Commodore 1084 monitor baffling intermittent flickering

[terryfi]

Hello everyone,

I have an original model Commodore 1084 made in 1987. These are with original Philips design with HR 6489 fly-back and external focus unit on neck-board.
The monitor works with no issue when it turns on but after a while when it gets warmer; every few minutes I see a flicker on screen and then image goes back to normal.
It seems that monitor gets out of sync momentarily and recovers.

The issue does not happen when the cover off. With cover off, I can only make it happen using a fan-heater to increase ambient temperature. To look for any arching I observed the boards and screen in a dark room no arching only a small static noise for resyncing. I have reflowed the solder on fly-back pins and checked for any voltage leakage in HV areas with a probe.

I would like to get some advice how to narrow down the faulty part. I have an oscilloscope and was thinking to check horizontal and vertical sweep signals while this happens; but I am not sure where to start and how to set the trigger for waveform capture.

I am suspecting the fly-back might be the issue and thinking of replacing it however since focus model is outside of fly-back I would need to splice high voltage focus cable which can be tricky.

I look forward to your hints or suggestions.

Terry

 

[betwixt]

Intermittent faults are always difficult to trace but a monitor that runs hot and is 35 years old will almost certainly have dried out electrolytic capacitors in it. For their relatively low cost, they are the first things I would change.

Brian.

 

[terryfi]

Intermittent faults are always difficult to trace but a monitor that runs hot and is 35 years old will almost certainly have dried out electrolytic capacitors in it. For their relatively low cost, they are the first things I would change.

Brian.

I have replaced all big caps on all three boards (main, psu and neck); only smaller capacitors I haven't touched yet. Surprisingly all big caps were ok.

 

[terryfi]

Here is a photo and video of flicker at 00:02.

 

[BradtheRad]

The issue does not happen when the cover off. With cover off, I can only make it happen using a fan-heater to increase ambient temperature.

I once read a story about a technician being skeptical when a tv owner said he aimed a hair dryer at his tv while it was opened up in the shop...
thus re-creating a problem which only occurred while the cover was on. Sure enough the technician used freeze spray to confirm that a certain component was faulty.

Likewise you get a heat-related malfunction once the set warms up. Somewhere a component is borderline faulty. Try to locate it with a concentrated hot airstream while you watch for the screen to flicker. Then aim cool air at the component, and the set should work again.

Or perhaps you can apply ice held in a dry bag, quickly before condensation drips into the circuitry.

 

[terryfi]

Thanks for suggestion guys.

I tried using a hair dryer and heating parts of board. I tried warming up other areas with ICs and did not recreate issue.

I could only recreate the issue by heating the flyback area with high speed, high temperature setting of dryer. Still it would take another two minutes for next flicker to show up. The flyback has a large heat capacity and will quickly warm up many of its surrounding parts; making it impossible to know if either flyback or neighboring parts are the issue.

Using freeze spray might not work for same reasons, it takes a lot of spraying to lower flyback temperature; and if I spray on neighboring parts they will quickly get warmed up due to high heat capacity of flyback; remember I have to wait two minutes to see the next flicker. I am also not comfortable using the spray in high voltage area as moisture conducts electricity.

I have only one other 1084 monitor that it also has the same issue. So this flickering must be a very common issue. People either notice it or ignore it; or maybe they use their monitor less than half hour every time. If we assume this is a common age related issue, the aging flyback issue comes back to mind.

I understand the analog nature of CRT and circular dependencies of signals makes it very hard to diagnose such intermittent issues but I was still hoping someone had the experience of diagnosing same types of issues with some systematic steps maybe with help of oscilloscope.

 

[BradtheRad]

After many years it's normal for a layer of dust to gather on circuitry, more so on high-voltage components such as a flyback. Heat buildup is the result, increasing gradually.

Besides which there's the attraction for carbon particles (from sources in the tv or the air), creating a mildly conductive avenue for electrons. Not necessarily to the point of arcing, but able to draw down voltage thus hampering proper operation.

Cleaning is difficult around congested circuitry. A tiny brush vacuum is on the market. High pressure canned air is another thing to try.

You may get lasting success if you install a fan. Space is probably scarce so you may need to mount it outside. Drill a hole in the cabinet perhaps.