Electronics > Repair

Analog oscilloscope repair / JFET confusion

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Tokipoki:
Hi!

I need some help with a faulty analog oscilloscope. Long story short, CH2 was working as intended, CH1 was dead. After poking around for a few minutes, I found the issue, it was the JFET Q101 at the input stage, it is a 2SK55D.
(Out of curiosity I moved Q201 from CH2 to CH1, and voilà, CH1 works, so everything's fine except the original JFET.)

Here is the issue: I need a replacement FET, but the one I bought doesn't seem to work. Sadly, at the store I usually buy my stuff this exact part wasn't available, and they only had like 4-5 different JFETs, and that's it, so I checked the datasheets to find something similar.

I chose the BF256B.
Basically CH1 doesn't work with the new FET at all: AC/DC switch does nothing, GND does switch nothing, I can't get that 1kHz square wave to show up, there is nothing on the screen, except at certain vertical and horizontal settings I get a really faint, wide vertical green blob moving sideways.


Am I missing something? Are those replacement FETs no good, or is it some calibration issue, or something like that?
I attached the two datasheets and the related part of the circuit. (Edit: I wasn't paying attention, so the schematic I uploaded is for the second channel, but they are identical, except the component numbers start with a "1" instead of a "2", so in this case Q201 is the part I am talking about.)

Kim Christensen:
I think you should have chosen the BF256A instead because the IDSS spec of 3-7ma is a match to the 2SK55D IDSS spec of 3-7ma... Whereas the BF256B has a IDSS spec of 6-13ma.

I have to say that those two datasheets are a bit confusing with their typos in their IDSS specs:
2SK55 sheet shows Vds = 10A whereas I'm sure they mean Vds = 10V
BF256 sheet shows Vgs = 15V, Vgs = 0 ( They probably meant Vds = 15V, Vgs = 0 )

Note: You may be able to adjust it so the B version works, but you'd need to look in the manual for that procedure. Mark the adjustments with a felt pen before starting so you can get things back easier if you mess it up.

MathWizard:
I'd put the fets on a breadboard with a couple of resistors and with a PSU and DMM, see what the JFET's are really like. They may just need a little mod to the original circuit to get working somewhat or fully even.

IDK those circuits good at a glance, but yeah it still could need re-calibration, and who knows if it would work the same over the whole operating range.

But surely not a lost cause just over a JFET

But yeah, what was was this scope, what year ?And people were probably solving by hand how to bias these things. So what year of EE do you learn to make complex multi-level potential ladders all over the place to fir together just right at DC and AC

Tokipoki:
@Kim Christensen
Yeah I noticed that difference, but they only had the "B" version available. Is the parameter IDSS critical?

@MathWizard
It is a Goldstar OS-9100D, probably from the early 90s. Also Goldstar is the old name of LG, and as far as i know they changed the brand name around 1995, so they made this a little bit earlier.



To be honest I didn't do a deep dive into the circuitry or the service/operation manuals, since I expected it to be "plug and play". And I didn't had many options so I just grabbed a fet that was reasonably close... Seems like it's not that easy.

I will probably order the proper replacement parts... The reason I didn't do that originally, because my only options are ali or ebay, and at this point of the year it will be like a 1 month wait. I guess in the meantime I will look up some FET theory  and I do some measurements to compare the two channels

Kleinstein:
The Idss itself is usually not directly critical. It is more the gate source voltage for the intended operating current that matters. However the 2 parameters are correlated. So a high Idss also means a higher threshold. With somewhat different FET types there can still be a difference.  The BF256 in principle is a reasonable substitute and not a bad choice and not that much difference expected.
The circuit looks like it can compensate for a certain range of threshold, but the range is limited.
Even within the groups there is still quite some variation in the FETs. It may be worth checking the threshold range of the JFET - is also a crude function test, in case one has a bad one or wrong pin-out.

The test could be something like gate and source to ground with something like 100 K and drain to some 12 V.  The measured gate voltage should be near zero (the resistor is mainly for protection in case of a wrong pinout). The source voltage gives an idea on the threshold (depending on the resistor a bit lower, but OK for matching / comparing types).
Especially with parts from ebay a functional test can be a good idea - to  make shure it is at least a JFET and the pin-out is correct.

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