Softening captive leads on vintage gear? - Page 1
Forum section: electronics / testgear

author: deafen
author_rank: Contributor
timestamp: August 30, 2021, 01:07:12 am
content: I just purchased an HP 412A, and all three leads are stiff as all get out. I don't want to have to replace them, seeing as they're apparently quite specialized. So how can I soften them back up? In the past, I've softened hardened old rubber bits by soaking them in silicone spray lube in a baggie, but I don't know what plastic these are made of and whether that would cause damage.The components date it to early 1968, and it's been sitting unused for heaven knows how long. Obviously I'm going to recap it, but just for fun I brought it up on the variac over a couple hours. It wasn't behaving at first (meter to full scale on every setting), but when I opened it up, one of the chopper bulbs wasn't lit. A little contact cleaner and everything settled in nicely - even without calibration, it's only off by about 1%. No signs of any previous servicing, although the caps on the modulator board had me fooled - this is definitely the earliest piece of gear that I've seen using orange drops.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 30, 2021, 01:28:21 am
content: If they're PVC, you can get plasticizer I guess (DINP etc.), probably just rub it on until it's back to normal. Assuming it returns to normal at all of course, and it hasn't been fractured or anything.The usual way DINP disappears is hydrolysis, releasing phthalic acid that corrodes the copper. This won't have stopped, and it's a catalytic reaction (the low pH, and maybe something about the copper ions, increases the rate of hydrolysis). At least, as far as I know it should be.Those really aren't banana plugs or something? Screw originality, that's just not good design; I'd replace them with such. If they're actually like shielded or something, then coax or triax or whatever, again with appropriate connectors. (Obviously, N/A if this is supposed to be a resto.)Tim

author: deafen
author_rank: Contributor
timestamp: August 30, 2021, 02:07:09 am
content: Nope, not banana plugs, and yes, shielded coax (DC and COM) and triax (MA/OHM). DC ranges down to 1mV full scale, so shielding is pretty important. And the ohmmeter ranges down to 1R center scale, so it uses a quasi-4-wire setup that uses triax on that lead. Granted, I don't need those ranges. Most of my work is on tube amps, so the lowest I tend to go is 1V full scale. And I've already got a bench DMM that does real 4-wire. The real benefit for me is the 200 megohm input resistance, so I can actually measure high impedance points (e.g. tube grids) accurately.The conductors themselves look to be in good shape. I stripped back the DC lead (the probe is long gone, I'll be making a new one). The strands are silver in color (tinned copper, I assume) and don't appear to be corroded at all. I have no idea if the insulation is PVC, or EPDM, or something else. Does DINP damage materials other than PVC?

author: bob91343
author_rank: Super Contributor
timestamp: August 30, 2021, 03:41:08 am
content: It's possible to find a junk unit to get the cables but most of the ones you find don't have cables at all. I have one of these as well as a junk unit I cannibalized for several parts to get the good one working. Nice meter.

author: james_s
author_rank: Super Contributor
timestamp: August 30, 2021, 05:33:42 am
content: I've dealt with a lot of various plastics shrinking and getting hard and brittle with age. I've found that soaking in various concoctions or even just brake fluid or automatic transmission fluid can soften it right up again but it isn't permanent, it will dry out and harden up again after a while. What you're trying to do is a bit like trying to un-cook an egg.

author: factory
author_rank: Super Contributor
timestamp: August 30, 2021, 12:43:04 pm
content: Quote from: deafen on August 30, 2021, 01:07:12 amI've repaired two of these, a standard one and the rack mount version, both have the original leads which are in the same condition as yours, I may have to replace them with modern stuff if they get worse. NOS leads do appear from time to time, but are expensive & get snapped up quickly, also no guarantee they won't also have gone hard with age.Despite being on different continents for 40 to 50 years, both had the same 1500uF low voltage electrolytic cap fail open circuit, all the others tested fine & remained. The Sprague bumble bee caps had slight leakage and were all replaced.One had contact problems with the chopper lamps and the other a tube had failed with grid leakage.I don't know what the switch wafers are made from in the later one you have, but the service noteswarnthat certain plastic types will be damaged by switch cleaner. In at least one of mine they are cracked, maybe a previous owner didn't know about this.The service notes are on the hparchive site;http://hparchive.com/hp_equipmentDavid

author: RoGeorge
author_rank: Super Contributor
timestamp: August 30, 2021, 12:56:36 pm
content: Quote from: james_s on August 30, 2021, 05:33:42 amThough this is possible, in a way. I've had a bag of zip ties that after about 2 years became brittle and they were snapping when bent like spaghetti, wanted to throw them away, but forgot about them. Few more years later, found the same bag of zip ties sitting on the same shelf full of dust, and this time they were as good as new.Not snappy any more, they recovered by themselves. No idea what happened. There is no doubt it was the same bag of zip ties.

author: deafen
author_rank: Contributor
timestamp: August 30, 2021, 03:14:21 pm
content: Quote from: factory on August 30, 2021, 12:43:04 pmYeah, I wouldn't use NOS to replace these anyway, they're just as old. That would be like buying NOS vintage electrolytic caps.But I'd love to get my hands on an original probe!I've been toying with the idea of replacing the leads with BNC connectors - coax for the DC and COM leads, and twinax for MA/OHMS (triax is just too expensive). I'd just have to drill out the chassis holes a smidge, and I think there's room to fit all three without interference.Quote from: factory on August 30, 2021, 12:43:04 pmAfter poking around some more, I think my plan is to replace the 1500uF/10V can and leave the rest of the electrolytics unless they actually show that they're failing. The little 50uF/6V ones at C107/C110 look to be a pain in the butt to get to, the 50uF/25V meter stabilizer (what the hell does "semi-polarized" mean, BTW??) is in an awkward spot, and getting at the terminals of the double 20uF/450V can looks to require fairly major disassembly. I mean, come on, HP, why didn't you anticipate someone needing to service these things 53 years later?Since mine is a pretty late model, the paper Sprague caps are the later style (labeled with words, not color bands). I'll test them of course, but they seem to be working fine for now. Or maybe not; I've got .47uF on hand, so I guess there's no reason not to replace them anyway. I was surprised to see orange drops on the modulator board - I'm pretty sure this is the earliest piece of equipment I've seen with those. They appear to be factory (uh, HP, not you - just noticed your username), because the solder joints are clean and identical to all the others.Quote from: factory on August 30, 2021, 12:43:04 pmI'm staying far, far away from that switch. It's a different color in mine, but the Army manual that covers my version has the same dire warning. In fact, I cleaned the contacts on the chopper bulbs by spraying the pins, working the bulb in the socket, then repeating, just to avoid any drips.Quote from: factory on August 30, 2021, 12:43:04 pmThanks, I appreciate that!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 30, 2021, 04:54:53 pm
content: Quote from: deafen on August 30, 2021, 03:14:21 pmDon't Fear Factory. Be a Self-Bias Resistor.Quote from: RoGeorge on August 30, 2021, 12:56:36 pmNylon (predominantly/all zip ties are?) absorbs moisture, maybe it was a particularly dry day? Is very suspicious that they were ever brittle at all. Not sure I've seen that myself, outside perhaps of extensive sunlight exposure, which should definitely not be a recoverable mechanism(?). But it's no desert here, YMMV.I've handled nylon zip-ties, connectors (familiar early Molex types!) etc. in 1960s TV sets that still looked as good as the day they were installed; truly a marvel of the space age, at least for the most part. Have also seen PVC in varying states of decay; if it doesn't turn hard and crispy, it might turn sticky-gooey instead. But again considering the age versus the design lifetime, they did pretty well indeed.Tim

author: bob91343
author_rank: Super Contributor
timestamp: August 30, 2021, 06:39:14 pm
content: As mentioned, the switch is a major problem. One of my units has a bad one. I sort of repaired it but not that well.They used some plastic that didn't work out but we restorers are stuck with it.

author: factory
author_rank: Super Contributor
timestamp: August 31, 2021, 09:22:49 pm
content: Quote from: deafen on August 30, 2021, 03:14:21 pmI remember the standard cabinet 412A was quite tricky to work on, I had to do a lot of disassembly to get to the meter (glass required gluing back in place), the rack version was a little easier to work on.The semi-polarized capacitor is possibly what is now known as a non-polarized capacitor, I had a look at the pictures of my 412A, but that one has been replaced with an ordinary polarized cap at some point in it's working life. Can't find a picture showing the back of the meter in the rack version.David

author: TurboTom
author_rank: Super Contributor
timestamp: September 01, 2021, 11:20:37 am
content: Quote from: RoGeorge on August 30, 2021, 12:56:36 pmThat's typical for polyamide. As long as there's some water dissolved in the material, it's very flexible and will be very hard to crack. If stored for some time in very dry air conditions, it gets brittle. But that effect is reversible. I remember when flying model aircraft with those grey nylon props, the more experienced fellows on the field told us (back then) youngsters to wrap the props of our planes in a moist rag for a day or two when we re-activate our planes from the attic after the winter pause. Then they won't shatter as easily during less than optimal landings...It worked!Just noticed: @T3sl4co1l already commented on this basically with the same information.

author: deafen
author_rank: Contributor
timestamp: September 01, 2021, 04:32:20 pm
content: Quote from: factory on August 31, 2021, 09:22:49 pmThe cap on mine looks identical (metal case, indent on one end), so yours might be factory too. I found an explanation of semi-polarized in a 1964 book - basically it will allow small currents to pass during polarity reversal.https://worldradiohistory.com/BOOKSHELF-ARH/Sams-Books/Sams-Understanding-Capacitors-And-Their-Uses-1964-Mullin.pdfThanks for the gut shot, btw. Interesting to see how the older layout differs. When I crack mine open again, I'll see if I can get something similar.Putting it back together yesterday, two of the bulbs had gone dark again - guess I didn't clean the contacts well enough. Well, when I went to do so, I cracked the base of one of them. I borrowed the pilot lamp for the time being - I'll probably replace that with an LED anyway.