Lafayette KT-135 EXPLOR-AIR radio kit

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From spring 1969 catalog.
The Headphones Jack
Notice the fiber and shoulder washers on the headphone jack. They are both made of the same fiber material and insulate the headphone jack from the chassis. I've found two sets with the shoulder washer missing. Without the shoulder washer, the common side of the headphone jack is connected to the chassis, which means that one side of the speaker and audio output transformer are also connected to the chassis.

Depending on how the non-polarized plug is inserted into the outlet, the chassis could be "hot." When the chassis is "hot" it's sitting at 94 VAC in reference to earth ground. Without the shoulder washer the headphone jack is now also "hot." The current is limited by a 100K resistor, so it won't kill you, but what would it do to your headphones?
I used the washer from the antenna connector as the shoulder washer for the headphone jack.

Noisy Antenna Tuning Capacitor
The Antenna Tune capacitor can be very noisy. This one was so noisy it was ruining the reception. It also wobbled and the knob looked crooked.

The capacitor is a standard compression type, mounted in a bracket. The bracket is grounded to the chassis. It has a 1/4 " diameter shaft attached to a screw that projects through a brass threaded collar. The value is 5 to 80 mmfd. In series with the variable capacitor is a disc capacitor with a value of 270 mmfd. This causes the total value to be 3 to 25 mmfd.

The capacitor is totally isolated from the chassis bracket and it can't go intermittent or short out by the nature of its construction. So what is causing the problem? It's the connection between the mounting bracket and the brass collar! An ohmmeter was connected between the chassis and the collar, and the meter swung wildly when the knob on the front was turned.

So what? There is no connection to the actual capacitor. Apparently, since this is where the antenna comes in, the intermittent causes a small electric current that the antenna picks up and is then amplified thousands and thousands of times. That's my theory, at least.
The fix is to polish up part of the brass collar and the mounting bracket (I used a knife to scratch it up) and connect the bracket to the collar with a blob of solder. This also stiffens the capacitor in the bracket so it won't wobble. Don't solder the screw to the brass collar!
Another example of the "solder blob" fix. This capacitor was so bad the radio was almost unusable.
Scrape the steel bracket, scrape the brass collar, then solder them together. I have a 100% success rate with this fix.


I found this picture somewhere on the world wide web. Someone has replaced the antenna tuning capacitor with a standard trimmer. This seems like a very good idea. Unfortunately, one side of this trimmer is now electrically connected to the chassis. (The capacitor needs to be isolated from the chassis.) Notice Coil "A" is missing! What could have gone wrong?

[Across the top (or bottom if you flip the picture) it says, "Copyrighted work licensed by WorthPoint." Give me a break. Do you think "WorthPoint" spent a hundred bucks to copyright that picture?]
Using this type of capacitor shorts out all the other tuning capacitors by creating a new chassis connection. Both sides of the main and fine tuning capacitors are now connected to the chassis. Talk about unintended consequences!

Follow the schematic and you'll see that the cathodes of the 12AT7 are now connected to the chassis via the coil. Not sure what that would do, but maybe that's why the coil is missing.

If you are going to replace the antenna tune capacitor, be sure to isolate it from the chassis.

Rebuilding The Filter Capacitor.
The KT-135 uses three power supply filter capacitors encased in a single cardboard tube. They are rated 30 MFD at 150 volts.
The filter capacitor bundle is encased in plastic. To remove it from its housing it is necessary to use a heat gun. You must get it very hot. Put the three red wires in a vise and heat it evenly with the heat gun. When it looks like it's "sweating" it's almost ready. Grab the capacitor with a rag and pull gently.
 If nothing happens, stop and apply more heat. When it's hot enough the casing will pull off, leaving the guts hanging out of the vise on the three red wires.
Once again, if pulling on the capacitor body does not give the intended result, STOP pulling on it, and apply more heat. Otherwise the red wires will pull out, or worse, as attested to by the above photograph. Just let it melt. On the other hand, don't start the thing on fire.

When you realize how hot the capacitor needs to get before the plastic melts, you'll wonder how it survived being encased in the molten plastic to begin with. How did they inject the hot plastic into the cardboard tube?

Replace the guts with three 30uf or 33uF caps rated at 160 volts. They may fit perfectly inside the casing. The voltage rating is higher than the original 150 volts. 250 volt caps won't fit. Observe the polarity carefully. A modern filter capacitor connected backwards will be instantly destroyed when power goes through it.

If the replacements are too large when stacked together, try it this way.

Capacitors with axial leads are more of a pain to connect together.
Use cardboard disks at either end, push everything into the empty casing, then seal the ends. Use pieces of beeswax, then heat the wax gently with the end of a hot glue gun to melt it. Also try 5 minute epoxy, which does a very nice job and makes the capacitor look like it's right from the factory. You can even use white glue.

If you acquired a set where the capacitor has already been replaced you can just make your own. Instead of trying to find capacitors to fit the cardboard tube, find a cardboard tube to fit the capacitors. Be sure to make a nice label. This one states that inside is 100% SNAKE OIL

OK, have it your way. Safety first and all that. At least try to make them look nice, not like the photo on the right.

All the capacitors and resistors needed to restore your radio can be found at
If you need tube sockets or other components, try

Stations drift, regeneration requires constant tweaking
These pictures, of two different radios, show the underside of the 12AT7 tube socket.  They show the 2.2 meg grid leak resistor (R2) and the 470 pF capacitor (C6). The leads are very short but somehow survived the heat from the soldering iron. As the tube heats the resistor and the capacitor, the values of these components will change, causing poor performance.
To improve the set, replace the resistor and capacitor. Get the resistor away from the socket and use a silver mica capacitor.  Silver mica caps are basically immune to temperature change. Don't attempt this without a spare tube socket at hand, unless you are very brave. You will also need to cut some of the blue wires going to the band switch control to get to the socket.

Don't forget to take the tube out of the socket before you start working on it.

This value of this 2.2 meg resistor was actually over 5 meg after being cooked by the soldering iron. You can tell it's bad just by looking at it. Guess who did that. ME. This is my KT-135 from 1972. The radio worked anyway! In the center photo the resistor has been replaced. The instructions say to cut each lead to 1/2 inch long. Cut them to 3/4 of an inch long instead.

Weak 12AT7
12AT7 vacuum tubes are getting expensive. A 12AU7 will work in this radio.
A 12AX7 does not give as good results but was used from 1959 till 1961.
Other designations for these tubes are: 12AT7 = ECC81 and 12AU7 = ECC82. If you find a tube marked "JAN", that designation stands for "Joint Army Navy." The tube was made for the military and may be superior, as well as cheaper.
The 12AT7W and 6201 are military tubes and may also be superior to the standard 12AT7.

Replacing the power cord
A new power cord will be polarized. The wide prong is "Neutral" and the narrow prong is "Hot". (Very useful to know when rewiring a lamp!) The Neutral wire has a thin stripe molded into the rubber so you know which wire it is.

Page 16 of the instructions say to turn the Explor-Air on, then plug it in. Without touching any part of the radio, check for a voltage between the chassis and ground. If a voltage is detected, reverse the plug.

To wire it correctly so that no voltage is present in the chassis while the unit is on, observe the picture above. Note that the instructions specify the set must be ON while checking for voltage. When wired correctly, you'll get no voltage while the set is on, but you WILL get a voltage while the set is OFF. However, it won't be directly from the AC outlet, it has to go through three capacitors before it gets to the chassis.
Basically, the plug will be wired this way. If it's wired the other way, the hot side of the AC outlet will be connected to the chassis via the 100K resistor. You WILL feel it if you touch it while grounded.

In any case, this is not a "hot chassis" where one side of the AC cord is connected directly to the chassis, so you don't have to worry about it too much. If you feel something, reverse the plug.
If the new cord is thicker than the original, carve some of the plastic out of the strain relief to make it fit. That way, you don't chew it up with the pliers trying to get it back in the chassis.
If the old cord is in good condition you may just follow the instruction in the manual to mark the "hot" side of the plug. This would plug into the narrow side of the AC outlet. I used red paint because a year later I'd forget and think, "Dolp, I wonder whut thet dot means." Now I can just think "red hot."
Click on photo for larger version.
This 1960 Lafayette catalog page states "the chassis is completely isolated from the circuit, so there is no shock hazard." This could be accomplished if nylon screws and standoffs were used for the tuning capacitors and a few extra wires were added. However, there is no indication that this was ever done. How the chassis was isolated from the circuit is a mystery, if it was even true.

Thanks to Jim Hale for catching this.
Truth in advertising! How it appears in the ad and how it would actually look.
Here's an ad from 1961. It, too, states the chassis is completely isolated from the circuit. Click on the ad for a larger version.
I replaced the cord on my soldering iron with an old KT-135 cord. I got shocked several times because I had to have the soldering iron and the new cord plugged in so I could use the soldering iron to solder the cord to the soldering iron. Solder conducts electricity while you're holding it.
    Here's the manual in high resolution, complements of Katie Wasserman.

Katie Wasserman's parents threw her KT-135 in the trash while she was away at college. According to Katie, the trash can was the near certain fate of 99% of them. A guy named Karl Keller had one that was burned up in a house fire. How many are left out there?

Note: Under cover of darkness, I carefully packaged Art's radio and sent it to Karl Keller. I told Art I'd give his radio a good home, and I did.
Next, some other Lafayette KT-135 kits found on the web.