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Old 2nd Apr 2024, 10:47 am   #1
Xyience
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Default TV18-2KT diode, test/replace

The pictured AEG TV18-2KT diode comes from a late 1970's B&W CRT monitor. It is connected between the flyback and anode cup.

I cannot get a current to flow through the diode in either direction when tested with 17Vdc or 37Vac in series with an ammeter, a current limiting resistor and the diode under test.

Have I done enough to prove the TV18 is no good?

I can find few references to the component, including this forum, but it seems to be rated 5kV 1A

Are there modern or period equivalents, or a good source of NOS?
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Old 2nd Apr 2024, 12:18 pm   #2
Maarten
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Default Re: TV18-2KT diode, test/replace

Make that 18kV 1mA.

I'd think 37VAC would be sufficient, but let's wait for conformation on that.
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Old 2nd Apr 2024, 2:44 pm   #3
Marconi_MPT4
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Default Re: TV18-2KT diode, test/replace

A quick check with a silicon BY8420 24kV EHT rectifier confirms that a current of 3mA is obtained at 19v....

However the TV18 is a selenium(?) type which typically requires a much higher forward voltage to begin conduction.

Unfortunately at the moment I can not find one to test for comparison.
Rich
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Old 2nd Apr 2024, 7:32 pm   #4
Maarten
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Default Re: TV18-2KT diode, test/replace

It's indeed a selenium stack, a tube filled with selenium discs so quite a number of them. I'm on the fence as to whether 37VAC is sufficient. I don't have one handy right now to test it.
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Old 2nd Apr 2024, 10:13 pm   #5
Joe_Lorenz
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Default Re: TV18-2KT diode, test/replace

Hello,

I think I still have some in my storage from salvaged TVs. As far as I remember I've never had a bad one in those days, but that was 40+ years ago. I can dig one out.

Joe
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Old 3rd Apr 2024, 10:41 am   #6
Xyience
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Default Re: TV18-2KT diode, test/replace

Many thanks for the responses.

So, perhaps my paltry 37V is too little to switch on the multiple selenium elements.
If I had a variable bench power supply with a higher voltage I could verify - but this is just one of many items on the wanted list.
If any one could test an equivalent diode I would be entirely grateful - as would this broken bootleg 'Space Invaders'.

There is no sign of life on the CRT itself (apart from neck glow), but the monitor chassis is showing various low and high voltages, and the transistors are nicely warm, so I am troubleshooting the obvious areas.
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Old 3rd Apr 2024, 11:59 am   #7
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Default Re: TV18-2KT diode, test/replace

I looked up the Wikipedia entry on selenium rectifiers which gave a very low value around 30 Volts for the reverse voltage of each element and 0.5V for the forward voltage drop. If you take a more optimistic 50V for PIV per element that is 360 elements for 18kV and a 180V forward drop so you would probably see little forward current below 100V or thereabouts.

I have no idea of the reliability of the Wikipedia numbers but they tie in with their illustration of a 160V selenium rectifier, presumably a US market.

Roger
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Old 4th Apr 2024, 7:28 pm   #8
Xyience
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Default Re: TV18-2KT diode, test/replace

Roger - thanks for the research. I will now assume that I have not used enough voltage to properly test the diode, so I will assume it is working and leave it alone.
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Old 4th Apr 2024, 9:39 pm   #9
mickm3for
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Default Re: TV18-2KT diode, test/replace

Hi i tested a tv11-2kt at 42v it read oc both ways tested 4 of them also test a tv20-16k80v also at 42v oc both ways all of them new . this sujests wou cannot test with a lo voltage. If you want a tv11-2kt or a tv20-16k80v drop me a pm Mick
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Old 6th Apr 2024, 10:52 pm   #10
Xyience
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Default Re: TV18-2KT diode, test/replace

Quote:
Originally Posted by mickm3for View Post
Hi i tested a tv11-2kt at 42v it read oc both ways tested 4 of them also test a tv20-16k80v also at 42v oc both ways all of them new . this sujests wou cannot test with a lo voltage. If you want a tv11-2kt or a tv20-16k80v drop me a pm Mick
Thanks for the check Mick
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Old 8th Apr 2024, 7:10 pm   #11
factory
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Default Re: TV18-2KT diode, test/replace

I've checked a few unknown spec high voltage diodes I have here, the Megger was used on the 1000V range and the volt drop measured with a DMM, open circuit voltage was 400V.

The first diode is marked US10/1 and the forward voltage measured at 85V.
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The second diode is a Varo H1455, this one is probably silicon, the forward voltage was lower at 11V.
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Hope this helps with testing unknown high voltage diodes.

David
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