Classic 1 Buzzing

[finkmac]

Searched the site, but I couldn't find any information on this.

I recently acquired a 2nd Mac Classic.

It's missing its case screws, so I opened it up... The little board at the end of the CRT was disconnected, so I reconnected it.

When I tried powering it up, it made a buzzing noise, which lasted 'til I killed the power. I tried powering it on without the case, and I noticed blue (flames? electricity?) surging upwards from the (not sure what it is, it looks like a vacuum tube... just above the net of copper wires)...

I was quite surprised, so I pulled the plug...

Is this display broken?

 

[uniserver]

wow could you plug it back in and take a picture?

 

[finkmac]

Take a picture of what? The tube? It scares me... :O

 

[uniserver]

come on i wanna see the blue flames.

i wonder if one could make a Jacob's ladder with a CRT

oh never mind,

 

[finkmac]

What if it blows?! :O :O :O

 

[uniserver]

It sounds to me like your CRT lost vacuum,

the nipple on the end of my SE FDHD tube got broke off in shipment. it was pretty much doing the same thing.

 

[finkmac]

So, basically… The CRT is shot?

I don't think I would be able to replace it… CRTs are too implode-prone for me…

Of course, the drives/mainboard seem fine… That's kind of what I bought it for…

 

[krye]

I would not power that CRT up again. Cut your losses.

 

[onlyonemac]

The board which was disconnected was probably disconnected because the previous owner found that the CRT was not working, so for safety reasons they disconnected it.

 

[uniserver]

Or like in my case with the FDHD, that video board was disconnected and hanging, and the glass nipple on the end of the tube was broken off.

Sometimes in shipping these things can be thrown off the back of a truck.

was this an eBay purchase?

 

[James1095]

Uniserver is almost certainly correct, CRT is gassy, the EHT is arcing. While it can be startling and visually spectacular, it's not particularly dangerous, just don't touch it. Also there is risk of damaging the analog board, especially if it's arcing to the deflection yoke.

A CRT that has lost its vacuum is a fancy looking glass jar with zero implosion risk. They're actually somewhat interesting to dissect, at least the electron gun in the neck is, there isn't much else inside a monochrome CRT. Just wash your hands after handling the leaded glass and don't eat any of the parts. Swapping in a new tube of that size is fairly safe too. Handle it as you would handle a large light bulb, vase, etc. You have to really abuse one that small to get it to implode, as in whack the bell part with a hammer or drop it on concrete. That glass is quite thick and very strong. It doesn't hurt to wear safety glasses but it really is not something to fear.

 

[finkmac]

@uniserver, no… I got it locally.

Glass jar eh? Looks like I have yet another repair project on my hands… Of course, this one may be more complicated…

What about discharge? Would that "pull the plug" trick work here?

 

[James1095]

Look at the little glass sealing pip in the center of the circle of pins on the neck, it's probably broken off. The sealing pip is a real weak spot and the reason larger CRTs usually have a plastic cap to protect it. When disposing of dead CRTs, it's commonly advisable to break off the pip, releasing the vacuum and eliminating the implosion hazard.

While it *should* self discharge within a short while, it's always wise to discharge it manually just to be sure. While the shock itself is unlikely to cause long term harm, it can certainly cause you to jerk back in surprise and rip your hand open on something or drop the thing. What I do is wrap the bare end of a wire around the metal shaft of a screwdriver and connect the other end to the metal chassis or ground wire on the tube. Then poke the end of the scredriver under the suction cup looking anode connection until it touches the contact. If there is any charge left you might hear a small snap like static electricity. At this point you can disconnect the anode and remove the tube.

 

[uniserver]

it's just the 128k / 512k / 512ke that you have to worry about discharging.

Oh and IIe and IIgs psu's lol

(witch is funny because even the LISA PSU has a bleeder resistor.)



Pretty sure any compact mac, Plus and forward has a bleeder resistor. Just let it sit for 5 min…

If you want to be sure, Just get a alligator clipped cable , one end to the chassis and one to a screwdriver, and slide it under the suction cup… it's easy cheesy.

 

[James1095]

It's just a good idea to discharge it manually, whether it has a bleeder or not. Always assume a wire is live just as you always assume a gun is loaded. It's so easy to discharge it that there's really no reason not to.

 

[uniserver]

Yeah, always assume a gun is loaded... for real.

 

[finkmac]

I examined the sealing pip while I was removing the logic board… Didn't look like it was broken, of course... The yoke did look like it was somewhat squished…

Hmm, Looks like an opportunity to make one of those Amber-displayed Classics…

 

[finkmac]

Unrelated to the Buzzing CRT, but…

My OTHER classic was having… issues… (I suspect the board needs recapping), so I swapped it's logic board with the borked CRT Classic's one…

Now, the internal floppy drive claims that every disk inserted is unreadable, and the drive is making sort of a swishing sound.

 

[Brett B.]

Sounds to me like you have one or more floppy drives in need of a good cleaning and lube job.