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On 2021-11-29 1:21 a.m., HF via groups.io wrote:

Hi Toby,
Thanks for your suggestions!
I had read of references to the 2N5454 and 2N4416 elsewhere in this forum. Once I looked in the document Michael referenced, I found out where those came from. The table describing the matching technique says it's from the 2N4416 "family" and a later page refers to the part itself as a 2N5454. I was able to find specs on both of these devices, but I'm not sure that those are the specs that pertained to the era when Tek was building these instruments. For example, the TO-92 package isn't mentioned, and the 2N5454 is described as 2 devices in one can, not individual devices. But it's all I have, so I put these two at the top of my table for checking other JFETs for potential substitution.
I just looked up Linear Systems; I hadn't heard of them until I saw your post. It looks like that company is geared towards providing devices for which mass-produced devices are inadequate. I'm afraid I would be pestering them if I were to call them with a hobbyist need. But I might still do that, once I study the circuit some more and think I understand it well enough to know why the devices need to have the characteristics on my list.

Hi Halden

It is weird that Linear Systems is so obscure on the web.

I found them very helpful and accommodating with a small/hobbyist order, so I wouldn't hesitate.

It looks like I'm experiencing mission creep. At first I just wanted to get this 'scope working. But now I see an opportunity to learn something about JFETs and JFET circuits.

Same! There's a Teledyne book online that might help:


"The Art of Electronics" also covers them in some depth.

--Toby

Cheers,
Halden

If it's Q522/524 and Q622/624 that you are referring to those parts compare to a 2N5245. The Vgs cutoff is a whimsical -1.0 V max. A 2N5454/2N4416 are not even remotely close to being that small. Since this is a trigger/switching circuit app the 2N5245 makes sense in a zero crossing detector.

I am just quoting what one rev of a 465B manual as I heard there are several? Comments?

Fabe

Here’s an update on the 465B trigger circuit JFETs. First, I’ll provide details for those interested. Others, just skip to the last couple of lines for the conclusion.
Reading the schematic from the “missing pages” document at BAMA and the theory of operation in the service manual helped me understand how this part of the circuit works. It’s a voltage follower to provide a low-impedance signal to U7331 and protect it from high voltage signals. It seems that the main requirements for this part are low input capacitance to maintain the ability to detect and trigger upon VHF components of the signal, and the ability to provide Id in the 10-15mA range. Low noise might be helpful. Per the spec sheets and comparison to those of the 2N4416 and 2N5454 cited in the Tek documentation, I found that the J310s and J211s I had on hand might be suitable. To measure the Id characteristics near the operating point in the circuit, I built a similar circuit on a prototyping board with 100 ohm and 20 ohm resistors, plus a scrap of wire to change Vgs. I applied 6V via another 100 ohm resistor to measure Id. With each FET type, I adjusted the power supply voltage upwards and back a little bit to verify that I’m to the right of the knee on the Id/Vds curve. I recorded Id for drain resistances of 0, 16.7, and 100 ohms. 16.7 (100 and 20 ohms in parallel) was the closest I could easily get to the 15 ohms that the Tek circuit uses for the resistor between G and S.
The original unmatched JFETs had Idss of 11.3 and 12.2 mA, a difference greater than the 0.5 mA matching spec in the Tek documentation. Id fell to 7.5 and 8.1 mA with a 100 ohm resistor between G and S.
The currents in all 3 columns for the J211 were closer to the originals than the J310 numbers. I chose matching J211s and installed them in the ‘scope. The voltage at U7331’s input is now -15 mV. Id in the JFETs is now 11.3 mA. Vgs is about -0.17 V. It triggers on my strong 40 MHz sine wave signal. I haven’t tried more-challenging signals yet.
BTW, I noticed a discrepancy between a voltage in the circuit and its marking on the schematic. The +8V line was actually at 8.6V. I tracked down its source on the schematic. It’s derived from a 6.2V zener on a 15.0V supply line. It appears that this is the intended voltage, as there is no provision to adjust it. And in this circuit, it doesn’t matter anyway.
In conclusion, the J211 may be a suitable substitute for the 151-1042-00. The input capacitance is not clear on the datasheet, but with an intended application including VHF, it’s probably quite low. At 10 for less than US$ 4 from Mouser or Digikey, it’s pretty cheap to buy 10 and match them one’s self.
I hope this helps the next person. Many thanks to the people on this forum who helped me explore options and think this through!
Halden VE7UTS