| Electronics > Repair |
| Agilent E4421B RF generator restoration project |
| << < (3/9) > >> |
| nctnico:
--- Quote from: Aurora on December 27, 2014, 10:00:49 pm ---The Output amplifier MMIC is available as a tested used part from China. I had a Google search some time ago. --- End quote --- Any pointers to a reputable seller? The output amplifier is definitely a key part of getting a clean output signal. I have seen them as well but mostly as part of a board which was cut from an output board. |
| tautech:
Wonderful work. :-+ --- Quote ---The fixed pallette conversion allows any color scheme. Who's afraid of red, yellow and blue? >:D --- End quote --- Na keep it white on blue. :-+ |
| nctnico:
Conclusion All in all the generator is working useful again except for AM and probably FM modulation as well. Both these modulations are generated on the reference board so there still are some problems lurking in there. My primary use for this generator is tinkering / learning HF stuff and that usually involves sending sine waves through amplifiers and/or filters. Yes, I also changed the battery but that was an easy job. |
| nctnico:
Fixing modulation I couldn't resist taking another look at why the modulation wasn't working. First I tried to apply an external modulation signal to make sure the internal modulation signal generator on the motherboard wasn't the problem (according to the service manual the internal modulation signal is generated on the motherboard). The modulation also didn't work with an external signal so the cause of the problem must be somewhere on the reference board. So I opened the reference module again and I noticed a few opamps where running hot. Both these opamps where connected to different DACs. DACs are prone to failure in general but two broken DACs is still unlikely. I tried to find out what the opamps had in common and I ended up with the power supply (+15V and -15V). I could trace the +15V back to the power supply connector but I could not find -15V. I remembered that the board had a blob of solder when I got it (inside the red circle): I removed it hoping it would not be harmful but appearantly it shorted -15V to ground which burned a filter inductor. I replaced it with a ferrite bead and presto: FM and AM modulation working again! The hot opamps are circled in green, the removed solder blob in the red circle and the replacement ferrite bead in the blue circle. And yes, I removed the resistor across the 10V reference. I didn't measure the current but I doubt it will be needed. |
| G0HZU:
--- Quote ---@G0HZU: Let's just say the board came with the generator. It's part number is E4400-60003. According to the service manual this can be used in both the ESG-A and ESG-D series. --- End quote --- Interesting... On Mike's youtube teardown his HP4421B A had the areas for the IQ modulator blanked off in the artwork and replaced with a stripline section to bypass this unused area. So he has a different output board. So maybe HP/Agilent decided at some point to use the E4400-60003 output board (with IQ modulator) on both A and D models? In my opinion the IQ modulator is one of the best features of this generator series. I wonder how much extra work it would be to make use of your IQ modulator? That was impressive work on the screen replacement by the way :) |
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