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 Post subject: Tuning Capacitor Restoration
PostPosted: Mar Sat 10, 2012 5:55 am 
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Air variable capacitors come in many different shapes and sizes. Single, dual, tipple, and even quad sections are commonly seen.

http://www.g3npf.co.uk/gang.gif

But the tuning capacitor most commonly seen by vintage radio enthusiasts is the dual section unit that simultaneously tunes both the RF stage and local oscillator. In the early days of radio, separate capacitors were often used. First the oscillator capacitor was tuned to select the receiving frequency, then the RF capacitor was adjusted to maximize the signal strength. Since modern capacitors offer dual sections, this task has been greatly simplified to just one adjustment. To insure proper tracking between RF and oscillator stages, paralleled mica trimmers are found on the side and adjusted by screwdriver. The smaller section tunes the local oscillator while the larger tunes the RF stage. During operation, the tuned circuit of the RF stage and local oscillator are combined in the mixer. The result is four (4) signals consisting of the frequency of the station being received, the frequency of the local oscillator, their sum, and their difference. When tracking properly, the IF stages “see” only the difference which is always 455 KHz and are sharply tuned to this single frequency. Since the capacitor sections rotate in tandem, the offset is always maintained at 455 KHz thus giving the receiver it’s selectivity or ability to separate stations.

As ingenious as this system is, an air variable capacitor is quite the dust and dirt magnet. Over the years, variable capacitors may become contaminated with dust, dirt, grease & grime, mouse droppings, corrosion & rust, spider cocoons, bird droppings, you name it. The plates may accidentally become bent, loose, or bound up. The ball bearings may become encrusted with grime or corrosion preventing the shaft from rotating smoothly. When you turn the tuning knob and the dial pointer refuses to move, often the tuning capacitor has become jammed.

Tuning capacitor frames can also be pulled out of alignment by the dial string when their rubber mounts become brittle and cracked, often carbonizing and turning to black particles. The large pulley can then rub against other metal parts and cause intermittent static or the capacitor frame itself becomes grounded to the chassis.

When a variable capacitor is in need of remounting or cleaning, the job is not all that difficult if you how the correct procedure. If not, getting one back together can be a real nightmare.

Tools Needed: ¼” nut driver, 5/16” nut driver, small screwdriver, medium screwdriver, soldering gun, solder wick, 1-gallon jug of [white] vinegar, 16 oz glass measuring cup, plastic meat tray.

Tuning Capacitor Removal

1. Be sure you have a dial string diagram, then remove the dial string.
2. Unsolder the wires going to the capacitor sections.
3. Most AA5/AA6 radios have three rubber mounts accessed from below. Using a ¼” nut driver, remove the 6-32 mounting screws and metal feed through bushings.
4. Lift the capacitor frame off the rubber mounts. If badly deteriorated, they may stick. Grommets having the same diameter and thickness can be used as replacements.
5. Heat a 12 oz glass measuring cup of vinegar on high power for two minutes.
6. Open the capacitor plates and soak overnight.
7. When finished, rinse vigorously in tap water.
8. Blow dry to remove all moisture using a hand held hair dryer.

Tuning Capacitor Breakdown

When a tuning capacitor becomes bound up, is excessively dirty, or no longer rotates smoothly, it becomes necessary to separate the rotor from stator to facilitate cleaning, lubricating, and adjustment.

1. Using a small 2” screwdriver, place the blade in the open space at the center between gang sections and pry upward on the brass spring clip. Do not force or bend the clip; it should slide directly up and off the center of the shaft.
2. Rotate plates fully open.
3. Working over a plastic meat tray, remove the shaft hex locking nut using a 5/16” nut driver and slowly back out and remove the adjustment screw. Ten ball bearings will fall loose, 9 from the front and 1 from the rear.
4. With the plates fully open, lift the shaft at the rear and pull the rotor section away from the frame.
5. Using WD-40, clean the ten ball bearings to remove all grime & corrosion.
6. Likewise, clean the brass spring clip, hex nut, and adjustment screw.
7. Place the stator and rotor sections a heated vinegar bath and let both soak overnight.
8. Remove the sections periodically and gently brush between the plates with a soft toothbrush. Continue this procedure until capacitor deposits are gone. Do not tamper with the mica trimmers.
9. When finished, rinse vigorously in tap water.
10. Blow dry to remove all moisture using a hand held hair dryer.

Tuning Capacitor Assembly

Directions must be followed to the letter or you’ll never get the capacitor back together. It’s very easy when you know the proper procedure.

1. Work over a catch tray or hand towel as parts will repeatedly fall.
2. Hold the adjustment screw upright and place the ball bearing on the top.
3. Hold the capacitor frame above the adjustment screw and turn adjustment screw part way in so ball bearing is trapped within the frame but not so far that it falls out the other end.
4. Put the rotor into the fully open position and insert the front of shaft into the frame hole.
5. Align the rear of rotor shaft with the adjustment screw and trapped ball bearing.
6. Keeping the rotor plates in the fully open position, tighten the adjustment screw until the end of screw with ball bearing engages the rear of rotor shaft.
7. Once the adjustment screw & ball bearing are joined to the rotor shaft, slowly back out the adjustment screw so the end of rotor shaft touches the frame. Very important!
8. Slowly turn the rotor so the plates mesh. The plates will be offset with one extra plate at the rear of frame. This plate misalignment allows space at the front to insert the ball bearings.
9. Hold the capacitor frame so the front of shaft is pointing down and place the 9 ball bearings into the shaft hole. Do this over a catch tray or towel as bearings may fall.
10. Once the ball bearings are in place, turn the rotor so the plates are fully open.
11. Tighten the adjustment screw so the shaft moves forward trapping and retaining the 9 ball bearings. Careful the rear ball bearing does not fall away from the tip of the adjustment screw.
12. Turn the adjustment screw so plates are in proper alignment and there is no play in the shaft. Do not over tighten.
13. Replace the locking hex nut and check for smooth rotation of shaft.
14. Spread the prongs of the brass center spring so they are separated ¼ inch.
15. Squeeze the ends of the center spring clip together using two hands and slide downward into position. It should snap down into it’s detent. The center spring clip should not move when shaft is rotated. If it does, press it down into it’s detent area.
16. Apply a drop of sewing machine oil to the rear, center, and front bearing points.
17. Wipe away any excess oil and check shaft for smooth rotation. If there is any binding, back off the adjustment screw a tad. There should be no front to back play in the shaft.


Attachments:
File comment: Cleaned and reassembled
7 Cleaned and Reassembled.JPG
7 Cleaned and Reassembled.JPG [ 123.88 KiB | Viewed 19834 times ]
File comment: Rotor and Stator Separation
6 Rotor Stator Separation.JPG
6 Rotor Stator Separation.JPG [ 126.81 KiB | Viewed 19834 times ]
File comment: Removing Rotor from Frame
5 Removing Rotor From Frame.JPG
5 Removing Rotor From Frame.JPG [ 127.99 KiB | Viewed 19834 times ]
File comment: Lifting Brass Clip
4 Lifting Brass Clip.JPG
4 Lifting Brass Clip.JPG [ 126.07 KiB | Viewed 19834 times ]
File comment: Adjustment Screw and Ball Bearing
3 Adjustment screw and ball bearing.JPG
3 Adjustment screw and ball bearing.JPG [ 125.6 KiB | Viewed 19834 times ]
File comment: Mounting Hardware
2 Mounting Hardware.JPG
2 Mounting Hardware.JPG [ 129.68 KiB | Viewed 19834 times ]
File comment: Tuning Capacitor Before Restoration
1 Dirty Tuning Cap.JPG
1 Dirty Tuning Cap.JPG [ 126.88 KiB | Viewed 19834 times ]


Last edited by R. Jepsen on Mar Sat 10, 2012 11:34 pm, edited 1 time in total.
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 Post subject: Re: Tuning Capacitor Restoration
PostPosted: Mar Sat 10, 2012 8:32 am 
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Thanks for the tutorial, I would never have done one, to that depth.A question, have you done any of the ones with out a adjusting nut? I have two that have lost ball bearings, and was going to take a stab at putting some replacements in.Thanks you your input.

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 Post subject: Re: Tuning Capacitor Restoration
PostPosted: Mar Sat 10, 2012 3:04 pm 
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Thank you. The tutorial was for those who have never removed or split a tuning capacitor. This is important because almost all of them accumulate dirt and eventually, the mounts go bad.

The best way to see if the mounts are bad is to look under the capacitor using a bright 9-LED flashlight. If you see any odd compression, cracks, missing chunks, telltale particles, or if the capacitor frame is being pulled crooked by the dial string, it's time to put in some new grommets.

But no, I have never worked on one that was missing the adjustment locking nut. Of course, I have not seen all types but would assume they all pretty much have the same basic design except for the single-gang types designed for communications equipment.

I'm guessing you plan on obtaining the missing ball bearings and locking nut from another unit? I'll tell you, in spite of exercising extreme caution by working over a catch tray, one of the ball bearings got away from me. They are so tiny that they don't make a sound when they hit the floor so no clue as to their location. Luckily, I found it under the computer desk and hadn't rolled to some obscure place. Nothing functions well with a missing ball. LOL


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 Post subject: Re: Tuning Capacitor Restoration
PostPosted: Mar Sat 10, 2012 4:47 pm 
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Very nice, detailed write-up and excellent photo-documentation. Thanks!

Don


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 Post subject: Re: Tuning Capacitor Restoration
PostPosted: Mar Sat 10, 2012 4:48 pm 
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Excellent tutorial. Thanks. I've had to remove a couple to replace the dried out grommets, but I was afraid to do much more than blow them out with low pressure compressed air or use some strips of paper wetted with alcohol to clean the plates, which often doesn't do all that well.

If the unit isn't so bad it needs to be disassembled, can you just soak the whole thing in vinegar? The vinegar and water rinse doesn't hurt the mica caps?

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 Post subject: Re: Tuning Capacitor Restoration
PostPosted: Mar Sat 10, 2012 9:19 pm 
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Yes, correct Ed. The reason I tore this one down is because the ball bearings were gummed up and weren't moving. The ball bearings should move freely in their track as the shaft is turned.

To answer your question specifically, for tuning capacitors that are in pretty good shape, I would first blow them out to remove all the loose hair & dust before removal to replace the mounting grommets. Then put the capacitor (plates open) into a 16 oz glass measuring cup full of hot vinegar. I microwave the vinegar on high power for 2 minutes. Every couple of hours you can run a soft toothbrush between the plates if necessary. As for the mica trimmers, the pressure applied by the adjustment screw against the outer plate seems to ward off any moisture. Plus the drying process gets the capacitor frame too hot to touch. My educated guess would be that any moisture in the air gap below the mica is driven out. A visual inspection shows no difference before and after treatment. I suppose you could remove the adjustment screw and mica slab before blow drying but that would destroy it's factory setting. Also the mica is pretty fragile stuff and best avoided.

In photo, mica trimmers appear to be undaunted by their dunking experience. Notice the cracked rubber mounts that need replaced.


Attachments:
P1000005a.JPG
P1000005a.JPG [ 102.86 KiB | Viewed 19740 times ]


Last edited by R. Jepsen on Mar Sat 10, 2012 10:31 pm, edited 1 time in total.
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 Post subject: Re: Tuning Capacitor Restoration
PostPosted: Mar Sat 10, 2012 9:36 pm 
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Am I right in assuming we're talking WHITE VINEGAR?

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 Post subject: Re: Tuning Capacitor Restoration
PostPosted: Mar Sat 10, 2012 11:17 pm 
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Yes, white vinegar. So far, no radio parts seemed to have a bad reaction to the acid but my long nose pliers tarnished.


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 Post subject: Re: Tuning Capacitor Restoration
PostPosted: Mar Sun 11, 2012 11:36 pm 
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Here is a tuning capacitor on an RCA 66X11 chassis. It's a perfect example of what can happen when the rubber mounts become defective. In the photo, you can plainly see the front of the capacitor being pulled up by the tension of the dial cord. This can cause the large pulley to intermittently rub against nearby objects creating static. Left unchecked, the mounts can deteriorate to the point where the tuning capacitor frame grounds against the chassis. The condition is remedied by replacing the rubbers with similar diameter & thickness grommets.


Attachments:
File comment: Tuning Capacitor Displacement
66X11 Tuning Cap_235K.jpg
66X11 Tuning Cap_235K.jpg [ 235.55 KiB | Viewed 19703 times ]
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 Post subject: Re: Tuning Capacitor Restoration
PostPosted: Mar Mon 12, 2012 6:09 pm 
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Guys: I purchased a small ultrasonic cleaner some years back, it even has its own tank heater within. I use a solution of Dawn, ammonia, and distilled water that I store in a glass jug when not using. The mica trimmer sheets are removed from the variable cap and the entire cap is then submerged into the ultrasonic tank and allowed to clean for 30 minutes or so. It emerges just spotless and the bearing races etc. are ultrasonically "scrubbed" and ready for lubing. I also used this technique a while back to clean a zenith "radiorgan" switch assembly that was extremely dirty and the outcome was excellent. I also use it for periodic cleaning of my devilbiss spray guns. You can pick up an ultrasonic cleaner at places such as harbor freight, ebay, etc. for less than $100.


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 Post subject: Re: Tuning Capacitor Restoration
PostPosted: Mar Mon 12, 2012 6:55 pm 
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Great suggestion! I'll look into it.


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 Post subject: Re: Tuning Capacitor Restoration
PostPosted: Mar Tue 13, 2012 7:21 pm 
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Nice tutorial.

Some guys throw them in the dishwasher, some have used the "Chem Dip" carburetor cleaner (me) and they come out great. Most of the time I wash in strong detergent, hot water and use long bristled paint brushes, etc. to get them clean.
I do remove the mica sheets from the trimmers. Murphy's law kicks in if I leave them installed :shock:
I will also use the air compressor to blow out any moisture, and don't want to hit a mica sheet with the air. Remove them to be on the safe side.
Just find out how many turns it takes to bottom each screw out, then you know where to set them to get you going, before alignment.

I have disassembled tuning caps, such as on a Philco 20, those come off easy. Ball bearings ones, no. Working them around in cleaner/detergent gets out any gunk out, old grease etc.

With the tuning caps without the end screw, they are not worth disassembling. You'd have to bend the frame apart. It would be overkill to do that.

Take care,

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 Post subject: Re: Tuning Capacitor Restoration
PostPosted: Mar Tue 13, 2012 8:36 pm 
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Quote:
With the tuning caps without the end screw, they are not worth disassembling. You'd have to bend the frame apart. It would be overkill to do that
That is what I was wondering...thanks.

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 Post subject: Re: Tuning Capacitor Restoration
PostPosted: Jul Wed 17, 2013 10:15 pm 
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gary rabbitt wrote:
I do remove the mica sheets from the trimmers. Murphy's law kicks in if I leave them installed :shock:
I will also use the air compressor to blow out any moisture, and don't want to hit a mica sheet with the air. Remove them to be on the safe side.

Let me second that. I just completed a tuner with very similar methods using a sonic cleaner and chose not to remove the mica sheets, and let the drying process handle it. After a few hours of drying and polishing, I was re-assembling it all, and used an air can to blow out a few bits of fuzz that had gotten between the capacitor plates during the cleaning process. Well, I mis-aimed and it blew across the mica sheet and out came a snow flurry of silver flakes... presumably, the end of that mica sheet. Guess the water loosened the mica from the plastic coating? Lesson learned! :mrgreen:


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 Post subject: Re: Tuning Capacitor Restoration
PostPosted: Aug Mon 11, 2014 5:36 pm 
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I know I'm on an old topic but when ever I run across this on the internet I comment . One of the posts states he warms vinegar in the microwave for 2 minutes . I once heated vinegar in my microwave in an attempt for the vapors to help clean it . I had it in to the point of boiling , probably a minute and a half or so . For some reason when I opened the microwave the hot vinegar somewhat exploded up out of the pyrex bowl it was in sending hot vinegar all over . Fortunately none of it went on me . This was plain white distilled vinegar . When ever I can comment on vinegar in the microwave I do it , because I suppose that could happen and possibly go in someones face ?? I would suggest heating the vinegar on the stove and stirring it regularly . I did see something once about surface tension on the smooth pyrex on the show mythbusters that explained why this happens .
The reason I found the thread I have a variable capacitor (single gang with 5 blades ) and three of the blades fell off . I'm trying to figure out what holds the blades to the grooves in the shaft ? I've just stuck mine back together but I'm almost sure it will fall apart again ?? I have to go buy a diode and I'm going to take it with me and see what the part guy says and / or possibly buy a new one . Is it possible to measure the value of these . I tried with my dmm and I did get different readings depending whether it was fully meshed or not but the readout was pretty jumpy . I have a vtvm and an oscilliscope , would those somehow work better ? Thanks .


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 Post subject: Re: Tuning Capacitor Restoration
PostPosted: Aug Tue 12, 2014 1:10 am 
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I will repeat a previous post I did on ultrasonic cleaners:
Do NOT use the harbor Frt. ultrasonic cleaning fluid.
It will damage aluminum.

I just used Dawn detergent (But the addition of vinegar would give it a bit more action).
I might try the next one with Dawn and Jet Dry.

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 Post subject: Re: Tuning Capacitor Restoration
PostPosted: Aug Tue 12, 2014 1:16 am 
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I had a very nasty 1920s cap just the other week that I wanted to send to a friend for his loop. I soaked it 24 hrs in vinegar and it came out very nice. Call me amazed.

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 Post subject: Re: Tuning Capacitor Restoration
PostPosted: Aug Tue 12, 2014 9:05 am 
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Quote:
. For some reason when I opened the microwave the hot vinegar somewhat exploded up out of the pyrex bowl it was in sending hot vinegar all over . Fortunately none of it went on me . This was plain white distilled vinegar . When ever I can comment on vinegar in the microwave I do it , because I suppose that could happen and possibly go in someones face ??


When a liquid is heated in a microwave, without agitation, it can be come "super-heated" in the center, way above the boiling point, and then, "explode" out of the cooler mass.

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 Post subject: Re: Tuning Capacitor Restoration
PostPosted: Sep Thu 18, 2014 4:18 pm 
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Great Article, I also came to it from a google search. Planning to do this with a tuning cap that has the oscillator side short. Is there any article to restore the mica trimmers, so they will look original.

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 Post subject: Re: Tuning Capacitor Restoration
PostPosted: Sep Fri 19, 2014 2:21 am 
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You can take the mica sheets used to isolate transistors and such and use them to replace the damaged mica's on the tuning caps with.
They may be too thick, but generally its easy to remove a few layers -very- carefully to get them to the original thickness.
You can get the mica's from MCM, NTE, Dalbani, or even try the local TV shop. The shops usually have some extras laying around. I would imagine that even if the mica is a little bit thicker, it may not make a large amount of difference, as you will be re-aligning the radio anyway. You may also be able to take the mica from air trimmers or old IFT's and use that also.

Note: The mica insul. for Xistors and IC's come in MANY different shapes and sizes. I have used good sharp scissors to cut them down to the correct size and shape.
If you are not bothered by appearances, you might even be able to use the SilPads that NTE sells. I bought a 12 X 12" piece for 21.00 + tax NTE part # TP0012A. (Yeah, I literally had it sitting right in front of me here, as I needed a piece to use replacing a Mitsubishi DLP chip in my TV.) That's enough to make replacements for a long time. I just cut it to size, and use a paper punch to make the hole with (There are several sizes of hand paper punches available and the smallest one I found is about 1/8" hole- which is just what I have needed).

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