fixing an iMac computer

Hi

My old iMac (manufactured in 1999) hasn't been used for some time. Yesterday when I turned it on, the monitor wasn't on. The power indicator light was still on, the keyboard gets recognised (the CAPS LOCK/NUM LOCK keys are still functional), the mouse gets detected (this a wireless optical mouse and the status light on the transmitter unit indicates 'Connected'). When I turned it on, I can hear the Apple's 'boot up' music, the sound of the CPU fan running and of the hard disk spinning. It's just that the screen isn't on.

It was an iMac and everything is integrated inside that big 'monitor' box. There is no plug for external monitor. I opened it up today, trying to spot some obvious problems, i.e. loose connections, leaked capacitors, but couldn't find any.

Does anyone have any ideas on how to fix it, or at least feed the monitor output of the iMac into a VGA monitor, or a TV (if the Mac has compositive video output) and check if the thing is still working?

Just a wild guess, but does the screen backlight work? If not then maybe the supply for that has died. Bad caps are the usual suspects there also.

Doesn't it use a CRT monitor?, it sounds like the computer part is OK, and the monitor is faulty.

Nigel Goodwin said:

Doesn't it use a CRT monitor?, it sounds like the computer part is OK, and the monitor is faulty.

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It uses a CRT monitor. However, I could not find any removable plug from the 'computer' part to the monitor - everything is soldered directly onto the circuit boards. Some plugs are removable, but those are for hard disks and motherboard components. The monitor and the motherboard are sharing the same power supply, and the monitor's power supply cable is removable. There is no plug for external monitor. The monitor backlight is also not on.

I find 2 pins connected to the monitor which I suspect to be the video output. I connected it to my TV. The TV then complained about "illegal video input" and switched to Auto mode (i.e. blank blue screen). Still, am I right to say that that it is possible to get VGA signal, or at least composite video signals, from the 'computer' part then feed it into a normal VGA monitor, or a TV's video input?

I suspect the supply to the monitor is not working and I am trying to fix it. I will post a picture soon.

It's EXTREMELY doubtful you can feed it to a normal TV, the spec is probably completely wrong. It may well be wrong for VGA as well?, is it colour?, in which case it needs more than two pins.

As it's a CRT there won't be a backlight, that's an LCD component only.

It's a color monitor. Wonder why the Mac doesn't have an option for an external VGA monitor?

mybad, missed the 1999 part. only current iMacs have LCD

mdanh2002 said:

It's a color monitor. Wonder why the Mac doesn't have an option for an external VGA monitor?

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Probably because it's not VGA compatible?.

mdanh2002 said:

It's a color monitor. Wonder why the Mac doesn't have an option for an external VGA monitor?

Click to expand...

hi,
You say its circa 1999, as a test, if you can get the use of an oscilloscope, it should be possible check the two video wires that you have identified.

The video waveform can be checked using the scope, if the waveform can be seen at least there's a good chance the video drivers are OK.

Some simple checks you can try, is the heater in the crt tube base lit, [bright glow],
can you hear the 'high pitch whistle' , coming from line transformer.
Does the 'brightness control' show any brightening of the screen.

Unless you are experienced engineer, you must take care when doing any measurements on a crt.

The most likely causes of failure are the eht power supplies, loss of crt heater drive, line flyback transformer and osc stages.

ericgibbs said:

hi,
You say its circa 1999, as a test, if you can get the use of an oscilloscope, it should be possible check the two video wires that you have identified.

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They can't be 'video wires', two is nowhere near enough, you need red, blue, green, and either one or two syncs (probably two for a monitor), plus ground of course.

Nigel Goodwin said:

They can't be 'video wires', two is nowhere near enough, you need red, blue, green, and either one or two syncs (probably two for a monitor), plus ground of course.

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The VGA interface requires those R, G, B, HSYNC, VSYNC, etc. wires but my TV video input requires only 2 wires.

mdanh2002 said:

The VGA interface requires those R, G, B, HSYNC, VSYNC, etc. wires but my TV video input requires only 2 wires.

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If we are talking TV, two wires, say Video signal and ground, then the Video signal will a be a 'composite' signal carrying video and sync information.

VGA and TV A/V are not compatable signals.

mdanh2002 said:

The VGA interface requires those R, G, B, HSYNC, VSYNC, etc. wires but my TV video input requires only 2 wires.

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Yes, but a TV isn't a monitor - a composite video signal is completely unusable for a computer colour monitor - resolution of a composite colour signal is pathetic!.