iMac G3 service and rebuild (with MacEffects inner bezel)

Hi all,

I've had my late nan's indigo iMac G3 DV sat disassembled on my office floor for the past 4 years. Like many who have tried to service these old machines, I had the extremely brittle inner bezel crack into a million pieces during disassembly.

I managed to catch MacEffects' Kickstarter 18 months or so ago, and my new replacement inner bezel arrived yesterday!

Now, since so much time has passed, I don't even really remember the condition of this Mac I have in pieces. I know I suspected some potential flyback weirdness, so I picked up a second machine that's in pretty rough cosmetic condition to use as a spares machine. That one died after a few hours of use (no signs of life at all), but hopefully it'll serve as a standby should I need to borrow any parts for my main machine.

Anyway, I suppose I'm posting just to record my adventure! And to be open to any tips or suggestions that might help along the way should I hit any problems (I'm sure I will!).

So, at the moment, the state of the machine is:

• Fully disassembled.
• All plastics cleaned and polished.
• CRT lightly dusted and put somewhere safely away from any chance of accidental knocks.
• Analog, down-converter and logic boards cleaned and inspected. No visible causes for concern, but I reflowed the bigger pads on the analog board around any hot-spots just in case.
• No apparent signs of cap leakage on any of the three boards. No bulging caps.
• No battery leakage. Removed and will be replaced with one of those cool little button-cell replacements.
• CRT neck-board looks ok at a glance, but I've not yet removed it from its aluminium shell.
• Fuses test OK, caps will be tested in-circuit soon (not ideal, I know, but at least I can tick some off as being good)
• Flyback, HV cable and anode cap all in physically good condition -- no external signs of arcing, heat damage.

Very much hoping I can bring this back to life as it has a fair bit of sentimental value. There weren't many septuagenarians using their own iMac to do their primitive early-2000s online banking and ordering Christmas presents for the family from Amazon back when it only sold books. She loved her computer and was so excited when we got our first cable internet service (a blistering half-megabit) and no longer needed to tie up the phone line! It came with OS9 and 10.0.3 dual-boot. Nan used OS9 whilst I often rebooted into X -- painful though it was, it was new and exciting.

Anyway, will keep this updated for anyone who's interested!

(edit: oh! I actually found an old thread I made on here when I first started the disassembly four years ago! https://tinkerdifferent.com/threads/imac-g3-dv-refurb.463/ -- I must've given up as soon as the bezel disintegrated.)

Right! Well, I've got it reassembled as far as needed to power it on. No way I'm putting that tricksy outer shell on until it's working 100%!

Results are:

• No pops, bangs or flames after connecting AC!
• Does make a high-pitched whine after supplying AC. The whine varies a bit before settling, but doesn't go away.
• On pressing the power button:
  • LED illuminates
  • Familiar clonk sound of the degauss ring
  • Staticky noise heard and... the CRT comes up with the Finder/Question Mark icon! Excellent!
  • The display's got a significant yellow gradient, lighter on the left and quite yellow indeed on the right.
  • No weird whistles, noises, crackles!
• On power-off, the high-pitched ascending whine starts again, just like when AC was originally connected.

Interestingly, it won't fully start up again without having power completely disconnected for a few minutes. Until that's done, the power LED will turn on and it'll sound like the CRT's started up (static noise, degauss thud), but no picture appears.

So.... I think this feels caps-y to me. Maybe on the down-converter board?

(there's so many capacitors on this machine -- I'd like to at least narrow it down to a board before I start pulling and testing caps!)

As for the weird tint... any ideas appreciated!

Cheers,
Chris

(photo of the discolouration for reference)

Seems the tint problem isn't uncommon. For example, https://discussions.apple.com/thread/8000298?sortBy=rank -- which looks identical to mine.

I've had my iMac sat running for a few hours and it does develop very quiet 'crackle' sounds. Initially, I thought, "uh-oh... flyback..." But the sound is coming from either somewhere on the assorted gubbins around the back of the tube or the neck-board itself. The image remains completely stable.

The flyback itself is just making a very quiet continuous buzz, which I think is healthy.

Also, I hooked up some headphones since the speaker pods are currently removed. Even when the display fails to appear after a power off/on (without unplugging AC), there's still a startup chime. So I'm thinking the down-converter board's probably OK.

I'm starting to suspect the neck-board, which might be the source of these issues:

• Video doesn't start after a power cycle -- only starts when AC's been disconnected for a while.
• Odd crackling noises from that general area
• Screen discolouration -- maybe weakness in driving one of the guns?

This still doesn't perhaps explain the ascending whine when AC's first connected (assuming the neck-board isn't powered until the power button is pressed?), so I still wouldn't be surprised if the down-converter or PAV board are also struggling.

I found some green crustiness on the little EMI pad at the back of the divider frame, which is just under where the neck-board assembly sits. I hadn't realised the neck-board sat right there when I had the machine in bits, so I didn't associate the corrosion with potential capacitor leakage. But given their proximity, I wonder if a cap or two on the neck-board has let go.

If I've got the inclination today, I'll strip the thing back down (ugh), disconnect and disassemble the neck-board and see what mysteries it reveals. Desoldering gun and LCR meter at the ready!

Ok, didn't get to tear the thing down today to look at the neck-board (I believe its proper name is the IVAD board?).

But I thought for completeness I'd work through the service manual's voltage checks in both the 'working' (i.e. CRT came up) state and the 'faulted' (machine powered on, but no display) state:

iMac G3 DV diagnostics:

Faults:

• Ascending whine when AC applied, stops when machine powers up, returns when powered down.
• Yellow hue to CRT
• Quiet occasional crackling sound after warmed up around neck-board (IVAD)/yoke.
• After power cycle (via button), CRT does not start up again (although system chimes). Needs removal from AC for a few minutes to return.

Tests:

• Trickle power (J9 pin 1 on MLB) with unit off, AC connected)
  • Looks like high inrush ~20V?
  • Settles at 4.88V (OK)
  • Holds for ~8 sec after power removed, falls quickly after.
  • Drops to mV when system started (whine stops), returns to 4.88 when turned off (whine resumes).
• Result: OK? (+5 a little low? Plus that whine certainly seems to be trickle-power related - goes away when trickle shuts off.)

• Down-Converter On (DCO) Voltage (C10 of MLB, pad closest to GPU)
  • -1.1V when AC connected, system off
  • +25V when system started
  • Returns to -1.1V when system off
• Result: OK.

• J7 Down-Converter/MLB Connector Voltages
  • Pin 2 (12V): 12.02V - OK
  • Pin 4 (5V): 4.96V - OK
  • Pin 14 (3.3V): 3.29V - OK
• Result: OK. Voltages correct when booted normally and when faulted with no display.

Not all that surprising really, I suppose! The machine never fails to chime even when the CRT won't start.

I did find this section from the service manual interesting:

Power is controlled in the iMac system by the power/analog board. Once the unit is plugged in, power flows from the AC outlet at the wall to the AC inlet on the power/analog board. From there, the power flows through the main cable on the power/analog board, to the video neck board, down converter board, logic board, and all its attached components

Click to expand...

...which I'm interpreting as power going this route: PAV -> IVAD -> DCO -> MLB. Which again makes me suspicious of the IVAD (neck-board), since it could be implicated in all the problems: CRT not starting sometimes, trickle-power whine, discoloured display.

Anyway -- I think I've done all the poking I can do -- I just need to get that IVAD off and disassembled!