Is it dual gate or dual FET? If the latter then just tie 2 basic ones together with some white goo between would probably be adequate for all but the most stringent of uses.
-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on
Hello all, I am repairing a Keithley 617 that has a blown input JFET Q308 which is no longer available. Looking at the service manual, I am unable to determine what a good sub would be, as the part # is a Keithley house numbered part (617-606) The manual is here:
In this application, I don't think that this approach would work well... at least, not without some preparation. It looks as if this dual-JFET is being used as a high-Z buffer in a sort-of-differential input stage. Matching of the two JFETs in the package (for Idss, transconductance, pinchoff voltage, etc.) is probably important.
The original Q308 part probably had good matching between the two JFETs. It may also have been hand-selected by Keithley for a specific range of Idss.
There's enough process variation in JFET manufacture that two single JFETs, with the same number and from the same manufacturing batch, are likely to vary in Idss by quite a bit (tens of percent). This might unbalance the input stage enough to affect its accuracy. Tying the JFETs together thermally will certainly minimize thermal drift, but if they aren't already well-matched this might not help matters.
JW - are both sides of the existing dual JFET blown, or did just the left-hand size (pins 1-3) get fried? If the right side is still intact, you could measure the Idss, pinchoff voltage, and transconductance and thus get a better sense as to what you'd need to replace it.
I'd offer two practical alternatives to replacing it:
[1] Try what N_Cook suggested - use two single JFETs glued together. Grab a bagful, measure them for Idss and pinchoff voltage, and pick a pair which are well-matched. [2] Try a U440 or U441. These have gate-voltage matching to within 10 or 20 millivolts. Newark has both types in stock... the U441 with its 20 mV matching is less painfully priced and I suspect it'd do the job.
--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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There are other things in this circuit that are supposed to die before Q308. Q311 is a transient supressor. Replace this first, and keep it clean.
RL
Hello Legg,
Indeed they did! Q301, Q303, Q306, and Q307 were all shorted; CR316 was about 90 ohms measured either way, And U309's output was also blown. All those parts have been replaced. Now I think that the right side of the dual FET is also blown (pins 5,6,7.) When the meter is in zero check mode (K307 closed) I can pretty much zero the meter; when I turn off zero check it goes into over range.
Given the above, would you still recommend this? I think the original failure was in one of the shorted transistors above, and this took out the others as well as the OP-AMP and diode.
Thank you for your help!
Hello Dave and N_Cook,
Thank you both for your helpful replies. I think just the right side, as I can zero check the meter when closing K307. Please see my other post about this.
I think I'll try option 2 with a U441. :) Now for my dumb/ignorant question: The schematic does not show source and drain - how do I determine which is which when I replace the part?
Thanks again.
Sounds like it could be anything, but I'd be looking for reasons for the initial semiconductor failure, first.
Relays or interconnecting control harnessing/connex.
With K307 closed, can you non-zero the meter through R314 ?
RL
Yeah... that's certainly the sensible thing to do, unless you find that the device *really* needs very specific selection for various parameters.
For an N-JFET, the source goes towards the more negative voltage... in this schematic, it looks as if the downward side would be the lower-voltage side per the normal convention, and thus the drain's on the top and the source is on the bottom.
JFETs tend to be fairly symmetrical, and will often work if you swap the source and drain, but since the source/gate and drain/gate capacitances may not be identical you should probably keep the part in its preferred orientation.
--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
I went to the web site of my first thought:
and found:
DUE TO A BAD ACCIDENT DAN'S SMALL PARTS AND KITS IS CLOSED UNTIL FURTHER NOTICE!!! DAN WILL BE BACK
--
Britney Spears\' Guide to Semiconductor Physics
Seconded. Dan is (has been, at least) a great source of interesting obscure parts - a really wonderful resource.
I hope that he and his are OK!
--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
Hello Dave,
Excellent. Many thanks for your help and valuable lesson. I've ordered the part from Newark and will reply back and let you know how it went.
Hello Legg,
I made a mistake in my original post - Q301 was not shorted, but Q305 was. That might explain the initial reason for why all those parts were blown, it being the -210VDC source. I was working from memory and did not have the instrument in front of me. Sorry about that.
Well, on the 200V range, the meter reads over-range. If I press zero check, it drops down to about 1.5 volts. At this point, if I adjust R314, it has no effect whatsoever on the meter readout or the voltage at U309 pin 2. This is what leads me to believe that Q308 is blown.
Thanks again for your help.
Looking at the schematic, the 210V supply rails are only capable of delivering about 55mA average into a short. A bit of an energy surge available on first failure, but there shouldn't have been much follow-on current. Just one of the benign features of capacitive multipliers.
RL
Well, got the device in and installed in the circuit. It was pretty close, but I couldn't make it through the input stage balancing procedure on pg.
143 and 144 of the service manual:I followed the procedure for setting jumper W303 (in my case position C was required) and could only get the display to read about 18mV when adjusting R314. So, I tweaked R351 a bit lower and managed to get it down to 0.0mV when adjusting R314, but it wasn't stable - it would drift around one or two millivolts or so.
At this point, I took a "known-good" instrument from stock, and verified it's stability. I found it to be rock solid give or take ten microvolts or so. I removed the JFET from the known-good unit and dropped it into the unit I was repairing. I put R351 back to it's stock resistance of 16.5K. This fixed the unit. Since the customer was getting (very) impatient and my boss was willing to sacrifice our unit for the time being, I finished calibrating it and shipped it off yesterday. Of course, now *our* "known-good" needs a replacement Q308 dual JFET. :(
Does anyone have any other possible subs I could try?
Thanks again, I appreciate the help.
Hello Legg,
Are you saying that Q305 shorting out shouldn't have caused the other devices to fail? If so, I'll be worrying for a while now that the instrument is on it's way back to the customer. Especially since the sub didn't work - I'd hate to blow another good one.
I was able to repair the unit by stealing the part out of out of one of our working units. (Boss's decision)
Actually, 50mA is easily enough to kill a forward biased fet. A gate resistor would have minimized this likelihood in the design, without affecting electrometer performance.
Subbig the entire assembly might have been safer, but at least you've pinpointed the final obvious defective component, and have more time to find a suitable sub for the back-up unit
With another working unit on hand, the matching requirements are more easily assessed, also. I'm surprised Keithley hasn't been of more assistance in this - at least to spell out the matching requirement and initial component type used.
The schematic doesn't show a voltage on the junction of R335 and R336. this would give you a biasing consideration to start with. In circuit, I think W303 is supposed to be an initial crude of trim +/- 12mV, so close gate voltage isn't expected in the part.
RL
The U411 wasn't specified for low gate leakage (~200pA), and may be in the wrong package (TO71 - 6pin TO18) to provide this. Most of the low-leakage duals ( 1 to 5 pA) are in the larger TO78 (6pin TO5) case sizes.
U421 through U428 are all below 5pA. If you move to the smaller can size, the lreakage pec automatically increases by a factor of 5.
For a static match less than 25mV, you'd be restricted to U421/2/5/6.
The jedec low-leakage types are 2N5902-2N5909 inclusive (2 to 5pA), all below 15mV match.
RL
Hello Legg,
Given the specs you mention WRT leakage, I managed to get a couple of samples of an LS5907 from Linear systems that is rated at 150fA. This is apparently a cross or the same as a 2N5907. The nice folks there sent 'em to me for free.
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