IBM 5151 Problem

[per]

I got a minor problem with my 5151 Monochrome display: Some time after I've turned on the computer, the contrast of the display is starts flashing. There is also sometimes a high pitched (about 17-19 KHz:crazy sound coming out of the monitor. It is not critical, but extreme annoying at it's worst.

Do any of you know what the problem might be?

 

[frozenfire75i]

Greetings I have the same noise on my 5151 but it go's away after a while!

 

[per]

The noise is problably the phenomen of "Yoke noise".

but do you have any idea about what the contrast noise migt be?

 

[nige the hippy]

1) wiggle the mains plug, and the monitor connector, just to make sure it's not anything silly.
2) Anything intermittent, especially something that happens when it "warms up" is likely to be a dry-joint problem. So (after waiting for the capacitors to discharge) whip the thing to pieces, and start looking for crystallised solder, and little "rings" around component legs. It's best to just suck, and re-solder anything that looks like it gets hot.
3) Clean round the EHT cap, now....IN CAPITAL LETTERS!!!! AFTER THE MONITOR HAS BEEN TURNED OFF FOR A WHILE, SLIDE A WELL INSULATED FLAT BLADE SCREWDRIVER WITH AN LEAD EARTHED TO THE CHASSIS CLIPPED TO IT UNDER THE CAP, UNTIL YOU ARE TOUCHING THE CONNECTOR, be prepared for a bang as the tube discharges, then still being a bit careful, clean round the cap with some alcohol.
if that doesn't cure it, say your prayers that it isn't the tube shorting out, if so, I can't help you!

 

[modem7]

It could also be a bad capacitor somewhere - temperature sensitive.

Whenever I hear of temperature dependent symptoms, I think, freeze spray.
You can buy freeze spray designed for use in live electronic circuits (e.g. non-conductive, non-corrosive).
You may see where I'm going with this.
BEING VERY CAREFULL, you would power up your 5151 without the cover, then after the symptoms have appeared, use the spray to see if you can work out what the temperature sensitive component is.
Don't overdo it on any one component as some electronic components can fail due to thermal shock.

 

[Druid6900]

Start by checking around the horizontal oscillator with the freeze spray.

As Modem7 says, a one second shot should do it and then wait a bit while it cools down the component.