Wrong Super Graphic Capacitors--Need advice

This weekend I dealt with a mirror in my Super Graphic that had come unglued. While I had the viewfinder out, II took the opportunity to replace the solenoid capacitors, since they tested bad on my Triplett 630. Being a radio amateur, I reached into my parts stash for 2 new 50v 47mfd caps to replace the 50v 50mfd caps. Then I went about the business of reassembling the camera. THEN, it occurred to me. I can't use those capacitors. NO WAY are the new ones I used going to be able to stand up to the repeated discharge to operate a solenoid. I'd e surprised if they don't blow up the first time I trip the shutter. See the photo. What sort of caps should I buy to replace the originals? Would Digi-Key have something? Thank you.

 

Caps have gotten incredibly smaller, but I'm not sure those wouldn't work. It comes down to internal resistance and the ability to dissipate power. The newer caps probably have lower internal resistance. (I don't use the term ESR because it's measured at 100 kHz and probably isn't the right thing to look at.) They obviously have much less ability to dissipate heat, but how often do you fire the thing? You might think about putting five 10 uF parts in parallel because the surface area would be smaller. Also look into multi-layer ceramic caps. 50 uF should be easy to make up but remember that they lose a huge amount of value as the voltage goes up. Me? I'd install the new ones and see how it works. Not sure audio caps would be my first choice, better to look for a standard commercial grade part with a decent ripple rating and long life.

BTW, the solenoid resistance is probably a good few ohms, which will keep current under control. It's not like you're shorting out the cap with a screwdriver. :D

I just looked at my own post and saw the 80mfd printed on one of the old ones. So that means 2 x 80=160. I don't know where I gt the idea they were 50's. But since all I put in was 2 off the 47's, that isn't going to be enough anyway. Since nobody else seems to be answering, I'll accept Conrad's advice and put in 7 or 8 new 22's. That ought to be a reasonable remedy. I don't think I have 16 of the 10mfd's in my stash. If I do, I might take the time to solder them all together. that way I don't think there would be any question. Since the capacitors are in the same small area as the mirrors, I'd hate to think of the damage that could result from blowing caps in there. Although somewhere on one of these sites, I saw a post telling about capacitors meant for this purpose, years ago.

The only caps "meant for this purpose" that I can think of are photoflash caps designed to dump their energy quickly, but those are invariably rated at a higher voltage and not really the right choice. I'd just buy some 100 uF @ 50V commercial grade caps. Digikey shows about 100 types to choose from, under a buck a piece. You could special order some 80 uF 50V 600D parts, which are probably about what was in there, but they cost a fortune and won't work any better. Whatever you get, check the dimensions to be sure they fit! Edited October 13, 2021 by conrad_hoffman

I was recently reviving an old Heathkit power supply that I bought.

Before powering it up, I was buying some parts from Digi-Key, and decided

to add a capacitor like the biggest filter capacitor to my order.

 

I later powered it up and it works fine. I was surprised, like you, though

when the new capacitor came, it was so much smaller.

 

The actual dielectric in an electrolytic capacitor is a layer of anodic

oxide on aluminum foil. Thicker oxide for higher voltage but lower capacitance,

so the size is approximately proportional, all else constant, to capacitance

times voltage. It seems, though, that over the years that constant has changed.

 

Among others, they etch the foil to roughen the surface, which makes the surface

area, and so capacitor area, much larger for the same amount of foil.

 

But also, old electrolytics have a wide tolerance, maybe +/- 50%.

I suspect that there is a good chance that new 47uF would work.

I run into this with old audio equipment all the time. The new filter caps are so small you often have to fashion new clamps or make spacer rings so they don't fall out! That "constant", and I use the term loosely, has certainly changed. By whatever magic, they've increased the surface area dramatically. I have a theory about values. They can control the final value so well now that they usually make the parts near the lower tolerance limit. That way they save money on high purity aluminum foil. Almost all filter caps I buy come in a bit lower than the stated value.

So here's the plan, after visiting my new parts stash. The original was 2 x 80mfd =160. I have 5 of the 33's at 50 volts. I was happiest with them after putting them on my Triplett 630na Type 3, which uses a special 30v battery on the top range. What I was checking for is to see if the needle would return back to zero or close to it after charge up. This is a pretty fair way to check electrolytics. With experience you get a "feel" for cap condition by observing how fast it falls. In the case of the 5 33mfd's I selected, I was reasonably happy. Since the Super Graphic has no switch (other that the trip), that means as long as batteries are in the camera, the circuit is alive. Leaky caps would run down batteries pronto. The capacitors ARE your switch. I realize capacitors are much smaller these days, but a solenoid is quite an instantaneous drain. Not like a flash, but still pretty hefty. Think of 6 shots rapid fire with a Grafmatic holder. That could heat up little bitty caps pretty good. Maybe with 5, the heat will be spread a little thinner amongst them than if it were just 2. Edited October 14, 2021 by henry_finley|1

I doubt 6 shots will stress anything, but more smaller caps should dissipate heat better. There are polymer and hybrid caps with much higher current ratings, but they have much higher leakage as well, so not a good choice for this. You can get low leakage caps (UKL) but most standard caps will beat their leakage specs by a wide margin once they sit under voltage for a while. Regardless, it's not something I'd leave batteries in for long periods of non-use. If it would work, I'd also use Eneloop rechargables (from B&H of course) to avoid chemical leakage damage.

I'm an Eneloop guy too, in radio, RC plane, and photography, and anything else. But in this case, because of the restrictions discussed above, I plan on the remedies mentioned. I think they are a good plan, as opposed to frittering 40-some-odd dollars away on a pair of 22.5 volt batteries. I believe all I need is proper capacity with minimal leakage current, without risking blowing up firecrackers in the mirror box. I don't want to set the camera aside for a week and come back and discovered leaky caps drained my batteries. If I have to be a slave to removing batteries after every use, then the entire affair is something I'm not willing to be a slave to. As for the quality of Chinese capacitors (from any source), I have very little regard for them. I've already discovered the pitfalls of cheap rice caps in radio. It's ALL crap in my book. We'll see. I'm going to finish this project this weekend. If I come back in a week and discover my batteries drained, I'll stick a block of wood in the battery bay to keep the cover from getting lost, keep the camera as a collectors piece and use my Pacemaker Speed.instead.

Ah, forgot that the voltage was that high. I'm a measurement snob, so I'd probably measure the leakage of those caps just to be sure. I'd also reform them at their full voltage rating for a good few hours before installation. That would insure the leakage was as low as possible from the get-go. It would stay low with the voltage of the battery present.

 

I don't know where the major brands of caps are made, but they're mostly Japanese companies and that's all I use. If it's a brand that hasn't been around for a couple decades I want nothing to do with it.

I thought i'd wrap this up with a happy ending (so far). The above battery advice worked out. I replaced the two 80 mfd.caps with 5 ordinary 33mfd radial 50v caps from my stash of new radio parts from Digikey. Then I soldered up 4 of the 23A, 12v batteries I got for $8 and Home Depot. It took me 2 hours to solder up the batch and make the contact board out of a piece of 3/16 balsa from my model airplane supplies. I might note that the 23A batteries were too long for the battery compartment, by trying to install them lengthwise. So I hot-glued them and soldered them up crosswise, side to side. The attached photos show crude work, but I was only trying to prove the idea. Since everything worked out, next time I'll just fabricate a little box or tray where new batteries can just be dropped in like any ordinary device, like a tape recorder or something. The project worked out well, and the red button now trips the shutter. We'll see how long the batteries last. Since there's no switch, I guess I'll find out how long they last sitting there keeping the capactitors charged the whole time. I suspect they will be dead in a few days or a week at most. I'd be HIGHLY surprised if I come back in 2 months and the batteries are still good.

 

 

I'm betting they last longer than that.

Sorry, double post.

Make that a triple post! :eek:

No idea why that happened.

And now the actual post has disappeared. Great!

I was just going to add that the original capacitors were probably non-polarised and maybe also non-electrolytic to withstand the back EMF from the solenoid.

 

Nowadays the back-EMF can be easily snubbed and absorbed by a diode (backwards) across the capacitors. A 1N4007 rectifier would do the job. Back in the day of that Super Graphic it would have needed a copper or selenium rectifier nearly the size of a beer can.

I was just a few days ago, working on a power supply I bought.

 

It starts with a GE Volt-Pac, which is what everyone else calls a Variac.

But then there is a transformer for 0 - 12VAC output, and a selenium

rectifier and capacitor for 0-12VDC out.

I thought that the capacitor was leaky, but then when I get

farther in, find that there is a resistor across it.

 

I replaced the capacitor, but the rectifier seems to work just fine.

 

The ones in the picture have a + sign on them, so I suspect

not non-polarized electrolytic.

Well my last post on Oct 17 expressed doubt about how long the batteries would last. Here is is Oct 28 and they still work. The camera has mostly sat because I haven't finished up my Strobonar 770 project to go with it. I'm expecting those 4 sub-C's any day so I can get that going. And in the meantime I'm fabricating a flash cord. For that, I'm turning down 1 inch pieces of 10 gauge copper wire from 10/3 Romex, to fit the camera sockets. That idea seems to be working out so far too. I chose the Strobonar because the rings from a Graflite fit it perfectly. I suppose that's what Honeywell had in mind when they invented the Strobonar.

The ones in the picture have a + sign on them, so I suspect

not non-polarized electrolytic.

Yes. Only just 'clocked' that. Plus they're crimped at one end, which would further indicate polarisation.

 

Maybe the solenoid back-EMF didn't even cross the mind of the Graphic's designer? Or maybe the inductance is negligably small?

 

Whatever the case, a reversed diode will cause no harm, and even without one I suspect those tiny modern electrolytics will last a long time.

Yes. Only just 'clocked' that. Plus they're crimped at one end, which would further indicate polarisation.

 

Maybe the solenoid back-EMF didn't even cross the mind of the Graphic's designer? Or maybe the inductance is negligably small?

 

Whatever the case, a reversed diode will cause no harm, and even without one I suspect those tiny modern electrolytics will last a long time.

There is a diode in there. I tested it and it tested fine, so I left it in.