Gates Radio M-5078 Commercial SW AM/CW Transmitter

Restoration Details

By John LeVasseur, W2WDX

Copyright March 05, 2012

Gates Radio M-5078 Commercial SW AM/CW Transmitter

Restoration Details - Part Two

The Care and Feeding of Variable Capacitors
Dated: March 16, 2012


In going through this transmitter it appears there may be some modifications. One may be in the addition of a slightly different plate tuning variable capacitor. This capacitor is located directly underneath the roller inductor. It is possible a slightly larger one was put in at some point possibly to add more tuning range to allow the transmitter to tune down onto 160m.

The issues that made me think this was the fact that the roller inductor was moved up by about 1/4", with new mounting holes being drilled into the panel it is mounted to. Which also led to an upward movement of the turn counter mechanism on the front panel. The alignment of the inductor and the panel mounted turns counter is not perfect. Even though there is a nice ceramic coupler between the inductor and the turns counter, the misalignment leads to a binding in the inductors rotational movement. Further research & work will need to be done in this area.

Things to Consider

In regards to restoring variable capacitors, several important things should be considered. First and most importantly, do the plates show any type of corrosion or even arcing burns? Either of these conditions can make a substantial change in the operation of the capacitor. If there is a great deal of corrosion it may actually add capacitance overall, making the capacitor unable to reach its lowest value, crucial at higher operating frequencies in most applications. In addition, corrosion is not always consistent, resulting in an inconsistent curve of capacitance. This means that capacitance will not vary in a constant increasing or decreasing amount as one goes through the rotation, but instead be erratic. These are only some of the possible effects from corroded or burned cap plates.

So how does one clean and service the "bread slicer" type caps? There have been many threads with some good (and some poor) suggestions on this forum and others. The idea here was to find a way to make the process simple, remove corrosion without using destructive methods, and end up with a smooth and consistently functioning cap.

I have heard people taking these caps apart and scrubbing the plates down with sandpaper and Scotchbrite pads Egads man! By trying to remove corrosion with this destructive and invasive method accomplishes the task, but creates other problems. Like marginally increasing the gap spacing and therefore the capacity slightly. It also prepares the surface for future large scale corrosion. This method is not dealing with the chemistry of corrosion at the molecular level and leaves you with untreated bare metal to corrode. The surface scratches that remain also increase the surface area at the microscopic level. This increased area allows more area to oxidize. You may be removing corrosion, but also creating an ideal surface for new corrosion to form.

There are two things to consider in caps of these types. Removing the corrosion without changing the electrical characteristics by mechanical misalignment is one aspect. The other is maintaining the moisture resistance of the ceramics. The latter is not easy to perform in the smaller type caps used in this transmitter. Unlike the larger caps, used in high power tuners for instance, you cannot really disassemble these caps easily. This makes it difficult to clean and treat the ceramic parts separately. However,  misaligned parts are a non issue; since you cannot take it apart. So the plates and the ceramics have to be dealt with at the same time, but treated separately (does that make any sense ... it will.)

If you have simple surface corrosion you are lucky. This is readily fixable. If the pitting is so severe where it has made impressions or holes in the cap, total replacement is preferred. Fortunately, many of the common capacitors in use in most transmitters are obtainable in good or even new condition. It takes a little searching, but they are out there. If you have slight corrosion and are confident the surfaces will clean up nicely then you can proceed as follows.

One of the best ways to remove oxidation or corrosion is with acids made specifically for the metal in question. Acids will break down the corrosion and release it from the metal. This is why tomato sauce (no kidding) and things like vinegar, and dishwasher methods are suggested in this application. While they do work,  they are not the best solution (figuratively and literally.) Most of the plates used in variable capacitors are aluminum or some alloy. Phosphoric Acid and especially Hydrofluoric Acid are probably best for removing aluminum oxides and other oxides as well. They are used in "chemical etching" for various metals and alloys, which is good for us and this application. Not all caps use aluminum, but this method works well on most alternative metals used.

Now a little chemistry is necessary here. Either or both of these acids, alone in full concentration are nasty to the extreme. Hydrofluoric Acid will melt plastics (even some Teflons) and glass, so low low concentrations in diluted & buffered formulations are usually used. (BTW ... Formula 409 uses this acid which is why it fogs plastic lenses and removes paint and lettering on front panels. Never use this crap for general cleaning of electronics.) "Wheel cleaners" for cars also use this acid in a slightly higher concentration. Do not use wheel cleaners on caps. The higher non-buffered concentrations may break down the ceramics if they are exposed for long periods and not thoroughly rinsed. It may also make the ceramics more porous allowing moisture to take hold more readily over time. Especially if you live in a humid climate. However in the right formulation, with the correct stabilizers and buffering agents, this acid can be quite useful in this application.

Phosphoric acid is used in dishwashing machine detergents, which is why this method works well and has been suggested. This acid (and Ascorbic acid) are contained in Tomato sauce, BTW. Yummy? My only complaint about the dishwashing machine method is the silicone agents added to the same detergents to help make dishes shine. I am not comfortable in applying these foreign materials to electronic components or leaving it on the aluminum surface. However I do not have any science to support any negative results from this. It's just an opinion.

There are chemicals that use these two acids designed specifically, in the correct formulations, to break down oxides and nothing else. You will see this answers the question of how to clean the plates and not negatively affect the ceramic parts.

A Good Restorative Methodology

So what do I use? I apply a solution used to chemically clean metals specifically. It's formulated to remove aluminum & other oxides and not effect other materials, like glass, ceramics, plastics or other inorganic materials. The brand name is Turco Alumiprep 33. The original ingredient for one of these acids is actually in the chemical form of Hydrogen fluoride, which is immediately converted to Hydrofluoric Acid once water is added. In addition to the two acids mentioned here, it contains buffering agents (Potassium phosphate) to help retard some of the effects of the acids employed. It also contains a form of alcohol (2-Butoxyethanol) and an emulsifier/surfacant (Alkylphenol ethoxylate) to help lift away oils and reduce surface tension.

This solution is used most often in the cleaning of aluminum fuselages of aircraft in preparation for painting or stabilization with clear Alodine. A process sometimes referred to as "chemical cleaning" or "molecular cleaning". It is used on aircraft because it will not fog navigation lighting, damage glass or plexi windows, or electrical devices. It will however remove all aluminum oxides and leave a pure non-stabilized aluminum surface. Below is a link to the Material Safety Data Sheet for this product.

Info on Alumiprep 33

Two things. Use latex or rubber gloves and eye protection. This stuff is nasty and it will burn your skin and make it peel like sunburn in few days, and it will take out an eye in short order. BE VERY CAREFUL!!! (Disclaimer: The author will assume no responsibility or liability for any injury resulting from a lack of intelligence on the part of any Hammy Hambone who does not heed this warning or follow smart ways to prevent personal injury to himself, his spouse, kids, the dog or cat, fish, shrubbery or any other person either living or dead.) Seriously, be smart!

Good ventilation is also recommended, unless you like the acrid smell of acid doing its thing.

Sidenote: The only drawback of this solution is it does not work well with aluminum castings. It will turn them black. So it does destroy the aesthetic nature of cast pieces, but otherwise does not harm them. Most caps do not have cast aluminum parts, but be aware of this if you use this product elsewhere, like on aluminum knobs.

OK ... so what's the methodology? Glad you asked!

First remove the cap from the chassis. Using a purer isopropyl alcohol (91% is fine), clean all of the "gummy bears" and other greases, oils & dirt from the cap. Use a long haired soft paintbrush for this. Do not skip this step. Removing as much dirt and especially oils allows the Alumiprep to do its job much better. As Turco says in it technical bulletin, "Good results start with cleaning. A clean surface is a "water break-free surface". The rinse water sheets out over the aluminum surface, while oil remaining on the surface will cause the water to bead up. Chemical cleaners will lift and break down oils on the surface and assist in rinsing them from the surface on the metal." Basically, if you do not really clean the oils off well you will leave spots, so clean the cap well with alcohol first.

Now that most of cap is clean of goo and "gummy bears", it is a good time to inspect the contact points near the bearings of the cap. Make sure no parts are broken or show excessive wear. Make sure the parts are seated properly. If you are confident the mechanical contacts are in good working order, move on.

Next prepare a weak solution of Alumiprep 33 in a plastic container large enough and deep enough to completely submerse the cap in question. I use those plastic storage containers for shoes, like the one made by Rubbermaid. The formula I use is 1 part acid to 6 parts water. This seems to work well on removing mild surface oxidation.

Submerge the cap completely in this solution, moving it around slightly (but not vigorously) to keep fresh formula on the surfaces. You will see the chemical reaction in the form of micro-bubbles foaming up on the surfaces. This is good. Do this for a few minutes. Do this by a sink or a source of fresh running water. Have a small clean unused very soft paint brush handy as well. (Not the one you used to clean the oils with alcohol earlier BTW!) Make sure the hairs are longer than the depth of the full plates. After a few minutes of submersion use the paint brush to lightly rub the surfaces of the cap with particular attention to the insides of the plates. Do this while the cap is still submerged, removing the black deposits that from by the chemical conversion of the oxides. Then remove the cap and immediately put it under running water, continuing to use the brush. DO NOT allow the Alumiprep to dry before rinsing it off. Rinse & brush immediately for several more minutes.

Note: Do not use natural sponges with Alumiprep. It will turn them to goo in short order. Soft natural hair brushes seem to be OK, but soft synthetic materials are better.

Inspect the cap. If you still see corrosion, repeat the process. Once you have finished and the cap looks clean enough, submerge the cap in water for several more minutes to help leech the acids from the ceramics. Inspect the cap once more very closely and remove any hairs that may have come off from the paint brush. They always get stuck in there somewhere!

You can save this solution and reuse it. Just put it in a plastic container. NO METAL! At $18 per quart, it's not being "to cheap" to save and reuse this stuff.

A further optional step I recommend, but not necessary, is to stabilize the metal and help it resist future corrosion. Dip the cap in a chemical solution like Alodine 1001. Alodine 1001 is a nonflammable, chromic acid based, coating chemical that will produce a chrome conversion coating on aluminum and its alloys. The coating formed by Alodine 1001 is clear in color and it becomes a part of the aluminum surface. However, it will not change the electrical properties of the surface in this application as a capacitor. Many of these capacitors were treated in this way originally when manufactured. This chrome conversion coating offers the best surface for corrosion resistance. Alodine 1001 is used when it is desired to retain the silver white aluminum. Basically it is a form of anodizing. It is clear in color (unlike Alodine 1201 which has the yellow/bronze color like you see on the chassis of a R390a for instance.) It will stabilize the metal surface and help retard any corrosion in the future. The process for applying it is the same as the one described above for the Alumiprep, although a different water/chemical mixture ratio is used. Follow the recommended ratios for immersion. And the rising is very important since incomplete rinsing will produce a powder on the surface. Here's the data for Alodine 1001:

Info on Alodine 1001

A good place to find Alumiprep and Alodine would be any aircraft maintenance supply places. One in particular who usually has these in stock in quarts and you can order online from is Aircraft Spruce. They have locations on the West and East coasts of the US and also one in Canada. A link to the page for these metal prep chemicals (and others) is here:

Now that it's clean and corrosion free ...

After you have finished this cleaning process, you need to bake the cap and remove the moisture from the ceramic parts.. This is done simply by putting it in a conventional over (not a microwave.) You must set & pre-heat the oven to the lowest temperature it can go. Set the cap in the oven for about 15 minutes to drive the moisture out of the ceramics. Remove from oven and allow to sit until it has cooled to room temperature. It is not recommended to dry a ceramic part by using a heat gun like the type used for heat shrink tubing. The inconsistent & rapid heating may cause expansion stresses in the material and cause it to crack. Slow, consistent & mild heating and cooling like in a low temperature oven is all that needed to drive out moisture.

Now, the next step is to prep the ceramics so they do not retain moisture over time. This can cause arcing across the ceramic in certain conditions (especially in circuits that have high voltages) and is a good practice in general for caps. Several things can be used for this, but any silicone based liquid designed to protect against moisture can be used. This is one of the few circumstances I recommend WD-40 or similar products. Now do not just go spraying willy-nilly all over the cap. Apply the stuff minimally to a clean lint free cloth and rub it into the ceramic only, taking care not to get any on the plates. Use a small foam cleaning brush to rub in the silicone on the inside parts of the ceramics you cannot reach with the cloth. Wipe it down so it appears dry. Now the ceramic will be reasonably protected from collecting moisture.

Now lubricate the bearings and use Deox-it on any contact surfaces as describe earlier. You will find your capacitors will turn smoothly, and operate consistently over time using this cleaning method.

Next time we will talk more about the Gates power supply/modulator. This will include some simple modifications for improved audio, refurbishing the transformer housings (including duplicating the original Triad decals), and other specifics of this restoration. Stay tuned ...


John LeVasseur, W2WDX

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