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


Tom Lee
 

I second the suggestion to use a J310. That's been my standard sub for general-purpose JFET buffers for a long time.

That said, I'm surprised that you didn't find joy with a 3819. The circuit is quite accommodating (necessarily) of a wide range of IDSS. The NTE133 should've worked, too; maybe it was just dead.

It's been some time since I took a close look at the 2215A's buffer, but a quick glance at the schematic says that the upper limit on acceptable IDSS is roughly 30mA. One that is around 10-15mA would be in the center of the zone, but the value is not particularly critical.

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 11/8/2020 20:29, charlesterrebonne@hotmail.com wrote:
HI All...

Got a Tektronix 2215A. Channel 1 was bad, no signal coming through. Channel 2 fine.

Found Q122 Jfet on channel 1 was a J210 part # and bad... already replaced a while back perhaps? should be orig J300 Jfet there?

After trying to sub Q122 without success with several Jfets on hand, including a NTE 133 that didnt work, decided to move Q222 (channel 2 same Jfet) to channel 1. This was found to be a J300 Jfet (never replaced?) and this made channel 1 work perfect and confirmed only prob on Channel 1 was indeed Q122.

Channel 2 is now without Q222. I tried to sub with several other Jfets I have on hand...k19...NTE132...Best result I had was a 2n3819 that works fine for about 1 minute then signal dissapears slowly. Freezing bring it all back. I suspect not a bad 2N3918, just freezing it make it behave more like a J300...that the circuit wants...

Can anyone confirm a few Jfet #s (still available? or not...I have large inventory of 70s-80s NOS parts...but no Jxxx Jfets) that will work reliably in that circuit? I think pretty much all that series of scopes used the similar input circuit. The Tek part # was 151-1124-00...

I have ordered some pn4392 Jfet but now thinking they will be no good in that circuit too...

Thanks



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