G3 AIO “Molar” issues - Flyback? Caps?

[Durosity]

Did you say the PSU was recapped?

Yes, fully recapped now.

What is it actually doing when the analog board is connected? Which rails are dropping down?

I'm gonna try and make a video of this.

My knowledge isn’t advanced either but perhaps you could start by using a multimeter in continuity mode to check any ceramic caps and diodes for shorts. Place the leads across each component in turn and see if it beeps (it shouldn’t).

Maybe map out the analog board beforehand by taking a photo, printing it out and marking off each component you’ve tested as you go along.

That's a good idea.. I never really thought about the ceramic caps or diodes. I've already taken pics that I'm gonna be sending to @3lectr1cPPC for their MacDAT database (along with a whole load of other stuff).. I've also documented every electrolytic cap in the PSU, Analogue Board, and Neckboard along with their measurements, so people know what replacements to buy.

Is it worth reflowing all the joints as another starting point if not done already?
Sorry, my knowledge is limited here too, but its always good to rule out the easy things.

Yeah done that, although I may go over it and do it again to be sure I didn't miss any!

Perseverance is the main key.

To an extent.. mental energy for it has dipped!

analog boards and power supplies are hell to work on (at least from my perspective, as a bit of an electronics noob), unless you are really accustomed to how they actually work and have some of that core electrical engineering skillset - I attended electronics class at school but I wouldn't say I remember a lot of it, and I've never properly sat down and spent the time to learn the skill in a serious way.

Yeah.. I mean.. I learned a fair bit about this back In electronics class at school.. but that was in 1993.. It's something I've always wanted to learn more about and frankly I have actually learned a great deal so far whilst doing all this, so that is at least a nice silver lining!

Also, having no schematic available doesn't help matters at all...

Yeah.. I actually contacted LG to see if they happened to have anything, the agent I spoke to said they get quite a few requests for that kind of older information, but alas it's not something they have access to (and he's not even sure if it still exists). I fully believe that all electronic equipment should be required to come with a full schematic by law.

things can get spicy...I know everyone knows this, but please stay safe when doing the poking.

Oh my yes. I'm being super super careful!

From what I've been reading, that's also why the flybacks fail, apparently the lower quality LG made flybacks when overheating, will cook the lacquer off the windings, which then causes the internal arcing which presents itself as unpleasant "zapping", which causes the image to distort as it zaps.

If the internal shorts are bad enough, that can cause the situation to become worse, to the point that the machine won't power on at all, shutting off the second the flyback receives power and immediately shorts itself out.
That's the case for iMac G3s, but also the Molar Mac, which has a strongly related CRT chassis from what I understand.

Yeah it's a brand new flyback, although to be fair I have no way to know that it isn't also defective. However that was useful info, I knew that FBTs failed, but I didn't know why so that's actually quite illuminating!

This is where I would start from, maybe seeing what other stuff is connected to R726.

Yeah I might move back to that, see what it finds

Did you use a known good flyback?

Well brand new but it's impossible to say if its not also defective.. though seems unlikely 2 would have failed in exactly the same way.

That does make this a more difficult to solve issue, I fear.

Alas yeah.. that's the problem. I'm pretty certain that someone who's experienced in CRT repair woulda worked this out in 5 mins.. but alas there's so few of them around these days.. If only I had a Time Machine..

 

[3lectr1cPPC]

That's a good idea.. I never really thought about the ceramic caps or diodes. I've already taken pics that I'm gonna be sending to @3lectr1cPPC for their MacDAT database (along with a whole load of other stuff).. I've also documented every electrolytic cap in the PSU, Analogue Board, and Neckboard along with their measurements, so people know what replacements to buy.

I can’t wait!

 

[falecore]

Any updates on this? Currently in Molar Mac hell! Did you find out which output transistor to buy?

 

[Durosity]

Any updates on this? Currently in Molar Mac hell! Did you find out which output transistor to buy?

Unfortunately other life issues got in the way so the project is on the back burner. What’s happening with yours?

 

[falecore]

Unfortunately other life issues got in the way so the project is on the back burner. What’s happening with yours?

Powers on, no LED, no chime. Replaced LB an reflowed AB and PSU with no effect, now in process of replacing caps on AB. Also, the laser diode and resistors at L710 on AB look suspect. Was going to buy replacement flyback but also unsure of which output transistor to choose.

 

[computercloset]

Alas I’m not having much luck with getting this working. One of the resistors on the analogue board gets super hot (R726), but I’ve not been able to work out why. I disconnected it and that then burned out R788 but replacing that didn’t help either. I’ve replaced the flyback, the C5404 transistor that’s known to fail, recapped most of the power and analogue board (I tested each cap and replaced all but the ones that tested 100% which was like 85% of the board. Unfortunately that’s about as far as my current knowledge takes me and any requests for assistance elsewhere has been unfruitful.

Anyone have any ideas of next steps?

I am having the exact same issues states here. R726 is super hot that the board looks burnt, and R788 is completely burnt out. I’m not sure how I go about finding the values of these since the markings are no longer legible. Any suggestions? It let out the magic smoke after I moved the SW701 switch to the upright position (can be seen to the right of the flyback in photos). It wasn’t giving a picture before I did this either though. I’d really like to get this machine up and running! I have not replaced anything yet.

 

[obsolete]

I don't have any more ideas on how to troubleshoot this, but I have a partially disassembled AIO that I can get those resistor values from for you. Do you have any idea what SW701 is supposed to do?

Edit: Found this in the 5400 service manual. The board must be close enough to the same that some of the component labels match up:

Switch SW701 has three positions. You can control the horizontal centering of the display either by using SW701 together with the horizontal shift control (VR701) or by using VR701 alone.

Based on that, I don't think that moving SW701 was related to burning up those resistors.

 

[falecore]

Here you go.

 

[obsolete]

Hey, you beat me to it. Thanks. So R726 is 3.3Ω, 5%, and R788 looks like 75Ω, 5% (unless that third band is brown instead of black, then it's 750Ω).