Oscilloscopes CRT

Dear Radiophiles,

Can somebody tell me if it is possible to test the CRT of a oscilloscope,the same way that tests are made with valves and TV video tubes? My problem is that I have an old Nordmend oscilloscope UO963 that I should like to recover,but to start with I am not sure wether or not the CRT is OK once the heather is OK.. Thanks in advance for any help on this matter that   would be very wellcome.

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The main difficulty is the rather high voltages 1,000V to 10,000V.

Conventional testers only do up to 300, 400 or 600V depending on model. You can use High Voltage power supplies (with current limit) and then measure the cathode current.

One possible test is to pretend it's a diode and connect HT to grid (which is often a cylinder with a hole and measure cathode emission that way, just a variable bench PSU and current meter.

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Thank you Mr. Watterson for your sugestion.I shall try in place to measure the cathode emission.

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One should be very careful when measuring the cathode emission of a CRT, regardless if from a TV or an Oscilloscope! The usual operating beam current from a CRT varies between 100 µA and, let's say, 500 µA (0,5 mA). Colour CRT's in the early days were limited to 1 or 1,2 mA with a protection circuit. An emission test should therefore not exceed a few mA's for a short time!

A suitable test setup can be made by using a normal TV tube measurement/rejuvenator. Since the matching socket adapter is not available in most cases, the CRT pins of the DG7-74 shall be manually connected to the CRT tester.

• Filament connect to 6,3 V
• Cathode connect to common (ground) potential
• Wehnelt cylinder (G1) to the variable neg. voltage -80 ... 0 V
• G2 + G4 (the electrode just in opposite to G1) to 250...300V positive voltage.
• You will not need a high voltage (acceleration potential)!

Start the measurement with heater on and apply the G2 voltage. Turn the G1 grid control from -80 towards zero (0) volts. A beam current should be measured, which increases with lesser negative voltage at the Wehnelt.

A rough method to see if the CRT cathode is working: Just measure the initial voltage (or current) which occurs when the "space-charge cloud" encompasses the cathode.

• Heat the filament with normal 6,3V
• Connect a Milliampere-meter (or Millivolt) between K and G1
• Do not apply any other extern voltage than the filament

You will measure a voltage of 1...2 V (high input-R) or a current of 100 to several 100 µA's if the CRT works. In most cases this will show up a functioning CRT if the filament is not broken or the vacuum got lost.

Good luck!

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Thank you very much Mr.Nagel,for your most detailed explanation,as it is usual from you..Once I have one of those tester/rejuvenators,now I should be able to check if my CRT could be working.Also the rough method you tell me seems to be very helpfull.But I have another doubt concerning this special Nodmende UO963.On several oscilloscopes that I have had before,the EHT used to be NEGATIVE to earth.On this one,it is POSITIVE to earth,although the cathode is negative.Is this OK?.Thank you in advance for an explanation.  A.B.Regada

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