Commodore 1084 monitor baffling intermittent flickering

[eeprom1]

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.

Immediately after I turn it on, the monitor works with no issue but after half an hour or so when it gets warmer; a flicker/jump happens on screen and image goes back to normal. This happens only once every few minutes. Artifact looks like monitor getting out of sync momentarily and recovers.

The issue does not happen when the CRT cover is 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 the tube in a dark room, but did not see any arching, the only thing I can hear is a very soft static noise due to resyncing. I have reflowed the solder on fly-back pins and checked for any voltage leakage in HV areas with a probe.

Many similar post suggest replacing the fly-back, however I rather be sure before before purchasing the new fly-back and do all solder/desoldering work.

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 look forward to your hints or suggestions.

[james_s]

Well you've determined that it is temperature sensitive so that gets you halfway there. Can you put it in a warm place and wait for the problem to start happening, then carefully squirt a bit of freeze spray on suspect areas? A can of air duster held upsidedown works well for this. You could also try heating selectively with a hair dryer.

[inse]

I also recommend Jame's approach.
Check for cracked capacitors near the FBT as well.
It might very well be an isolation issue with the FBT with some internal arcing not (yet) noticeable from outside.
Anyway exclude every other cause as I heard the FBTs have turned rather expensive.

[eeprom1]

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 don't 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.

[SiliconWizard]

Could it just be a bad solder joint?
https://forum.digitpress.com/forum/showthread.php?67499-commodore-1084-problems-intermittent-power-issue!

[decoder_82]

It seems like cold solder joints on the FBT are not the issue:

I have reflowed the solder on fly-back pins and checked for any voltage leakage in HV areas with a probe.

But maybe there is a cold solder joint somewhere else.

Personally I would start by checking or replacing the caps in the area of the FBT and near the heatsink.

[james_s]

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.

I don't think an oscilloscope is going to be useful here, you already know more or less what's happening, everything is fine most of the time but then there is a temperature sensitive intermittent fault. Even if you manage to capture a glitch with the scope it's not really going to tell you what's going on. Intermittent faults can be a pain to chase down, but on the other hand the fact that it works part of the time tells you that most of the parts are good and working correctly. 

[BrokenYugo]

Can we see the video of this glitch?

Without further info, I'd concur with the others, go over the solder side of the whole board with a magnifying glass. 

[inse]

Or, continue using the monitor until the problem intensifies, making it easier to track down, maybe?

[eeprom1]

I attached video of the glitch.
Flicker is at half way at 0:00.50. The constant flashing is due to refresh rate difference of camera and screen.

[james_s]

That does look a bit like an arc in the HV, it's hard to say for sure though.

[eeprom1]

Sometimes flickers are more pronounced like this one.

[BrokenYugo]

That one is a lot more informative. I would guess something in the vertical area.

[inse]

Have a look at the vertical deflection when the flicker occurs, the manual shows the according test points.

[mbarszcz]

Did you ever figure it out?  I have a 1084 that does the same thing.

[m k]

Short videos are difficult.
Fast happenings even worse.

If the problem is what is in the picture and is visible once in the video it's vertical deflection issue.

Do a similar video but put something meaningful on screen.
Something where general horizontal and vertical positions are easy to figure out, like a big X.

Not moving mouse pointer in the video means that the blanking lookalike is not that.
If it is then mouse pointer is also positioned below it.

You can also start by looking cracked solders.