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Old 15 December 2020, 17:03   #1
Retrorepair
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M1438 Pictures

Hi all,

Would anyone with one of these monitors be willing to pop it open to take some decent pictures of the chassis?

I have a 31khz only model here but am 100% certain it's a M1438 that's missing a bunch of components (even the mounts for the speakers are there and the PCB has a position for the missing volume and audio in sockets).

If anyone was kind enough to list the component values vs their PCB designations it would be amazing.

I contacted Microvitec but unfortunately, even they don't have schematics anymore, they were apparently all lost in a fire years ago. No one who now works there were around back in the day either so this information is lost to time now.

I found some pictures online which help to identify a few of the absent components but not many. I also wonder if some of what's installed are different values on the multisync models..

Anyway, if anyone can help I'd very, very VERY much appreciate it
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Old 15 December 2020, 20:20   #2
jbenam
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I do actually have two of them!

One of them is faulty and turns purple after a short while. I have tried to diagnose this issue for years - it seems thermal related. I have resoldered literally everything on the neck PCB to no avail.

Anyway, let me know which kind of pics do you need and I'll crack one of mine open. I have two different models. They are both M1438s. If you can post the picture of yours, I can open the respective model.

I was actually thinking of adding the speakers to mine and upgrade it to a M1438S...

BTW, if I do forget to watch this thread, feel free to send me a PM as a reminder!
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Old 16 December 2020, 10:10   #3
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Hi jbenam, excellent news

I'll take some pictures this evening, they should indicate which components are missing on mine. I do wonder if some of the shared components will be different values but I think identifying those which are missing would help a lot.

I do actually repair CRTs so would be happy to take a look at your purple chassis since the tube in mine is likely compatible. Could kill two birds with one stone

It sounds like dry joints or a failing colour driver (green). At worst it's the tube but I can let you know if you want

Thank you for the offer anyway, it's very much appreciated!

*EDIT* If you are thinking of adding speakers I think you will need to also add the amplifier circuit unless your models have these populated. Either that or add a small amplifier inside the case. You will also need to drill holes for the audio input jacks but (on mine at least) there is a guide where to cut for these, as well as the volume control. I plan to 3D print a volume knob for mine too

Last edited by Retrorepair; 16 December 2020 at 15:51.
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Old 16 December 2020, 16:01   #4
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Great stuff!

I have literally resoldered everything as I suspected some foul play with dry joints as well, but unless the issue is on the bottom PCB, I have literally reflowed everything, both with hot air and manually and nothing changed.

I have also tried with a freezing spray, nothing changes - so it doesn't seem to be thermal related, even if it starts after a while. If I leave it turned off for a night, it works for a minute or so before turning purple. Maybe it's dry caps?

I have a bunch of tools except for an Oscilloscope (I should buy one soonish), recently also got an ESR meter, so testing should not be an issue.

As long as the PCB has the spaces I just need to populate, it sounds doable
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Old 17 December 2020, 17:10   #5
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When you say bottom PCB do you mean the main board? I'd say that's likely the culprit yes. Old caps could be the problem, it can't hurt to replace 25 year old caps anyway

It's hard to tell without it in front of me but happy to help trouble shoot any way I can.

I will actually get the pictures tonight !
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Old 17 December 2020, 22:00   #6
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Pictures!

Interestingly, even though this model is 31khz only, the pots to adjust 15khz geometry still exist!

I have a feeling the forum has killed the resolution so I may find somewhere to upload them externally if so.
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Old 18 December 2020, 07:59   #7
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Quote:
Originally Posted by Retrorepair View Post
I have a feeling the forum has killed the resolution
Open an image in a new tab or save it, and you'll see it in the full resolution.
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Old 18 December 2020, 08:09   #8
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I was concerned due to the file size but they look fine on inspection
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Old 18 December 2020, 14:19   #9
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Thanks, I did not think that the issue could be originating in the bottom PCB. I'll purchase a bunch of caps and recap everything, then.

I will try to take some pics during the weekend.
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Old 18 December 2020, 17:41   #10
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It could still be a driver on the neck board but I'd go after dry joints on the bottom PCB where the ribbon cable connects the two or around any transformers/power resistors.

Great thanks, I look forward to the pics
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Old 27 December 2020, 13:19   #11
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I've found some photos of a multisync chassis online which has given me some idea of what to change on my 31khz only chassis, though there could easily be more that I've missed or were not obvious from the pictures.

I think mainly I need the values of the large caps near the flyback. I think these are the E/W correction caps so currently would probably result in a very narrow image.

jbenam - If you can verify any of this it would be greatly appreciated.
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Old 27 December 2020, 18:10   #12
jbenam
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Hi,
I've got your pictures.

Also the caps near the flyback are:

C5 - 100uF 400V
C17 - 47uF 250V
C213 - 100uF 25V
C215 - 47uF 250V
C314 - 100uF 25V
C20 - 470uF 35V
C214 - 470uF 35V
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Last edited by jbenam; 21 March 2021 at 00:22.
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Old 27 December 2020, 18:15   #13
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More photos (max 8 per post, it seems).

If you need anything else, please let me know.
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Old 28 December 2020, 11:47   #14
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Great thanks for that, though it seems yours is a later model so most of it is pretty different.

It does seem the sync generation circuit is mostly the same though, any chance you could take a picture of the underside of the main board?

I did make some changes last night as per other pictures I've found online (anyone know how to get hold of the owner of http://ataricomputers.altervista.org ?)

All I seem to have achieved is a super wide image in 31khz mode. I'll keep plugging away, if I find the magic formula I'll report back.
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Old 28 December 2020, 12:13   #15
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Actually the value of the large blue caps would help, I think these relate to brightness in different modes. I can see the bigger one is 3.3uf but can't make the other big one out.

Thanks again for this, it's very much appreciated
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Old 28 December 2020, 17:31   #16
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Quote:
Originally Posted by Retrorepair View Post
Actually the value of the large blue caps would help, I think these relate to brightness in different modes. I can see the bigger one is 3.3uf but can't make the other big one out.

Thanks again for this, it's very much appreciated
You're welcome!

Whoops, I must have not noticed them last night!

C501 - 3u3 160v
C503 - 1u0 250v
C216 - 4n3 1600v
C500 - .015uF 1kV

Also, attached pics of the underside of the PCB.

EDIT: Please let me know if you need any more pics, as as soon as you are done I am going to close it up and see if I have fixed the missing green channel issue
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Last edited by jbenam; 28 December 2020 at 18:50.
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Old 12 January 2021, 00:58   #17
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Bookmarking this thread as I am hoping to recap my 1438 one of these days. Be great to pull in your expertise lads!
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Old 15 January 2021, 14:53   #18
Retrorepair
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Quote:
Originally Posted by jbenam View Post
You're welcome!

Whoops, I must have not noticed them last night!

C501 - 3u3 160v
C503 - 1u0 250v
C216 - 4n3 1600v
C500 - .015uF 1kV

Also, attached pics of the underside of the PCB.

EDIT: Please let me know if you need any more pics, as as soon as you are done I am going to close it up and see if I have fixed the missing green channel issue
Sorry mate, we had a baby between my last post and now so my attention has been elsewhere

I have made the modifications which appear to be all that differs between my model and the model found here:

http://ataricomputers.altervista.org...1438S_MOBO.jpg

However, both low and high resolutions seem to be twice as wide as the should be and therefore double up. The vertical size seems fine though.

I'm going to investigate a bit more but without seeing that exact models board and the values of the caps on that model, I don't know that I can complete this project. I'll go over again though to try and rule out any mistakes.
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Old 16 January 2021, 08:54   #19
jbenam
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Originally Posted by Retrorepair View Post
Sorry mate, we had a baby between my last post and now so my attention has been elsewhere

I have made the modifications which appear to be all that differs between my model and the model found here:

http://ataricomputers.altervista.org...1438S_MOBO.jpg

However, both low and high resolutions seem to be twice as wide as the should be and therefore double up. The vertical size seems fine though.

I'm going to investigate a bit more but without seeing that exact models board and the values of the caps on that model, I don't know that I can complete this project. I'll go over again though to try and rule out any mistakes.
Hi,

No worries and congrats!

My other M1438s (which is now my only one, since I gave the purplish one to a friend, since I couldn't keep it - too many CRTs!) is a slightly previous model, which might be the one you have got.

I will do a tube swap between the two 1438s in the next 2-3 weeks to see if the purple issue is a tube issue. I will sneak some PCB pics in and see if those help you.

Please let me know which part of the PCB you exactly need and I will do my best
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Old 16 January 2021, 10:13   #20
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That would be great as I believe mine is a slightly earlier model (date on the label).

I recon there's going to be a variance in the values of some of the components or maybe I've just missed something critical I couldn't see in the low res photo above.

And thank you, he's amazing, and tiring
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