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


Ed Breya
 

It may be good to rethink and summarize what's going on with these parts. Here's my take on it, presuming I haven't missed something.

1. There is one "good" original J310 that works properly in either channel. This indicates the circuits for both are likely OK, as long as the "right" JFET characteristics are present.

2. The main issue is finding another JFET - a J310 or similar one - that has the "same" characteristics as the good one. Alternative part trials so far have been unsuccessful.

3. The input circuit of the 2215A is of the opamp-stabilized type, so should be quite tolerant of JFET DC characteristics.

4. The "proper" part is 151-1124-00 = Siliconix J2400, which apparently is not a standard commercial part.

5. You have to get the pinout right, and we're assuming a TO-92 package.

6. Measuring Idss

Recommendations:

1&3. You can be pretty confident that both channel circuits are OK. However, it's possible that one may have a minor fault or part out of tolerance such that it's OK with the good J310, but another part that's close and should work, doesn't quite. For now I think it's safe to say both are OK, and can be determined once a "right" part is found.

2. All along, the discussions have been mostly about getting Idss right - this is still the case, for DC conditions, one way or another. If you can find a VHF-class JFET (more on this in #5) that you can select to be close to the good J310, then it should work.

4. Let me explain a little about Tek and selected parts. In the old days, when particular characteristics were needed, if they could be selected out from stocks of generic parts at reasonable yield, Tek often did this in-house, and a dash-number was assigned to the base part number. This assumed also that the generic parts were used in sufficient quantity that those not selected out (usually the -00) would be used up elsewhere. Later, as the real cost of all the in-house selection and inventory and tracking complications became evident, there was a big push to reduce this sort of stuff. One way is to have the vendor pre-select for the desired characteristics, out of their much larger volumes, and assign a custom part number. This can be done if you're a large enough customer, and you pay a little more for the parts to cover the vendor's cost of doing it. The benefit is that you get what you need, under a single part number, and you know the true arm's-length cost. So, the supposition that J2400 is a custom part number is likely correct - it's selected by the vendor from one of their generic types, and sold only to Tek.

5. The pinout is important - more than just getting it connected right. What you want is a TO-92, with the source lead in the middle. VHF parts typically have this arrangement to minimize proximity and capacitance between gate and drain leads. The exception is if it's for common-gate topology, where you'd ideally want the gate in the middle, and it's RF-grounded, and shields the drain from source. So, in choosing possible candidates for substitution, first they should be listed as "VHF amplifier" in the application highlights. Then look at the pinout, and pick only those with the source on the middle pin. The drain and gate on the outside don't matter - you can always flip them around, but of course be sure what's what when installing.

6. For Idss, the easiest is to use a curve tracer and measure the original good one - it's the gold standard, so take good care of it. If you don't have a curve tracer, you can rig up a simple bias circuit for checking and comparing. The best would be to measure the DC bias conditions in the actual working circuit. The gate is assumed at zero, so all you need are the source and drain voltages, and you can estimate the drain current. Then set up a resistor bias scheme that gives the same conditions if the same JFET were present. Note that this is not an Idss measurement (unless the source happens to also be at zero) - it's even better - an actual in-circuit test that can be done on the bench to sort the best parts to try in the real thing. In reality, the vendor likely sorted for a certain Idss range at a certain drain voltage, which you don't know, but an in-circuit equivalent should be as good or better. There is a simple proxy for Idss, that you can use to pre-sort parts, rejecting those that are very unlikely to land close. Just measure the "on" resistance of the drain-source (with gate tied to either), with an ohmmeter, compared to that of the gold standard. I think you'll find that the switching type JFETs will be quite low (10-50 ohms), and the small, slow ones (like for high-Z DC amplifiers) quite high (over about 200 ohms). The VHF and HF ones are typically in the mid-range around 50-200 ohms.

The idea of putting in a temporary socket is good too, but a little awkward working down in the guts. That will certainly tell what works and what doesn't.

That's all for now. Good luck.

Ed

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