CRT Monitor Repair/issues

[pigpen]

Hi, just looking for some advice on wether to repair or bin a CRT i've had in storage for many years. It's a Sony HMD A400 Trinitron unit that I used before I moved onto LCD. It was pretty nice in the day.

I just dug it out of the attic and gave it a try but it's not in good health. I get a picture but it will cut out for a fraction of a second with an audible "click" when it does so. I'm thinking a failing/shorting flyback transformer? I'm not an expert on CRT repair by any means though, I googled the symptoms and they seem to match. There's other issues too but this is the obvious one to start.

I certainly can't find a replacement part if it is the flyback. Is it worth keeping and hunting for spares or should I sell it on in the unlikely event anyone wants it?

Thanks

 

[Hugo Holden]

Hi, just looking for some advice on wether to repair or bin a CRT i've had in storage for many years. It's a Sony HMD A400 Trinitron unit that I used before I moved onto LCD. It was pretty nice in the day.

I just dug it out of the attic and gave it a try but it's not in good health. I get a picture but it will cut out for a fraction of a second with an audible "click" when it does so. I'm thinking a failing/shorting flyback transformer? I'm not an expert on CRT repair by any means though, I googled the symptoms and they seem to match. There's other issues too but this is the obvious one to start.

I certainly can't find a replacement part if it is the flyback. Is it worth keeping and hunting for spares or should I sell it on in the unlikely event anyone wants it?

Thanks

It is very odd the way, when anything goes wrong with a CRT TV or VDU, that people jump to the conclusion it is the flyback transformer.

Working with VDU's and CRT's for over 50 years, it is seldom the case (though it can happen). The usual suspects for VDU failures are the electrolytic capacitors, followed by some of the semiconductors and resistors and inductors and sometimes the EHT rectifier which is built into many of the flyback transformers.

It is easy to test if the flyback transformer is ok, but to do it it , requires some knowledge of the flyback transformers resonant frequencies and how to test these with a signal generator and scope in circuit.

Your problem sounds like the power supply is shutting down, or it could well be that one of the output stages in the set is drawing too much current.

It is true that the most likely stage to overload the power supply is the H output stage. The reason for this is that when the H output transistor is switched on, the collector current rises at many thousands of amps per second. So it relies on the drive voltage to the horizontal output transistor turning off quite quickly in the range of 40uS after being turned on. So the whole system can fail and overload the power supply, simply because the H output transistor is turned on for too long per cycle. In other words the problem can reside in the signal circuits, not the power output circuit.

So, if you want to diagnose the problem, and have any realistic chance of fixing it, you need to become familiar with how CRT VDU's work.

The first step here is to post the schematic, and then post the recordings you have made at various points on that schematic with the scope any meter. If you don't do that, somebody would really require a Crystal Ball to help you fix it.

 

[Timo W.]

If you already thought about binning it and you are not an expert in repairing CRTs, please give it to someone else. Sony Trinitron CRTs are very valuable and sought-after by many.

And yes, that sounds very much like failing caps than anything else. When the picture kicks in, current draw goes up. With bad caps, voltage will drop without compensation and the PSU will do a safety shutdown. That's the click you hear. Moreover, a flyback transformer does not go bad when being stored away, but caps do.

 

[pigpen]

I'm happy enough to take it to bits for a look, I just lack the equipment and space at home to do much about it. I can borrow the basic kit from work for a weekend if I have to poke about. I'm fully aware of the high voltages floating around, but living by myself I don't really fancy poking about in its guts when its live. I'm not totally unfamiliar with servicing electronics but I'm not at all comfortable working on high voltage power supplies.

If there's something I can diagnose by eye then thats easy enough to swap obviously failing parts. Given its manufacture date I should probably expect some dodgy caps, all the other computer stuff I have from that era is suffering the cap plague.

The flyback thing is my own confirmation bias kicking in, there's certainly enough information on google about it but I agree, in the bunch of videos i've watched here and there basically nobody has replaced one, its always been other issues.

As I mentioned, it has other issues like an overly bright screen (an issue with this model, "fixed" through the WinDAS calibration software) and it does look a little out of focus, but that might be my poor memory and my eyes just being used to pin-sharp LCDs these days.

 

[andy]

I get the impression the picture is just blanking out for a split second every so often. This could be caused by some HV arcing due to dust, and dirt. When I see a monitor doing that, I disconnect the HV connector, and clean the side of the CRT, and inside of the rubber cup, then apply some silicone dielectric grease before reconnecting it.

It could also be arcing inside the CRT. This can happen after many years of storage, but usually settles down after some use. It seems more common with Sonys than other brands.

If it's arcing inside the flyback, which isn't very likely, it will fail completely before long.

 

[daver2]

Yes, there are spark gaps in the monitor, and these could be covered in dust. The application of a vacuum cleaner to suck up the dust may be all that is required.

Dave

 

[Hugo Holden]

Often the spark gaps in many VDU's are built into the CRT's socket assembly & difficult to get at, not always, other times you will find objects on the CRT's base board, and sometimes main board, that resemble a ceramic capacitor with a saw cut in the top of them. You can run a piece of paper through that cut to clear them of duct particles.

 

[jlang]

I once was working on a ticking monitor. I just could not find the arc.
One of the old grey beard engineers walked into the shop. I explained the problem.
His response was "Did you turn off the lights?" Boy did I feel dumb.

 

[modem7]

His response was "Did you turn off the lights?" Boy did I feel dumb.

Oh, I get it. By turning off the lights, there is more electricity available for the spark, and the spark becomes louder.

 

[pbirkel@gmail.com]

Oh, I get it. By turning off the lights, there is more electricity available for the spark, and the spark becomes louder.

In the dark ... see the spark ...

 

[Hugo Holden]

In the dark ... see the spark ...

Can work, that is if the discharge is in a visible location. If it inside a CRT socket assembly, where sometimes the spark gaps are located, you won't see it.

Often people wonder what a spark actually is.

It is an interesting physical structure that has phases to its development.

Initially in phase 1 a thin stream of gas is ionized by a high voltage. This excites adjacent gas molecules. The plasma then grows in diameter and as it does the voltage across the terminals drops because the spark plasma has a negative resistance. As the current increases, the resistance drops, and the spark tends to adopt a constant voltage drop irrespective of the current, because as the current increases the spark plasma gets larger in cross sectional area. In many ways it acts like a Zener diode.

In the automotive industry in the field of spark generation, Zener diodes are used as dummy spark plugs, to assess the power output of various ignition systems. A typical spark in air for example, across a gap of about 1 to 2 mm tends to have a fixed voltage drop around 500V (once the plasma is established), though it may have taken about 1000 to 5000 volts or more initially, to start the spark. Automotive spark plugs can be modeled with a 1000v bidirectional Zener diode. (usually a series array to spread the power). These snub off most of the early transient seen with the real spark plug, so it is a close simulation but not exact.

In any case, one good definition of a spark plasma is; "A structure that has the physical properties of a gas and the electrical properties of a metal"

Due to the ionization though, electrons are pushed up into higher energy shells and when they fall back down release Photons, (in air) you can see the blue -white light, easier to see in the dark as noted.

Because the spark tends to have a relatively low and nearly fixed voltage drop, compared to the supply voltage that initiated it, say it could be a 15kV or more supply, you can regard the spark, when it occurs, as being an effective short circuit of the power supply. And, if that power supply has filtering capacitance of a few thousand pF, as most CRT EHT anode supplies have, if a spark occurs the initial peak currents can be extremely high.