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failed trimmer capacitors, was Re: [HP-Agilent-Keysight-equipment] 8566B - notches in response

Dave McGuire Member Profile All Messages By This Member #92868 On 11/25/18 5:01 AM, pianovt via Groups.Io wrote: It would be good to see pictures of the notches. However, even without seeing the pictures, I would say that this looks like a problem with the SYM adjustment on the 3 MHz filter board. If the problem appeared suddenly, it may be a failed adjustable capacitor. If it showed up slowly, the filter probably needs adjustment. You will need to build the 3 MHz crystal bypass networks in order troubleshoot and adjust. Still, it would be good to post some pictures before you take my word for it. Speaking of failed adjustable capacitors...My new-to-me 8568B had been sitting unused for a long time; I picked it up as part of an EMI receiver setup (preselector, QPA, TEM cell, etc) and had just started putting the final pieces together a couple of weeks ago. I knew it was way out of calibration but I didn't have time to worry about it until I got the other components for the system, which finally came together recently. Finally, being able to perform FCC pre-qualification testing on my prototypes in my own lab, rather than paying other labs to do it, is within reach. So, after building my bypass networks, I started calibrating the IF section, and it was behaving very poorly. I observed very unpredictable behavior in response to the rotation of those trimmer capacitors. It just didn't make any sense; filters just don't act that way. Perplexed, I pulled out some of the boards for inspection. I came across no less than four trimmer capacitors on those two filter boards whose rotors had either cracked or shattered. In one case, pie-slice-shaped pieces of ceramic littered the bottom of the subchassis well. The only thing I could think of that could've caused this is some ham-handed oaf jamming a screwdriver into the holes too hard, or torquing the adjustment screws off to the side, placing pressure on the hole in the rotor with leverage from the top subchassis plate. After replacing all failed trimmer capacitors and starting over, it calibrated fine. Watch out for this problem. -Dave -- Dave McGuire, AK4HZ New Kensington, PA
Kuba Sunderland-Ober All Messages By This Member #92875 Thank you, Dave! I had a 7L12 Tek spectrum analyzer module on the “to do” pile. I decided to check for the integrity of all of the trimmer caps, just to cross that newly-added item off the list, and what you know: two were mechanically damaged just as you described! There are other problems with that unit, but this brings it closer to resolution. Cheers, Kuba toggle quoted message Show quoted text 25 nov. 2018 kl. 11:33 skrev Dave McGuire <mcguire@...>: On 11/25/18 5:01 AM, pianovt via Groups.Io wrote: It would be good to see pictures of the notches. However, even without seeing the pictures, I would say that this looks like a problem with the SYM adjustment on the 3 MHz filter board. If the problem appeared suddenly, it may be a failed adjustable capacitor. If it showed up slowly, the filter probably needs adjustment. You will need to build the 3 MHz crystal bypass networks in order troubleshoot and adjust. Still, it would be good to post some pictures before you take my word for it. Speaking of failed adjustable capacitors...My new-to-me 8568B had been sitting unused for a long time; I picked it up as part of an EMI receiver setup (preselector, QPA, TEM cell, etc) and had just started putting the final pieces together a couple of weeks ago. I knew it was way out of calibration but I didn't have time to worry about it until I got the other components for the system, which finally came together recently. Finally, being able to perform FCC pre-qualification testing on my prototypes in my own lab, rather than paying other labs to do it, is within reach. So, after building my bypass networks, I started calibrating the IF section, and it was behaving very poorly. I observed very unpredictable behavior in response to the rotation of those trimmer capacitors. It just didn't make any sense; filters just don't act that way. Perplexed, I pulled out some of the boards for inspection. I came across no less than four trimmer capacitors on those two filter boards whose rotors had either cracked or shattered. In one case, pie-slice-shaped pieces of ceramic littered the bottom of the subchassis well. The only thing I could think of that could've caused this is some ham-handed oaf jamming a screwdriver into the holes too hard, or torquing the adjustment screws off to the side, placing pressure on the hole in the rotor with leverage from the top subchassis plate. After replacing all failed trimmer capacitors and starting over, it calibrated fine. Watch out for this problem. -Dave

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