1. TekScopes@groups.io

Re: Q122/222 Jfet J300 subs for 2215A scope ?

Tom Lee
 

As I've said a couple of times, that circuit is very tolerant of FET parameters. It was designed to avoid the expense of matched FETs, so I very much doubt that any selection was involved here.

Are you certain that you are inserting these various JFETs correctly? Pinouts are not standardized. Perhaps you are assuming that they are.

Again, that circuit is just not fussy. Your problem lies somewhere else.

Tom

Sent from my iThing, so please excuse the terseness and typos

On Nov 16, 2020, at 6:35, charlesterrebonne@hotmail.com wrote:

thanks to all who replied - the 2215A scope is off the bench for now but Ill get back to it soon and will report when I get this working...I am almost at the point of sticking a high quality transistor socket there and just trying some other close to J300 specs Jfets I have on hand...FYI all the parts I have tried in the Q122/Q222 position in the scope : 2N3918, NTE133, k19, NTE132 tested good out of circuit...i have some more Jfet #s in my stock I need to try.

From the Tektronix replaceable parts reg. : 151-1124-00 = Siliconix J-2400...would'nt be surprised this was not a custom design but more likely tested/hand picked J300s jfets and assigned a Tektronix custom part# from Siliconix...searching Siliconix j-2400 brings up nothing...Id suspect most input boards from that 22xx series scopes came off the assembly line with "J300" marked Jfets in those Q122/222 positions..thanks
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