Help!! Imac CRT Screen wobbling

The first of these is power supply. Ah, but which? A Snow iMac 500 gave me a nasty time for a month last year, even to the extent that I left it off while I digested the nasty taste before a CRT replacement (ie, never). The display had been jittering randomly, and narrowing by up to 5mm at intervals. Then phhht! and blackness.

Having forced myself into making a systematic exploration, I broke and remade the power board AC connection. No startup. I broke and remade the mains-to-Mac cable connection. Boing! Several days later, the same jitter, ending in phhht! and blackness. It was one of the rainbow cables, with what I take to be a silicone-like casing. With time, the female plug end had softened and flowed enough to give unreliable contact with the male socket. I replaced the mains cable with a bog-ordinary black one, and 18mo. later there has been no repetition of the fault.

Pray that it is so with your sister's iMac, because the next suspect is incipient failure of the flyback transformer, and that is no joke in a CRT iMac.

de

My daughter's rev. A iMac is exhibiting this same goofiness, and that after I had resurrected the thing with a new flyback transformer. I was worried that there might be something else wrong with the power supply or analog board, but this at least gives me hope for an easy fix.

I tried the power connection cable at the back and thought that it helped.....until I started it up the next day and the problem returned.

It seems to only happen when the computer first starts up for the first 30 minutes or so.

Any ideas??????

Do you shut the iMac down between uses, or leave it in Sleep mode? If you use Shut Down, do you leave the mains (wall or powerboard) switch on? If you turn the mains switch off, how long is it since the backup battery was replaced? The machine (and the original battery, if still present) is now roughly four years old. Was the battery replaced at the same time as the HDD?

de

I don't know when the battery was last replaced, but the date and time are correct.

The date and time on Macs running newer Mac OS (8.5 and up) use a network time server to set the clock. I have several Macs here with dead PRAM batteries and they always display the correct date and time. If your iMac is connected to the Internet, it is probably setting its clock from an Apple time server, regardless as to whether the battery is charged or not.

Secondly, I believe that if you open your iMac up, you will discover that there is a monitor port inside. Plug an external monitor into that port and try your iMac with the external monitor.

If the problem persists on the external monitor, you can safely conclude that:

a) Your flyback transformer is just fine.
b) The CRT in your iMac is just wonderful.
c) Your problem lies elsewhere.

Give it a try and report back.

Sorry, the imac in question is a 350MHz Bondi with no vga socket to connect an external monitor to.

The imac is not currently connected to the internet so it isn't using the apple time server.

Oh, okay... I was thinking that you were using an upgraded Bondi Blue iMac of the first generation. I'll check my service material and see what it says.

EDIT:

Too bad your sister opted for the good option as opposed to the better and best options. I looked in the service manual, and it says that your USB problems are most likely due to power problems. Take a look at the block diagram below:

I would guess that your monitor problems also are the result of power problems, although the service manual does not expressly indicate this.

Here's what you should try, and in what order:

1) Reset the PRAM
2) Reset the PMU
3) Check voltages on the down converter board. Pins 2, 4, and 14, should read +12, +5, and +3.3 respectively. If the voltages don't match, replace the down converter board.
4) Reseat the CRT video board (Located at the back of the CRT)
5) Check Power/Analog board
6) Replace internal RGB cable
7) Check logic board.

That's what the Apple service manual says about the situation.

Good luck,

Victor E. Smine

I don't know if this helps, but when the Imac is shut down with the plug switched on, there is occasionally a noise comes from the speakers. It sounds like somebody is scrumpling up a piece of paper, kind of like "fffffffffffffffffft!".

What does this mean?

I've got a slotload 400 that does the same thing, but it doesn't do any thing else when powered up but turn the power LED green. I know the mobo and DCB from it are fine. I'm suspecting the sound may be coming from the DCB right behind the left hand speaker.

Hi Jon, thanks for the reply. You mentioned the DCB, I take it that is a down converter board?

If so, would that explain my monitor problems?

One thing I tried on my quest was adjusting the voltage on the thingy (sorry cant remember the name of it) that allows the altering of the focus and voltage,(black box thing)......Anyway, when i turn the voltage knob high, the screen switches off. Is this normal??

That'd be the flyback transformer, I'd guess. If adjusting it makes it just shut off, I'm guessing there is a short in it somewhere. Replacement flybacks aren't too pricey, about $25-$40, but doing the repair is quite involved.