Some dead treasure to repair - your help will be needed.

[Bignumbas]

I finally manged to get my "treasure" picked up I bought earlier this year for AU$100.
3 x CTM644 monitors a 6128 and 464 all apparently dead all filthy and been stored unprotected in an outside shed it looks like..
Well one CT644 fired up so that was a win.
However.....
I am working on the 6128 (MC0200B board) which does nothing, blank screen, no keyboard beep if you hold a key down.
The 464 does exactly the same thing mysteriously although nothing gets hot so I am thinking maybe someone use the wrong powerpak to test both of them long ago.
Both boards are in good condition although the 6128 has some slight moisture damage, a bit of green copper here and there which I have cleaned up.
Thus far I have swapped the Z80, the 40010, 40015, 40025, Pal and found all of them are working and were not damaged.
The memory was getting really warm, not burning hot, so I replaced all the ram anyway, it still gets a little warm.
The CRT.C - IC108 Chip seems to be getting warm to, and the protection? relay has tripped a few times.
I did see a green screen flash on momentarily when I first swapped the Z80 but have not been able to repeat this..
I only have a logic probe and MMeter, no fancy stuff here, I do work on gut feelings plus the odd chicken sacrifice.


Your thoughts would be appreciated

[Bryce]

Have you cleaned all the port pins thoroughly? Checked for dry joints? What voltage is the 5V rail at?

Bryce.

[Bignumbas]

Yes and yes.
I have magnifying glasses for looking over boards for cracks/dry joints etc.

I forgot to say that the power led lights up when switched on.
The CRT and ram chips do not warm up on my other CPC's.
The CRC outputs sync.
I have replaced the ram??
The power supply I am using is a Cisco 12v/5v which works fine on my other CPC's.
The Amstrad troubleshooting flow chart suggests faulty ULA(tested ok), Z80A(ok), 6845(?, warm), Ram(ok) or Crystal(?)
The 6845 is the warm CRT chip.
I think I will try that next and the crystal, I love desoldering very therapeutic......

[Bignumbas]

I have checked the 74ls153 chips and compared the results to a functioning 6128.
It would appear that 2 of them are not workinging like the others.
Don't know it this is because of the 6845 not working properly so I am going
to desolder and test them, I have also ordered some new ones.

[Bignumbas]

Now I have 3 boards with a 40 pin socket where the 6845 crt chip was.
I used a known working one to try in the 2 dead cpc's

I now have 2 dead 6845 after testing them in a working cpc.
Now I have a 464 and a 6128 both with the same problem, a grey box on screen, success!
Thats progress, one hurdle at a time.
I also found that the 464 had a dead Z80 cpu as well.
Now I have to wait for chips to arrive on the slow boat from China.
I took out the 40015 in the 6128 and it still stops on the grey box.
I also have a fully working CTM644 now, it only had a broken video cable which I swapped out, just 2 more to fix.
And I discovered that one of the monitors was a CTM640 when I was wondering why it did not have a 12v socket, duh!
I am up to 470 chip legs unsoldered and resoldered now   

[Bryce]

Luckily the 6845 was so extensively used that they are still very easy to source even today.

Bryce.

[Bignumbas]

Yes and cheap to.
AU $9 for 5 x z80 and the same for 5 x 6845

[Bryce]

Just be careful which version of the 6845 you buy.

Bryce.

[Bignumbas]

Well I have ordered  68B45S which according to the datasheet is a 2Mhz capable chip and the the 6845S is a 1Mhz.
Apart from that identical.
What do you think Bryce?

[robcfg]

Which brand? Because the difference in behaviour comes from each manufacturer.

If you search for CRTC on the wiki, you'll learn of the different types of them.


Sent from my iPhone using Tapatalk

[Bignumbas]

Hitachi


http://pdf1.alldatasheet.com/datasheet-pdf/view/139476/HITACHI/HD6845SP.html

[Bryce]

Hitachi is a type 0. you can check out all the details, advantages/disadvantages of each type here: http://www.cpcwiki.eu/index.php/CRTC

Bryce.

[Bignumbas]

Ok, working on ctm644 monitor.
no 12v, no screen.
ic503 had 16v going in and 0v out so replaced ic503 still no 12v.
Looked at circuit only one cap c523 between ic503 and socket, checked cap - was behaving like a piece of wire, replace cap -have 12v out. 


Now for nothing on screen, check voltages given in manual .
ic401/402 all over the place.
ic502 good.
q405 ok, maybe, but 109v on collector
q404 bad, 109v on collector, should be 34v.


Should I replace q405?

[||C|-|E||]

Independently of the current problem of the board, for the screen you probably will need to replace all the caps and the thermal paste. Many things in there run very hot and the ESR increases quite a lot over the years. Moreover, the caps have an impact in the final image you will see. If you plan to use the screen for the years to come, I would go for high grade caps  On the other hand, it should be much easier to diagnose a board with new caps, at least you know what is not failing.  Maybe Bryce will kill me for recommending you to fix something that perhaps is not broken but... 

[Bryce]

Independently of the current problem of the board, for the screen you probably will need to replace all the caps and the thermal paste. Many things in there run very hot and the ESR increases quite a lot over the years. Moreover, the caps have an impact in the final image you will see. If you plan to use the screen for the years to come, I would go for high grade caps  On the other hand, it should be much easier to diagnose a board with new caps, at least you know what is not failing.  Maybe Bryce will kill me for recommending you to fix something that perhaps is not broken but... 

Nope, that's all sound advice. And it IS broken, so general cap replacement would be a good place to start.

Bryce.

[Bignumbas]

I am waiting on the slow boat from China for parts, Q404, Q405 & Q502.
Looks like I might  not need them.
I discovered these two articles today and this eevblog that almost perfectly describes my problem with both screens.
https://www.eevblog.com/forum/repair/amstrad-ctm-640-crt/
https://www.youtube.com/embed?listType=playlist&list=PLKDTtm8adgHmHN_2Tcc6R_MnRANVDZnYX


I can get 10 LA800 from aliexpress for $10
I do not have voltages past Q502 on the ctm640 so I will use that and see if the IC502 and 5v comes to to life.

[Bignumbas]

Slow boat from China arrived this week with HD68B45SP chips (2mhz version of 1mhz HD6845)
Tried one in working 464 all good.
Inserted into 6128 grey screen with black borders.
Inserted into 464  random screens/colours with characters or plain blue screen
See photos below.
The first photo is the 6128 which has new ram and hd6845, the ram is still getting warm, not really hot, just warm.
The other 5 photos are the 464 after switching it off and on, different each time. This needed a new z80 and HD6845..so far.
The screen is one of the three I bought  with the 2 CPC's. Still waiting for chips to arrive for the other two screens.

[llopis]

I know it's been over a year, but did you ever get anywhere with this?


I'm going through old threads reading up on cases similar to the ones I'm working on, so I'm curious how/if people resolved them.


Thanks!