Re: 465B won't trigger

Fabio Trevisan
 

Hello Lorn,
Great news indeed!... So your problem is narrowed down to exactly the same cause of mine: One, or both, of the input FET Transistors bad.
And I did exactly what you did, meanwhile I couldn't source another pair... I cannibalized the B sweep trigger.
In fact, there's very little impact to it, because 90% of the times, we use the B Sweep in the "STARTS AFTER DELAY" mode... and this mode doesn't use the triggering input buffers or the trigger amplifier whatsoever...

As for the source of the transistors you wrote:
Any ideas anyone?
Manufacturer part number: SF50031, which comes back to 2N5245 N channel JFET, which is obsolete. It's late and I haven't tried to cross reference that yet.
Not quite right!
I don't know from where you got the 2N5245, but according to Tektronix_Xref_sm.pdf available at Archive.org (https://archive.org/details/tektronix_Tektronix_Xref_sm), part number 151-0042-00 is found at page 7-7.
This part number is a characterized and matched pair of 2N4416 (which is probably also obsolete, but not that hard to find).
From the "stock" 2N4416 whose IDss can vary from 5 to 15 mA, those ones are selected to a narrower IDss range of 10 to 15mA, and the pair need to be matched within 0.5mA between each other.

Even if you cannot get a matched pair, as long as they're not too different, you can zero them out by tweaking the value of R7325 or R7327 (or both).
This will compensate for the different Vgs that will develop across each one of the **not matched** xsistors' source resistors, in order to get the output voltage as close to 0V as possible,
The fact they're not matched will have the down side their IDss will also drift differently with temperature so, tweaking the resistors solves the matching problem at one given temperature but as the temperature changes, they will drift apart from each other...
But since this is **only** triggering (I can see Howard Vollum revolving in his grave now), it's not that critical!. And you can always put the non-matched pair on the B sweep trigger, which is rarely used.

Good luck with your 465B, This is my dream 465 scope, with Alternate Delayed time base instead of the least useful "MIX" mode that's present on the regular 465s.
I have a 464 with DM44 multimeter (it has a weak CRT) and just recently got a 7623A with 7B53A timebase, so I'm probably married to the "MIX" mode for the foreseeable future.
I'll probably sell my 464 and watch out for a good 465b, or even a 468 (which is a 465b + digital storage).

Rgrds,

Fabio

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