Aligning the Advantest R3132 Spectrum Analyzer 2nd RF Lo

I came across an Advantest R3132 spectrum analyzer from my favorite local eBay seller. The seller deals with a lot materials discarded for recycling and they were selling it for little more than scrap prices, I jumped all over it. I did not know its history, the eBay listing showed a picture of the analyzer powered on but little else. I figured if nothing else there would be some great parts for salvage, it turned out salvage would not be necessary.

I brought the analyzer in to my lab at work and set it up. I powered it up and at first glance it appeared to work with the exception of a dead backup battery. Once I discovered how to run the self test (Shift-0), I learned there was a failure in the RF section. I figured the best way to proceed was to connect a reference signal and make some measurements. I connected up the calibration reference and dialed in the expected frequency. Quickly I could tell that yes there was a problem, the calibration frequency was expected to be 30MHz but was reading 28.95MHz. A google search confirmed my suspicion and fortunately this is common with the A3132 and there was ample info on the problem.

 

Aerial shot of the open RF board

There are at least two types of RF boards in the R3231, this guide covers the earlier appearing model.

 

The Steps
  1. I desoldered the little blue wire by U303 and attached my power supply directly to the PCB. I wanted to make sure that I was only powering the VCO as I didn’t check out what else the 6.5V supply was powering.
  2. I soldered a ground wire to TP501 as that is the varicap control voltage line.
  3. I connected the test port to a spectrum analyzer and powered on the bench supply.
  4. I went looking for the Lo and I found that it was some 40Mhz lower. Uncovering the VCO had a major effect on the frequency as expected. I got a chunk of aluminum to cover it between adjustments.
  5. I tweaked the tuning “fork” a bit, measured, repeat until I was under 500kHz from 3840MHz
  6. Found out that tightening the adjustment screw caused the frequency to jump so I aimed a but under 3840 then when tightened it was nearly on target.
  7. Disconnect everything, reconnect the blue wire to the MMIC power supply near U303, pop on the cover and put in ~50 screws. Slide the RF deck back in the analyzer and reconnect.
  8. Run self test (shift-0) and now RF passes.
Photos

Click the images for a larger view with description

-Ty

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