HP 180A oscilloscope - neon bulbs in power supply

[JonnyBoats]

I picked up a HP 180A oscilloscope at a hamfest this weekend for $5 because it has issues ;-)

Basically it is super clean and looks generally well cared for. The one issue I see is with the high voltage power supply, I hear an occasional popping sound accompanied by flashing of the two neon bulbs (V301 & V302) on the high voltage power supply board. When this happens the beam jumps on the screen before returning to the normal position.

I have seen comments on the web that HP used to use these NE-2 neon bulbs as voltage regulators in their power supplies. Is this how they are being used here?

If so, could someone please describe how they are intended to function? Should they be on at all times during normal operation?

Also I hear they degrade with age, and these would be about 45 years old. Is it likely that one or both of these could be the source of my issues?

[edavid]

I picked up a HP 180A oscilloscope at a hamfest this weekend for $5 because it has issues ;-)

Basically it is super clean and looks generally well cared for. The one issue I see is with the high voltage power supply, I hear an occasional popping sound accompanied by flashing of the two neon bulbs (V301 & V302) on the high voltage power supply board. When this happens the beam jumps on the screen before returning to the normal position.

I have seen comments on the web that HP used to use these NE-2 neon bulbs as voltage regulators in their power supplies. Is this how they are being used here?

If so, could someone please describe how they are intended to function? Should they be on at all times during normal operation?

Also I hear they degrade with age, and these would be about 45 years old. Is it likely that one or both of these could be the source of my issues?

If I recall correctly, they are used as voltage clamps.  They should not be on.  They are not the cause of the problem.

It sounds like arcing inside the CRT, but maybe not.

[JonnyBoats]

Thank you edavid, it makes perfect sense that they could be used as a voltage clamp. It also makes sense that that they would flash at the same time as I hear what sounds like arching. I opened the case and powered the unit in a dark room to check if I could see any arching but I could not.

How would one check for arching inside the CRT?

[JackOfVA]

May be something as simple as an accumulation of dust on the anode power supply that can be fixed with a brush and careful cleaning. Over time, the HV attracts dust that builds up and the 180A has been around long enough to attract a fair load of dust.

Just be careful to discharge the anode power supply before you go poking around - they can hold a charge for a surprisingly long time.

[lowimpedance]

Since you are getting a trace, my gut feeling would be the CRT is probably okay, and as JackOfVA suggested it could well be a build up of grime and dust due to its age.
Old CRT televisions suffered from this problem and when the air humidity was higher it could cause similar symptoms to what you describe for the 180. Check for the dust build up but also any black sooty looking layer near the tube anode connection, quick wipe of the finger to see what gets deposited on your skin, if 'black-ish' then clean with a mild soapy solution on a rag.   ****( MAKE SURE THE TUBE ANODE POTENTIAL IS DISCHARGED FIRST BEFORE POKING AROUND THIS AREA!!) ****. If that does not fix it - well time to mull over the manual.

[ElectroIrradiator]

I had a look at the schematic for the 180A, and the neon bulbs are used as voltage limiters between the CRT cathode and control grid (g1).

If they light up during normal operation, then I'd expect if to be fairly weakly. When they flash, then it is because you have what you describe: A spark jumping on either the -2.9KV or the slightly less negative control grid rail (supplied by CR302), temporarily causing a huge voltage difference across the neon bulbs. Could be in any of the associated components, or maybe just due to dust buildup, as others have suggested.

[ElectroIrradiator]

If you get nowhere with the usual debugging techniques, then there is a way of checking if it is the CRT doing the sparking. Find a third neon lamp from any source, and in turn connect it between:

*) The junction of R330 and R331 and ground.
*) Wiper arm of R347 (but not R344) and ground. Edit: Use a 47K current limiting resistor in series with the Neon bulb for this position.
*) The junction of R332 and R333 and ground.

In normal operation it shouldn't light up when connected to any of these points. Any internal flashover in the CRT ought to be noticeable at at least one of these terminals, causing the Neon to flash when the discharge occurs.

Only do these tests, if you have ruled out other potential causes for your troubles. Otherwise you may get a false reading from these tests.

[PA4TIM]

First check all powerrails for voltage and ripple. Including the HV testpoouint. If that is not perfect all kind of strange things can happen. This is the first thing I do for every repair.

But I agree with the rest, I have seen arcing CRT's caused by dirt/soot at the CRT connection and on the HV board/section.

[ElectroIrradiator]

Yep, check the rail voltages first, particularly the -2.9KV rail.

If the HT voltage regulator, Q301-304, is faulty or improperly adjusted, then that could be your fault right there.