Really need to find obsolete J-Fet 2SK508G-k52, what to do?

[rwgast_lowlevellogicdesin]

I blew  channel 1 on my Siglent SDS1104X-E and I really would like to repair it, I locked myself in to the Siglent 11x series by buying the logic probe and a Siglent AWG. If anything id like a new GW-Instek MDO that has python built in as a second scope, but I wont have any money for that if I have to buy another Siglent scope to replace mine!

Anyways I combed through the front end of the channel 1 and I found some bad transistors, diodes, and a bad J-Fet. Hopefully that is the extent of the damage, it seems like an easy fix although the soldering has been challenging for me, with components being so small and so close. I managed to pull all the bad parts out of the scope for testing without damaging anything thankfully.

With some help in another thread I managed to identify all the components I need to order. I went to order them today and found out the J-Fet is already an obsoleted component! What the hell? Thus isn't an old scope!!!!

I need to find a SOT-23 J-Fet by Unisonic, 2SK508G-k52-AE3.  I can not find anyone selling these except a few surplus sellers I guess, the websites I found them on all want you to message them for a quote. I filled out forms at one or two of the sites to get a quote on 20 and have not heard back! There are people selling the 2SK508G-T1B in sot23 on eBay but I don't think they have the same Idss current as the SK52.

Im not sure what to do, or if I can substitute the original J-Fet with a different one, this is a scope front end so I would assume everything is pretty tight tolerances. I have a few J310s in sot23 the Idss is similar and there drain and gate voltages are 10 volts higher than the 2SK508G-K52. I just do not understand J-Fets enough to determine if something will work as a substitute in a scope front end!

[Kleinstein]

The J310 is not a very good replacement. It has a lower Yfs and thus more noise. The different gate voltage may cause an offset problem too. The J309 would be slightly better due to the lower gate threshhold, but still not good.

A better match could be the MCH3914-7. It comes in a slightly smaller case, but for a single unit one can usually still solder it to SOT23 pads.

[DeepLink]

Renesas does also make 2SK508-T1B (K52)
These can be found at Rochester - rocelec.com

Alternative - still active
Onsemi 2SK932-23-TB-E (SOT-23)

/Hans

[rwgast_lowlevellogicdesin]

I checked eoelec, and a search brought up nothing.

The onsemi part has a bit of a lower Idss than the k52, how much effect will that have?

I found this on ebay

https://www.ebay.com/itm/233257758605?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=6pNTEShvSa6&sssrc=4429486&ssuid=javOCFC9QkO&var=&widget_ver=artemis&media=COPY

Any reason to doubt the parts valitity?

[rwgast_lowlevellogicdesin]

I would much rather use the onsemi part, the 24 rank has an Idss of 14.5-24 while the k52 is 15-30.

Here is the onsemi part sheet, i can buy the 24 rank from digikey

https://www.mouser.com/datasheet/2/308/1/2SK932_D-3150145.pdf

If anyone can tell me if the 2sk508 k52 vs the 2sk932 24 should give me the same results id appreciate it!

[Kleinstein]

The SK932 has a much higher input capacitance - this can be an issue with the high frequency performance / BW or adjustment range for the input capacitance. How relevant the lower Idss is depends on the circuit and point of operation. It can be from no issue at all to not working (out of current / adjust range).  The bigger point could be having about 2 x the capacitance.