J2907-TR3 Alternative - UK Vintage Radio Repair and Restoration Discussion Forum

veedub565 · 23rd Jun 2022, 8:42 pm

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

Radio Wrangler · 23rd Jun 2022, 9:35 pm

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

Diabolical Artificer · 24th Jun 2022, 7:07 am

These might do the job - https://www.ebay.co.uk/itm/263573919...sAAOSwjShdenex they are from a reputable seller.

Andy.

veedub565 · 24th Jun 2022, 8:02 am

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

TonyDuell · 24th Jun 2022, 10:50 am

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.

veedub565 · 24th Jun 2022, 4:06 pm

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

Radio Wrangler · 24th Jun 2022, 9:13 pm

Quote:

Originally Posted by veedub565

The J112 only seems to be rated to 35v and I need 60v

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

23rd Jun 2022, 8:42 pm	#1
veedub565 Tetrode Join Date: Sep 2015 Location: Walsall, West Midlands, UK. Posts: 81	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 Attached Thumbnails Last edited by veedub565; 23rd Jun 2022 at 8:56 pm.

23rd Jun 2022, 9:35 pm	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,864	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 __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

24th Jun 2022, 7:07 am	#3
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,657	Re: J2907-TR3 Alternative These might do the job - https://www.ebay.co.uk/itm/263573919...sAAOSwjShdenex they are from a reputable seller. Andy. __________________ Curiosity hasn't killed this cat...so far.

24th Jun 2022, 8:02 am	#4
veedub565 Tetrode Join Date: Sep 2015 Location: Walsall, West Midlands, UK. Posts: 81	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
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,208	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.

24th Jun 2022, 4:06 pm	#6
veedub565 Tetrode Join Date: Sep 2015 Location: Walsall, West Midlands, UK. Posts: 81	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