Rebuilding Dried Out Capacitors

If you’ve ever torn into very old equipment for a little refurbishment, you’ve seen ancient capacitors among tube sockets and carbon resistors. These caps are long past their life expectancy and are dried out. Putting a brand-new metal can cap in a piece of equipment from the 40s just seems wrong, though. Luckily, [unixslave] posted a nice cap rebuilding tutorial on the Hack a Day forum.

To get inside, [unixslave] melted the resin and wax plug at the base of an old cap with a soldering iron. After cleaning out 70-year-old goo with the tip of a flathead screwdriver, he drilled a hole through the core of the cap.

[unixslave] took the old wax-pressed tube of the old cap and put some modern electrolytics inside. The result reminds us of a shady practice happening in a cap factory somewhere, but [unixslave] is keeping everything on the level. The repaired cap has the same value as what’s on the label, just enough to get that old tube amp working.

31 thoughts on “Rebuilding Dried Out Capacitors

  1. Electrically equivalent…..but to some of my audiophile customers, it is blasphemy bordering on the obscene.
    I maximize my bottom line with these individuals by keeping them happy with the older look of the parts.

    It should also be noted… with times being what they are… You can command a little bit more for the units you choose to sell on ebay or whatever when no current technology parts are visible.

    So, the reasons he states for the hack is to scam buyers. :(

    1. We ARE talking about audiophiles here – and ones that don’t understand the placebo effect at that. Read: the ones that deserve to be scammed at every opportunity. They’re the reason that $500 ethernet cables can exist.

      And, he’s delivering a device that is cosmetically and functionally identical to what they expect.

      1. Technically, these caps do have different properties due to their different construction.

        One can think of an electrolytic capacitor as a coil, a capacitor, and a battery in parallel because of the electrolytic charge retention effect, and the dielectric relaxation effect, and two resistors, so you actually have at least six variables which change depending on the materials and the design.

        It’s a different matter how much these affect the operation of the final circuitry, for example the dielectric relaxation can have a 5-6% non-linearity effect over 1-100 kHz, but then again it’s impossible to measure since nobody has any original parts to measure against.

        Still, an audiophile can always claim superiority due to some esoteric thing like that.

    2. As several others have already noted, transparency is a significant part of ethical behavior. There is nothing wrong with stuffing an old can with new components, and there are several reasons why this is actually a *good* idea. As long as you disclose to a buyer that this work has been done, I think you’re on solid ground.

      That said, I never cease to be amazed by the audiophile crowd. The capacitor in question appears to be a two-segment filter cap out of an old tube radio (note the IF cans and oscillator coil in the background). If replacing it with a contemporary part results in any detectable change in “sound” at all (and that can be debated,) it would surely be for the better. A new filter cap with the correct capacitance and reduced ESR will result in a stiffer B+, a supply that won’t sag during transients.

      I understand why a person, for aesthetic reasons, would insist on preserving the vintage look of their electronics. However, the idea that a piece of audio gear “sounds better” because it’s filled with dried up caps and cracked carbon resistors is lunacy.

      Having said all that, I have a large collection of vintage capacitors I’ve collected over the years. They are dried out,shorted, or electrically leaky. I’d be happy to sell them to any interested audiophile for $50 –each. ;)

      1. “A new filter cap with the correct capacitance and reduced ESR will result in a stiffer B+, a supply that won’t sag during transients. ”

        But it’s exactly these transients that give tube amplifiers their characteristic sound.

        It’s actually the distortions to the sound that make the “warm” quality of tube sound, as ironic as it is.

  2. Still, if he tells them that it’s ‘all original parts’ or something, he’s misrepresenting it. That would be a scam, to me. And even audiophiles don’t deserve that.

    If, on the other hand, he just lets them draw their own conclusions, I wouldn’t mind much. If they can’t tell the difference, and don’t ask about the condition it’s much of a problem.

  3. Caps of those values at those voltages are awfully hard to find, and extremely expensive. A cheap source of high voltage, high capacitance electrolytics is discarded switching supplies. They usually have a couple of high voltage caps in the primary. Matching the exact capacitance is not usually necessary, just try to come close. Getting it to fit in the same footprint will be the challenge.

    1. Not true on anything with tube rectifiers. Use a high value cap and you run the risk of causing the rectifier tube to flash over. This is especially true on radios.

      If it’s using a selenium (or modern silicon diode) rectifier, then you may be ok going a bit up there.

  4. So harsh fellow commenters!

    There’s so many old radios and such with no back or windows into the electronics so you can see the tubes, it’d be a shame to throw in modern electronics and spoil the look.

    I suppose if you did misrepresent it during a sale that’d be a little dastardly, but even if I knew I was getting modern caps carefully masqueraded as originals, I’d probably still want to pay more than for just modern caps undisguised.

    Nice Idea! I’ve always wanted to throw in some tubes with the heater turned on just to make my projects look a little cooler.

  5. As radio tech and a ham the only reason I would ever consider doing something like this is for a museum display that had to actually function. Otherwise function > form on the component level.

  6. Hardcore stuff. He has no choice: it is either a new cap, new cap in an old case, or nothing. He is surely not expected to make new electrolytic capacitors?

    Good work I say, and be careful of toxic stuff.

    1. This isn’t an uncommon practice. I don’t know how he markets his goods, if he says it’s all original and leaves it at that then I can see people’s points about ethics. Otherwise I don’t see anything wrong with this at all.

      He is just giving it the best of both worlds, vintage look but new performance. And.. as someone else pointed out, if he leaves an old cap in and it fails it COULD damage some other part that has been out of production for decades!

      I want both form and function in my projects. I have done this same thing before and I plan to again although I don’t have any plans to sell it. I suppose my radios will be sold someday, if not by me then by my wife or children.

  7. Decent method for keeping an authentic appearance. Pointless in terms of practicality though. Selling the kit as containing original parts is a bit off though but buyer beware and all that.

  8. Several years ago, I spent a lot of time with my grandfather in his basement shop repairing old tube radios. He was a technician in the air force, and working on these things really brought him back.

    For most things, we just used little yellow axial-lead capacitors. They didn’t look too out of place, and they worked fine. For a few really special radios, we used the technique shown above. Sometimes that look is important, and it’s a very common technique in the antique radio restoration community. It seems that a lot of people here are up-in-arms about it, but there’s really no other alternative.

  9. Was intrigued by the article title, thought it would be a pretty technical writeup.

    Drilling a hole and sticking in a modern cap – a stretch to call this rebuilding, but understand you do what you need to do in a restoration project and that usually includes crafting something that looks original but isn’t.

    Thumbs down if this is being done for reselling without disclosure – otherwise, good job! Be sure to *proactively* pass along the change to whoever you hand it off to. If you don’t disclose and someone peeks in, they believe it’s a real part and pay the premium. That person would pay a much different price if they know parts are not original and not to spec.

    Consider throwing an arcade machine up on eBay and forgetting to mention that the original PCB isn’t in there – it’s actually a PC running MAME. I’d pay a very different price with that disclosure. (Maybe the analogy isn’t a good one, but I have over a dozen classic arcade machines and would be very angry of that discovery and would find myself shelling out for the real parts in addition).

  10. I’ve repaired / recapped several old amplifiers, and consider this deceptive.

    Rather than just replacing, it would take much more time and effort to “fake” the caps in the chassis. They aren’t visible, so what’s the point? Other than deception…

    If you faked a “bumble bee” cap in an old Les Paul, would it have the same “mojo?” Only if the player thought it was real… (you make your own mojo).

    Now, gutting and “reloading” a can cap is different–those are usually visible, they might have a weird footprint or visual appeal. Plus replacing with multiple caps instead usually requires adding additional terminal strips that might not fit, need drilling, etc. That’s a permanent change rather than a restoration.

    1. I must agree. I’ve even seen it in QST. This is STANDARD PRACTICE in the radio restoration area. Save the outer shell for appearances, even the leads, if they are cloth insulated wire (though, I’ve made cloth insulated wire from mini-paracord for some other things, it does have advantages in some projects), stuff the shell with comparable modern parts, re-seal the original shell.

  11. Nice hack, and I can’t see anyone commenting “not a hack”;) However the recondition cap should be labeled reconditioned, if only to inform future techs that may have to repair the equipment. The only problem there may be with the hack might be with heat dissipation for the actual replacement caps.

    The cap chosen to document the hack was mounted under the chassis. No doubt their are purists who are smug enough to pay more for what only repair techs will see, that’s their right power to them. The only way it could be a scam if the repair tech claims they installed a NOS/OEM cap, and Charges considerably more than the tech’s labor, and modern parts are worth. As far as future buyers are concerned caveat emptor Malaya applies, and implied warranty rarely does with items as this.

    1. Hell I reported my own comment, when my intention was to reply to it to say please replace Malaya with always. Evidently I clicked on the wrong spell check suggestion. I tell ya December has been a bah humbug month for me so fa, with so sign of improving.

      1. It’s a bitch ain’t it? They put the report link where you’d expect the reply link to be, in a dark font that’s hard to read against the green color of replies.

        And there’s no confirmation or way to reverse it either. Great system, guys.

  12. Fair enough, but remember “they don’t make ’em like they used to”. Good quality electros used to last for decades even in varying climates. Then it was determined that the paste was carcinogenic, and the caps couldn’t even be manufactured safely. So, now we have a different chemistry, and a lot of equipment that only lasts 2-3 years, especially if it’s always on (even in standby).

    1. True that. The very first thing I do when opening any failed piece of recent electronics is visually inspect all electrolytic caps. And that frequently reveals the problem without picking up a single piece of test gear.

      So although I understand why this kind of “rebuild” is done, denying direct visual inspection of such failure-prone parts still seems wrong.

  13. I’m curious about the picture of the cap-within-a-cap (fake.jpg) Was this done by an individual or company doing restoration work? Have you ever seen examples of this done in *original* equipment, possibly to obscure the real circuit design and frustrate attempts at reverse engineering?

  14. Interesting hack. My dad still runs the 6550 A/B push-pull mono blocks he built in the 50’s and early 60’s with the original pre-amps of the same era. Somehow the caps and resistors and even the tubes have lasted more than 50 years. He’s a bit of an audiophile but he doesn’t fall for the esoteric BS. If he lost a cap, he’d replace it with a reasonable substitute, especially if it was part of the power supply.

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