• Please review our updated Terms and Rules here

Model III CRT Issues

That's not good. Others may want to chime in, but it sounds to me like a heater-cathode short. That would explain the effect.

Try your lowest resistance setting and see if you can get a value for the resistance. Since you still have the original B&W jug, compare the same measurement. It should be very obvious what's wrong.
 
You can even go one step further and plug in the old CRT and see if the problem goes away.

Sounds like you got the $40 CRT instead of the $75 one. ;)
 
Uh, this is weird. So when I test resistance on 2-3, 2-4 on the plug, it pegs out.

But when I test continuity on 2-3, 2-4 on the actual prongs of the tube, I get nothing. That goes for the B/W and Green.

Am I doing something wrong?

KC9UDX, let's not even go there... ;)
 
Last edited:
With the socket off, test (high ohms reading) between pin 3 and 2 and 4 and 2. If you get anything but infinity, you have the problem solved. If not, I'll have a look at some of my OEM monitor schematics and see what I can come up with.
Click to expand...

Argh--with the socket off, test the tube. Okay so, it says open circuit on both tubes, right? That's okay.
 
Okay, let's go back to your video. If you turn the contrast all the way down, the display should disappear--take a look at the monitor schematic (PDF page 78 of the Sam's Computerfact document) One end of VR1 is fed with the video input, where the other end is grounded. The wiper goes from full input to ground--i.e. nothing.

So we've got to ask if the bottom leg of VR1 is really grounded. If you take a hunk of wire and ground the center lug of VR1, does the video go away?

Power off tests: If you do a continuity check between the end lug (try both) and ground, is it grounded? If it is, can you check the resistance between center lug of VR1 (contrast) and ground as you rotate the knob? It should go from about 500 ohms to 0.

It could be that the "fuzziness" of your original CRT might be related to this.
 
When I ground the center lug of the Contrast knob, nothing happens. When I do the same to the Brightness knob, the text goes away.

Also, not sure on the exact numbers here, since my multimeter needs a new battery, but... When I measure the resistance on the center lug of the BRIGHTNESS knob, when it's all the way off, it's 2.5k. Then, as I turn it up, it goes down to 2k then as I reach full brightness, it goes back up to 2.5k!

I'm away from home right now, and when I get back I'll get you some more info. Right now though, I'm kinda guessing that it might be faulty pots, considering two things: The fact that the resistance goes from high to low to high again, and that the knobs are REALLY stiff.

What do you guys think?
 
How's your stock of resistors? We might be able to cobble something up for testing--the contrast should never read more than 500 ohms according to the service manual.

Otherwise, something's not right here--according to the schematic in the service manual, grounding the center lug of the contrast control should ground the video signal.
 
Last edited:
Darn it. I made a typo. My multimeter only goes to 1k, so it was reading 2 to 2.5 ohms.

Does that make more or less sense?!
 
Hi ardsleytank.
Do you have some sewing machine OIL or a Frypan spray pack to ease the Potentiometer, spray or oil the shaft as it comes out of the body of the pot and wait for it to free up.
Then after its free rotate it about 5 times from end to end and this will probably clean the contacts in the pot.
Do you have a Technical friend near your home who could come over and give you a hand ?.

Keep going, Good Luck
Ray
 
Well, repeat the measurement with the connector removed from the monitor board; that should isolate things.

Even a cheap DMM may be less confusing for you. Harbor Freight has them for less than $5--not wonderful, but good enough for this kind of work.
 
Just be awful dang careful never to measure voltage with a harbour freight meter in the resistance range. Unless you're wearing your Category 4 arc flash PPE, of course.
 
Yeah, well, you have to use your head. Yes, set the scale, check your setting, then measure.

Reminds me of folks checking line voltage with the DMM set to the 20A range...
 
I bought a shiny new meter a long time ago.

A buddy came over when I was working on my car. He decided to use my shiny new meter to see how many amps my car battery had. :mad:

Here's a promotional video which may be a bit biased, but shows a harbour freight meter in action
https://youtu.be/OEoazQ1zuUM
 
Story time:

I once ran into a Fluke field applications engineer who was a friend of my then-boss; they're both airplane guys, and knew each other both from work connections and from having adjacent hangars. He explained why Fluke created a special line of meters which specifically lacked any current measurement capability. It seems that many industrial customers got tired of their service techs blowing up meters by accidentally measuring AC line voltage with the probes plugged into the current shunt jacks. So they filled in the current shunt jack holes with epoxy, thus preventing the problem from cropping up again. All was well until they sent their meters in for calibration... which the meters failed, because the calibration routines required use of the current shunt jacks. So, Fluke responded by designing new meters that lacked any current measurement features, for those customers who didn't know better than to short out the AC mains with a current shunt.

Hang in there, ardsleytank. You'll get your screen looking great, and you'll learn a lot in the process.
 
Well, I broke down and bought a digital multimeter. Now I have some interesting measurements for you guys...

The brightness knob on minimum reads 0.00 ohms. On max it reads 575 ohms. However, the contrast on the min or max reads 0.4 ohms.

Bad pot?
 
Neat story NF6X, and thanks. I hope I can get it to look good.

Once I fix the screen, I'm gonna try to get TRS-BOX Serial up and running.
 
You must log in or register to reply here.
Back
Top