Author Topic: Seeking a JFET repacement for a Heathkit 512 Mhz frequency counter  (Read 890 times)

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Offline postfiveTopic starter

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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. 
 

Offline George Edmonds

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Re: Seeking a JFET repacement for a Heathkit 512 Mhz frequency counter
« Reply #1 on: November 19, 2024, 04:40:06 am »
E304 is a common 30V JFET

Replace 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
« Last Edit: November 19, 2024, 04:43:33 am by George Edmonds »
 
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Offline fzabkar

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Re: Seeking a JFET repacement for a Heathkit 512 Mhz frequency counter
« Reply #2 on: November 19, 2024, 05:15:06 am »
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=detail

https://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.htm

https://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.


« Last Edit: November 19, 2024, 06:11:40 am by fzabkar »
 

Offline floobydust

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Re: Seeking a JFET repacement for a Heathkit 512 Mhz frequency counter
« Reply #3 on: November 19, 2024, 05:41:14 am »
I would say it's a hand selected JFET for low IDSS. Because they have so much variance, some are 10:1 in IDSS and 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 range
Heathkit 417-828 is specially selected 417-802. Consider:
Interfet J305 (Mouser 106-J305; expensive) 1-8mA
MPF102 2-20mA
Linear Systems also might have offerings Digikey has them in the USA.
 

Offline fzabkar

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Offline postfiveTopic starter

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Re: Seeking a JFET repacement for a Heathkit 512 Mhz frequency counter
« Reply #5 on: November 20, 2024, 01:32:44 pm »
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.
 


Offline floobydust

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Re: Seeking a JFET repacement for a Heathkit 512 Mhz frequency counter
« Reply #7 on: November 21, 2024, 07:42:00 pm »
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" IDSS 5-15mA.

Lookup the freq. counter's history: https://archive.org/search?query=IM-2420&sin=TXT
The 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
 
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Offline fzabkar

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Re: Seeking a JFET repacement for a Heathkit 512 Mhz frequency counter
« Reply #8 on: November 21, 2024, 08:01:29 pm »
Look at the IM-2420 Heathkit docs, BoM and schematic ...

Are 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.
 

Offline floobydust

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Re: Seeking a JFET repacement for a Heathkit 512 Mhz frequency counter
« Reply #9 on: November 21, 2024, 08:31:26 pm »
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.
 

Offline JDubU

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Re: Seeking a JFET repacement for a Heathkit 512 Mhz frequency counter
« Reply #10 on: November 21, 2024, 08:56:56 pm »
Look at the IM-2420 Heathkit docs, BoM and schematic ...

Are 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.

Heathkit Schematic and Manual Archive:
https://www.vintage-radio.info/heathkit/
 
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Offline fzabkar

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Re: Seeking a JFET repacement for a Heathkit 512 Mhz frequency counter
« Reply #11 on: November 21, 2024, 09:35:38 pm »
Heathkit Schematic and Manual Archive:
https://www.vintage-radio.info/heathkit/

Thanks. I should have searched for them, but I presumed they were gone.
 

Offline David Hess

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Re: Seeking a JFET repacement for a Heathkit 512 Mhz frequency counter
« Reply #12 on: November 21, 2024, 11:29:05 pm »
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.
 
 

Offline Kleinstein

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Re: Seeking a JFET repacement for a Heathkit 512 Mhz frequency counter
« Reply #13 on: November 22, 2024, 08:59:05 am »
I would not consider the J109 a RF fets. That was likely a typo: a common RF type the the J309.
 


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