Hi, folks:-
What would you expect to see for JFET failure modes? I have some ~500kHz preamps with common source JFET front ends that are behaving quite strangely- the bias looks correct but they have little or no AC gain and appear to be loading the input. Could be something else- what say ye?
What's the drain voltage?
-- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Depends on the bias. If I_DSS is too high, it could be railed low, or if I_DSS is too low, you might be seeing a forward-biased GS junction.
Or of course you could have ESD damage.
Cheers
Phil Hobbs (at ATL going to DFW)
Sounds like gate junction failure.
More explicit data would be helpful ;-) ...Jim Thompson
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| James E.Thompson | mens |
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| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
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I love to cook with wine. Sometimes I even put it in the food.
About 1.1V drain to source with gate grounded. Funny thing is that the 'bad' channels are not significantly different bias-wise from the good channel, so it can't be a shorted gate junction.
A sketch would help.
Jfets vary wildly in Idss and drain curves; some parts may be running in their ohmic regions. 1.1 volts is pretty low.
-- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
I can lift the gate connection.
The gate junction does indeed act as a diode, so it looks fine. I can't see how a JFET would fail is such a way...
Okay, it appears to be a damaged (or possibly incorrectly manufactured) impedance matching transformer. Custom part **dang it**.
Thanks, gents.
This reminds me of a problem I encountered some 25 - 30 years ago when much of my time was spent repairing TVs. It was a 20-inch Sony and the screen remained dark while there was sound. The heaters glowed, G1 and G2 voltages were OK and HT was presemt. Obviously a cathode drive problem, and the voltages *were* at the
200V rail but it wasn't as easy to isolate the culprit as I first expected.After working back through a long chain of control circuitry, I narrowed it down to a JFET which seemed to be either leaky or had faulty associated parts. But each component seemed fine when tested individually - resistors were within tolerance, JFET and capacitors showed no detectable leakage at the most sensitive range of my analog multimeter.
I put all the parts back in and switched the TV on while keeping the meter probe at the point where there was an abnormal voltage drop. The voltage drop came up gradually while the CRT was warming up (it wasn't a fast-start tube). By the time the tube should have warmed up enough to show a picture, the cathode voltages were high enough to keep the screen dark.
I didn't have a freeze spray, so I removed the JFET again and clipped the meter probes to the base and drain while I warmed it with my fingers (using a soldering iron would have been too heavy-handed to be immediately conclusive). Sure enough, the leakage showed up slowly. I replaced the JFET with the only type I had - a BFW11, and everything worked fine.
How about a sch drawing, maybe we can reverse engineer it. And find a replacement JFET if that's what's needed.
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Thanks,
- Win
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