Author Topic: Help needed with Rat clone - JFET not passing signal  (Read 1118 times)

Catacomber

Re: Help needed with Rat clone - JFET not passing signal
« Reply #20 on: February 25, 2021, 07:24:21 PM »
Well, I thought it was closed, but for grins I thought I would try something. Since a diode test on these 2N5458s showed a .6v drop from center pin to end pin with positive on the center pin and the same result for either pin, I had assumed this was an NPN transistor. But I started testing other NPNs just to see if that is how they react but I could not match my results. Then I started to wonder if this really is a JFET but maybe with the pins out of order. I bent the center pin down so it would fit into the 3 hole on my socket. It actually works and seems to match the sound of the J202 I tried! See photo.

So here is the question - is there any way this could still be a BJT? I am not savvy enough to say yes or no. Any thoughts?



Catacomber

Re: Help needed with Rat clone - JFET not passing signal
« Reply #21 on: February 25, 2021, 07:47:28 PM »
I’ll add ne more thing - I also tried swapping the two non-gate legs by bending the pins and it still passed signal but probably about 20dB down from the other way. I could see a JFET doing that but wouldn’t a BJT just do nothing at all if C and E were reversed?

iainpunk

Re: Help needed with Rat clone - JFET not passing signal
« Reply #22 on: February 25, 2021, 08:08:36 PM »
I’ll add ne more thing - I also tried swapping the two non-gate legs by bending the pins and it still passed signal but probably about 20dB down from the other way. I could see a JFET doing that but wouldn’t a BJT just do nothing at all if C and E were reversed?
most Jfets are symmetrical, work the same both ways.
old Ge BJT's are symmetrical as well, but as the quality and HFE improved, the "reverse beta" mode became worse than the forward operating mode, not that it really matered to the manufacturers. with the advent of Si transistors, that became even more. the most transistors work alright in reverse, but the gain is greatly reduced, depending on architecture. the 2n2222 loses most of its gain, while the BD139 only loses 1/3rd. some transistors are way worse, going as low as 1, and darlington transistors don't work in reverse.

cheers, Iain
half man - half snail - 6 feet to scale - Snail man's - not frail - He's been - to jail - snail man is fuccing real
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Catacomber

Re: Help needed with Rat clone - JFET not passing signal
« Reply #23 on: February 25, 2021, 08:20:55 PM »
Thanks, Iain!  Sounds like I may have a BJT if swapping D and S on a JFET is normally symmetrical.  And with a center base pin.  I will be contacting the seller for sure!

PRR

Re: Help needed with Rat clone - JFET not passing signal
« Reply #24 on: February 25, 2021, 08:26:31 PM »
I answered this here recently.

One pin is 0.6V to either other pin, yes. That could be a Base or a Gate.

Leave the Base/Gate pin hanging in air and measure Ohms across the other two pins. A BJT will be infinity. A JFET will be hundreds of Ohms.
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Catacomber

Re: Help needed with Rat clone - JFET not passing signal
« Reply #25 on: February 26, 2021, 07:26:41 AM »
Ah yes, thank you PRR!  I do remember that but I failed to make a note of it.  I just tested across the other two legs and my DMM did not budge off of infinity, so I guess that nails it - BJT with a pin 2 base.  (I did the same test on my lonely J202 and sure enough it was about 500 ohms.)

I think this is finally it for this thread.  Thank you all so much for sharing your knowledge!

Catacomber