EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: scopeman on January 05, 2023, 01:23:37 am
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Hi Guys,
I am working on a Tek Telequipment D34 small portable analog scopes. There is a JFET, TR302 in the CRT circuit that is a Tek Part 151-1076-01 or a 151-1076-00 aka WN537B N Channel JFET
I can't find any data on this part or a replacement for it. I have tried 2N5457's and 2N5459's and they work well but only for a few hours. Being in an HV circuit it's hard to quantify what is going on other than perhaps breakdown. It seems that JFETS, especially higher voltage ones are rare these days.
I was thinking about putting some 20V or so zeners from the Gate to Source and from the Drain to source to protect the 2N5457 parts but am unsure of how that would affect the normal circuit operation of the current source circuit.
I have included a schematic of the HV Section showing the TR302 and a circuit description and a picture of a working one from the web.
So if anyone happens to have any of these parts I would appreciate getting one or two. The scope was made by Telequipment in England a subsidiary of Tek and overall is one of Telequipment's better efforts as it has a really sharp Tek ceramic bell tube and regulated power supplies (something the earlier Telequipment scopes were not known for).
Thanks for your time!
Sam
W3OHM
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With 150V on the drain, limited or no offerings for high voltage JFETs or depletion mode MOSFETS. Pretty rare bird.
It does not look hand selected though and estimating the operating point, you can calculate gate voltage but drain current is a bit complicated.
You could hit up Sphere (https://www.sphere.bc.ca/test/tek-parts/tekparts2.html) for the part and see what they could dig up, talk to the bank for financing.
Maybe consider LND150 (https://ww1.microchip.com/downloads/aemDocuments/documents/OTH/ProductDocuments/DataSheets/LND150-LND250-N-Channel-Depletion-Mode-DMOS-FETs-Data-Sheet-20005454A.pdf) VDS 500V, VGS(OFF) -1 to -3V, IDSS 1-3mA.
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Wow thanks for that. Now if I can find someone that has the To-92 version of that LND150 I think I'll be set.
Sam
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Unless you have a reason to go with TO92 beyond ease of installation/soldering (such as thermal dissipation), if I were you, I would order a few of the SOT23 version right now while it is still available : https://www.digikey.com/en/products/detail/microchip-technology/LND150K1-G/4902831 (https://www.digikey.com/en/products/detail/microchip-technology/LND150K1-G/4902831)
You should be able to install it with small wires. Or if you want to get fancy, a small PCB adapter.
PS: You can keep searching for a better option and replace it later on. But at least it secures a solution immediately. With the current chip shortage and factories pivoting away from certain exotic parts, I think your best bet is really to secure that solution while you can.
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You should be able to install it with small wires. Or if you want to get fancy, a small PCB adapter.
Search on Aliexpress for "SOT23 TO TO92" gives this:
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Well I believe that the other issue is the Vgs/Vgd breakdown voltage. The LND150 is +/- 20V and I think that is going to be a problem. I found some other depletion mode MOSFET's but they would have the same issue. I am trying to simulate the circuit but that is a PITA as well. I guess what I might do is to install another one of the JFET's that worked for a while and make some measurements if I can dig out my DMM HV probe that I have somewhere. That may help make the analysis easier. Alternately I could just reference the DMM to the -1630 volt supply.
Sam