iMac G3 Potentiometer

[Greg890]

Hi all,

I am replacing the VR 901 potentiometer on my iMac 400 Mhz DV in hopes of fixing the strange geometry on the CRT. When I took a reading of the potentiometer outside of the board, the total resistance of it is 380 ohms, with the divider set at 150 ohms and 220 ohms. However, the only potentiometers I found online was 500 ohms and 200 ohms. Is it possible that the resistance of the potentiometer degraded over time? If so, what would be a viable replacement and where should I set the new values on the potentiometer? Any advice would be greatly appreciated especially if someone has done this replacement before and happens to have the values they set their potentiometer at.

 

[MacTech68]

These potentiometers have two legs tied together, so the actual originally set value is somewhat different.

IIRC they are 300 Ohm.

If you look at the tracks it is soldered to, you can work out which two pins are tied together - (which two is important for measuring the value).

When replacing these I would warm it up (hair dryer or soldering iron) and then measure the value, ensuring that you short the correct two pins together, AND avoid touching the pins or probe ends with your fingers.

Of course, this pot adjusts the B+ rail, but I've yet to see a definitive answer as to what the B+ should actually be.

 

[Greg890]

Hi, thanks for your response MacTech68.

I warmed up the potentiometer and received similar readings, so I took an educated guess and set the pot to 200 ohms with the two pins tied together. However, when booting back up with the iMac put back together, no video appeared

When I tried to debug the issue, I noticed that the board that plugs directly into the neck of the CRT is extremely loose and hard to tighten fully. I'm not sure if it is 100% on the CRT, so I've added images.

https://imgur.com/a/dOoe3u1

The logic board of the iMac works perfectly when the external VGA port is connected, however the screen does not show any sign of life. When the machine powers off, I can hear static noises, which I assume is the CRT discharging. If any one has any ideas on how I could repair this, it would be greatly appreciated.

 

[MacTech68]

They usually do have a slight ability to be wiggled, and as far as I can see, it looks to be fitted correctly.

However, as difficult as it might be, it could be worth checking that the pins on the back of the tube are not bent.

Also, the RGB cable that is screwed into the motherboard connector, must use the screws with the partially non-threaded shaft. These screws should have a dot stamped into the head if they are the correct (revised) type.
They are also used towards the front for the Audio/DownConverter connector.

You may also want this if you don't have it already:

http://tim.id.au/laptops/apple/imac/imacdv.pdf