Can cause problems for someone who isn't aware of this when replacing a ROM. Great diagnostics btw :)
Yes! Good eyes. Someone else mentioned that. I can't believe I didn't catch it. Probably because it came with the EPROM already on it and I missed during the initial inspection.
I am stumped with my one I just had in here, I got this one for myself as it was kept in a case in pristine condition! It is a 1983 model with no ribbon cable ports at all the motherboard is a Z70100. When it's switched on you get a garbled glitch image, switch it off and then on and a sort of chequer board screen shows with some artifacts and that is all. The chips feel all warm except for the AMSTRAD 40009 chip which is stone cold. Do you have any advice for this one? thank you
Is the glitch image still in the center and you see the border the way it should be? If so my money is on a RAM chip (even if none of them stand out). Pop in the diagnostics ROM and it'll tell you almost for sure.
@@NoelsRetroLab It was one single ram chip that was bad! I replaced it and all is well. Connecting the diagnostic rom would only display a glitched screen, error beeps and then giant numbers on the screen with coloured bars until this ram chip was replaced. thank you for your help in determining this!
@@martinsamuelsson2322 If the chip has an internal pull up resistor then it should be OK. Otherwise it can act as an antenna and oscillate. It's a bad thing to have unknowns inside a computer, that's when they start doing weird illogical things. If there is one thing that computers must be it's logical. That does not happen by magic or good fortune, it happens because the design forces them to stay logical. That's why there is no state between 0 and 1. By design.
