Apple Color Display M1212 - No picture after recap

[makerorama]

Hi,

A while ago I acquired a Color Display M1212 in pretty decent cosmetic condition. Unfortunately, the display's image was not always very sharp and would occasionally flicker or turn on and off by itself. The degaussing was also very loud when powering on the monitor. Since I have some experience reparing vintage power supplies and have successfully recapped all my vintage macs before, I decided I would try to recap the CRT control board in an attempt to fix the issue.

After replacing all electrolytic capacitors with high quality Nichicon and Rubicon replacements from Mouser, I am unable to see any picture showing up on the CRT. The CRT does turn on (LED goes green), but I see no picture and also no static on the screen and no visible glow from the "neck" of the CRT tube.

After doing some serious ChatGPT-ing, I was able to isolate the problem to faulty or missing Horizontal output. According to sources in the web, the monitor will refuse to turn on if it doesn't receive any Horizontal output signal. I am not a CRT expert and cannot be sure that this is accurate, so after days of getting nowhere I am requesting the support from the community to try and solve the issue.

What I have verified so far:

- All capacitors are of right value and have been placed in the right orientation
- I checked the soldering joints are solid by verifying continuity (I used this schematics: https://usermanual.wiki/Document/AppleM1212MonitorSchematic.2035905086/view)
- I checked voltage at the H.O.T and got 118V at the Colector and 0V at the Base (according to sources, Base should not be 0V)
- I am hooking up the monitor to a known good machine

Also, I found that there is a 4.7KOhm resistor soldered to PIN 39 of the Jungle IC and leading up to jumper (see picture below). I have another one of these monitors, which I opened up and oddly I don't see it on the other monitor. Im not sure this is stock and was probably added during some repair work in the past?

Any help saving this monitor or a tip that could lead me in the right direction would be very much appreciated.




Capacitor list: https://tinkerdifferent.com/resources/macintosh-color-display-m1212-capacitor-list-1-645-346-13.25/
Schematics: https://usermanual.wiki/Document/AppleM1212MonitorSchematic.2035905086/view

 

[croissantking]

It almost sounds like a dry solder joint somewhere that was marginal before the recap and is now completely broken.

 

[bibilit]

I can see something strange near RV33, in the bottom of the board, probably a ripped solder joint (solder tip is missing, you can see the board underneath) (new cap fitted ?) and maybe a broken solder as well.

 

[croissantking]

I can see something strange near RV33, in the bottom of the board, probably a ripped solder joint (solder tip is missing, you can see the board underneath) (new cap fitted ?) and maybe a broken solder as well.

Can’t see what you’re referring to, please could you indicate it on the photo?

Edit: oh yes. I assume OP has scraped back a bit of the trace and soldered the lead to it?

 

[croissantking]

These joints look like they could use a reflow.

 

[makerorama]

Can’t see what you’re referring to, please could you indicate it on the photo?

Edit: oh yes. I assume OP has scraped back a bit of the trace and soldered the lead to it?

View attachment 96403

Hi,

Thanks so much for looking into this, and yes you are correct. This contact was already broken when I opened up the monitor and despite me being very careful, it came completely lose during the recap. I did scrape a bit of the trace and soldered one leg of the new capacitor to it. I have payed careful attention to test the joint here and I get a very strong reaction when testing continuity from this point to the next component it traces into. Yesterday, I redid this a little bit and got even a better bond in order to prevent it from coming lose in the future. Nevertheless, I still get the black screen.

 

[jmacz]

Not sure if you noted the cap specs in your monitor as you removed the old ones or whether you just used the list from TinkerDifferent. I found that list from TD had an error in it. I noted it here. You might want to check.

Post in thread 'Wanted: Cap list for M1212 Macintosh Color Display'
https://68kmla.org/bb/threads/wanted-cap-list-for-m1212-macintosh-color-display.40953/post-499900

 

[Callan]

Something to watch for when you rebuilding / capping this monitor. There are 4 compression leafs (2 at the top, and 2 at the bottom). Their function is to tie the tube to the chassis ground to discharge the static from the tube. The leafs in the one I rebuilt were all broken due to metal fatigue. This needs to be fixed if yours has done the same. You can run a ground braid from the top left (or right) corner to the opposite bottom side. Then 'pull' the braid to the opposite bottom side (see pic). I used a spring to pull it across (pull it taught against the tube), but another piece of wire or a tie wrap will work. Tie a ground wire to the braid and use that as your ground point.
As for your picture issue. This monitor will not produce any high voltage without a video signal. Everything i'm recommending to check is based on the fact you stated it worked before you capped it. First thing I'd do is go through everything that was capped and check for broken traces at the pad. Depending on how you desoldered them it's easy to break the trace from the pad and not be able to see it. I'd also check for any possible solder shorts. It's easy for solder to be 'flicked' off of the tip. Just give it a good once over to be sure. I would also pay close attention to everything on the input side of 1y1. Also make sure you dont have any issues at connector 9v1 (your video input connector).
I did a compete cap list when I rebuilt my monitor. It's on my garage comp, so don't have it handy. I'll post it a little later if you want to compare it to the others