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48-440 Hz AC power compatibility option (HP 8660B OPT 003) - acoustic noise (hum/buzz) from the unit?
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My HP 8660B (subject of the instability issue now resolved) includes option 003 which is compatibility with 48-440 Hz power sources.
The HP 8660B has a linear power supply (ca. 1970s, predates switchers), presumably to improve the ability of the unit to serve as ballast on a large ocean-going vessel, among it's other capabilities.) Question is this - the machine emits an acoustic (audible) noise after being on for a few minutes. I didn't pay attention to this before, but realized that the noise may be associated with the wide-range AC mains frequency compatibility - I figured it was just plates in the power transformer rattling. In my (now obsolete) work life (mostly in cable tv engineering), "ferroresonant" power transformers (used to adapt 60 Hz local mains power to the 60-90 Hz pseudo-sinewave power used in the cable tv hardline infrastructure to deliver power to inline amplifiers), the noise was similar. Does this (acoustic) noise ring a bell with anyone? Should I be concerned? The machine doesn't appear to exhibit any operating problems related to this (and I doubt has anything to do with the now-mitigated PLL issue). Thanks Dave |
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Do you mean that the audible noise is there when operating on 400 Hz, or that it's there at 50 or 60 Hz now? You are right to suspect the core laminations buzzing. On transformers intentionally built to run well up to 400 Hz, the laminations are usually quite a bit thinner than for 50/60 Hz, so tend to be more pliable and can rattle against each other if anything is too loose. Usually the whole transformer would be vacuum impregnated with some kind of varnish or other goop that damps the motions at all frequencies, but after so many years it can deteriorate or get rusty, or the core mounting may loosen up. This all can happen in a regular 50/60 Hz unit too. Also, a regular unit can work at 400 Hz too, but the core loss goes up, so it would tend to run hotter, and you would want a little derating of thermal specs - usually no big deal.
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You can look at the condition and try tightening or loosening it up first, then look to see if a particular area is emitting most of the power. If it's an E-I core, the corners can be quite tight due to the mounting screws, but the rest in between tends to bow and form gaps that allow motion. Sometimes it can be mostly caused by a gap around a single lamination that's just right to resonate and be irritating. If there's room enough, you can poke around between laminations with a razor blade and maybe find spots where it can jam in and greatly change the sound situation. If you ID any really bad ones, you can jam a paper or plastic shim in there.
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The noise is harmless to the machine, but you can get eddy currents that circulate in and around the chassis depending on the electro-mechanical character and condition of the core mounting scheme, so there can be some signal interference. I wouldn't worry too much about it in this old gear, unless it seriously affects the specs or is very irritating.
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Ed |
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My AC mains 60 Hz (however, good question!)
Other than irritating (and not really all that bad, seeing as the cooling fan is about the same total dBa), it was more of a concern that it might be indicating something (else) wrong with the unit. I could see running one of these at 400 Hz on an aircraft, but the aircraft probably couldn't get off the ground with this thing aboard. Thanks for your description/info, much appreciated! Dave |
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Years back I had a problem in my 8660 where I had 60 Hz sidebands on the CW output that exceeded spec.? After extended troubleshooting and consultations with the (at the time) Yahoo group it came out that the problem was not electrical in the power supply itself but related to the transformer which was creating an external magnetic field which was inducing current into the instrument frame rear section which formed a pickup loop and this 60 Hz ground potential shift was getting into various electronics.? Mechanically breaking the loop temporarily would eliminate the output noise and I considered a mechanical solution involving a machined FR4 piece to replace a small aluminum section but eventually was able to find a replacement transformer which solved the problem.? It didn't eliminate the problem but attenuated the 60 Hz to where it was acceptable and within spec.
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The new transformer was audibly quieter as well.? IIRC it was the 400 Hz capable one, as was the replacement.? I believe most of the instruments available at the time were military surplus. I disassembled the old transformer with guidance from at least one person from old HP who was intimately involved with this generator and even posted photos but there was no obvious faults found such as an unsoldered or cracked copper sheet shield. One conclusion was that it was entirely possible that some instruments left the factory out of spec due to issues in their transformers. I still have the instrument and it still works but needs alignment and the low frequency plug-in needs repair at some point.? Much of the alignment can be done without extenders but some adjustments cannot be done without and that's kept me from working on it in favor of other projects.? Perhaps I can borrow a set from someone at some point. Peter On 3/3/2025 10:34 PM, David Feldman via groups.io wrote:
My AC mains 60 Hz (however, good question!) |
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Well I have run into the laminates?also. But on a HP8640b it was the transformer and it finally let the smoke out. This happened some 20 years ago and on that generator I plumbed in a external linear supply. It lives in the basement and is a backup backup backup... So trying to find a replacement never made sense. Paul |
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David,
I worked on many 8660's and only once did I come across a buzzing power transformer.? The customer complained about it and as it did uncharacteristically buzz.? ?(no recollection if it had opt 003) I replaced it and have it in a home made power supply that I built at least 25 years ago.? ?I use the 5V winding, full wave bridge rectified and filtered it is capable of > 20 A at 13.8 V to run a 100 W HF ham band transceiver.? ? The 8660's power transformer is sealed in a gray case (mu metal?) and I thought it might have been potted.? Maybe there are voids in the potting or it was not potted in a sufficient vacuum.? ? In my homebrew power supply it doesn't seem to buzz loud enough to be a problem but I did place it under a bench so maybe it still buzzes:)
Pete |
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