CRT circuit Tektronix 465

[Grapsus]

Hello !

I think your HV multiplier box might be good. You are looking for some kind of intermittent issue around the tube. It might be a failing capacitor or even a cracked solder joint that changes with temperature.
I would wait for the scope to break again then carefully and systematically inspect everything around the tube:
 - filament current (pins 1 and 14)
 - isn't the Wehnelt disabling the beam (pin 2)
 - do we have acceleration voltage (mesh, pin 12)
 - is T1420 oscillating ?
 - if not measure everything on the primary until you debug it

I'm not a specialist on CRT but I think having current in the filament and -1700V on the mesh is enough to have a dot on the screen. If anything is getting in the way and deviating it, you can just disconnect it until you get the dot.

Good luck !

[vangheli]

The high voltage multiplier is not difficult to make from parts.

Thank you. I have read the method used by GRAPSUS in an old post .I appreciate more advice ,I am inexperienced on HV.
Today I was able to access the multiplier , the transformer and the diode CR1421 .
Is it possible that an high voltage ceramic capacitor is bad despite an almost perfect measurement of capacity?
Thanks

[vangheli]

Hallo Grapsus ,
thank you for your help . I spent the day to disassemble the vertical board from the interface board :it was really a pain ,even following step by step the manual. I think that you already targeted the problem.At the moment I do not have an high voltage probe to check the voltages you said. But I can do the low voltages measurements. In my opinion the oscillator is working: the frequency is 50 khz , but I checked with an other oscilloscope a sinusoid of only 2 volt peak to peak on the collector of Q1418. Perhaps this is caused by a failing capacitor (C1421,C1422,C1423) on the side of the secondary of T1420 ,but their value measured with my DMM is perfect.The diode perhaps is the culprit , but at the moment I do not have the equipment to really test it.    By the way my diode is different from the one I saw on your pictures . Mine is anonymous , no marks for the cathode , no part number : perhaps somebody changed it.
Do you know what should be the correct   value of the resistance of the filament of the crt ?
I need time to do all the tests do debug the oscilloscope . I hope to profit of your knowledge.
Merci beaucoup!

[David Hess]

Is it possible that an high voltage ceramic capacitor is bad despite an almost perfect measurement of capacity?

It is very possible.  High voltage capacitors need to be tested at high voltage to reveal if they have failed.

[floobydust]

Another way to test the HV multiplier is using a signal generator and inject a 50kHz sine/square wave of at least 10-20Vpp (because multiplier HV diodes have high Vf). Discharge the CRT (using a resistor, a few in series to make say 1MEG) and then disconnect the HV wire from the CRT. You could leave it connected, the CRT anode then just acts as a capacitor.
Just to measure the multiplier's output, it should be around 6x and DC only coming out. I'm not sure- but the multiplier failures are likely a shorted diode inside.

Ceramic disk capacitors - if the dielectric is failing at voltage, there's corona hissing and a loud arc you will hear repeating over and over, but not in the potted module.

The other mystery HV diode CR1421 is "VG-12X; SILICON 12,000V 5mA" 250nsec 35mA and specs are Vf 22V to 30V, so it would read open circuit with a multimeter unless it shorted.

[vangheli]

Hi everybody.
This is a very late post about the repair of my scope Tektronix 465 , due to the late finding of the required part.
Well , the failed part was the high voltage multiplier. I changed it , and now the scope is working perfectly.
I arrived late to the correct diagnosis , because I was convinced that ,in order to isolate the circuit from the high voltage multiplier , it was enough to disconnect the ground wire of the multiplier ( this was also the more easy and lazy way to proceed ,because it does not require the disassembly of the board). I do not know the failure modes of the high voltage multiplier , but I can affirm that the only way to avoid interference on the circuit from  a faulty multiplier , is to completely remove it. As soon I did the removal , the high voltage oscillator resumed normal operation , the beam came back (with reduced trace length due to the missing post-acceleration and everything seemed promising for a successful repair.
Thanks to  all the contributors who helped me.