Seeking a JFET repacement for a Heathkit 512 Mhz frequency counter - Page 1
Forum section: electronics / repair

author: postfive
author_rank: Contributor
timestamp: November 19, 2024, 04:15:07 am
content: I'm repairing a Heathkit model IM-2420 512 Frequency Counter, and have identified a bad N-Channel JFET at Q1 shown in the manual as Part Number 417-828 and labelled as a "selected E304 transistor". The transistor has Heathkit's part number stamped on to it, so no help in identifying it further. I cannot find anything relating to a E304 transistor when using Octopart. The schematic shows the input signal feeds from the BNC jack to a .15 microfarad poly cap, to the input of a parallel 51pf cap and 100K resistor combination, and then to the gate of the JFET. A couple back-to-back diodes (FD777 / TD 777 per the manual) connect to the gate and go to ground. Thus this JFET sees the input signal and must have a flatline response beyond 512 MHz. Does anyone have any clues what this part actually is, or can someone recommend a suitable replacement for this JFET given the frequency requirements? Power is applied via a 10 ohm resistor to +12VDC.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: November 19, 2024, 04:40:06 am
content: E304 is a common 30V JFETReplace with BFS80, BFW11, 2N4418, 2N5245,If selected by Heathkit it was probably a below manufacturers spec device that was used.In the front end of a counter the performance is not critical.G Edmonds

author: fzabkar
author_rank: Super Contributor
timestamp: November 19, 2024, 05:15:06 am
content: An NTE cross-reference search for "417-828" ...https://www.sourceresearch.com/store1/quickstore.cfm?do=search... produces NTE312 -- "N-channel 30V JFET TO-92 case VHF/amp mix"https://www.sourceresearch.com/store1/quickstore.cfm?ProductID=54110&do=detailhttps://web.archive.org/web/20240621184924/www.nteinc.com/specs/300to399/pdf/nte312.pdf(datasheet)2N5245 and 2N5245A both cross to NTE312.https://www.farnell.com/datasheets/46878.pdf(2N5245 datasheet)Edit:I should have searched for E304.E304 crosses to NTE312.Two more possibilities are 2SK104 and 2N3819:https://elecurls.tripod.com/nte.htmhttps://bitsavers.computerhistory.org/components/siliconix/_dataBooks/1977_Siliconix_FET_Data_Book.pdf(E304 datasheet on page 113)http://www.bitsavers.org/components/national/_dataBooks/1977_National_FET_Databook.pdf(page 23 -- E304 = J304, J304 datasheet on page 124)The Idss range for E304 is 5 - 15mA, E305 is 1 - 8mA.

author: floobydust
author_rank: Super Contributor
timestamp: November 19, 2024, 05:41:14 am
content: I would say it's a hand selected JFET for low IDSS. Because they have so much variance, some are 10:1 in IDSSand the circuit is DC-coupled into NPN Q2.It seems to be 1.7mA which is at the lowest end of the part's 2-20mA rangeHeathkit 417-828 is specially selected 417-802. Consider:Interfet J305 (Mouser 106-J305; expensive) 1-8mAMPF1022-20mALinear Systemsalso might have offerings Digikey has them in the USA.

author: fzabkar
author_rank: Super Contributor
timestamp: November 19, 2024, 05:51:51 am
content: MPF102 crosses to NTE451.https://assets.rs-online.com/v1698949606/Datasheets/8fda5560042eeecafa9d65a357a81797.pdf(NTE451 datasheet)

author: postfive
author_rank: Contributor
timestamp: November 20, 2024, 01:32:44 pm
content: Thanks everyone for the replies. I need to improve my ability and skillset when doing parts replacement searches, so am wondering what tools you used to look up the part? I commonly use Octopart for specific part numbers but came up dry on this one. Google search didn't get me anywhere. And a specific query on DigiKey came up dry. So I'm thinking there must be some specialized electronic part search tools I'm not aware of.

author: fzabkar
author_rank: Super Contributor
timestamp: November 20, 2024, 06:46:46 pm
content: https://www.google.com/search?q=filetype%3Adoc+OR+filetype%3Apdf+%22absolute+maximum%22+E304+JFET

author: floobydust
author_rank: Super Contributor
timestamp: November 21, 2024, 07:42:00 pm
content: Nowadays you don't to be smart at all, aside from your search engine queries.Google search results are poor - its focus is more on shopping, based on their profile of you. Use other search engines.You went to electronic parts sites that are about selling you the part... but it's very old and their databases will be useless for that.Some of what I'll do, for the fun:Look at the IM-2420 Heathkit docs, BoM and schematic -> Q1 is an N-ch JFET source-follower, good to 500MHz and DC-coupled into Q2.Shows Heathkit p/n 417-828 with footnote "selected 417-802". Original is "E304" IDSS5-15mA.Lookup the freq. counter's history:https://archive.org/search?query=IM-2420&sin=TXTThe IM-2420 announced 1980 for $249 kit, $300 assembled. Kit almost $900 in today's dollars.Same front end as the IM-2410 but that is biased to 3mA.Gets lost in old magazines.Search for "E304 JFET". This is a transistor from 45 years ago so Digi-Key won't list those, the ones that were not popular.Also I found Q2 417-293 is 2N5770 RF NPN 30V 50mA fT=600MHz TO-92 see PN918, maybe MPS918 as well.https://www.d8apro.com/https://www.qsl.net/w6ovp/heathkitparts.htm

author: fzabkar
author_rank: Super Contributor
timestamp: November 21, 2024, 08:01:29 pm
content: Quote from: floobydust on November 21, 2024, 07:42:00 pmAre these available anywhere that doesn't want money for them? ISTR that somebody purchased Heathkit's IP and inventory, and then began asserting his copyright over the manuals. Previously, they had been freely available for download.

author: floobydust
author_rank: Super Contributor
timestamp: November 21, 2024, 08:31:26 pm
content: I find the kit manual posted on a few sites but the schematic is the same 100dpi fuzz. Many people must have just copied the .pdf from one post or something.

author: JDubU
author_rank: Frequent Contributor
timestamp: November 21, 2024, 08:56:56 pm
content: Quote from: fzabkar on November 21, 2024, 08:01:29 pmHeathkit Schematic and Manual Archive:https://www.vintage-radio.info/heathkit/

author: fzabkar
author_rank: Super Contributor
timestamp: November 21, 2024, 09:35:38 pm
content: Quote from: JDubU on November 21, 2024, 08:56:56 pmThanks. I should have searched for them, but I presumed they were gone.

author: David Hess
author_rank: Super Contributor
timestamp: November 21, 2024, 11:29:05 pm
content: Based on the faded schematic, it looks like 1.7 volt Vgs at 1.7 milliamps Id.Ft of a JFET source follower is roughly Gm/(2 Pi C), so candidate devices have high transconductance and low total output capacitance.I would pick up a couple of different available RF JFETs, like the BF256, J109, and J304, and see which works best. There is not a lot of choice about pinchoff voltage and drain current at zero gate voltage.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 22, 2024, 08:59:05 am
content: I would not consider the J109 a RF fets. That was likely a typo: a common RF type the the J309.