Electronics > Repair
Seeking a JFET repacement for a Heathkit 512 Mhz frequency counter
postfive:
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.
George Edmonds:
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
fzabkar:
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.
floobydust:
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.
fzabkar:
MPF102 crosses to NTE451.
https://assets.rs-online.com/v1698949606/Datasheets/8fda5560042eeecafa9d65a357a81797.pdf (NTE451 datasheet)
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