How to identify a failing Cathode Ray Tube?

[Tom Vaughan-Mountford]

Hi everyone - first time newbie here! I've had an interest in electronics since I was a kid, but broke more stuff than I fixed. Now trying to make my interest of practical use...

I've acquired a broadcast video monitor, probably 30-35 years old, and I'd like to have a bash at tracing the fault and trying to repair it. However, the symptoms it's displaying, and it's age, make me think it has a failing CRT - and there's no way in hell I'm ever going to get it running again. I'm figuring it's either the tube that's about to fail, or a (potentially replaceable) component elsewhere. Check the video of the raster I put on YouTube, and let me know if I'd be better just scrapping this and finding a new project. Thanks.

https://youtu.be/9Md0j7WGCSo

 

[Les Jones]

I dont think the fault is the CRT. I suspect the power supply to the line and field timebases. It could be the power to the line timebase as this also generates the EHT supply for the tube. If the EHT voltage drops the beam is easier to deflect so it can increase the width of the picture. It could be caused by decoupling capacitors on these power rails. I find testing capacitors in the scanning department with an ESR meter that gives good results with the capacitors in circuit a quick way to eliminate faulty capacitors before taking a more logical approach to the problem.

Les.

 

[Tom Vaughan-Mountford]

I find testing capacitors in the scanning department with an ESR meter that gives good results with the capacitors in circuit a quick way to eliminate faulty capacitors before taking a more logical approach to the problem.

Les.

Cheers Les, that's really helpful - I'm also rather pleased that the next thing on my list was failing caps somewhere, perhaps there's hope for me yet! I'll make the effort to get this opened-up and get the probes on it.

 

[cowboybob]

Old as it is, and given the probability that during its lifetime it was on 24/7, weakened, faulty caps (and other components), especially in the power supply(s), would be high on my list of culprits.

And, no doubt, there will be a ton of them for all the various HV circuits.

Keep in mind that there are lethal voltages inside that box, even well after it is turned off, especially around the anode.

Just out of curiosity, are there any electrical sounding pops or "sizzles" while it is on?

 

[JimB]

On good old CRT based monitors, the last thing to consider is a faulty tube.
Yours appears to show good brightness and focus, tube is probably OK.

As others have suggested, suspect the power supplies, especially the electrolytic capacitors.

JimB

 

[Tom Vaughan-Mountford]

Thanks JimB, thanks cowboybob.

I left it switched on for an hour whilst I shot the video, there weren't any crackling or popping sounds - I lifted the lid, no nasty smells coming from inside, nothing seems excessively warm. Thanks for the heads-up about the voltages, how long do caps remain charged, and what's the common service procedure - is there a way of discharging them before working?

Interesting to hear the CRT might outlast the rest of the components - long assumed it would be the first thing to burn out.

 

[Les Jones]

Hi Tom,
The capacitors can stay charged about as long as a piece of string ! Check the voltage across them with a meter before touching anything. Discharge them trough a suitable value resistor and confirm that tey are discharged again with a meter. Also the coating connected to the EHT connector on the tube acts as a capacitor and can stay charged for many days. To discharge this connect the blade of a screwdriver to ground (on the monitor.) via a high value resistor (Say 1 meg) and poke the end of the screwdriver under the rubber cover of the EHT connector until it touched the metal part of the connector. hold it on for a few seconds. Sometimes the tube can charge up again when the EHT connector has been off for a long time so discharge it again before touching it. Also I should have mentioned that you need to be sure thet the capacitors are discharged before testing them with an ESR meter. If you do not discharge them you could destroy the ESR meter. If it is a bought one rather than a home made one the instructions will probably warn you about this.

Les.

 

[Tom Vaughan-Mountford]

Thanks Les, I've not bought an ESR yet - so far I've got myself a soldering iron, electrical and ceramic screwdrivers, a multimeter and cap checker. ESR meter is going on my Christmas list!

 

[spec]

Hi TVM,

As the others have said, it is unlikely that the tube is faulty.

My advise is to strip and clean everything. The high voltage components including the CRT, EHT cable and connector, and CRT shield attract fine dust and grease from the atmosphere. This can act as a resistance and upset the voltages on the CRT. I have never figured out where the grease comes from.

The other main area of trouble is the EHT generator, which will have a large ferrite-cored inductor. The EHT generator is normally in a screened metal box. This must be opened up and given the clean/inspect treatment.

Once everything is clean, go on to lightly burnish all contacts, the valve base on the rear of the tube for example.

Then do a visual inspection and check for dry/corroded joints and cracked printed circuit tracks or corroded wires, especially where they connect to components.

In addition to capacitors, another suspect is potentiometers, both hand-adjustable and presets.

The only faults I have ever come across with CRTs have been open circuit heaters (no picture) and low emission cathodes (dull picture).

Have fun

spec

 

[cowboybob]

TVM, some well lit and nicely focused pix of the case-off guts of the monitor would be very helpful. Also, if possible, a Model#, or other identifier would be useful for trying to track down a schematic (maybe a pipe dream, but worth a shot).

A little factoid about CRTs (of any kind): the phosphors on the inside of the screen face will, whenever they're exposed to light, generate voltage potentials that will, over time and exposure rates, increase to lethal levels at the anode connection, especially when the CRT is disconnected from all its various wires. Bleeder resistors in the HV circuits reduce the risk, but they can fail in older units.

Point being that you should always be very cautious when handling a CRT or anything attached to it.

<EDIT> Corrected CTR to CRT ...

 

[Tom Vaughan-Mountford]

Fascinating stuff, I didn't know all this about CRTs. I probably should have, I used to dismantle TVs and VCRs all the time as a kid... seems my luck held out!

Spec, thanks for running me through the procedure. The inside isn't too grubby - no dust or fluff - I suspect it's been in a rack in a machine room all its life (an asset label shows it came from ITN in London), but definitely evidence of heat and grease, all the high-voltage stuff looks quite brown, faded colours on all the components.

I'll get some photos of the innards, and see if I can locate a model number.

 

[Tom Vaughan-Mountford]

Hi all - Interior shots as promised. I've noticed evidence of past repairs - some new resistors soldered in, and the large potted green thing (I think it's something to do with HT/focus??) has a date-stamp of 1991 - and this monitor looks 10+ years older than that. There's a rectangular component with cracked outer casing (slightly discoloured) and it's sitting a little proud of the board - I reckon that's also worth looking at. Other than that the caps here look visually okay on first inspection. I can see the mains PSU encased in a metal box underneath the tube, and through the vents I can see a couple of very large caps inside.

I'm waiting for some croc-clip cords to arrive so I can discharge the tube and dismantle further to get access to the PSU and take a look inside.

Model is Vistek GM 7137 - no reference to it on Google, and Vistek Electronics no longer seems to be in existence.

 

[cowboybob]

Good pix, TVM, but as displayed pretty hard to see any detail - can you enlarge them a bit and post them individually?

... There's a rectangular component with cracked outer casing (slightly discoloured) and it's sitting a little proud of the board ...

That sounds like a transformer (possibly a HV flyback ) that's gotten hot, causing the crack(s) and perhaps compromising the insulation of the wire(s) inside. If so, it could be arcing internally which might explain the whack screen display. I said could...

 

[Tom Vaughan-Mountford]

Definitely sounds like something to add to my list of things to inspect. Hopefully these are all fairly generic, simple components I can source. Larger pics attached.

 

[ericgibbs]

hi Tom,
The outlined resistor pad looks as dry as old boots, I would remake the joint.

Eric

Also R16 , give it a good tug.

 

[Tom Vaughan-Mountford]

Cheers Eric, good spot - I think I'll take a keen look at all the joints throughout - it's not the only one which is in that condition.

 

[alec_t]

The top end of C10 doesn't look too happy. And as for R18....what's going on there?

 

[JimB]

In addition to what others have said, it would be worth checking all the solder joints on the big components in this area marked in red.

The large components, and the ones which get hot, the resistors and the transistors on heat sinks, are all candidates for breaking their solder joints.


And as for this capacitor,

Is it just my eyes or has it bellied out and pushed the seal out of its bottom end?

JimB

 

[cowboybob]

TVM, the electrolytic cap that JimB pointed out above, might be the place to start. By proximity alone, it looks to be a component of the HV circuit for the CRT, i.e., next to the tall green transformer (X1, probably the flyback, which also has a single, heftily insulated gray wire headed towards the front of the CRT - where the anode is).

It only has 2 leads. Removing a cap from a PCB is relatively easy, but will require the use of a "solder sucker" or some solder removing copper braid.

Google "youtube PCB solder removal" for some examples.

Of course, getting the PCB out might be a bear. Although it also might be accessible through the outside panel...

 

[KeepItSimpleStupid]

Agree with CBB. CRT failures are rare. Broken filaments, cracked tubes, poor brightness, destroyed phosphors are a few such symptoms. I fixed a computer terminal with an internal short. You could see the retrace lines.