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8970A Performance Problem ...

Question asked by zialabs on Mar 27, 2012
Latest reply on Oct 9, 2012 by Uky
I purchased an 8970A that had one bad 2400uf/40v filter cap in the +15v power supply. Replaced the cap and the instrument came alive. I went through the box doing the Adjustments defined in the manual. All seemed well, everything was with in the range of the adjustments. However, after turning the instrument off for a few days and then turning it on again some strange behavior exists and I can't seem to find the root cause. When I first turn the instrument on it calibrates and operates what I would consider normally. Then after about 20 to 30 minutes the NF and Gain displays began to move up and down, meaning the numerical values of the NF for example drift from -18.00 to +18.00 and continuously up and down (the values are just an example, the actual values are different with time but the effect is the same). Same is true of the Gain display. Eventually the NF display goes blank and displays - - but will eventually come back on it's own. Yet, in the uncalibrated self NF mode the value displayed meets the requirements in the manual. However that value too has a similar constant periodic drift but much smaller, in the 0.5 to -0.5 range. The drift time from -18 to +18 is about 20 to 30 seconds or so from peak value to peak value. Eventually, after about 2 or 3 hours the displays somewhat stabilize although they never remain at zero. I have three other 8970's, two A's and a B and none of them exhibit this problem. Does anyone know what might be causing this? I have looked (o'scope) at all the power supplies, they are rock solid. The 1750MHz LO does drift and was originally about 2.5MHz off, but it is right on now. like I said, the other adjustments worked fine in accordance with the manuals adjustments instructions. Any help would be greatly appreciated! -Bob Simpson