MELDAS 520A Amber CRT screen dims back and forth!

[CNC Viking]

Hi,

I have a VMC with a Mitsubishi Meldas 520 AM control fitted with a Totoku Electronics 9" BW (amber) screen. PNo: MDT962B-1A. Manuf. year: 1995. Powered by 100VAC.

My problem is that when the NC is on the CRT suddenly dims to 50% brigthness or less, making it very difficult to read the small figures as those found in the work offset area etc.

When I tap gently on the control box, it sometimes goes bright again. It will mostly go bright by itself, just to dim down again after another while. And this goes on and on during a work day.

I suspect some bad solder points on one of the CRT's PCB's and would like to have some guiding from someone that has experienced this- and fixed it.

Where will I most likely find any bad solder points?

Besides resolder, any more tweeks that I need to perform?

If I can't fix it, I will go for a TFT conversion rather than buy a NOS CRT.

[VOLFAN]

I am not of any help to you but maybe you could help me. I'm looking for a sample tapping program for this control so I can see what I am doing wrong. I have tried every method I know of and get a new error each time. If you could help it would be a blessing.
Thanks in advance.

[EPTech]

Hi,

You should especially look at the HV side of the PCB. The components in this arear are usually the largest. Hence they cause the most stress to their solderings. Things like the HV transformer + HV cascade, large capacitors, large resistors. If this does not solve the problems there may also be leaking capacitors, causing the powersupply to fluctuate. In that case you need to replace them.

greetings.

[CNC Viking]

VOLFAN,
I will see if I have a tapping cycle ready to go. Just make sure your machine is empty of any tools and that the tool no. you are using in the program has a high offset in the register so the program is executed high above the table.

I will pm you when I find one.


EPTech,
Thanks for the tips. I will check it as soon as I find a "time window" between jobs. Will report back when done...

[VOLFAN]

cnc viking
Thanks I'll be looking for it.

[CNC Viking]

Updating the thread.

Last October things got worse and worse, so I finally removed the screen and opened it for inspection. I looked at the components to see if any of them showed signs of damage etc.. Not really. Then I looked at the different circuit boards. Some of the solderings looked a little dodgy, so I re-soldered them just in case. Then I unplugged and re-seated all cable connectors at the control end as well as the CRT end. I have a hunch that this was the most important move. Anyway, aftyer my "service" it worked brilliantly and have done so ever since. Occasionally I have the machine on for full 8 - 14 hour days, and have an "as new" crisp picture all day.

Looked at the components for signs of damage, but everything looked OK







Same with the boards and solderings. I resoldered a few just in case