Restoring Eico’s ST-Series Knobs

Dealing with the myriad of knobs used in Eico’s latter day tube amps.

The intro to the ST-84 Mods article works well here, too: Around the year 1961, when NASA first launched a man into space, Eico introduced a new line of stereo amplifiers and tuners, designated the ST-series. It had distinctive extruded aluminum front panels and side pieces, as well as two-tone custom designer knobs. An example is the ST-70 integrated amplifier, seen above. It is those custom designer knobs that we cover here.

Why write an article about something so mundane? I have noticed that many of the ST-series products which appear on eBay are what I call “snaggle tooth.” They either are missing knobs, have substitutes or are missing inserts. [Inserts are the brown and spun metal decorative circles in the middle.] Also, there have been requests on the forums from people who are trying to find replacement knobs. I have wondered about the possibility of having some made. However, having so many different types and each having two-pieces, makes the plastic fabrication difficult. That goal is a ways off but in that direction we will cover here:

• What knobs existed
• Ways of refurbishing existing ones
• What it would take to make new ones.

What Knobs Existed?

This survey concentrates on Eico’s hifi electronic products produced after 1961 but before the later Cortina line. As bookends, we will include a glance at the preceding and succeeding knobs. Prior to 1961, their hifi electronics used what I call the Chocolate knob series, exemplified by the HF-20 knob shown at right.

With the ST-series product line, they started producing the knobs shown at the top of this page. I have given each type a name to facilitate discussion. Along the top row are one-piece knobs used for things like Balance or a Selector switch. (Sorry that I don’t have a single-piece example of the ST Double Pointer type.) The bottom row is for two-piece knobs, used for Bass and Treble.

I have chosen to use the two-piece pointers in the knob type names because that is the most distinguishing characteristic among the types. Note though, that except for the ST Slotted Pointer type, the single piece knob flavor did not have the distinctive pointer on it.

In the picture of all the knobs, I have them from left to right in my best guess at their chronological order. Certainly, the Classic Series was the last of the four. I also know that the ST Slotted ones existed in 1962, so that puts it early in the group. Since it looked different than the others, that suggests it was the first.

The ST Slotted-Pointer Type

These had a paper insert, with brown paint on it. The first time I tried to clean one of these was very early in my restoration experience and I foolishly used alcohol. This quickly rubbed some paint off. I advise you to be very careful in cleaning these, sparingly using only a mild solution, like Windex.

Unfortunately, I have seen these pretty dirty at times (right), so sometimes it’s a pretty thin line between cleaning and damaging. Going through the paint that one time did reveal something interesting: The paper has a thin foil layer, under the paint. Not sure why that was there; maybe to hold out the paint solvent. One worries about paint adhesion to the aluminum foil, though.

The pointer of the white part of the knob was formed by a tab of the paper insert lying in a slot in the plastic. Needless to say, this wasn’t very sturdy, as you can see in two examples above. A solution for those would be to use a brown paint marker to fill-in the slot. Decocolor Acrylic Paint Markers, available at Dick Blick are an example.

Make a Two-piece from a One-piece?
One issue that arises is that the two-piece knobs seem to be less available than the one-piece. I speculate that it is because it’s easier to lose one of the two pieces than it is to lose the larger one-piece knob. For one thing, the one-piece attaches more firmly to the shaft. Also, it is much more difficult to replace the two-piece knobs with substitutes because they must be compatible with the particular concentric shafts used in the controls. Anyway, for these reasons I have wondered whether it might be possible to separate the two pieces of a one-piece knob and convert it into a two piece.

There is a gap between the white and gray sections of the one-piece, which invites one to pry. Haven’t tried that yet. As you can see in the picture at left, there is a difference in the shaft coupling for the gray sections, which would have to be addressed.

Two claws would have to be added, which engage with the outer sleeve of the control. It’s possible that a silicon rubber mold could be made to allow a small amount of epoxy to be formed into the gray part coupling. For the white part coupling, (at right) it appears that only a metal insert would need to be formed, making the D-shape. That could be done with a small scrap of spring steel, like a speednut.

All that considered, it seemed too difficult to attempt the one-piece to two-piece conversion but it wouldn’t be impossible.

The ST-Double Pointer Type

This knob (seen at right) seems to be an intermediate form, because it retains the paper insert detailed above for the slotted-pointer type, yet sports a double pointer, similar to the later styles. However, the double pointer does not have the extra small bump in the middle of the projections, which the later styles do.

Unfortunately, the unit on which I found this style in two-piece form, did not have matching one-piece knobs. I suppose it is possible that the slotted one-piece knobs which were on it are original but that seems unlikely, since there would be a mismatch of the gray parts. It is more likely that the one-piece double pointer knob has a single pointer bump, as used in the later single-piece knobs. If anyone can shed some light on this, I would greatly appreciate it, if you would contact me about that. 

The ST-Triple Pointer Type

As seen at right, both the white and gray parts of this style change from the double pointer version.

Not-So-White Section
The white part now has a brown plastic insert, which is far more robust that the previous paper ones. Unfortunately for modern collectors however, the plastic used for the white part has yellowed with time. I once tried various strong solutions (including bleach) to see if I could improve the yellowing; no dice. Nothing I tried helped at all.

However, recently, I have become aware of a hobbyist community developed process for eliminating the yellowing, without otherwise affecting the part in any way. (Example at left.) The process is known as Retr0bright and it has been donated to the public domain by its developers, a very generous move.

They discovered that what is causing so many plastics to turn yellow has to do with the flame retardant which is sometimes added, rather than anything about the plastic itself. That suggests that flame retardant began to be added to the Eico knobs, with the triple-pointer style.

More than one formula is offered for Retr0Bright but basically it involves hydrogen peroxide, a catalyst and ultraviolet light. You can read all about it here. It seems highly likely that Retr0bright could eliminate the yellowing seen in the triple-pointer knob style.

Gray Section
The gray part is similar to the one for the double-pointer style, except for two things as seen in the detail at right: (1) A pointer bump is added between the pointer horns, hence the “triple” designation. (2) The front edges of the pointer horns are painted white—that is not just light reflecting.  Some of the white paint has worn off of one I have. The same paint markers mentioned above with the slotted-pointer style are also available in white, so that can be used to touch-up this style.

Unlike the one-piece knob in the slotted-pointer style, the triple-pointer one-piece knob (at right) has only the little pointer bump on the gray section. Thus, there would be no possibility of converting the one-piece knob to a two-piece, as was loosely discussed above.

The Classic Series Type

Eico introduced the new Classic Series of hifi products in their 1964 catalog. It included no fewer than four receivers, three integrated amplifiers and another tape deck. Exemplified by the 2080 below, the front panel styling was simplified, compared to the ST-series; no more extrusions. However, Eico upgraded the styling of the knobs, exchanging a spun aluminum, champagne-gold insert for the previous brown plastic.

As was their practice, Eico still kept the older ST-series products in their catalog, however those products received the new Classic series knobs. (Note-1)

Aside from the shiny new insert, the formerly white part of the knobs becomes beige and there is a plastic underlay for the insert, having a charcoal color. That matches the new darker color of the gray part of the knob. The pointers didn’t change much (stayed triple), except for dropping the white paint on the front.

Replacing Missing Inserts
As was often the case with other metal knob accoutrements, quite a few of this series of knobs today have lost their aluminum inserts. Fortunately, there is an company that is making new inserts (at right) for this series of knobs: JTNS Products. They also offer inserts for Fisher, Scott, Harmon Kardon, Sony, Eico and special ordered custom sizes. Not only that, they produce many other restoration products as well. You can find them at their eBay store here: http://stores.ebay.com/JTNS-Products.

It would be easy to blame the lost inserts on poor adhesive. However, it could be due to heating and cooling cycles, causing the aluminum to expand and contract. Under such stress, any inflexible adhesive might lose its bond, over a long period of time. That suggests that it is the kind of adhesive, rather than its quality that was the issue. For joints subject to thermal expansion, it is important to use an adhesive which remains pliable after it cures. Examples are rubber cement, contact cement, Pliobond® and 3M’s F9465PC.

That last item has become one of my favorite adhesives. It is a 5-mil thick, pressure sensitive film, which comes on a roll of transfer tape, as seen at right. Its benefits are that it forms an instant, permanent, flexible bond, sticks to most materials and is designed to work with most plastics. That last feature is important here, since the knob is plastic. To be compatible with plastic, an adhesive must be able to hold out the plasticizers which migrate out of plastics. That is a particularly acute issue for adhesives which remain pliable.

The only downside is price/availability. I bought a roll for $35 from Micro-tools.com back in early 2005 and the 1-inch x 60-yard roll has easily lasted ever since. It hasn’t shown any signs of deterioration over time. Unfortunately, I can no longer find it on their website. Many vendors (including the 3M online store) only sell a 36-roll case. I did find it at Rshughes.com for about $70/roll. Please let me know if you find a lower-cost source. The per-roll price for a 36-roll case from 3M is about $50.

End of an Era

When Eico’s Classic Series of hifi products came out in 1964, it was, in a sense, already obsolete. By then, Heathkit was already featuring its second generation of transistor hifi products, with the AR-13 receiver, a predecessor of the legendary AR-1500. Eico would introduce its first solid-state hifi product, the 3566 receiver, in the 1965 catalog. I don’t think it fared well competitively though, and didn’t propagate into a product line.

They introduced the Cortina line in 1967 and that is what I think of as Eico’s solid-state hifi. The line was designed for price-performance, where the 3566 was weak. They dropped the concentric tone controls and went with the simpler knob, seen at right. By 1969, all of the original series, ST-series and Classic series products had been dropped from the catalog. (A moment of silence, please ;)

Many of the knobs on Cortina amps appearing on eBay seem to have lost their caps, no-doubt suffering from the same adhesive issue discussed above. Alas, I have a Cortina 3070 on its way to me as I write this, missing all of its caps. Fortunately, Tony at JTNS Products has agreed to fabricate replacements at a very reasonable price.

How to Make Your Own Plastic Knobs

Plastic injection molding is only practical in large volume applications, so there is no way that you could custom-make plastic knobs, right? Well, I’m here to tell you, that it isn’t necessarily so. I used a Resin Casting Starter Set from Micromark.com to duplicate some intricate mechanical parts in plastic. [There are cheaper ways to get started, documented around the Web.]

A full explanation of silicon rubber mold making and plastic casting is beyond the scope of this article but here is basically how it works: You make a mold by pouring a two-part silicone rubber compound around the part to be duplicated. You do that in two steps. First you pour enough to cover half of the object (a knob, in our case) and let that cure about four hours. Then you coat that section with mold release (to keep the second pour from sticking to the first) and pour the second half.

After that cures, you use a hobby knife to cut a channel (called a sprue), through which plastic resin will be poured-in You cut another channel for air to escape. One of the molds which I made is shown at left. After coating the mold with mold release, so the plastic won’t stick, mix a two-part polyurethane compound and pour that into the mold. Just 40-minutes later, you can pull the finished part out of the mold. A completed plastic part which I made is shown on top of the mold. It worked very well, by the way.

If you look carefully at the picture, you can see one of the problems of making plastic parts this way: bubbles. Although one tries to do things to avoid them, I ended up with quite a few in the silicon rubber mold itself. You can touchup some of those, but to really do better, a vacuum pump is needed. For the mechanical parts I was making, appearance was not important, so it wasn’t an issue. For a knob though, bubbles would need to be eliminated. In the article on rewinding a power transformer, I actually did employ a small hand-operated vacuum pump for removing bubbles in the impregnation step. It seemed quite effective.

Besides bubbles, other reasons that I haven’t tried to make ST-series knobs are:

• Each knob requires two parts (2-molds)
• There are two different flavors (one and two-part) of each style (now 4-molds).
• Color is required for at least one part of each knob. That takes either careful pigmentation of the polyurethane or painting.
• An insert would need to be made. For the slotted-pointer and double-pointer types, it could be paper but it would need to be painted and precisely cut to an exact circle. For the Classic Series, it can be purchased, though.
• Making each knob set requires coating the mold, mixing polyurethane, pouring, demolding and then hand work of cutting-off the sprues and any bubble remnants.

All things considered, it just didn’t seem practical to try to cast ST-series knobs from scratch. It gives you new appreciation for what all goes into something as mundane as a knob.

So there you have it: The Eico ST-series knob odyssey. I hope that this helps explain what the knobs are and what can be done to handle problems with them. Hopefully someday, there might be a technology with which new replacements can be made efficiently. Until that day comes, the efforts at careful cleaning, replacing inserts and caps, touching-up with paint markers and Retr0bright yellowing treatment, seem to be our best bets for keeping the ST-series, Classic series and others looking as good as they sound.

Copyright © 2011 by Stephen H. Lafferty. All rights reserved.

Reader Comments

Posted by Steve L.

December 02, 2011 - 08:53 am

Hi Sam, I'm not sure if you were addressing Art (the poster) or me (the author) but I will reply, in case. I don't know of anyone selling the ST-series knobs but I have seen them on eBay, on rare occasions. Since there are four variations, it might be hard to find the one you want. An alternative is to try to replace the whole set. For example, the ST Triple Pointer version seems to crop up more often than some. The yellow cast is a downside but see the article about that.

Posted by Sam

December 02, 2011 - 08:40 am

Hi Art, Do you know of anyone who sells the two piece double pointer knobs? I need the top section of one, of course, Thanks in advance.

Posted by Steve L.

September 02, 2011 - 07:57 am

Hi Art, Thank you for the leads. I'm always on the lookout for new adhesives. Yes, thickness is sometimes an issue but it sounds like the 3M 03609 would be good where it isn't a problem.

Posted by Art G

September 01, 2011 - 11:05 pm

3m offers an acrylic based tape that could be a substitute for the pressure sensitive film that you describe. It comes in 1/2" by 5 ft. rolls and carries the 3M number: 03609 It sells for about $6 in auto supply stores and Amazon. The only issue that may not make a good substitute, is that it may be a little too thick to be useful in this application. It adheres extremely well however, and is used by auto manufacturers to attach moldings on new vehicles. Another possibility is SEM "Bumper repair" #68422 which is about $12 at auto body suppliers and on the internet. This is an epoxy that is specifically designed to adhere to "problem" plastics such as ABS, PUR and PP. Conventional epoxies adhere very poorly to these and ofter plastics. This stuff works! Be aware that once mixed, it sets in three minutes.

Posted by Steve L.

July 12, 2011 - 08:30 pm

Hi SteveK, Thank you for posting the link. That is indeed a very impressive video. The fact that they can "print" an assembly, already put together, with moving parts is awesome. When I wrote, "someday there might be a technology," I had in mind the crude rapid prototyping and 3D printing of a few years ago. Already it has advanced far beyond what was available then. I look forward to a day when short-run plastic items have become cheap and commonplace!

Posted by Steve K

July 12, 2011 - 08:37 pm

Hi Steve: Given your interest in vintage electronics, I thought you'd be interested in this video. http://www.youtube.com/watch?v=ZboxMsSz5Aw&feature=youtube_gdata_player 3D printing has been around for awhile, but the video suggests it's becoming more practical.

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