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Old 20th Aug 2018, 10:28 am   #1
mole42uk
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Default Typical EHT power supply faults?

I'm working on a Gould oscilloscope power supply. Most of it is working now, but extended testing revealed that the screen is a little dimmer than it should be and there was some cracking sounds from the EHT board. This board mostly contains 12kV diodes and 4kV ceramic capacitors.

I can see no evidence of electrical flash-over, but I'm wondering if the 40-year-old ceramic capacitors may be suspect or whether I should suspect the diodes as well. The plan at the moment is to replace all the ceramics.

Any help would be appreciated!
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Old 20th Aug 2018, 1:01 pm   #2
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Default Re: Typical EHT power supply faults?

Cleanliness can help. It doesn't take a lot of dirt to cause tracking.
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Old 20th Aug 2018, 1:16 pm   #3
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Default Re: Typical EHT power supply faults?

Check the ohmic value of high resistance resistors. It's almost a given that you'll find at least one that has gone high resistance. That could explain the dim screen (insufficient voltage). And yes, the ceramic caps. are also suspect: do any have built-in spark gaps? That could be where the cracking sound is coming from. An inspection in a darkened room can be revealing. I doubt if the diodes require replacement. Finally, ensure that the EHT board is really clean.

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Old 20th Aug 2018, 1:58 pm   #4
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Default Re: Typical EHT power supply faults?

Most likely The ceramic caps are ok and good quality ones too. If you replace them all you might just be fitting inferior parts.

The diodes are almost certainly good if you have a reasonable EHT. THe suggestion about looking for gone high value resistors is a very good one as is making sure the board is clean as it doesn't take much of a surface film to encourage tracking.

Usually if the EHT was running low one might expect tracking to be less likely though.

Also note that if the EHT is low probably the CRT would not focus properly and the Horizontal calibration probably off a little due to increased deflection sensitivity, though in post deflection acceleration CRT's this latter effect is not as noticeable, but usually if the EHT is off there will be other signs related to the CRT function, not just the beam brightness. So it could be there is another cause for the CRT being a bit dim. You will need to check the CRT grid to cathode voltage , as that approaches zero, from a value where the grid is more negative than the cathode, the CRT,s beam current is maximum.

The best way to work out if there is a problem somewhere in the EHT voltage multiplier chain is with a proper high voltage probe such as a Tek p6013 or P6015. THat way if there happened to be a partially faulty diode or capacitor in that area you would be able to measure it, otherwise it's a bit of guesswork and good parts get needlessly replaced, but you might get lucky

Last edited by Argus25; 20th Aug 2018 at 2:25 pm.
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Old 20th Aug 2018, 6:09 pm   #5
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Default Re: Typical EHT power supply faults?

Thanks guys, that's given me a new set of tests to do! Much cheaper than buying new Vishay 6kV capacitors. The ones on the board do not have spark gaps. I only have a Heathkit EHT probe that has a small analogue meter and I don't knopw if that can be relied upon. There is dirt on the capacitors and the board generally.

I checked the high-value resistors just now, one board has four 33MΩ 5% resistors. Using my Keithley 175 they measure 23.4M, 36.2M, 34.6M and 16M so I'll clean the board and re-measure. The other board was a lot closer to the design values but that will get cleaned anyway. Looks like a dishwasher session plus several days in the airing cupboard.
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Old 20th Aug 2018, 9:10 pm   #6
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Arrow Re: Typical EHT power supply faults?

A nominal 33 MΩ resistor that, upon test, read 16 MΩ. Well, I suppose that that is possible, but I would expect that a resistor of that ohmic value would normally go high R. I trust that when you measured it that at least one end of that R was not connected to anything? Whatever, changing them all is a trivial and a cheap matter.

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Old 20th Aug 2018, 10:13 pm   #7
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Default Re: Typical EHT power supply faults?

I had a low EHT problem on a scope when I refiited the cabinet. It turned out that a short co-ax cable in the HV circuit that had gone leaky. Not the actual co-ax, but rubber sleeving on the end to "hide" the screening. The rubber was conductive.
Replace any out of spec high value (10Mohms plus) with the VR38 type. (Vishay nowadays I think).
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Old 21st Aug 2018, 7:42 am   #8
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Default Re: Typical EHT power supply faults?

I've had this on a few scopes, but surely the EHT if working hasn't got a lot to do with brightness (though all previous replies apply), it's the Vgk potential that dictates brightness, amongst other things. So if your CRT circuit is ya bog standard resistive divider I'd check there, starting with a grid to cathode measurement. You don't need a EHT probe here, as the potential difference shouldn't be too great.

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Old 21st Aug 2018, 7:47 am   #9
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Default Re: Typical EHT power supply faults?

After an overnight clean in the dishwasher, and an hour's drying time on top of the Rayburn, the 33MΩ resistors measure thus: 33.0M, 32.3M, 32.1M, 32.9M. I hadn't appreciated the significance of dirt on EHT boards! The board will spend the rest of the day in the warming oven, that should dry it nicely.

I don't have time to do more testing before leaving work, but the other thing I suspect is the cable that joins the EHT board to the tube neck board. It is several thin wires of thin insulation all contained in clear flexible tubing. There may be some crossover at 2.5kV in there.....
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Old 22nd Aug 2018, 6:27 pm   #10
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Default Re: Typical EHT power supply faults?

Testing today didn't bring any good news. Although the EHT board is clean and dry there is no longer any trace on the screen. The remainder of the oscilloscope seems to work as expected.

I have checked the transformer by checking continuity of the heater and EHT windings, the associated resistors in the circuit measure correctly. I checked the heater in the tube by a resistance measurement so I know that isn't o/c.

Argus25 wrote: "You will need to check the CRT grid to cathode voltage , as that approaches zero, from a value where the grid is more negative than the cathode, the CRT,s beam current is maximum."

I don't know how to check this, could you be more detailed? I'm quite accustomed to working alongside 600 volts on a valve anode, but I want to be more sure where I'm probing on an EHT board.
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Old 22nd Aug 2018, 8:03 pm   #11
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Default Re: Typical EHT power supply faults?

Generally the CRT grid runs at a negative voltage with respect to the CRT cathode. It should never be positive with respect to it. Normally the brightness control adjusts this value or at least the average value by manipulating either the grid or the cathode voltage. Generally when the grid to cathode voltage is zero, the CRT beam current is maximum.

In a scope though, there are often blanking pulses applied to take the grid much more negative than the cathode (or they could be positive pulses applied to the cathode) to blank the beam during retrace. Also in single trigger modes the beam is blanked and there are also inputs to the grid- cathode circuit from the Z amplifier or input. So there are many faults or even control settings that could blank out the beam.
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Old 23rd Aug 2018, 7:07 am   #12
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Default Re: Typical EHT power supply faults?

....of course there could be other reasons why you cannot see the beam. It might be there but deflected off screen if there is trouble or an offset in the deflection amplifiers. One thing to try is to switch the scope to X-Y mode. The retrace blanking is disabled (as there is no trace) and the beam should just produce a dot at the screen centre with the X & y position controls near the centre. Start with the brightness control at low and increment it up. If a super intense sharp focussed spot appears it can be a bit harsh on the screen phosphor.

Because there are a number of reasons a beam might not be seen, even in a working scope, Tektronix sometimes added a "beam Finder" button, but I'm not sure if your brand of scope has that.

The test the grid or cathode voltages you need a high impedance meter, but in some scope circuits they could be significantly above ground and it could be risky if you are not familiar with it, though testing the relative voltage between the grid and cathode is safer, but check the scope manual & circuit first.
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Old 23rd Aug 2018, 7:26 am   #13
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Default Re: Typical EHT power supply faults?

Thank you. This is a Gould 4070-series DSO which has a useful "Auto Setup" button that attempts to provide a useable display of the incoming signal, and it has a "Menu" button which displays the settings for the digital computer. I have seen a useable display for many minutes over several fault-finding sessions until the dirt on the EHT board made itself a nuisance.

Even the heater connections are described as 6.3V at -2.7kV and although my Keithley 175 DVM is quite capable, I don't think it would like 2.7kV near any part of it, let alone that my test leads are not kV rated.

My overnight thoughts revolve around a 260V supply which, from memory having not looked at the service manual since I woke, might have something to do with the brightness of the screen. I have had trouble with this supply, the main PCB tracked underneath an electrolytic and although I cut that part of the board out of the circuit, I might not have completely resolved the problem.
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Old 23rd Aug 2018, 7:30 am   #14
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Default Re: Typical EHT power supply faults?

"I don't know how to check this...."One way is to find the pin out for your CRT (might be on your schematic but which one is pin one ?? A CRT pinout helps here.) ID the cathode and grid and pop your DMM test leads on. Watch out, the cathode might be at around 1000v below ground, so it is a shock hazard.

If you have no trace you can short the X pins on the CRT or do as Argus suggested. Or you can check voltages, both the X plates should be at the same potential/value for the trace to be in the center, TB off, twiddling the X/horizontal shift should make one go up, tother down. The same goes for the Y/vertical position etc.

But first check Vgk, it should be about 20v ish, twiddling the brightness knob should alter it from 100v to 20v ish.

Actually, check you havn't made a boo boo first, check all your voltage rails.

Andy.
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