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Author Topic: When to recap  (Read 3344 times)
N0SOY
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Posts: 72




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« on: December 20, 2009, 09:21:58 PM »

I have three old shortwave radios, Zenith transoceanic, a Hallicrafters S-108 and a S-38e.  All of them work well (the S-108 needs a speaker)  and have a minimum amount of AC Hum.  It is hardly noticeable in any of them.  I do not know their history.  I read about recapping old radios and I wonder if I should do it on them.  But I do not want to do something that is not needed.  How would I tell if the radio needs it and When is it normally done?  

73
David
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AD4U
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« Reply #1 on: December 21, 2009, 06:24:13 AM »

Most any 50+ year old radio will benefit from a re-cap job.  If a PS filter cap goes, it may take the transformer (if one is used) with it.  

I have all 7 Trans Oceanics and the tube type Trans Oceanics do not have a fuse.  I recommend that you add one.  I am not sure about the other radios you mentioned.

IMO it is a testament to the Trans Oceanics that so many are still up and running even though some of the earlier ones are over 70 years old.  I often wonder how many of the modern computerized receivers will be working in 70 years - probably none of them.

Dick  AD4U
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N0SOY
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« Reply #2 on: December 21, 2009, 07:17:42 AM »

The interesting thing is that the Transoceanic is the most sensitive, selective and best sounding of all of my dozen shortwave receivers.
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K8AC
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« Reply #3 on: December 21, 2009, 11:37:15 AM »

<<snip>> How would I tell if the radio needs it and When is it normally done?  

If you open up the radio and look at the filter caps, that will give you some idea of when they were last replaced, if they ever were.  That's where some experience comes in handy.  For example, if the filter caps have a stiff paper wrapping, they've been in there a while.  If they're in aluminum cans mounted on the chassis, chances are they've never been replaced.  If the ends are bulged, replace them.  Personally, if a piece of gear is 25 years old or older, I'd always replace the filter caps before turning it on.  They DO deteriorate over time and there's no easy way to know if they are putting unnecessary strain on the transformer due to leakage.  You'll usually find some other electrolytic caps in the radio - often bypassing a cathode bias resistor in an audio stage and I'd replace those as well.  When restoring a tube radio, I also replace all the bypass caps (if they're tubular, not disc).  Some people wouldn't do that and question those who do.  It's been my experience that multiple bad caps can cause multiple problem symptoms that would be very difficult to resolve if you tried to troubleshoot them one at a time.  Why bother?  For a few bucks you can replace all the caps and avoid all those problems.  Plus, I can replace all the caps in the time it might take to troubleshoot a single bad bypass cap. 

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KE3WD
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« Reply #4 on: December 22, 2009, 12:55:56 PM »

"All of them work well ..."

Which is reason enough to leave well enough alone in my book.  

I'd just keep an eye on things and if it began to look like any one of 'em was getting into trouble, then I'd cross whatever bridge that happened to be at the time.  

Old radios and "preemptive strikes" like the much-mentioned-on-the-web filter capacitor changeout, when all functions are still working well, can become something much worse due to the invasive things necessary to get to and replace.  And you maybe would have never had THAT problem either in the first place.  

I know it won't stop the many, but perhaps it will stop the few.  

"If it works, for God's sake, don't FIX it!"


KE3WD said that.
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K9LJH
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« Reply #5 on: December 22, 2009, 01:46:15 PM »

Wait a minute here.  Doesn't that violate one of the basic tenets of Ham Radio "Never leave well enough alone" Smiley
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W8JI
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« Reply #6 on: December 22, 2009, 02:45:17 PM »

I have three old shortwave radios, Zenith transoceanic, a Hallicrafters S-108 and a S-38e.  All of them work well (the S-108 needs a speaker)  and have a minimum amount of AC Hum.  It is hardly noticeable in any of them.  I do not know their history.  I read about recapping old radios and I wonder if I should do it on them.  But I do not want to do something that is not needed.  How would I tell if the radio needs it and When is it normally done?  

73
David

David,

I'm a firm believer in NOT recapping radios unless they have a problem.

I also always, without fail, make sure the radio is properly fused. Many old radios, even the one you have, are not fused. Look at the power rating of the radio, divide that by 70, and use something close to that size fuse.

If the radio draws 55 watts, 55/70 = .79 so add a 3/4 or 1 amp fast blow fuse. This will almost always save a transformer.

When radios have a problem, I like to take the time to check things and change only what is really needed. I can't see replacing 50 components when only two or three are really bad.

Tom

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N4ZAW
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« Reply #7 on: December 23, 2009, 06:51:32 AM »

I've taken both approaches to the same piece of gear, the good ol classic, the Cobra 2000GTL. I already knew the production dates and that the lytics were in excess of 22years old. I read the advice online to change out all the lytics because they "dry-out over time, and in some cases, leak electrolite just like old batteries do".
That may be the case but---
Both radios, simular symptoms too (No TX/ due to blown final or driver).
The first 2000 I simply found the bad driver, changed it, reset the bias and put the cover back on it. As far as I know, it's still kickin.
The other one(after reading that advice) I found a blown 1969 final. I changed it out, re-adjusted the bias, tested to find good RF, and proceded to replace most of the 'lytics(23 of them as I recall). I had the time, and this one was my OWN dust-free shelf candy.
The problems I ran into afterwards requiered a total realignment. Sure, it didn't take long (only a couple hours, and changing all those caps took me half a day), but when I plugged her back in, the B+ was so high it smoked the meter lamps in 3seconds!
The IF's were so far out of adjustment that hiss could not even be heard until I hooked the signal generator up to the atnenna jack set on high!
Let's not go into frequency realignment, but it took a while. Sure, the old girl is 100%,and will probably remain so for years to come -- but that cured me of ever trying such a day and money-eating stunt again.
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N7IOH
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« Reply #8 on: December 31, 2009, 05:53:35 PM »

I too have some old gear, some Drakes that are ~35 years old and two Johnson Rangers that are ~50 years old.  I would normally side with Ton w8ji, but not this time.  Replace all the electrolytic capacitors, there aren't that many, they are cheap, and most likely the radio will work better.  It just isn't worth it to take a chance on damaging your valuable radios.  It can be a pain in the butt to trouble shoot them.  The old electrolytic capacitors are most likely out of spec and will just cause problems down the road.  I would install a fuse as mentioned as added protection.  Also I would change the power cords to ether a two prong polarized plug or a three wire plug.  Make sure that the fuse goes in the hot lead.  

My Drake R-4B receiver for example uses only nine electrolytic capacitors.  Not a big deal money wise or time wise to replace.  In being a member of several old radio web groups I feel safe in saying electrolytic capacitors, tubes and resistors that have gone high in value cause the most problems.  The electrolytic capacitors once replaced should be good for another 25 years.  Todays electrolytic capacitors are much better made than the old paper caps and you can upgrade some to give you a little better performance.  I would do a visual inspection of the old carbon resistors.  If they are cracked or look like they have been baked in an oven I would replace them too.  There may be some resistors that were originally install that are underrated and could use a higher wattage resistor.  I would try and find a web group for each of your old radios and see what the other guys have had to do to their gear to keep it working well.  Most of these groups have someone that is very knowledgeable and welling to give you some good advise.  Good luck.

Al, n7ioh
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KE3WD
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« Reply #9 on: December 31, 2009, 06:13:08 PM »

Damage the radio?

I don't think that's the mean failure mode when electrolytics go south.  

Not for the living operator, anyway.  The dead ones might not notice and let the thing get to the point where you might lose a resistor, choke or transformer.  

Old tubers are likely to just start motorboating if the filter caps go leaky.  You would notice that and the troubleshoot/repair cycle begins.  

In years of repairing all types of consumer RF and Audio gear, I've encountered some few cases where a leaky electrolytic actually managed to damage another part.  

The best thing you can do for an old radio is to install a fuse in it of proper size if it doesn't already have one.  W8JI gave you the formula already.  With the proper fuse in there, if the filter cap went to a dead short it still shouldn't damage another component.  That's what fuses are for.  

Those old designs can take a heckuva lot of motorboating anyway.  So it starts buzzing.  Then repleace the cap.  The radio won't mind.
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N7IOH
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« Reply #10 on: December 31, 2009, 06:49:17 PM »

Damage the radio?

I don't know about you, but if I lost a choke or a transformer, I would call that damaging the radio!  Just try walking into your local Radio Shack and telling them you need a choke or a transformer for a 50 year old radio.  For $25 I would guess you could replace all the electrolytic capacitors in all three radios assuming you buy them from someone like Mouser Electronics.  The other alternative is wait until something goes bad and then buy the needed test gear to test the radios.  No it won't be cheaper or faster to trouble shoot the old radios especially if you don't have the experience or equipment.  I think sometimes the guys that have all the test gear and experience forget that the guys asking the questions don't have their background or experience or equipment.  If they did they most likely would not be asking these questions.  

Just replace the old electrolytic capacitors, it's cheap insurance against future problems.  

Al, n7ioh
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WX7G
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« Reply #11 on: December 31, 2009, 07:48:20 PM »

Let's recap: We have advice to recap based on the ease, low cost, and that it can head off future damage. We also have advice to not recap until the radio tells us it is time.

My take:
I have seen many, many dried out 'open' electrolytic caps but few if any shorted electrolytics.

I have seen gear damaged by dried out caps but in my experience it is rare. This is very circuit specific and is a sign of a bad design.

In spite of the failure mode of 'cap aging leading to further damage' not being a common failure mode, it has been pointed out that IF the radio is damaged it can spell the end of an old and very fine radio. Gage your risk level and recap if it makes you feel better.  

If you want to retain that old time look of metal can chassis mount caps, new and smaller caps can be placed inside the hollowed out shells.
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W8JI
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« Reply #12 on: January 01, 2010, 11:20:52 AM »

Like you, I have never seen a bad capacitor take out a choke or transformer unless the device lacks a proper fuse.

The real solution is to add a fuse, not randomly shotgun all the parts, because even if you changed the capacitors (to eliminate a 10,000:1 shot that a filter capacitor would short) the radio still could be damaged by a shorted tube, shorted resistor, shorted wire, or operator error.

If you have a radio that can have chokes or transformers damaged from a seriously leaky or shorted capacitor, add a fuse! Nothing else makes any sense.

Tom
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AC5UP
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« Reply #13 on: January 05, 2010, 10:05:26 AM »

It has been my experience that when caps fail they fail 'open'. Little tendency to short, they just lose capacitance until the performance starts to go south. In the case of Twist-Lok electrolytics I tend to leave them alone until they test bad since finding a suitable replacement can be a real challenge. A quick & dirty test of power supply filter caps can be made with a voltmeter... Measure the AC and DC voltage across the cap. Should be almost no AC, DC should be within a few percent of the schematic.

In the case of cathode bypass caps & such a cheap hand held capacitance meter is a good idea and I have one that I use regularly. I've been spending my holiday time tinkering and last weekend replaced a 100 uf / 50 volt axial lead cap that measured a shade over 6 uf. It was a cathode bypass in an audio amplifier and the sound quality is much improved with the new part. I've had very few problems with ceramics and silver micas, but wax paper tubular caps get replaced on sight.

So... When to recap?

I think that waiting until it breaks is about as misguided as replacing parts just 'cuz they're old. But, an argument can be made that some parts are easier to find today than they will be 10 years from now when you really need one. In any case, it doesn't hurt to do a little R & R every five years or so. Remember that carbon comp resistors and small value caps tend to drift very slowly over time... Meaning, if you're relying on your ears to tell you when it's time to pop the chassis, you've probably become accustomed to listening to a fraction of what the rig is capable of.

Afterthots added 1/6/2010:

Another quick & dirty check of the power supply filter caps in a tube rig can be made by pulling the power cord out of the wall socket. With the rig operating normally, pull the plug and note how long it takes for the audio to go bye-bye. Good filter caps should float the rig for 2 - 5 seconds, extra good caps even longer. I currently have on the bench a Fisher 15 watt audio power amp built around 1960 that plays for 10 seconds after the plug is pulled. I think that speaks well for the original triple section Sprague Twist-Lok, which also checks good on the cap meter.

And speaking of electrolytics... They are kinda' rare, but you will see them in old test gear: Sprague Extralytics. Do not assume they're junk because they're old. They're physically much larger than a cap of the same capacity and voltage so they're not exactly plug & play as a repair part, but they do live long and prosper.

« Last Edit: January 06, 2010, 05:24:33 PM by AC5UP » Logged

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KE3WD
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« Reply #14 on: January 05, 2010, 03:27:43 PM »

If you are into recapping, the first thing you should buy is not a boxful of electrolytics, but a good ESR meter.  Then you might be able to get an idea about electrolytic health before just blanket replacing caps.  

There is one place where those who love the idea of soldering in new caps can have a field day, though.  And that would be in the switching power supplies.  Now there's a place where new electrolytics before the fact of failure can be a good thing!  Even so, I'd use the ESR checker on 'em first anyway, for if it is still healthy, I feel that the invasive nature of replacement is reason enough to leave well enough alone.
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