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| Video Teardown and Repair of an Agilent E4433B ESG-D Signal Generator |
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| free_electron:
who , knows , maybe after traveling through the cats digestive system that part may work again.. if only it were a cat-whisker diode ... :-DD |
| G0HZU:
I managed to dig out a really old Stanford Microdevices datasheet for this SHF-0189 device and it shows the Id vs Vgs vs Vds. Stanford were the company that made this device many years ago. You can see why the device will get fatally stressed if it loses the negative bias and Vgs falls to 0V and Vds is 8V. It will draw about 300mA from 8V and this will cook it. It will normally run at about Vgs = -1.3V for 100mA Id although this will vary from device to device. This is why it is wise to design in a cutout/interlock circuit for the 8V feed and I suspect that HP will have done this. It's a shame that they don't produce a schematic for this sig gen... |
| Hugoneus:
--- Quote from: free_electron on December 22, 2014, 09:51:48 pm --- if only it were a cat-whisker diode ... :-DD --- End quote --- Brilliant. |
| Hugoneus:
--- Quote from: G0HZU on December 22, 2014, 10:04:35 pm ---... You can see why the device will get fatally stressed if it loses the negative bias and Vgs falls to 0V and Vds is 8V. It will draw about 300mA from 8V and this will cook it. It will normally run at about Vgs = -1.3V for 100mA Id although this will vary from device to device. ... --- End quote --- Thanks for the info. It is possible that the biasing circuit is what has died. The new device does not need the input negative biasing scheme. I can measure the IV curve of the device I took out and find out if it still works! |
| G0HZU:
Probably the simplest check is to see if the gate has shorted to the drain with a DMM. Normally the device measures just a few ohms drain to source on a DMM but the gate to drain should he high impedance. If the device fails and the drain goes low Z to the gate then the device will override the biasing (active or passive) when in circuit and put a positive bias on the gate and it will probably cause the 8V feed to trip out a protection circuit as the device will take a lot of current in this faulty state. Maybe this is why you saw 0V on the drain and the gate? Because it tripped something? But I'm just guessing really... |
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