Electronics > Repair

E4433B RF sig gen repair

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microbug:
I picked up this sig gen from work before they threw it away ('beyond economical repair'...). I was hoping for the same MOSFET that was blown in many other people's to be the problem, and was very disappointed when I opened the output board only to find a sea of custom Agilent parts. The layout was different too so they aren't just rebadged. These parts all have the prefix 1GG (e.g., 1GG3/1GG7) and appear to be internally 50 Ohm matched.

The instrument fault is an 'unlevel' displayed error, and the output of the unit is fixed at 2.628GHz regardless of the frequency set on the front panel. The output attenuators seem to work (the signal is reduced in 5dB steps) but fine-grained attenuation doesn't work. The output drops slightly to 2.622GHz when the set frequency is 0-250MHz. These sig gens have a different signal path in this low frequency range -- the synthesiser board generates a 1GHz - f_set carrier, and then it's mixed down with a 1GHz reference on the output board.

I poked around the board by soldering coax to different points. The output power amp seems to be doing fine. There is all sorts of crap coming out of the IQ mixer (SOIC-14 part labelled 1GG7 4225): there is always a peak at 2.72GHz. The correct carrier peak is also there when the set frequency is above around 2.95GHz -- below that it rolls off quickly. I probed at the input to the 1GG3 amp after the mixer (i.e., after the two small transformers).

The IQ mixer input looks correct -- single peak at the carrier frequency, like the carrier output on the back of the unit.

This leads me to suspect the IQ mixer (ignore the missing DC block cap in the photos -- I damaged it but will replace). I'm still a bit confused about where this 2.72GHz signal is coming from though -- I don't see why the mixer would need a fixed reference that could leak through (and besides, the fixed reference available is 1GHz).

I'd appreciate any ideas or suggestions. I've attached some pictures. The block diagram of the output board is at page 35, http://www.doe.carleton.ca/~nagui/labequip/synth/manuals/e4400335.pdf.

I might try to get a copy of the CLIP (including the full schematic) from this guy. I'd have to pay for it, but it would easily be worth it to get the sig gen running.

R-1125F:
Have you seen this video of an E4433B repair?

microbug:
If you mean the Signal Path videos, yes, I've watched both of them.

R-1125F:
These generators have trouble with the frac-N chip in the synthesizer board.  Have you checked the synthsizer and reference board?  A bad frac-N can cause all kinds of problems. 

I have bought the CLIP.   I think it for an A model but it is still useful for troubleshooting.

microbug:

--- Quote from: R-1125F on April 01, 2024, 09:22:11 pm ---These generators have trouble with the frac-N chip in the synthesizer board.  Have you checked the synthsizer and reference board?  A bad frac-N can cause all kinds of problems. 

I have bought the CLIP.   I think it for an A model but it is still useful for troubleshooting.

--- End quote ---

I haven't checked the synth board, but its output looks good (as measured at the coherent carrier output and on the output board before the IQ mixer). Carrier between -5 and 0dBm, with some small harmonic peaks.

Edit: I've emailed Milan to buy a copy of the CLIP.

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