Erik S
Well-known member
I'm trying to test some JFETs in-circuit and make sense of the readings. These are part of the keyboard for a synthesizer I'm working on.
I'm working from a lot of guesses and limited understanding, so if I'm saying something that makes no sense, please call me out! I'm also hoping that taking the time to type this out will help me organize my thoughts a little.
I included a section of the schematic showing the two pieces highlighted in yellow. Each is a metal can dual package JFET (IMF3958). The keyboard generates a different Control Voltage for each key, and in the case that two keys are pressed, a second voltage indicating the difference between the two keys. These CV values are then stored in capacitors (C2, and C4) so the synth can continue to play those notes after you release the keys. The JFETs are working along with some op amps to buffer the voltage from those capacitors. (The manual has an explanation of how that works, I certainly didn't figure that out just from the schematic).
I understand testing anything in circuit is not ideal, but given that these have 6 legs, and are out of production, I was hoping to collect some data instead of yanking them out to test them, especially since I'm not at all sure these are the source of my problem. I tried shorting each Gate to it's Source with my meter lead, and tested the Drain to Source resistance, expecting the channel to open, and resistance to go from high to low. One out of 4 gave me about what I was expecting, but the other three are still showing pretty low resistance in the "closed" condition.
From looking at the schematic, I would guess that each pair would test the same.
Some things I would love to know, if you know them and have made it this far, and are taking pity on me:
Does my test procedure make sense?
Did testing these in-circuit yield any useful data?
Would you assume one or both of these is faulty?
Would you assume that each pair would test the same?
Would you assume that each of the 4 individual JFETs would test the same?
What the heck does the dotted line oval symbol mean between C2 and Q2?
Should I stick to assembling pedals since I clearly don't know what I'm doing? Probably yes on that one.
I'm working from a lot of guesses and limited understanding, so if I'm saying something that makes no sense, please call me out! I'm also hoping that taking the time to type this out will help me organize my thoughts a little.
I included a section of the schematic showing the two pieces highlighted in yellow. Each is a metal can dual package JFET (IMF3958). The keyboard generates a different Control Voltage for each key, and in the case that two keys are pressed, a second voltage indicating the difference between the two keys. These CV values are then stored in capacitors (C2, and C4) so the synth can continue to play those notes after you release the keys. The JFETs are working along with some op amps to buffer the voltage from those capacitors. (The manual has an explanation of how that works, I certainly didn't figure that out just from the schematic).
I understand testing anything in circuit is not ideal, but given that these have 6 legs, and are out of production, I was hoping to collect some data instead of yanking them out to test them, especially since I'm not at all sure these are the source of my problem. I tried shorting each Gate to it's Source with my meter lead, and tested the Drain to Source resistance, expecting the channel to open, and resistance to go from high to low. One out of 4 gave me about what I was expecting, but the other three are still showing pretty low resistance in the "closed" condition.
From looking at the schematic, I would guess that each pair would test the same.
| Part | "Open" | "Closed" |
| Q2-1 | 400 ohms | 20 M |
| Q2-2 | 400 ohms | 27 k |
| Q5-1 | 400 ohms | 27 k |
| Q5-2 | 400 ohms | 24 k |
Some things I would love to know, if you know them and have made it this far, and are taking pity on me:
Does my test procedure make sense?
Did testing these in-circuit yield any useful data?
Would you assume one or both of these is faulty?
Would you assume that each pair would test the same?
Would you assume that each of the 4 individual JFETs would test the same?
What the heck does the dotted line oval symbol mean between C2 and Q2?
Should I stick to assembling pedals since I clearly don't know what I'm doing? Probably yes on that one.
