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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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23rd Jun 2022, 8:42 pm | #1 |
Tetrode
Join Date: Sep 2015
Location: Walsall, West Midlands, UK.
Posts: 81
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J2907-TR3 Alternative
I need to replace a faulty J2907-TR3 JFET. It's long since discontinued with no direct replacement. And the only information I've been able to find is here
https://www.asapaviationstock.com/ns...5961013309267/ I've attached a picture of the part, and also a snapshot of the circuit it's used in (it's Q23, bottom left) Any suggestions ? I'm thinking maybe 2N7000 would work Last edited by veedub565; 23rd Jun 2022 at 8:56 pm. |
23rd Jun 2022, 9:35 pm | #2 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,864
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Re: J2907-TR3 Alternative
No, not at all
2N7000 is a MOSFET with an insulated gate. The FETs on the schematic are JFETs and they are only being used as diodes, diodes used to clamp inputs to stop them swinging too far. This is a trick used to get lower than usual diode capacitance if speed is important. You could just try some 1N4148, but buy them from a reputable source, even these get faked and substituted. David
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24th Jun 2022, 7:07 am | #3 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,657
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Re: J2907-TR3 Alternative
These might do the job - https://www.ebay.co.uk/itm/263573919...sAAOSwjShdenex they are from a reputable seller.
Andy.
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24th Jun 2022, 8:02 am | #4 |
Tetrode
Join Date: Sep 2015
Location: Walsall, West Midlands, UK.
Posts: 81
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Re: J2907-TR3 Alternative
Ahh yes of course, I missed that 2N7000 was a MOSFET. I might give the 1N4148 a try, just to see if it fixes the fault, or at least has an effect. The leakage current might be higher though.
The faulty JFET measures 168ohm Drain to gate, 258ohm Source to Gate, 320ohm Drain to Source. So seems to be acting more like a resistor than a diode, not good. The J112 only seems to be rated to 35v and I need 60v |
24th Jun 2022, 10:50 am | #5 |
Dekatron
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,208
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Re: J2907-TR3 Alternative
Some years ago I needed to replace a low-leakage diode in a digital AVO meter. Turns out the original part was a JFET internally strapped to make it act as a diode. Back then the Vishay JPAD50 was recomended, and it worked perfectly. This has now been discontinued, but there might be some around somewhere or perhaps similar parts are still made.
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24th Jun 2022, 4:06 pm | #6 |
Tetrode
Join Date: Sep 2015
Location: Walsall, West Midlands, UK.
Posts: 81
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Re: J2907-TR3 Alternative
Seems I may have been getting a bit hung up on the voltage rating. I'm told because of how they are wired up, they're just acting as protection diodes and not actually handling that much voltage. So the J112 or JPAD50 would probably do just fine.
I imagine when this instrument was designed Fluke probably just used what they had lying around. Anyway I tried a 2N3812 and nothing went up in smoke. Might order a supply of J112's though |
24th Jun 2022, 9:13 pm | #7 |
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,864
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Re: J2907-TR3 Alternative
As they are wired back-to-back, the only reverse voltage a junction will see is the forward voltage of its twin. So 60v is irrelevant as the voltage is clamped at about 0.7v.
David
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