HP 1740A scope input fet

[AB3FL]

I lost the 'B' channel on my HP 1740A scope.  The 'A' channel works fine.  I traced it to a dual jfet device with hp part number 1855-0217.  This part is made of pure unobtanium.  Does anyone know if I can replace it with 2 regular plain old mpf102 fets? or some other fet.  It is right after the input attenuator and HP refers to the section as the impedance converter.


thanks

Tom - AB3FL

[WA1GFZ]

look at Siliconix parts maybe they still make dual FETs. I think Vischay owns them now. I would think a pair would work but since they are not a perfect match you might have a slight DC offset. U310 is also a good choice.

[KD6VXI]

Did a yahoo search, and someone else has the same problem, on channel B, in espanol.

Replaced member: JFET DUAL N-CHAN HP part number 1855-0217
(like from maintenance handbook). Sul JFET is written 55-0266.
I'm assuming they mean they SuB not Sul is for substituted.

Maybe you can find that number, it seems to be what that clown did and got his up and running.

--Shane

[KB3DKS]

You might check with Sphere in Canada. A very good source for test equipment parts.
http://www.sphere.bc.ca/

Bill

[AB3FL]

I got 2 mpf102's from Rat Shack and put them in.They work fine, at least for looking at waveforms and modulation

thanks all

Tom - AB3FL

[WBear2GCR]

they may or may not make the requisite bandwidth...

the Vishay (Siliconix) is a good idea, since Tektronix used them in the front end of 100mHz scopes for sure.
The duals keep the DC drift down.

               _-_-bear