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Old 15th Dec 2015, 3:41 pm   #1
WaveyDipole
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Wink What to replace an OF619 with?

These are in the EHT section of my Hameg scope. They have no markings, but on the circuit they appear to be diodes, marked OF619. However, I cannot find any reference to this part number online. I have also checked DF619 just in case (the lettering on the diagram is difficult to read). They are in a circuit with a couple of 0.1 1kv rated capacitors, so I imagine these diodes are also high voltage and hence the ceramic looking coating. A quick search brings up 1N4007 and 1N5408. Would either of these plastic diodes be an appropriate substitute?
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Old 15th Dec 2015, 4:03 pm   #2
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Default Re: What to replace an OF619 with?

Those will be a stack of fast silicon diodes inside the ceramic package and will be rated at 3KV or so.
They will not read on a regular diode tester.
You will need to use the high resistance range on an analogue meter to check them out.
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Old 15th Dec 2015, 4:44 pm   #3
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Default Re: What to replace an OF619 with?

BY184 type comes to mind.
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Old 16th Dec 2015, 7:37 am   #4
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Default Re: What to replace an OF619 with?

You may be able to use a Vishay BYX10GP, I bought 5 for 3 ish off ebay some time back for a scope. The 1N4007 has a PIV of 1000v from memory, the above has a PIV of 1.5kv.

BTW, you can test HV diodes with a bench PSU and a resistor, look up testing HV diodes.

Andy.
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Old 16th Dec 2015, 8:11 am   #5
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Default Re: What to replace an OF619 with?

1N4007 are also very slow switching, which makes them unsuitable for supplies running at any frequency higher than 50Hz.

David
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Old 16th Dec 2015, 10:21 am   #6
WaveyDipole
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Default Re: What to replace an OF619 with?

Thanks for the helpful replies. Ok, so that's the 1N4007 (and 1N5408) ruled out but BY184 or BYX10GP are possibilities. The diagram shows them as a single diodes but it has been indicated that these may be a stack of diodes, so how do I determine whether that is the case?

I have attached the diagram which shows the diodes as D608 and D609. The main EHT stack is, of course, shown at the bottom. The output of the arrangement of the above and C626 and C627 goes to the focus and trace brightness controls. The quality of the text is awfull even in the original. In fact, having rotated the diagram so that it is the right way up and having blown it up a little, I think the numeric part may be 419 rather than 619, but still no joy with any information for OF419 or DF419 online.

PS. Gents thanks also for the information on testing these.
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Old 16th Dec 2015, 2:45 pm   #7
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Default Re: What to replace an OF619 with?

Eight diodes for 12.5kV means a lot of volts/diode. 2kV minimum?
The rectifier is presumably silicon, if this is so, then passing say 1mA through it and measuring the voltage drop, you could get an idea of the number of diodes at 0.6V/diode.
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Old 16th Dec 2015, 7:47 pm   #8
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Default Re: What to replace an OF619 with?

You could put a lid on by using a couple of BYX90 (6KV/0.2A) EHT-rectifiers.
Seriously overkill, but they're available on eBay for a reasonable price.
...AND you'd never have to worry about them again.
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Old 16th Dec 2015, 8:20 pm   #9
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Default Re: What to replace an OF619 with?

2CL70 series?

http://www.americanmicrosemi.com/pro.../2CLseries.pdf
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Old 16th Dec 2015, 11:01 pm   #10
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Default Re: What to replace an OF619 with?

Hello,

This (German) article suggests BY133 -

http://elektronikbasteln.pl7.de/hame...ersorgung.html

Michael
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Old 17th Dec 2015, 4:21 am   #11
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Default Re: What to replace an OF619 with?

If you draw the circuit a bit differently, you can see it's actually a negative voltage-doubling rectifier. If the capacitors are only rated for 1 kV, then the diodes will see at most 1 + sqrt(2) times that = 2.4 kV reverse voltage.

I know of a similarly-packaged diode, the BY8424; which is rated for 24 kV PIV (when I worked in an electronics lab, I once managed to convince a colleague -- about whom the words "drawer", "sharpest knife" and "not" sprang to mind -- that they were very static-sensitive, but that is another story), but only good for 3mA. However, there probably is a whole BY84xx series, which could include a lower PIV part with a higher current rating.
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Old 23rd Dec 2015, 1:09 am   #12
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Default Re: What to replace an OF619 with?

There is. I think a BY84xx with a somewhat lower voltage rating would be the best replacement. There are also other older high voltage rectifiers that may be suitable. The BY50x and BY71x series come to mind.

The original OF419 is a custom or selected part made by Philips. Maybe a BY84xx with a higher current rating or a voltage rating in between or below that of other types that were officially in the BY84xx series.
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Old 28th Dec 2015, 4:26 pm   #13
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Default Re: What to replace an OF619 with?

I tried using a multimeter on a high resistance setting with the result that the resistance climbed steadily to over 500Mohm (presumably the caps charging up) in both directions at which point I stopped.

I then tried the searching for 'testing HV diodes' as suggested by Dr Wobble and came up with the test below. I set this up using the AVO on the 25v DC range as a voltmeter and a 270ohm resistor. I tried it with a PSU DC voltage of 15v as suggested. I measured a voltage of about 14.25 (i.e. a drop of about 0.75v and hence much less than expected) at the point indicated on the diagram. However switching the diode the other way around I got PSU volts. I experimented with reducing the voltage to 10v which now made only a tiny difference of about 0.1v, but upping to 20v gave me a 2v drop.

Since there is a clear voltage drop on both diodes, although perhaps not as much as the diagram indicated, am I correct in assuming that the diodes are actually OK?
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