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


On 2020-11-08 11:29 p.m., 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...
The Tek cross-reference confirms Idss between 10 and 30mA. Vp max 4.5v.
Gm 6000 µMho.

Linear Systems site indicates J310 equivalent in TO-92, "U/J/SST308 SERIES":;partnumber=j310

They're easy to deal with & I'm a happy customer.


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


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