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| Video Teardown and Repair of an Agilent E4433B ESG-D Signal Generator |
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| Hugoneus:
--- Quote from: G0HZU on December 23, 2014, 12:22:49 am ---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... --- End quote --- Dammit! I can't find the part! I knew my cat would do something to it... |O |
| SeanB:
Look tomorrow in the litter box, it will be there somewhere............ But in any case you did in testing see a negative bias then nothing, so likely it was short circuit eventually. As is you probably could leave the MMIC in there as it does provide enough gain, and for most applications does the job. Might not be perfect at edge cases, but for everything else it works. |
| bktemp:
Great video! I always enjoy the teardown/repair of expensive equipment. A bit off topic, but I wonder how the IQ modulator works. A typical IQ modulator needs a 90° phase shifted signal, but how is this done in the signal generator over such a wide frequency range? |
| G0HZU:
I watched the video more closely this morning and I think I can explain why the device biasing seemed confusing during the DMM probing. At 42:53 you actually measured the gate bias at -2.1V (not the drain). The drain was close to 0V. So I think you accidentally got these measurements back to front and this caused the confusion. So I think the active bias was valiantly trying to bias off the (damaged,shorted) SHF-0189 device at 42:53, and maybe -2.1V is the most negative the bias generator can go before it hits its circuit limit and gives up. It was never going to succeed because I now think the HFET is indeed faulty. If we then fast forward to 45:50 when you remeasure the gate voltage we can see that it has risen from -2.1V to +0.2V. I think this is expected because you removed the bias choke in the drain and so there is no drain current. So the active bias circuit will raise the gate voltage to the other end of its range to 'try' to get the target drain bias current of maybe 100mA. Obviously it is going to fail because there is no DC path to the drain because the bias choke is removed and it probably hits its endstop at +0.2V gate bias voltage. So I think the active bias circuit may be healthy and it probably has an operating range of -2.1V to +0.2V at the gate. This should be more than enough range to bias a healthy HFET to the target bias current (probably 100mA) with enough range to cover the spread in device charactersitics across a large batch of SHF-0189s. The other thing I noticed was that the final HFET device at the sig gen output looks like it may have already been changed in the past as it looks like it has been hand soldered. See 28:09 in the video because Q2600 (H1) looks like it has been replaced/resoldered in the past and it also doesn't have the extra red dot on it like the other H1 devices. So maybe these devices fail quite often in these sig gens. It's a bit of a concern for me because I now have two of these 4GHz E4433 sig gens here at home. I bought the first one earlier this year and bought another one a few days ago. They can be bought quite cheaply if you are patient. I bought the first one for £780 and the second for £1000. Both have lots of options fitted and they probably cost over £20k new. |
| SeanB:
If that then at least we know you can replace some with MMIC devices with little drop in performance. |
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