1942 Monitor goes dark

[desantii]

Hi, a have an issue with my 1942 monitor, sometimes, frequently in fact, the screen will go dark to the point that you can barely see the screen. If I turn off and on it will be ok, or if it get a "hard" tap goes back to normal. Any thoughts on where to look? anyone had a similar issue?
Issue seems random, sometimes happens other times works great for several hours

thanks

[JimS]

There are a lot of things that can cause this. One thing I've seen a lot is bad solder joints on the flyback transformer. The flyback vibrates a bit and can sometimes crack the joints. They break and arc & burn. It's easy to resolder them... but you have to be careful of the possibly lethal voltages in there which can hang around long after it's been powered off.

[Ral-Clan]

Yup, I believe this happened to my 1942 and I had to retouch all the solder points on the board at the back of the CRT.

As mentioned, you have to be careful though because there are some lethal voltages in there.

If you plan to open up your monitor then read about the dangers of flyback transformers. (<----something is adding an ebay link to this word)

I'm not sure how long they hold their charge - days?

[JimS]

Yup, I believe this happened to my 1942 and I had to retouch all the solder points on the board at the back of the CRT.

As mentioned, you have to be careful though because there are some lethal voltages in there.

If you plan to open up your monitor then read about the dangers of flyback transformers.

I'm not sure how long they hold their charge - days?

At least days... It's the CRT that holds the charge. If you're careful, you can discharge that voltage with a grounded screwdriver under the HV lead at the CRT.... I don't recommend it unless you're experienced with it.

[spaceman88]

Check both the flyback and the CRT board for poor solder joints. Most likely it's the Heater circuit (may be marked "H" or "heater"). Make sure it's unplugged!!! Usually you won't get shocked unless you lift up the high voltage lead going to the picture tube. It's the picture tube that holds the charge. Probably won't kill you, but I don't recommend it (personal experience :-). If you leave it plugged in and touch the wrong thing then it probably WILL kill you!!

[Zac67]

With very low air humidity, the voltage can stay for weeks but it usually decreases substantially overnight. Actually, any larger capacitor can store serious charges for quite a while. Big caps in power supplies can be much more dangerous than CRTs (lower voltage but much higher energy stored) - but better safe than sorry.

For discharging, don't use a plain screwdriver (resistance too low, so discharge is immediate) but rather an earthed (monitor chassis, water pipe or such) resistor of 1-100 kOhm to avoid damage or you getting startled.

PS: fixing bad solder points in HV areas is highly recommended rather than just slapping the monitor: I've had a case where the solder had already crawled along the arc gap and started to build a 'bridge' - ~2 millimeters - to a neighbor ground lead. The resulting short would probably have destroyed several components. :eek: