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General Category => Technical Support => Hardware related => Topic started by: Socali on 16:29, 05 January 21

Title: [SOLVED] CPC 664 - Slow HSYNC Signal
Post by: Socali on 16:29, 05 January 21


I finally got my CPC 664 from my parents house, cleaned up the board, replaced the disk drive belt, got SCART cable and HDMI converter, got everything hooked up and... no picture! :(

I then followed @llopis (https://www.cpcwiki.eu/forum/index.php?action=profile;u=1728) great videos and read lots of insightful messages here from @Bryce (https://www.cpcwiki.eu/forum/index.php?action=profile;u=225) and others, and here's what I found out so far:
- Z80A, 40010 Gate Array, CRTC and RAM all getting good voltage (4.88v - 5.0v)
- Gate Array has signal on Pin 24 = 16Mhz, Pin 19 = 4Mhz and Pin 14 = 1Mhz (and has activity on other pins)
- Z80A has signal on Pin 6 = 4Mhz, Pin 19 = 2Mhz, Pin 27 and Pin 28 = 1Mhz (and has activity on other pins)
- All RAM chips have signal on Pin 2 (and has activity on other pins)
- Only Z80 and CRTC get slightly warm, all others stay cool

Now the CRTC (HD6845SP) is having Pin 39 = 4.25Khz (or 4.18Khz) instead of 15Khz, and Pin 40 is pulsing slowly.
It also has signal on Pin 21 = 1Mhz.
About 1 out of 3-4 times that I turn it on/off, the HSYNC and VSYNC will not output anything.
Another interesting point is that the address Pins 4 - 17 (MA0-MA13) have signal going down like this:
Pin 4 = 500Khz, Pin 5 = 250Khz, 130Khz, 65, 35, 17, 8, Pin 11 - Pin 16 = 4.25, Pin 17 = 0.

I'm good with soldering, but not so much with desoldering PCB components, so I rather ask before trying to desolder a 40pin chip.. :)
Since the CRTC does have signal but slower than normal, I'm not sure if it's actually dead or there's something else causing it to behave like that. Any ideas?

Title: Re: CPC 664 - Slow HSYNC Signal
Post by: Bryce on 19:27, 05 January 21
Hi Socali,     
      my first thoguht would be that the CTRC is defect, but... What did you use to measure the frequency of Pin 39? Inexperienced scope users often end up measuring harmonics or alias frequencies. I'm not saying that you did, but how sure are you that you really measured the correct signal?

Title: Re: CPC 664 - Slow HSYNC Signal
Post by: Socali on 21:09, 05 January 21
Hi Bryce,
Thank you for your prompt reply!
I am actually pretty rusty with measuring electronics. Last time I used oscilloscope was in school a few years after I got my CPC, so I appreciate your help.

I got the YEAPOOK ADS5012h scope (I know, not the best, but I figured will be good enough for the task). I set the probe and the scope to 1X and AC, connected the black wire to the disk drive ground and touched with the probe the different pins. I then pressed the Auto button for it to zoom to the right display. Most of the pins look very much active like what llopis had on his youtube video fixing CPC 6128.
Attached are the measurements of the Z80 4Mhz clock and CRTC Pin 39 4.25Khz from my scope.
Please let me know if I should measure anything else, or do anything differently.

Attachments Displayed Below Message Body

Title: Re: CPC 664 - Slow HSYNC Signal
Post by: Bryce on 23:32, 05 January 21
Sounds good, but one tip: You should always use the 10x setting (I assume you have probes that can switch). This will give you much cleaner and accurate measurements. 1x seriously limits the bandwidth. I've never heard of YEAPOOK, must check it out.

Title: Re: CPC 664 - Slow HSYNC Signal
Post by: Socali on 05:52, 06 January 21
I just tested again with 10x settings on the scope and probe, still same results, just a bit more squarish waves.
I ordered new CRTC, DIP socket and some things to help with desoldering. Will let you know what happened when I get it all in place.

This scope is pretty cool for hobby users. It's fully handheld and portable (with rechargeable battery), has beautiful screen and reacts fast to buttons and measurements. The best thing is the price - something I could justify paying for fixing a 35 year old computer. :)  Only thing I wish I could do is take the saved screenshots out of it via USB.
You can find it under several different "brands" if you look for ADS5012H, the YEAPOOK one was the most popular on Amazon US so I got that one.

Thanks again for your help. I've been reading through a lot of material on the wiki and the forum, and the amount of knowledge you share and time you spend is very much appreciated!

Title: Re: CPC 664 - Slow HSYNC Signal
Post by: Bryce on 10:23, 06 January 21
I would highly recommend you use this method to remove the CRTC: http://www.cpcwiki.eu/index.php/IC_Repair (http://www.cpcwiki.eu/index.php/IC_Repair)

Trying to remove the IC in one piece pretty much guarantees that you will damage the PCB unless you have many many years of experience.


Edit: Just checked out your Scope. I know it as the FNIRSI ADS5012H. It has a few serious drawbacks, so you might want to watch this video before trusting anything you measure with it:

Title: Re: CPC 664 - Slow HSYNC Signal
Post by: Socali on 05:19, 11 January 21
So I got a desoldering iron with manual pump and some fast chip alloy. I worked for over an hour to carefully desolder the CRTC without damaging the board. First time ever I'm using these tools and I was super happy with the results. Everything came out nice and clean.

Then I soldered a 40pin socket, put the new CRTC in and turned on the computer. Measuring Pin 39 - and I see about 6 Khz signal. A little higher than the 4 Khz before, but still not enough. I was almost in tears.. I spent a lot of time and money to make this thing work, and nothing. I was sure that since quite a few PCB traces look like they're corroding from the inside (dark stains), I'm out of luck. :'(

Although I'm not using the oscilloscope to measure anything above 40 Mhz or using any special technique, your advice about trusting it made decided to plugin to the SCART-HDMI converter again and check. RetroTink showed nothing as expected. Then I used the generic black metal converter and something interesting happened! It detected signal this time. But it was all black screen with some random white dots jumping around (kind of like when UHF TV didn't get any signal, but not as much).
I started measuring everything again, everything measures pretty much as before, except for CRTC pin 39. I was done.

Now, when I ordered the CRTC I figured it wouldn't hurt to get a new Z80A while I can, just in case it'll die at some point later. So, I decided to pop it in and at least measure a few pins to make sure I got a working one. I put it in, turn it on, and I could not believe my eyes! The beloved blue Amstrad screen showed up!! :o
I measured pin 39 out of curiosity, and sure enough it's at 15 Khz this time!
Picture quality was kinda lousy, but I was so happy. A complete U-Turn from just a few minutes ago! I then plugged in the RetroTink instead and I was in heaven! Clear and steady picture, just like I remembered it. ;D

Now, I don't know if it was the CRTC, the Z80A, or both of them, and as much as I'm curious to replace them back and see, I do not dare touching anything while it's working..

Thank you again for everyone posting in the wiki and the forums! I could not have done this without this amazing community.