HP8903b Questions - Page 1
Forum section: electronics / testgear

author: rdenney
author_rank: Regular Contributor
timestamp: July 01, 2025, 09:22:06 pm
content: My old (but new to me) HP8903b is doing some strange things and I'm curious if someone has run into the same issue.When I do a loopback test, I'm getting distortion readings in the -89 to -91 dB range, which meets spec but isn't quite as good as most people report. But after the is on for a while--20 or 30 minutes--the signal generator starts to become a bit flaky. The oscillator output will start to glitch occasionally and then become somewhat more frequent to average ever few seconds or worse. The period between the glitches is random.I connected the signal generator to the input on my HP339a analyzer and also to my analog scope. When it glitches, the needle on the 339a jumps and pegs for a moment. But the sine wave on the scope is as pristine as ever--whatever extra distortion there is more than 10 or 15 dB but not enough to become visible on a sine wave. When I'm looking at the distortion output of the 338a in the other scope channel, the distortion noise will shout for a fraction of a second during the glitch. So, it appears not to be a dropout or distortion in the oscillator, but rather an injection of noise.The input measurements seem solid as a rock--no glitching at all and accurate when reading the signal generator of the 339a (instead of its own generator) for extended periods.Just as a matter of general hygiene, I measured the voltages coming out of the power supply. Voltage levels are all stable and well within the service manual specs. Ripple, on the other hand, is a bit high on the +15 and -15V outputs, while well within spec +5 and +12. Meaning: +5's ripple is 0.67 mV RMS (estimating 1.9 mV p-p) and 0.54 mV on +12 (~1.5mV p-p). Those are well within the 2 mV p-p specification. But the 15-volt rails show 2.7mV RMS on the positive rail and 2.9 mV on the negative rail. This result confuses me a bit, because the 12V rail is fed by the bridge rectifier that supplies the 15V rails, on the +15 side, downstream from the large filter cap C1. I would think that if C1 is a problem, it would affect both +12 and +15, and not -15.I've ordered replacements for the three large filter caps as a matter of course, even though those puppies are pricey. But I suspect the old ones are fine.I'll be doggoned if I can find the 12V regulator on the schematic in the service manual and I'm going by the text and the block diagram.It may be that something in the analog circuits that uses the 15V rails is feeding noise back into the power supply test points. Next step for that is to pull all the boards and test the power supply outputs again. Didn't think of that last night.I will also reseat all the grounding screws on the main board, which is how it is grounded to the chassis.In terms of what gets warm--transformer is cool and everything on the motherboard is cool. But the regulators on the back wall of the case are pretty darn toasty. One may be toastier than the others--I couldn't tell. Those are the regulators for +12, +15, and -15. Again, I wouldn't expect the two 15-volt regulators to be bad in the same way, or the +12 to be good if the +15 is ripply.I decided to look at the distortion waveform, and ran a BNC cable from the 8903b analyzer output (post notch) to the second channel on the scope. The pattern had a 2nd-harmonic wiggle to it that remained synced with the signal generator output. But at some point I was testing the operation at a range of frequencies and output voltages, and when that distortion output was hooked up, the signal generator started cutting out every second for half a second. The period was precise--it was timed to the pulse test LED #4 that hangs off the CPU on the Processor card. I think I was driving high-frequency output from the oscillator at the time--20 KHz at 4V--but cranking it back down to 1 KHz and 1V didn't solve it. When I removed the BNC connector, it was fine. The input impedance of the scope is 1Mohm, so I wouldn't think that particular output of the analyzer would be overdriven by just being connected even without a 600-ohm terminator. I don't think this is related to the glitchy signal generator, but maybe that's just my ignorant gut making a guess.One guy on YouTube apparently had the same problem with the oscillator glitching after warmup, and he resolved it by replacing a 0.1 uF tantalum C46 cap in the sample and hold circuit on the oscillator board. I thought one of the solder joints on that cap looked dodgy and I pulled it for testing. The cap tests fine and I reinstalled it with fresh solder. The little 8.2 pF cap next door to it that acts as an overshoot snubber on sample and hold circuit FET switch Q15 looked like it was cracked and manipulating it caused all sorts of issues. I replaced it with an 8.2 pF ceramic capacitor with no change in behavior except that now it doesn't react to physical touch. I have a replacement for the 0.1 uF tantalum cap on the way--just because it tested okay doesn't mean it is under heavy warmup. Can't hurt and I'm always willing to be lucky.So, to sum up: 1.) The oscillator works great for about half an hour, and then starts getting glitchy. 2.) The distortion output, when hooked up to a scope, causes the oscillator to cut in and out according to a precise one-second period, probably based on some sort of error condition the controller is detecting. 3.) The ripple is a little too large on the 15V rails, but not really in a way that points to the power supply filter caps C1 and C2.Anything jump out at anyone who has experience inside the 8903b?Rick "appreciative" Denney

author: rdenney
author_rank: Regular Contributor
timestamp: July 15, 2025, 05:50:32 pm
content: So, conducted a test: Removed the 8903b low-level boards one at a time and looked at AC ripple current on the 3456. Every removal slightly improved ripple, but the best the +/- 15V circuits got was about 1.5 mV RMS, still out of spec a bit.I then replaced the three large electrolytics on the main board. These provide basic bulk filtering. The replacements are Kemet ALS31's, 4700uF (+/- 20%) 63V to replace the 3600 (-20/+70%) 36V Spragues. And one Kemet ALS80, 15000 uF, 63V to replace the 13000 uF, 50V Sprague. These are screw terminals, so easy to replace, but believe me I checked the polarization twice--against how the old ones were installed and against the board layout in the service manual.With no boards installed, I powered up the main board to check the power supply. For the minute or so that it worked, the ripple voltage was about the same as before, checking only the -15V rail. Then, the main fuse blew. That fuse is upstream of the "Line filter" shown on the A14 Line Power board, which is itself upstream from the power switch (which switches both incoming hot and neutral configured for 120VAC) and then goes through the voltage selection jumpers to the upstream side of the transformer. Everything else in the box is downstream of that, and all the rail fuses on the output side of the transformer are fine.I'm hoping I bumped something that caused a short and that's what blew the fuse, but it's more likely that an excessive power draw is the culprit. The specified fuse is rated at 1.5 amps, and is a typical AGC package. I only had 5x20mm glass fuses in stock, so I've ordered some. The 8903 would have to pull more than 180W to blow that fuse, which seems to me unlikely, especially with all the boards pulled. There is no direct short, at least, but I'll investigate further after the fuses arrive. The 8903 was plugged into an isolation transformer, but not the dim-bulb tester. I'll measure power draw as a next step, and will also measure resistance between legs and to incoming earth with the power switch in the on position to check against something obvious.Meanwhile: Any opinions on replacing the C45 capacitor? It's a white can, about 6x30mm (eyeballing), axial leads, polarized. I'm assuming it's an earlier mil-spec 100nF 35V tantalum capacitor. I have two choices: A modern solid tant (a mil-spec Kemet T322, 100nF, 50V, polarized of course, and a fraction of the size of the original) or a Cornell-Dubilier Knowles WMF film cap of the same specifications. Even the film cap is smaller than the original. What say ye?Rick "comments needed" Denney