CRT monitor going "full brightness"

[roli]

Hi!
Recently I got my hands on an old Videotel terminal (basically an Italian version of Minitel) that I've been trying to use as an SSH terminal. The thing is a small-ish BW CRT made I think around early 90s.

When I first opened it up there were two loose AA batteries in there. Luckily most of the thing has survived without any damage. With the exception of the CRT neck board where a few traces were corroded. I think I managed to fix all of them before first even trying the first power up. And when I powered it on everything was working fine. Now, during the past few days I have been trying to interface an ESP32 with it, so I have left it disassembled on my bench until I can figure everything out and mount it somewhere. During this time I have plugged it in repeatedly and everything was fine. Until today. During some more testing I unplugged and re-plugged it into power a few times and suddenly the screen went really white. Pretty much like somebody turned the brightness all the way up. Adjusting the brightness knob does nothing (except maybe make the picture wobble a bit while turning it).

Obviously the first thing I tried is the brightness knob. I sprayed some contact cleaner in there and measured the resistance on the board where the pot contacts are - which seems to change as expected when turning the knob. While looking for anything obvious I was moving the thing around and for a second the picture went back to normal brightness and then back to being white. I tried some more wobbling and poking around the components with a plastic tool, but can't get it to change anymore. I tried looking for usual bad solder joints and stuff, I tried removing the CPU/mainboard. And even then the screen is fully white, even when nothing should be showing.

Now sadly I really don't know much about CRTs, so I have no idea what determines their brightness. Should I be looking more into the neck board? Or should I be focusing more on the power supply/analog board? So I am hoping for your expertise on this topic. I tried finding a service manual for this, but it's pretty much impossible even finding that it existed, let alone a service manual.

Image 1. Here is what it looks like (it's extremely hard to get a good picture of the problem)
Image 2. Here is how it looked like when I first got it
Image 3. Closeup of the neck board
Image 4. After some cleaning
Image 5. With "some fixing"

[inse]

I suspect it has something to do with the screen voltage.
The TV flyback transformers used to have two pots attached for focus and screen voltage adjustment.
Those HV pots also might have been affected by corrosion.
If the screen pot is embedded to the flyback or separate, maybe even on the CRT PCB, you have to find out.
Mark the position before adjusting it.

[George Edmonds]

Hi

The 1st Grid to cathode voltage, BUT the cathode is normally at a high voltage so accurate measurements are difficult.

G Edmonds

[strawberry]

SCREEN (~1kV) that thin wire next to thicker FOCUS (~5kV) wire (dont remember when its open circuit)

not necessary to use bodge wire in this case. clean oxide ,tin traces witch acid, clean, Nitrocellulose Lacquer(seems to work good enough) or liquid UV curable soldermask

[roli]

I suspect it has something to do with the screen voltage.
The TV flyback transformers used to have two pots attached for focus and screen voltage adjustment.
Those HV pots also might have been affected by corrosion.
If the screen pot is embedded to the flyback or separate, maybe even on the CRT PCB, you have to find out.
Mark the position before adjusting it.

Hmmm there aren't any pots on the flyback (which I am guessing is the big metalic can with the wire to the anode cap).
There are three pots around it though. They are marked: "V. LIN", "V. AMP", "FREQ". Any of those sound like what I am looking for?

SCREEN (~1kV) that thin wire next to thicker FOCUS (~5kV) wire (dont remember when its open circuit)

not necessary to use bodge wire in this case. clean oxide ,tin traces witch acid, clean, Nitrocellulose Lacquer(seems to work good enough) or liquid UV curable soldermask

So you are saying that you think that it's likely that this wire has a bad connection? If you can see it on the picture can you point it out for me? Just so that I am sure we are talking about the same thing.

[james_s]

Sometimes the pot is on the board instead of the flyback, this is particularly true of small monochrome monitors. Look for something called "Screen", "G2", or occasionally "sub-bright" or something like that. The thick wire is probably focus, another wire to the neck board will be G2, which is the second accelerating anode.

[Chris56000]

That's an Italian made Hantarex Monochrome Monitor Chassis, and a common cause of the screen going uncontrollably bright is dry–joints on the little panel that plugs on the back of the CRT neck – take GREAT CARE if you remove it as the CRT pins are very easily bent over and you can also accidentally break the glass pip off in the middle of the base!

If you look at the small CRT base you will see a circle of seven pins with a gap between two of them – counting clockwise from the gap, the base connections are :–

1 :– Grid (G1) – typically 0 to about – 20 V ;
2 :- Cathode (K) – typically +50 V to +80 V ;
3 :– Heater (H) – 0 V ;
4 :– Heater (H) – +11 V ;

These little CRTs use a heater rated at 11 V 60 mA !

5 :– Grid (G1) – Connected internally to pin 1 of the CRT ;
6 :– First Anode (G2) – typically + 90 V ;
7 :– Focus Anode (G4) – typically 0 V to + 90 V, varied by the "focus" preset if one is fitted !

It is normal for the preset control to have only a very slight effect on the sharpness of the text displayed, and with an old tube, you probably won't see any obvious effect of adjusting it !

I've attached the service manual for it for you !

Check choke B200 and R200 on the little CRT neck board for dry–joints, also R64 and R66 on the main monitor board for open–circuits, and resolder all the pins of the flyback transformer – the 15 kHz h.f. pulse voltages and the heat generated cause the solder to decompose round all the transformer pins !

You might have to replace the video transistor TR202 on the little CRT board if it has gone short–circuit !

If you're really, really unlucky, the CRT itself might be at fault due to a heater–cathode short, but in all my years of servicing CRT monitors, I only came across this fault once – it's much  much less prevalent than it was in the old days of black–and–white TV only !

Chris Williams
 

[jonpaul]

SAFETY WARNING

I hope that the OP knows the risks of CRT implosion and high voltage, stored charge in HV caps and CRT aquadag.

1. use gloves and safety glasses/ face shield, CRT implosion can case eye and other injury or death.

Old CRT are especially susceptible

2. Get Earthing stick with 10k 2w resistor, discharge CRT anode, cathode before any work.

3. even a small CRT and 5 kV anode can cause a shock that results in more damage.

INEXPERIENCE IS DANGEROUS

Bon courage

Jon

[james_s]

SAFETY WARNING

I hope that the OP knows the risks of CRT implosion and high voltage, stored charge in HV caps and CRT aquadag.

1. use gloves and safety glasses/ face shield, CRT implosion can case eye and other injury or death.

Old CRT are especially susceptible

2. Get Earthing stick with 10k 2w resistor, discharge CRT anode, cathode before any work.

3. even a small CRT and 5 kV anode can cause a shock that results in more damage.

INEXPERIENCE IS DANGEROUS

Bon courage

Jon

While safety is important, that's a little over the top. This CRT is not *that* old, it will have integral implosion protection. Gloves and a face shield are useful for 1950s TVs but not necessary for this. No need for a resistor either, just wrap a wire around the shaft of a screwdriver, connect one end to the ground strap on the aquadag and poke the other end under the "suction cup" and you might get a small static zap sound as it discharges. This is only necessary if you're going to disconnect the anode though, otherwise just leave it alone.

[jonpaul]

very sorry to disagree

1. even a small CRT implosion is very traumatic, the pictured CRT has no shield on the envelope, just the face band.

2. Stored HV charge in CRT Ultor is retained for many minutes or hours, the zap can cause a muscle kick that has other consequences.

The OP seems unaware of CRT precautions

Jon

[Chris56000]

The nature of the OP's fault means that he has no need to go anywhere near the final anode cap of the CRT or to attempt to remove it from the chassis to diagnose a bright white screen – it is OBVIOUS that adequate extra–high–tension voltage (10 kV typical for this type of CRT) is being supplied from the line–scan flyback transformer !

Modern CRTs have been "implosion–protected" for the last 50 years but that does NOT mean they're unbreakable – it is all too easy to accidentally destroy one if you are not careful by removing or replacing the base connector socket !

ALL reputable service manuals for equipment containing cathode ray tubes carry warnings about the high voltages employed to drive them and also about the lingering charge on the Final Anode, as well as warnings about the extremely high degree of vacuum to which they're evacuated and the implosion danger – in fact the small monochrome chassis can be more hazardous than colour ones, since the integrated diode–split type of flyback transformer in a colour chassis will discharge the EHT within about an hour to a much lower voltage, because of the built in EHT/focus potential divider, but SOME charge will be left, which must be discharged with a shorting lead to chassis or the outer conductive coating of the CRT BEFORE any attempt is made to handle it !

Monochrome chassis do NOT have a bleed/divider resistor built into the transformer so these MUST be discharged by short–circuiting the anode to chassis before any attempt is made to handle their anode caps or CRTs, but as I said at the beginning the OP has no reason to disturb either the anode cap or the CRT itself to diagnose his fault !

Chris Williams

[roli]

When it comes to safety I am all aware of the high voltage danger lurking bellow the anode cap. Though I haven't really considered the implosion/tube breaking part of it.

I've gone and re-soldered all of pins on the neck board, and I re-soldered all of the pins on the flyback as well. I've also removed the neck board and sprayed some contact cleaner to the CRT connector, because the pins were looking a bit ugly. No change.

I've also measured the two wires that were mentioned. Or at least I think those were the ones. The connections seem to be fine.

No pots named "Screen", "G2", or "sub-bright" anywhere. There are pots named "V. LIN", "V. AMP", "FREQ", "V. FREQ". On the neck board there are pots called "FOCUS", "CONTRAST" and "RV1".

Check choke B200 and R200 on the little CRT neck board for dry–joints, also R64 and R66 on the main monitor board for open–circuits, and resolder all the pins of the flyback transformer – the 15 kHz h.f. pulse voltages and the heat generated cause the solder to decompose round all the transformer pins !

You might have to replace the video transistor TR202 on the little CRT board if it has gone short–circuit !

If you're really, really unlucky, the CRT itself might be at fault due to a heater–cathode short, but in all my years of servicing CRT monitors, I only came across this fault once – it's much  much less prevalent than it was in the old days of black–and–white TV only !

There are no components with these designators on there.

I've posted some additional pics if anyone wants to point out anything that could be of interest. Sadly it's impossible to take a good picture because of horrible cable mess everywhere.

[james_s]

very sorry to disagree

1. even a small CRT implosion is very traumatic, the pictured CRT has no shield on the envelope, just the face band.

2. Stored HV charge in CRT Ultor is retained for many minutes or hours, the zap can cause a muscle kick that has other consequences.

The OP seems unaware of CRT precautions

Jon

In ~30 years of repairing hundreds of CRT monitors I have never once had a tube implode from working on it, the only time I had that happen was throwing a dead one into a dumpster and the bell of the tube came down on the hinge, ever since then I always would break the neck before disposing of a worn out tube.

The zap hurts, but there is no need to touch the anode during most servicing, if you don't mess with the anode it can't give you a shock.

[james_s]

No pots named "Screen", "G2", or "sub-bright" anywhere. There are pots named "V. LIN", "V. AMP", "FREQ", "V. FREQ". On the neck board there are pots called "FOCUS", "CONTRAST" and "RV1".

It could be RV1, where does that connect to? I know what the other controls you mention are, don't mess with those, they're not related to your problem.

[roli]

No pots named "Screen", "G2", or "sub-bright" anywhere. There are pots named "V. LIN", "V. AMP", "FREQ", "V. FREQ". On the neck board there are pots called "FOCUS", "CONTRAST" and "RV1".

It could be RV1, where does that connect to? I know what the other controls you mention are, don't mess with those, they're not related to your problem.

You can see it in the pictures. It's the black pot next to the large purple cap. It's the area that got most damaged due to corrosion.

I've marked the three legs of the pot with red circles. Cyan circles are where these eventually lead. One side goes through a diode and I think then lands on the big cap. The red wire coming from the analog board also connects here. It's marked W. The middle goes through a resistor and eventually lands on the middle copper area around the tube. The third pin connects to the black wire of the clear/black wire pair that comes together from the analog board and then splits into black/clear. Black one is marked W11 if I can read it correctly. Then continues to a group of three resistors (seen on the left side next to the clear coated wire (looking from component side).

[james_s]

That central ring of copper is ground, you can see how they've cut slots to make crude spark gaps to protect the electronics from internal arcs in the CRT. Somewhere I have notes with the pinout for small monochrome CRTs, every one that I've sen uses the same pinout, I'll see if I can find that. You can also look closely at the neck and visually follow the connections to the various elements of the electron gun.

[roli]

Well... some progress I guess. I tried rotating the RV1 which did nothing. So I put it back to where it was before. Then I went around with multimeter checking connections and measuring various resistors, diodes, the big transistor, ... Suddenly when I plugged it back in, the picture wasn't white anymore. Rotating the brightness knob now goes from... too dark to see almost anything to "just about right" at full brightness. Though it does seem to "flicker a bit" - goes a bit brighter and a bit darker on it's own. So, yeah. Something with the neck board, and most likely something physical. But what?

EDIT: Never mind, it is in fact RV1. I decided to try twiddling it a bit while the monitor was on. And it seems that a tiny bit of adjustment in one direction will instantly turn the monitor into a spotlight. I now got it "just right", but I think I need to find a new pot somewhere, because it is a lot twitchier than it should be.

EDIT2: Found a replacement. Everything is a lot more linear now. No more twitching. So I think the issue has been solved. I couldn't even get a consistent measurement out of the old pot when I pulled it out.

[james_s]

That actually sounds a lot like the issue with a TV I fixed the other day. The housing of the focus/screen pot assembly on the flyback had cracked releasing the tension on the screen pot wiper. Due to the fact that there is essentially no load created by the screen grid it was behaving like a sample & hold circuit. Turn the knob to whatever setting and then push in on it and the picture would snap to that brightness and stay there as the grid was charged up to the potential set by the pot. It sounds like that's pretty much what was happening with your monitor.

[DavidAlfa]

Those boards are terrible, very little solder, big holes, great tendency to cause broken joints.
I'd re-solder everything out! For example, this one.

[roli]

I already did. Those pictures were from when I first got this and before I resoldered everything on the neck board.