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IBM 5150 Monitor Issue

gbrownie

New Member
Joined
Apr 18, 2020
Messages
8
Location
USA
Recently I stumbled across a IBM 5150. With it came a Magnavox Professional PC Monitor 80 (monochrome). The display is super sharp and great...except for the horizontal line going through the middle that looks like the paint on a road (dashed horizontal line). The PC itself I think works perfectly. I am able to execute basic scripts, boot msdos 3 off either floppy drive (it has a hdd but I think that might be dead as it doesn't spin up). Because of this, I'm assuming this is an issue with the monitor instead of the computer.

The line itself only appears after the initial blinking cursor and the IBM/DOS logos appear and it always looks exactly the same. I have opened the back of it and looked at the caps and fuses and both at least visibly look fine. A strange thing I noticed is if I touch the "v. lin" dial (see 2nd pic right most) the text on screen very lightly vibrates.

Anyways, I'm wondering why this horizontal line is appearing and if there is any hope to saving it?

https://postimg.cc/yJmY4SMR

https://postimg.cc/FfWCKPXX
 

modem7

Veteran Member
Joined
May 29, 2006
Messages
7,920
Location
Melbourne, Australia
Refer to your first photo. Notice that the "Ok", compared to the text above it, is spread out vertically. That dashed line is Cassette BASIC's list of function commands (see bottom line of the screen shown at [here]) that is compressed vertically. So there is a vertical deflection problem. From the top of the screen downwards, vertical deflection is starting off okay, then going crazy. If you turn up the brightness, the raster may appear, and if it does, you may see that the raster only occupies the top half of the screen.

That behaviour is not what I would expect if the 5150's MDA card was putting out a bad vertical sync signal (bad pulse width and/or pulse frequency), but there are others on these forums with more repair experience than me, and they may possibly disagree.

A strange thing I noticed is if I touch the "v. lin" dial (see 2nd pic right most) the text on screen very lightly vibrates.
V. LIN = Vertical linearity. That 'vibrate on touch' symptom may be relevant (or maybe not). Look for a bad solder joint on that V. LIN pot. Try adjusting the pot slightly.

As a comparison, the vertical stage of the IBM 5151 is shown at [here]. Just a dedicated 'vertical deflection' chip and some supporting circuitry. Your monitor may be very similar. Sometimes the vertical deflection chip fails (fully or partially). But considering the age of the monitor, bad solder joints and bad electrolytic capacitors are, in my opinion, just as likely.
 

Hugo Holden

Veteran Member
Joined
Dec 23, 2015
Messages
2,622
Location
Australia
As noted by Modem7 this is a vertical scan amplitude & linearity problem.

From first principles. the scanning current should be in increasing in a sawtooth linear manner, from top to the bottom of the screen. Clearly the vertical yoke current is inadequate and the scan waveform driving the V yoke coils is defective.

The common cause would be the V yoke coil's coupling electrolytic cap has gone low uF and high ESR. The power supply voltage could be sagging down to the V deflection IC (or vert deflection circuit). The V deflection IC could be faulty, or a capacitor/resistor supporting it is defective.

This sort of fault is dead easy to sort out with a scope, checking the power supply rail first and the voltages in the V deflection system. So there is plenty of hope for your monitor.
 
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