WTB: Matched JFETs for Tektronix 2215 repair - Page 1
Forum section: electronics / general

author: giosif
author_rank: Frequent Contributor
timestamp: April 24, 2021, 09:11:18 pm
content: Hi all,I am trying to repair a Tektronix 2215 scope with a non-working horizontal sweep and I am in need of a matched pair of JFETs (Q630A and Q630B) bundled by Tektronix under the P/N 151-1042-00.If anyone happens to have one such matched pair they can spare or, otherwise, a spare A12 attenuator / sweep board, please let me know, as I am interested.I should mention I already checked in the usual places on the Internet (i.e. eBay, Qservice, Sphere), but there doesn't seem to be any in stock.Thank you!

author: bob91343
author_rank: Super Contributor
timestamp: April 24, 2021, 09:28:59 pm
content: What is the generic part number? In what way are they matched? With that info you can match a pair yourself.

author: giosif
author_rank: Frequent Contributor
timestamp: April 24, 2021, 09:46:38 pm
content: There is no generic part number mentioned in the manual, although I did see it mentioned as 2N5454 in a few places.In any case, you can see below a photo of the actual JFET.With regards to the matching criteria, the manual says "[...] the Idss (drain current with the gate-to-source shorted) characteristics of each are nearly identical."I won't pretend I understand this entirely.

author: floobydust
author_rank: Super Contributor
timestamp: April 24, 2021, 09:56:07 pm
content: It's the "2N4416 family" for Tektronix 151-1031-00,151-1042-00,151-1070-00,151-1144-00,153-0559-00 JFETs.Tektronix 151-1042-00 matched pair:Intersil ITS31259Siliconix J2012 (pair)2N5245Motorola SPF627M2National Semiconductor SF50031TI SKA5390Sphere Research Corp.in Canada lists them but as 2N5454 and no matched pairs in stock.I'd get get a bunch of JFETS and hand match them.

author: giosif
author_rank: Frequent Contributor
timestamp: April 24, 2021, 10:12:47 pm
content: Thank you for the additional details and the documentation on how to match them!Yes, I saw those non-matched ones on Sphere as well and I would consider manually matching a pair, but the question is: how big of a bunch do I want to order, so I have a decent chance of finding a matched pair?

author: TimFox
author_rank: Super Contributor
timestamp: April 24, 2021, 10:15:50 pm
content: Matching for Idss is simple: wire a socket with the gate shorted to the source and apply the desired voltage from a battery through a DMM in mA mode. You may have to allow warm-up time to stabilize, or carefully record the initial current.

author: giosif
author_rank: Frequent Contributor
timestamp: April 24, 2021, 10:22:46 pm
content: Quote from: floobydust on April 24, 2021, 09:56:07 pmPossibly silly question, but did you mean the above P/Ns may be interchangeable?I am asking as I found 153-0559-00 available on eBay.Thanks!

author: floobydust
author_rank: Super Contributor
timestamp: April 24, 2021, 10:46:20 pm
content: I included all the Tek P/N's for the search engines, basically the same 2N4416 family JFET but sorted/matched differently and three possible packages TO-92, TO-71, TO-72.The 151-1042-00 JFETs are matched within 0.5mA within a spread of 10-15mA IDSS, at 15V. So that is more important - that the pair is matched, and absolute IDSS can vary more.I would say the 0559 is a contender but have not analyzed the circuit and it's TO-72C package.For boutique JFET manufacturers, Mouser carriesInterFETbut USD $10 each seems outrageous.Linear Systemshas JFETs singles and duals, not sure about distribution or pricing in your country.

author: bdunham7
author_rank: Super Contributor
timestamp: April 24, 2021, 11:05:47 pm
content: Here you go. You can corner the worldwide market.https://www.ebay.com/itm/Tektronix-151-1042-00-Pair-of-Transistors-2N5454-N-Ch-JFET-TO-92-Pack-of-10-/283989822064

author: giosif
author_rank: Frequent Contributor
timestamp: April 24, 2021, 11:07:16 pm
content: Quote from: floobydust on April 24, 2021, 10:46:20 pmI suspected it might not be a simple matter of package...For Linear Systems, there is a distributed here in UK, but they don't show prices on their page (you need to ask via email).However, I might have an option to ask a friend living in the USA to order for me.For the dual JFETs, can I safely assume they are matched or that's only from the perspective of physical space economy (i.e. one package vs two for single ones)?

author: giosif
author_rank: Frequent Contributor
timestamp: April 24, 2021, 11:08:50 pm
content: Quote from: bdunham7 on April 24, 2021, 11:05:47 pmWow, 10 packs of pairs!I'm not sure I want to get that many, though.In any case, it is listed as out of stock...

author: Oldtestgear
author_rank: Regular Contributor
timestamp: April 25, 2021, 09:11:38 am
content: Linear Systems do ship direct from the USA for small orders OR Micross Components in Norwich are listed as an authorised distributor. I have bought from the company years ago ( when it was actually Mintech Ltd.) without any problems. No affiliation with either company.Phil

author: Kleinstein
author_rank: Super Contributor
timestamp: April 25, 2021, 09:43:41 am
content: For IDSS matching one would have gate conencted to source. For a known circuit one may get more relevant mtching by having a comparable resistor at the source and this match at a lower current, closer to the actual use.For 0.5 mA from a 10-15 mA range, I would expect something like 10 Fets as a good start for looking for a good pair. Quite often the parts from one batch / bag are already not that far appart, covering the whole range. With no other constraints it does not take so many to find a good match. It would take 11 Fets to be sure to find a good pair. Chances are good already with less as there are many combinations and a slim chance to have them all appart by chance.Today the problem is more finding the TO92 variant. The TO18 versions are often quite expensive and may not be a valid substitute because of added capacitance.

author: giosif
author_rank: Frequent Contributor
timestamp: April 25, 2021, 11:21:31 am
content: It looks I have a few options available, so I need to evaluate which one to go for.Thank you all for your inputs!

author: edavid
author_rank: Super Contributor
timestamp: April 25, 2021, 07:02:22 pm
content: I don't think the matching is very critical here. If you have 2 JFETs that are fairly close (within 2:1?), you could put the higher IDSSJFET in place of Q630B and add a source resistor (trimpot) to get it closer, or you could replace Q630B with a BJT current sink tuned to Q630A's IDSS.(If you just want to get it working, you can start by replacing Q630B with a resistor.)