HP 8350B Sweep Oscillator Power Supply fluctuation - Page 1
Forum section: electronics / testgear

author: tonycox01
author_rank: Contributor
timestamp: May 15, 2021, 11:32:56 am
content: HiI am bringing an HP 8350B sweep oscillator (with a 83952C RF plugin) back to life. It would not start up (kept blowing fuses) - which was down to a semi-seized cooling fan and an intermittent plugin/chassis connection.Having got it running stably I'm beginning to check it out. The first thing I looked at was a main power rails. I've let the 8350 (and HP 3456 I'm measuring it with) warm up for an hour and checked the -40V, +20V and -10V volt rails.All are close - but definitely out of spec because they are all "wandering" above and below the spec value:+20V: 20.007V - 19.990V-40v: -39.993V - -40.010V-10V: -9.9979V - 9.9984VThey all go through their fluctuation range at about the same rate (~1-2 seconds).I was hoping the kind souls on this forum may be able to give me some advice on where best to start. My thought was that there is some ripple in the PSU that may be causing this - but any pointers before I dive it would be very welcome.thanksTony

author: techneut
author_rank: Regular Contributor
timestamp: May 15, 2021, 07:36:49 pm
content: I don't know what the specs are but within 0.1% fluctuation looks very good to me.

author: srb1954
author_rank: Super Contributor
timestamp: May 16, 2021, 01:24:39 am
content: Quote from: tonycox01 on May 15, 2021, 11:32:56 amThe fact that all these supplies vary in a similar fashion suggests a common cause but, since the respective voltage regulator circuits are independent of each other, this can only be from a variation in the incoming mains supply or a problem with the measuring equipment.Check the raw input voltage to each regulator. Do these vary in a similar cyclical manner? If so then it could be a variation in your incoming mains voltage that is causing the problem.You should also consider whether your measurements are valid. Have you verified the performance of your 3456 with a stable voltage reference?How many power line cycles integration time have you set on your 3456? You should set at least 1 cycle, preferably 10 cycles to average out reading errors due to hum and ripple pickup. The default power on setting for the 3456 is 10 cycles integration time but you should be aware that there are different versions of the 3456 for 50Hz and 60Hz mains. If you have the wrong version you might be seeing slowly varying errors in the readings due to imperfect synchronisation of the readings with the power line frequency.

author: kipp
author_rank: Contributor
timestamp: January 07, 2024, 03:02:12 pm
content: Hi,These three voltages --- these three in particular --- are used only by the plugins. These supplies each has an independent return line, and independent sense lines leading back from the plugin. The regulator circuit regulates the voltage at the other end of the sense lines, not the supply voltage on the regulator board. The regulator, however, has diodes that conduct from supply line to sense line when there is no voltage on the sense line (when the plugin isn't installed), so that the regulator is always working. So, first, double check how you're making the measurements. Are you measuring between the sense line test points on the regulator board, or between the regulated output voltage and chassis ground? The latter won't give proper readings because it's not at exactly the same potential as the return path.But, yes, the three supplies are (sort of) coupled together. The +20 V supply is used as the reference for the -40 V regulator. The -40 V regulator sets its output to -2.0x the +20 V supply. In the schematic you can see the -40 V adjust trim pot whose wiper feeds the controlling op amp is part of a voltage divider between the +20 V and -40 V regulated output rails. Instability of the +20 V supply will be exactly mimicked by the -40 V supply *-2.0 (if that's happening it means the -40 V regulator is working perfectly).The -10 V supply uses the -40 V regulated output and some zener diodes to set up a reference voltage against with the output of its trim pot is compared (which is forming a voltage divider across the sense lines). So ... small fluctuations in the -40 V supply will produce small changes in the current flowing through the zener diodes, but shouldn't change the voltage drops across them to 1st order, should be a higher order effect. If you're seeing fluctuations at 1 part in 1000 on the -40 V supply I wouldn't expect to see fluctuations at that same level in the +10 V supply. Maybe 1 part in a million. There will be a coupling, but you should have to make a super careful measurement to see it.For that reason I think what you're seeing is due to measurements w.r.t. the wrong reference potential (e.g., chassis ground instead of the return line).PS --- you (or somebody else) wouldn't happen to have complete + readable schematics for the regulator? I'm trying to debug a problem in my own, but none of the service manuals floating around on the internet that I've found are both readable and complete. You can choose readable or complete, but not both.

author: srb1954
author_rank: Super Contributor
timestamp: January 07, 2024, 07:10:34 pm
content: Quote from: kipp on January 07, 2024, 03:02:12 pmI have a hard copy manual for the 8350B and will copy a few pages for you if required. I can't copy the full schematics in one piece as the fold-outs are huge, around about an A1 size, which my scanner can't handle. Also, the drawings are bound into the manual which makes getting them on the scanner difficult.My manual covers serial number prefixes 2309A and 2251A. What serial number is your unit?Otherwise, I suggest you purchase a scanned manual from Artek Manuals.

author: kipp
author_rank: Contributor
timestamp: January 17, 2024, 08:44:01 am
content: Quote from: srb1954 on January 07, 2024, 07:10:34 pmHi, thanks for the offer! I've actually figured out that the PDF I have *is* complete after-all, but the pages are in a somewhat random order. With enough flipping back and forth I eventually found all the parts of the regulator schematic :-). I can't remember where I got it from, but I know it's not the version on Agilent's web site, their own version is the unreadable one. I think the internet archive version is the same as Agilent's and also unreadable.I didn't know Artek has it, so I'll check that out. I've been very happy with their scans and am happy to send money their way to support them.