rf preamplifier *hp70000* - Page 1
Forum section: electronics / rf-microwave

author: coppercone2
author_rank: Super Contributor
timestamp: November 16, 2019, 07:56:43 pm
content: Ok, I had a previous thread on this but I am revisting the issue. My preamplifier (70620B opt 001) has a low frequency error that looks like a U shape focused around 500MHz. I believe what happened is I overloaded it with a nearby walki talki around that frequency years ago.I am going to take it apart and test it sub assembly by sub assembly on a VNA setup with external power, since there is no complex control circuits in the preamp other then some sweep related switching, with a transfer switch between two amplifiers (not a yig), I assume the culprit is the low frequency amplifier. (to 2.3GHz).But, how does the damage look like in a RF amp where its 'bowled' around the overload frequency? The performance is better at 100MHz and fine to high frequencies (say above 700MHz) in a parabolic manner with worst gain around 500MHz.Does the amplifier (smaller then altoids tin) have a few paths separated by filters inside of it (like a passive preselector going to several stages)? People previously thought it might have been a bad hardline or something, but is it even worth my time to test those? Can the switch itself be damaged in some way to cause this? what happens to the silicon(germanium) to cause this? I just need to build a table that fits inside of my rack so I can use the working part of the analyzer with a signal generator to test the components.I can't imagine a hard line being broke to cause this (relief but wishful thinking).. can that happen anyway, where a coaxial cable throws a fit in some small region but works above 700MHz? Keep in mind the internal sub assembly amplifier goes to 2.9 GHz where it works fine.https://nscainc.com/wp-content/uploads/pdf/A_70621A.pdfI want to limit my test so I don't accidentally damage another sub assembly working on it outside of the module.I am starting to think.. maybe its just the input capacitor on the RF preamplifier maybe?? and that active elements are not likely damaged to exhibit this behavior?

author: coppercone2
author_rank: Super Contributor
timestamp: November 16, 2019, 08:28:57 pm
content: Also I measured all the hardlines very carefully with a DC multimeter (took it apart once to look for obvious stuff) and sprayed deoxit on all the non RF connectors just incase.. that did not fix itbut I did not test them on a VNA. I don't imagine its a contact problem at this point..

author: German_EE
author_rank: Super Contributor
timestamp: November 17, 2019, 11:13:22 am
content: Are there any filters inside the box used for equalization?

author: coppercone2
author_rank: Super Contributor
timestamp: November 17, 2019, 05:46:52 pm
content: the reason I am confused is because it looks like a weird notch filter just randomly placed inside one of the amplifiers. How does that happen? I would expect a high pass filter.. but why a notch?could one damaged circuit element do this? or does it take two? A mixture of shunt and series devices must have been 'modified' some how? or in distributed circuit terms, a 'wedge' of damage?

author: jfphp
author_rank: Regular Contributor
timestamp: May 16, 2020, 11:05:08 am
content: The only way is to test on a VNA each module including the pin switches (they often fail). If everything OK a new cal is necessary.

author: coppercone2
author_rank: Super Contributor
timestamp: May 16, 2020, 02:18:55 pm
content: does that happen, where a PIN switch fails in such a way that there is a big dip (many db) across a narrow range (25% of span of amplifier), making the preamp basically useless in VHF?I was going to gamble on getting the right part.Since its the amp that broke, I was going to use the SA and a sig gen to try to figure out whats broken at least.The obstacle of calibrating that thing is ridiculous.I did not think that far ahead. Is a wide band expensive noise source going to be enough? Theoretically a replacement part and a noise source would be a better value then replacing the preamp because I would get a bitching noise sourcethats like finding 'build empire state building' on your summer backyards projects list

author: jfphp
author_rank: Regular Contributor
timestamp: May 17, 2020, 10:30:32 am
content: You need software + 8340A/B or 836XXX and power meter.

author: coppercone2
author_rank: Super Contributor
timestamp: May 17, 2020, 01:52:16 pm
content: yea I got a sweeper but I don't think I want to work on this shit until I get a 21 inch rack so I can put the sweeper and some amps on it and build a little shelf for the rack, that slides out, so I can take the parts out of the amplifier on a table, put them on a shelf, connect it nicely to the rack and run tests. I am sick of ghetto ass microwave work on wooden tables meant for hooking up banana clipsneed a little table fixed to the rack mount so I can service this stuff neatly, right now the idea of servicing it feels.. desperateneed to class it up. going to take a whileand get some torque wrenches