250 Miles for a Macintosh

[macq]

The title is a bit misleading, but I did travel 250 miles with a Macintosh! I picked up a Quadra and a ColorPlus display from Tim Colegrove (of The Byte Shop fame) in Boston and traveled back home to New Jersey. I took a bus from MA to NYC, hid the Quadra under my jacket, as my luggage policy only permitted one bag (which ended up being filled with clothes and the CRT). Once I made it to NYC, I lugged all 50 pounds on the subway, then to NJ Transit, then walked with it home when I got off the train.

I plugged them both in, and everything worked fine! I turned them off, went to eat dinner and came back an hour later. Tried to turn them on and there's next to no life from the monitor...

When I pressed the power button in, the green light came on for a second, but went out as soon as I let go of the button. I didn't hear anything from the CRT.

Where do I begin to repair this? I'm concerned about discharging a CRT this large, would this make it any safer? 112K ohms of resistance doesn't sound like its anywhere near enough to make it less scary. Could it just be an issue with the power switch or should I start by recapping?

 

[Phipli]

Where do I begin to repair this? I'm concerned about discharging a CRT this large, would this make it any safer? 112K ohms of resistance doesn't sound like its anywhere near enough to make it less scary. Could it just be an issue with the power switch or should I start by recapping?

That is designed for capacitors, which have much lower voltage. You'd want to do some sums and design your own for a tube, or do what most people do and use a wire with no resistor attached to the end of a screwdriver... but make sure you read detailed instructions on how, because these things can hold crazy voltages. A good starting point is to leave it unplugged for a couple of days before you take the case off. This doesn't guarantee that it is discharged (it should be, but if there is a fault condition, it might be still trying to hold a charge) - but it means that using a screwdriver to discharge the tube is just a safety precaution, rather than the thing you are depending on.

The screen might need recapping, or it might need more extensive repair - was the image twitching at all while you were using it? That can indicate a failing flyback converted.

Honest truth? I'm not a CRT fan - I recommend getting a Apple video to VGA adapter (get one with 10 switches for best results - not always needed, but saves buying twice - or ask @bigmessowires about the availability of his new fancy adapters) and use it with a VGA LCD, which you can generally get for free. Ideally go for something with a 1024x768 native resolution and a 4:3 aspect ration. They're small, use less electricity, sharper picture (at native resolution), generally easier to repair if they do fail (its usually a diode or capacitor on the power board, more of a pain if it is the backlight) and you can always find another easily.

I picked up a Quadra

Which Quadra?

 

[macq]

The screen might need recapping, or it might need more extensive repair - was the image twitching at all while you were using it? That can indicate a failing flyback converted.

Honest truth? I'm not a CRT fan - I recommend getting a Apple video to VGA adapter

No twitching when it was on, the only thing that made me nervous is that it felt like the power socket was a bit loose, I plugged it into the wall and was able to hear waves of high voltage if I pushed it a little too far in one direction or another.

I'm more interested in repairing in to learn more than to use it for the long run

You'd want to do some sums and design your own for a tube, or do what most people do and use a wire with no resistor attached to the end of a screwdriver...

How would that be any better than using the tool I linked? Even if I bought that to discharge high voltage caps, would it act similar to a screwdriver if I tried to use it to discharge the CRT (but with my hand farther away)? Or is there not enough insulation?

Also, I've heard about people doing that with compact Macs, but is it any more dangerous with larger CRTs like these?

Which Quadra?

A 610!

Thanks for your reply

 

[Phipli]

How would that be any better than using the tool I linked? Even if I bought that to discharge high voltage caps, would it act similar to a screwdriver if I tried to use it to discharge the CRT (but with my hand farther away)? Or is there not enough insulation?

Also, I've heard about people doing that with compact Macs, but is it any more dangerous with larger CRTs like these?

The resistor won't be rated to high enough power. The power dissipated is a function of the voltage, with a big cap, you might be saying 500V, while the tube might be about 20,000V or more.

Power = I^2.R, so...

Power = (V^2)/R... Your R was 112k...

With a cap, the resistor dissipates...

250,000/112,000= 2.23 Watts @ 500v

Put the same tool on a tube and...

400,000,000/112,000= 3,471 Watts @ 20,000V.

Which is about the maximum power you can get from a UK 240V mains power outlet. And 1,601 times more than the device was potentially designed for...

If you want to do it using a resistor, you'll need to calculate a correct one.

Someone check my sums.

 

[Phipli]

A 610!

Great machines. If you're comfortable with a soldering iron it is possible to get them up to 33 or even 40MHz.

Is yours a 20 or a 25MHz machine? Does it have built in AUI ethernet?

 

[macq]

The resistor won't be rated to high enough power. The power dissipated is a function of the voltage, with a big cap, you might be saying 500V, while the tube might be about 20,000V or more.

Power = I^2.R, so...

Power = (V^2)/R... Your R was 112k...

With a cap, the resistor dissipates...

250,000/112,000= 2.23 Watts @ 500v

Put the same tool on a tube and...

400,000,000/112,000= 3,471 Watts @ 20,000V.

Which is about the maximum power you can get from a UK 240V mains power outlet. And 1,601 times more than the device was potentially designed for...

If you want to do it using a resistor, you'll need to calculate a correct one.

Someone check my sums.

That makes sense..
I've read 50kV is the max for consumer CRTs, if 25W is a safe wattage:

25W = (50,000^2)V/R
25R = 2,500,000,000
R = 100,000,000

Is 100 megaohm resistors in series the safest option? Is there a risk of burning the first resistor? (sorry if this is a silly question)

Great machines. If you're comfortable with a soldering iron it is possible to get them up to 33 or even 40MHz.

Is yours a 20 or a 25MHz machine? Does it have built in AUI ethernet?

It's a 25MHz with AUI.

 

[Phipli]

Is 100 megaohm resistors in series the safest option? Is there a risk of burning the first resistor? (sorry if this is a silly question)

I'm not sure where you'd get such a resistor. I think reading up on how people normally discharge them is better and safer.

If nothing else, once you put a resistor in there you have to consider how quickly the charge dissipates. You wouldn't want to only half discharge 50kV.

It's a 25MHz with AUI.

Good, it's easier to overclock because it already has an independent ethernet clock.