my 1st monitor fix

[anunaki]

Guys, I'm very proud because I just finished fixing up my 1st monitor ever.

It's the MTC-900 from one of my zaccs I got from pauls50s. When I 1st put it on, picture on the screen was too small, wobbling and the colors were off.

I discharged the tube (well, nothing really happened), I did a capkit and I studied the manual to find out about the degaussing coil. I also found out about B+ and I adjusted it to 126V. Now the picture looks OK, just a little bit washed out in the lower left corner (when oriented vertical). No more wobbling.

I do have some questions for next time

- There are measuring points on the chassis for all kind of stuff, including B+, but what is the best place for GND (I used video ground, but there must be a better spot)
- What could be the reason the degaussing button isn't working all the time (after power off and up, it works again)
- Is there a way to test Vpp with a multimeter or do you always need a scope?

This is the result:

[level42]

Good job !

The degaussing works only when the monitor is cold (unless you have a manual degauss button). A posistor, the thing that looks like a little black box) lets mains voltage "through" to the degauss coil when it's cold. It quickly heats up by the current running through it and because of that starts to block the voltage because the resistance increases.

Usually I pick the point where the ground "enters" the chassis. On some monitors (not all !!!!) the heat sinks are on ground level too, but sometimes (old monitors/TVs) they ca carry high voltages.
If nothing is hacked, the metal frame of the monitor should be at ground level too.

A multimeter cannot measure Vpp. Vpp stands for Volt peak-peak, meaning the voltage between the Negative and the positive peak. This could be something like 300V, but the resulting energy, depending on the wave form, but let's assume a regular sinus will be lower. A DVM s designed to measure the resulting average voltage, not peak-peak. some can do RMS voltage measurements.

Others can probably explain better and you can do a shotload of mathematics on this stuff, but I forgot all about it

Here's a pretty good explanation:
http://www.learningaboutelectronics.com/Articles/What-is-peak-to-peak-voltage.php

[Etienne MacGyver]

Nice work Patrick !

Be sure to reflow all the connectors, they are known for cold solder joints..



the one i have had here, had the complete header loose because of it.

[Laszo]

Well done Patrick.

[anunaki]

tx

I put it back in the cab, that is fully working now.

One things concerns me: There still is a little hissing like sound. Not very loud, but it's coming from the HV section of the chassis.

Is this a risk? Or is it just the monitor working?

[level42]

Probably around the HV cup ?

The HV cup is made of transparent plastic and often becomes really stiff/brittle. It's hard to get it to seal off well......also check that the area around the hole is clean....if there is dirt/goo it might conduct some HV.

[level42]

I see you mention its around the HV section of the chassis......might be the Flyback nearing its end of life but on the other hand I've heard that on good working monitors too.....

[joeks]

Good job on your first monitor repair Patrick!

I still have several capkits lying around which I should install 

@etienne What connectors or solder joints do you mean exactly? I read about reflowing solder joints often, but what are the most common once on a monitor chassis?

[anunaki]

Etienne means the connectors from the degauss section

Andre: Will cleaning or so help?

I'd prefer to replace the flyback, but I never see MTC900 flybacks for sale. Only 9000s. I don't think these are interchangeable?

[Etienne MacGyver]

I don't think these are interchangeable?

You thought right