Macintosh Classic 9" monitor has got some curves - Page 1
Forum section: electronics / general

author: kevinateev
author_rank: Regular Contributor
timestamp: June 04, 2023, 08:34:42 pm
content: Hi all, I got this Macintosh Classic fitted with a PS2/ADB adapter and it is ready to go as word processor for the kids.The monitor behaves strangely though. On warm-up, it has a 1/2 hourglass shape to it. I re-capped the main board (they were all bad) .. I'm concerned I have to re-cap the monitor too. Any thoughts on what causes this? It is the only tube in the house these days.I hate to think that using it this way is causing damage.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 04, 2023, 09:13:20 pm
content: This is some kind of pincushion distortion on the CRT, a solder joint problem is not unlikely (especially if the image is ok at power-on but starts to distort when it warms up), or possibly a dried cap.You should inspect the analog board for either.Be extra careful when working with high-voltage stuff.

author: cruff
author_rank: Regular Contributor
timestamp: June 04, 2023, 11:32:43 pm
content: Just tell the kids that's the way it was in the old days?

author: AndyBeez
author_rank: Frequent Contributor
timestamp: June 05, 2023, 09:20:31 am
content: Although the monitor is horizontally pinching, the focus is spot on. So the coils/yoke are still in their factory geometryI was wondering if the horizontal size pot just need a harsh tweak as these pots often oxidise and dust over time.Similar too:https://archive.org/details/classic_ii_performa_200/page/n103/mode/2upOtherwise it's somethingelseon the power/sweep board. Maybe recapping might solve this.

author: Benta
author_rank: Super Contributor
timestamp: June 05, 2023, 07:31:59 pm
content: Never worked with a 9" Mac monitor myself.Based on TV experience, resoldering the whole high-voltage section usually helps. The components there are heavy, and there's a lot of thermal stress, which tend to crack the solder joints.Old solder away, fresh eurectic solder instead, and it suddenly works. If it doesn't, this job needed to be done anyway.CAREFUL: discharge the PSU caps AND the CRT and leave the unit for a day before working on it. Failing to do this is very painful and potentially lethal.

author: AndyBeez
author_rank: Frequent Contributor
timestamp: June 05, 2023, 08:25:12 pm
content: Quote from: Benta on June 05, 2023, 07:31:59 pmI agree. The Mac must be like a sauna on the inside.Reflowing has worked for me in the past.Look for hidden cracking, especially on the legs of parts with heatsinks, such as the power ICs.I should add that vertical or horizontal non-linearity does tend to point towards those pesky caps.btw I noted this from page 54 of the above linked service guide. Speaks for itself

author: kevinateev
author_rank: Regular Contributor
timestamp: June 05, 2023, 10:34:56 pm
content: Ok, thanks y'all for the input. I was hopeful that there was some capacitor re-forming going on but it seems to go through the same process every time I turn it on. Hourglass and then eventually full screen.Reflow-solder, and inspect/replace caps. I'll report the progress when done. I'd already cleaned the pots.

author: Benta
author_rank: Super Contributor
timestamp: June 05, 2023, 10:49:42 pm
content: Quote from: kevinateev on June 05, 2023, 10:34:56 pmReflow the joints can work, resolder with good eutectic is better.Do you know how to discharge the CRT?

author: kevinateev
author_rank: Regular Contributor
timestamp: June 05, 2023, 10:54:40 pm
content: Yah man. Have done before, old enough. Had CRT in pieces in my room in high-school under regular use, skipping school to watch the space shipColumbiaChallenger blow up in full Back and White.Don't think I've touched a CRT's innards since thenHave high tension screw driver and a wire ... SNAP! Not elegant but a little welt is no big deal.Found this BTW ...https://vintageapple.org/gamba2/images/plus_analog.PDF

author: Benta
author_rank: Super Contributor
timestamp: June 05, 2023, 11:05:46 pm
content: Yep, way to go.But I had to ask.

author: james_s
author_rank: Super Contributor
timestamp: June 05, 2023, 11:53:23 pm
content: Wow that's a weird one, I've worked on loads of these and never saw that.If you haven't re-capped the analog board though you should do that, or at least test it, CRT monitors are harsh enviroments for capacitors. Working on them is no big deal though, especially these piddly little 9" monochrome tubes. The EHT is only about 10kV and the tube holds very little charge, it's just a little static zap. Slip a grounded screwdriver under the anode cap to discharge it.

author: helius
author_rank: Super Contributor
timestamp: June 18, 2023, 02:26:25 pm
content: The FBT on this model has a bleed resistor, so it is not actually necessary.The large reservoir capacitors in the power supply (which is a separate unit from the analog board) do carry lethal charge, but this CRT is not a safety issue at all.

author: james_s
author_rank: Super Contributor
timestamp: June 18, 2023, 05:55:18 pm
content: Quote from: helius on June 18, 2023, 02:26:25 pmBleed resistors can fail, it's *always* a good idea to discharge a CRT, it's just a good habit to have. They will often pick up some charge just sitting there disconnected too so I usually keep a grounded clip connected to the anode terminal so I don't get any surprises. It's unlikely to be directly harmful but it could easily cause you to drop the tube or drop something on it and cause it to implode or at the very least be ruined.

author: helius
author_rank: Super Contributor
timestamp: June 18, 2023, 06:49:17 pm
content: I don't agree with *always*. Most adjustments or tests do not necessitate removing the anode cap in the first place.

author: james_s
author_rank: Super Contributor
timestamp: June 19, 2023, 04:56:06 pm
content: Quote from: helius on June 18, 2023, 06:49:17 pmI didn't think I needed to specify "when necessary to disconnect the anode", that was implied. If you don't need to mess with the anode then you don't need to discharge it.