Asymmetric VDR

[coquinho]

Ok, I'm trying to figure out how to replace brokens asymmetric VDR (polarized VDR?) like this one:



If I'm reading it right, its 12V, right?



Would a 12V Zener do the job?

There is also a 1.4V diode in other place, I believe it was a VDR also, replaced by a BZX75C-1V4, also a rare component. Is there any way I can make a substitute with other components?

Can't find anywhere close. Worse for the VDRs.

Its from a oscilloscope, PM 3226, from Philips. Service manual is here: https://www.electronica-pt.com/esquema/instruments/philips-pm3226-51780/

They dont say the values, just say that its a VDR on components list, also did not find any high res photos from this scope. Attachments below.

 

[srb1954]

Ok, I'm trying to figure out how to replace brokens asymmetric VDR (polarized VDR?) like this one:



If I'm reading it right, its 12V, right?



Would a 12V Zener do the job?

There is also a 1.4V diode in other place, I believe it was a VDR also, replaced by a BZX75C-1V4, also a rare component. Is there any way I can make a substitute with other components?

Can't find anywhere close. Worse for the VDRs.

Its from a oscilloscope, PM 3226, from Philips. Service manual is here: https://www.electronica-pt.com/esquema/instruments/philips-pm3226-51780/

They dont say the values, just say that its a VDR on components list, also did not find any high res photos from this scope. Attachments below.

You could try searching on the Philips 12-digit part number listed in the BOM. That might turn up some information although the format of the 12-digit part number may be an old one as it differs from that in the data books I have.

As a replacement for the 1V4 Zener you could use 2 silicon diodes in series, maybe in a SOT-23 dual diode package or a small infra-red LED.

[Kim Christensen]

Searching for info on part number "4822 116 20063" turned up this:

Hi, This is part you are looking for is indeed a VDR which is all the manual says. If you search the internet for philips component colour codes you will eventually find it is an "asymetric VDR" with a reference of 1.35V at 1mA Red top = 1.35V @1mA Black = 10% and the white spot on the brown body = cathode. Where mine failed on the vertical o/p stage (trace would not move up enougth) it was caused by this device, there are two in the vertical and two in the horizontal o/p stages. Fix was to put two diodes in series accross the faulty component (observe the white spot side = cathode k) To see which one of the two devices are faulty measure the volts accross them with the trace fully up for one or fully down for the other (same side to side if it is an horizontal scan problem) The volage should not go above 1.5V on mine it excedded this, leave the origional device in and fit the two series diodes accross faulty device and all should now work! Normal voltage accross this device is 0.3 to 1.4 volts. Wish you luck, work perfectly with my PM3226

I have no idea if replacing a VDR with two series diodes is a good idea for temperature stability. Not sure if you found this on your search.
I also did find a different color chart which confirms the guy's conclusion that it's a 1.35V @ 1mA VDR. I've attached it.

[coquinho]

Wow, did't know about this catalog. Nice to know.
So, the VDR that I think it was 12V is actually 1.35V... and the 1.4 zener that someone placed probably its not the same value as the VDR, because the VDRs next to the bunch of BD199 is a different part number! 572 and 573 are 4822 116 54012...

EDIT: No wait, I saw it wrong, its 4822 116 20063, same as you found, 1.4.





Its the same VDR, they just wrote it separately, don't know why.

EDIT 2: Now I just need to know if its a good idea two diodes in series, like two 1N4007, or there something else. I'm not find this 1.4 zener to sell anywhere.

[srb1954]

EDIT 2: Now I just need to know if its a good idea two diodes in series, like two 1N4007, or there something else. I'm not find this 1.4 zener to sell anywhere.

It is not actually Zener diode but is really 2 silicon diodes in series in the one package. The trade name for it is a Stabistor.

You would be better to use some low current diodes such as a 1N4148 rather than a larger 1A diode like the 1N4007. Since the Stabistor is run at a relatively low current the voltage drop of a pair across a pair of larger diodes might be a little on the low side compared to the Stabistor.

The smaller diodes will also have lower capacitance than the larger diodes, which is always preferred in a wide-band circuit such as a scope amplifier.

[Brianf]

It's worth noting that none of those 12NC numbers in the image above are standard numbers. I can't remember what the '53' prefix means but it certainly isn't usually found in front of a generic MR25 resistor.

[Kim Christensen]

It is not actually Zener diode but is really 2 silicon diodes in series in the one package. The trade name for it is a Stabistor.
You would be better to use some low current diodes such as a 1N4148 rather than a larger 1A diode like the 1N4007. Since the Stabistor is run at a relatively low current the voltage drop of a pair across a pair of larger diodes might be a little on the low side compared to the Stabistor.
The smaller diodes will also have lower capacitance than the larger diodes, which is always preferred in a wide-band circuit such as a scope amplifier.

I agree. Two small signal diodes like the 1N4148 in series should work. Stabistors like the "newer" 1N4156 appear to be spec'd similarly (1.35V @ 1mA) to the original one that the OP needs. A person could set up a small 12V power supply, resistors to limit current to 1ma, 10ma, etc, and bin a bunch of 1N4148 pairs using the 1N4156 spec as a guide... Probably don't have to get that precise about it but it would be nice to know that the diode pairs are at least a close match to the original VDR/Stabistor.