The results seem ok.
The voltages +5.4V/+48.8V at R915 seem to suggest that the INTENSITY control is working.
We need to look at the HV CRT BIAS circuit.

 

Thanks for the help.

I imagine the resistors are fine; but circled them just in case.

The maximum voltage my multimeters can handle is 600V.

 

By the way, I noticed that another component is getting quite hot -even though it has a heatsink- and the board is also darker around it. It's IC1121.

 

Adam, you might like to watch the video in post#11 here.

http://forum.allaboutcircuits.com/threads/testing.103403/

High voltage can easily creep horizontally and you have kV available within the scope.

Are you sure there is no HV tracking along the board?

 

With the CRT BIAS pot in the middle:
- 45.5V with the intensity knob at minimum
- 46.5V with the intensity knob at maximum.

The voltage corresponds to the movement of the intensity knob, but the brightness on the screen does not. It rises up to around 46V, and then drops if you continue up to the maximum at 46.5V.

With the CRT BIAS at minimum I get:
- 8V with the intensity knob at minimum and at maximum; there is no change in voltage or brightness on the screen.

And with the CRT BIAS at maximum:
- 75.6V with the intensity at minimum and at maximum; though the brightness increases towards the maximum.

 

Adam, you might like to watch the video in post#11 here.

http://forum.allaboutcircuits.com/threads/testing.103403/

High voltage can easily creep horizontally and you have kV available within the scope.

Are you sure there is no HV tracking along the board?

I'm not sure what to look for. If there is: should it also say it on the schematic?

By the way, thanks for the video; I got quite a spark the other day measuring a HV component by mistake with the multimeter. Thankfully nothing else happened.

Never got near again the HV side of the board. Anyway, most of it is in a metal case and marked as "HIGH VOLTAGE DANGER". I did open it to check the resistors though.

 

In the video when he tested the HV screwdriver, there was a buzzing noise before actual breakdown.

This means that the handle insulation was partially breaking down, but not enough for us to see a spark (you can sometimes see them in the dark).

This can also happen on a circuit board, or in an item of equipment and lead to the 'burning and blackening' you see.

We call this high voltage tracking. Sometimes the board gets wet or some other contaminent gets on it and the residue after evaporation provides a low(er) resistance path that allows this to happen.

 

I think I see what you mean; Dave Jones did a video about it recently.

On the top side of the board I can't see anything wrong, except NL1025 (right-top side of schematic 7) -which is like a little light bulb with the symbol of a capacitor inside a circle- and which sparks inside every time you turn the oscilloscope off.

I'll have a look under the board this week when I have a chance.

 

I think I got it!

On the bottom side there was like a powder residue around connector P1001; through which the HV goes from the CRT BIAS circuit to the tube. after cleaning it, when I touched it all 3 pads moved.

 

Sounds a good way to go.

 

I can't believe it; I was almost sure I got it. It's still failing.

The only other thing I noticed is TR852, which is the worse looking from the bottom side. When I measured it (in circuit) I got readings that I should get from a working NPN transistor; but since the measurements were done without disconnecting it I'm not sure.

 

I just saw w2eaw using a special probe to test the high voltages on his new video on how to calibrate an analog oscilloscope (great video as usual, by the way). Buying one just to use once would be kind of wasting my money, so I was wondering: is there any way to make an equivalent at home? Maybe not a probe, but a sort of voltage divider that I can attach safely when the oscilloscope is off.

 

Experienced constructors can make high voltage dividers, but the problem to overcome is to prevent the HiV tracking round the outside of the insulation used.

That is the purpose of the wide flanges in HiV probes.

Obtaining a sufficiently high resistance for the divider resistor is another.

Say you wish to divide 100kV down to 100V, a division of 1000.

To allow 1 microamp through the chain implies a chain resistance of 100,000 megohms.

 

To allow 1 microamp through the chain implies a chain resistance of 100,000 megohms.

... And with resistances, even the slightest stray capacitance in the divider will have an extreme effect on the bandwidth of the probe.

Here is how Tektronix did it -- ignoring a lot of details like the freon inside the probe.

 

Been looking at the prices of high voltage probes, and I would probably get it fixed for less by a professional technician.

... and on top I'm not even sure the problem is the high voltage.

The good thing is that I can still use it. Hope it lasts until I win the lottery to buy a new one.