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Video Teardown and Repair of an Agilent E4433B ESG-D Signal Generator
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Mosaic:
Well, a Hi Z probe will see ripple, but I don't see how a lo Z probe (coax  with a 6dB pad for  better matching?)  will improve matters here when looking for small signals. Certainly if signal integrity was  the subject a Hi Z would not be the way to go with VSWR all over the place.

If I knew what levels to look for I could use the coax/pad approach either with the TDS694C or a spectrum analyzer. Too bad Shah didn't quantify the amplitude of the stages for us.

As I recall anything under 250Mhz used a different signal path earlier on, so going with a sub 25Mhz signal is reasonable IF the unleveled indicator remains on with low freq. sigs.
My unit hasn't arrived yet so i can't say.

From a repair approach would it be simpler to pull the several RF SMD transistors from the module in question (once the coherent feed signal is alive), then measure all the bias  points for correct bias and measure all the transistors out of circuit on an analyzer or using a basic jig  @ say 25Mhz  to 1Ghz for quantifying their function? That way all the transistors and biases are tested in one pass.

I could do a custom jig to 3Ghz and use the VNA on it for the 1dB compression as a matter of interest.

If all those are good then the ALC loop is suspect.



nctnico:
In my generator the signals below 250MHz are made by mixing 1GHz with the generated signal but that part is before the final amplifier stage and after the auto leveling stage so testing the final amplifier at 25MHz is just as usefull as testing it at 500MHz to get a feel for the signal levels involved.
Jay_Diddy_B:
Hi group,

I realise that this is an old thread, but I thought it was a good idea to keep all the ESG-D repair information together.

I have an E4432B ESG-D with options 1E5 and UN4. The symptoms are No output and UNLEVELED indicator when the RF is turned on. Having read this thread I was expecting to be change a dead amplifier in the signal chain.

I have confirmed that the problem is on the A9 Output board assembly, by swapping the module from a good generator.


First the board is very different to the one in the video:






I have tracked the break in the signal path to the 'Burst Modulator'.
I did this by soldering wires to the board and running the ESG at 270MHz and monitoring the signals with a scope and 10x probes.

Here is a partial block diagram for an output board (option UN3 and UN4):



And here is a picture of the 'Burst Modulator' on the PCB:




I suspect that the circuit is something like this:




I have confirmed that the right end of the diode string has an inductor to ground. I would expect the that the left hand end would be positive about 6V if the "burst modulator' is on.

If I twist the wires together, to form a small capacitor, to bypass the 'Burst Modulator', the generator seems to work fine, except the 'Burst modulation'. The UNLEVEL indication is gone and I have output.

The questions is where on the board is the 'BURST MODULATOR DRIVER'?

Thanks for your help with this challenge.

Regards,

Jay_Diddy_B






nctnico:
I'd look in the block diagram where the burst modulation is coming from. A DMM with instant continuity is handy because you can swipe the probe along the pins quickly.
Jay_Diddy_B:

--- Quote from: nctnico on January 29, 2017, 06:41:10 pm ---I'd look in the block diagram where the burst modulation is coming from. A DMM with instant continuity is handy because you can swipe the probe along the pins quickly.

--- End quote ---

I followed this advice, and ended up in this area of the board:



I reflowed some of the soldering joints and the problems have gone away !!

I was lucky, I didn't need a single part, especially some of the unobtainium amplifiers.



Regards,

Jay_Diddy_B
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