Lafayette KT-135

Lafayette KT-135 EXPLOR-AIR radio kit

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In the 1960s, Popular Electronics hosted a monthly column called "Short Wave Listening." Occasionally a KT-135 would appear in the photos. On the left is Mike Byers of Shelby, NC whose primary receiver was a Lafayette KT-135 (from October 1965 issue.) The photo on the right, from August 1964, shows Robert Mladenka of Flatonia, Texas, whose "backup" receiver was a Lafayette Explor-Air.

Notice the newer style knobs on Mike Byers' 1965 KT-135.

From Electronics Illustrated, November, 1963.
This is John Sullivan of Greenwich, Connecticut. He's operating a
Hallicrafters S-120, but you can see he started with a KT-135.

From Popular Electronics, September, 1964.
Edward Jacobson of Westbury, NY is using a Lafayette HE-30 receiver,
with his KT-135 in "standby service." The HE-30 was also a kit.

Gary Confrey of Killingworth, Connecticut.
A Lafayette KT-135 sits on top of a Lafayette "Starflite" transmitter.
Photo from the May, 1966 issue of Popular Electronics.

Listening post of Grant Power, VE2PE6K of Montreal, Canada.
For some reason he isn't in the photo, but the KT-135 is.
Photo from September, 1963 issue of Popular Electronics.

Popular Electronics, July 1969.

Popular Electronics, May 1968.

These call signs were issued by Popular Electronics to shortwave listeners. They all started with the designation "WPE."

It is interesting to see the faces of people who built a KT-135. Makes you wonder what happened to them. Not the people, the radios! Well, the people, too, I guess.

Gary Confrey was about 15 years old in the photo above. The KT-135 was his first kit, purchased at the Lafayette store in New Haven, Connecticut. (The Starflite transmitter was also a kit.) The KT-135 got him started in electronics and he spent his life in that field. As of 2022 he is 72 years old and a senior technician in a lab at an electronics company. He is still an Amateur radio operator and lives in Monroe, GA.

From spring 1969 catalog.

REPAIR NOTES AND TIPS CONCERNING THE KT-135

Lafayette claimed that the KT-135 is so easy to build a child who can read could do it. This is marketing BS. Building or rebuilding a KT-135 is NOT easy. Some were built by a 10 year olds, but that doesn't mean it's a piece of cake because you're not 10. However, the instructions are practically fool-proof. Any wiring mistakes or a component installed of the wrong value will be picked up later in the assembly.

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?

If necessary, you can use the fiber washer from the antenna connector as the shoulder washer
for the headphone jack, since the antenna connector is plastic and has a shoulder molded into it.

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.

In the left photos I've removed the shaft before soldering because it was bent. On the right is the same capacitor put back in service.

Scrape the steel bracket, scrape the brass collar, then solder them together. I have a 100% success rate with this fix.

This radio appeared on the Worthpoint website in 2015. In April of 2024, this same radio appeared on ebay. The antenna tuning capacitor has been replaced with a standard trimmer. If it is the wrong type, 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?

Using the wrong type of capacitor here shorts out the tuning capacitors by creating a new chassis connection. Both sides of the main and fine tuning capacitors are now connected to the chassis. Worse, one side of the (unpolarized) AC cord is now connected to the chassis through whatever coil has been selected with SWITCH K, causing a dangerous shock hazard. If the chassis touched anything grounded, the missing coil probably went up in a bright orange flash and a puff of smoke.

If you are going to replace the antenna tune capacitor, be sure to isolate it from the chassis and do not allow the shaft to come into contact with the front panel. Do not use the type that mounts to the front panel using a nut around the threaded base of the shaft.

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 using a rag (NOT YOUR BARE HAND) and pull gently.

If nothing happens, stop and apply more heat, but don't scorch it or start the thing on fire. When it's hot enough the casing will pull off, leaving the guts hanging out of the vise on the three red wires.

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?

The three capacitors are made of layers of metal foil insulated with paper, rolled up very tightly. The paper should be damp or sticky with electrolyte, but in this case it was bone dry.

Replace the guts with three 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.

Capacitors with axial leads.

Use cardboard disks at either end, push everything into the empty casing, then seal the ends. Use pieces of beeswax (not paraffin), and heat the wax gently with the end of a soldering iron to melt it. Hot glue would work, and using beeswax or hot glue means you can take it apart later by melting it. 5 minute epoxy does a very nice job and makes the capacitor look like it's right from the factory. You can also 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 make them look nice, not like the dangerous mess in the photo on the right.

NOTE: With the radio on and volume turned all the way down, the radio has a slight 60 cycle hum. There is nothing wrong with the radio or your new filter capacitors.

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

Stations drift, regeneration requires constant tweaking

These pictures, of two different radios, show the underside of the 12AT7 tube socket, 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 a change in 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.

The 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.

Radio will not oscillate, regeneration control doesn't seem to work.
Weak 12AT7

If the 12AT7 drops below 25% of its rating, the radio will not break into regeneration no matter how high the regen control is set.

A 12AT7 vacuum tubes can be overpriced. Above is a 12AT7 and a less expensive 12AZ7.

A 12AX7 was sold with the KT-135 from 1959 to 1961. After 1961 a 12AT7 was used. It's easier to control the regeneration with a 12AT7.

Another designation for the 12AT7 is ECC81. If you find a tube marked "JAN", the designation stands for "Joint Army Navy." The 12AT7 JAN may be superior, as well as cheaper. The 12AT7W, 12AT7WA, and 6201 are military tubes and may also be superior to the standard 12AT7. The CV4024 is a 12AT7 made for the British military.

USE A 12AZ7 INSTEAD OF A 12AT7.

	12AT7		12AZ7
HEATER VOLTAGE	12.6 Volts		12.6 Volts
HEATER CURRENT	150 mA		225 mA
	MIN	MAX	MIN	MAX
PLATE VOLTAGE	100 Volts	250 Volts	100 Volts	250 Volts
PLATE CURRENT	3.7 mA	10 mA	3.7 mA	10 mA
PLATE RESISTANCE	15000 Ohms	10900 Ohms	15000 Ohms	10900 Ohms
CATHODE-BIAS RESISTOR	270 Ohms	200 Ohms	270 Ohms	200 Ohms
GRID VOLTAGE	-5 Volts	-12 Volts	-5 Volts	-12 Volts
AMPLIFICATION FACTOR	60	60	60	60
TRANSCONDUCTANCE	4000	5500	4000	5500

The chart above compares a 12AT7 with a 12AZ7. Except for an increase in the amount of current needed for the filament, they are exactly the same, but the 12AZ7 is half the price of a 12AT7. Both the 12AT7 and 12AZ7 were made for use in television sets.

Above is the pinout of the 12AT7 / 12AZ7. It is shown as though you were looking at the bottom of the tube. If the pins on the left are designated 1, 2 and 3, why is it considered triode 2 and not triode 1?

The answer, which is not an answer, is that the Radio Electronics Television Manufacturers Association (RETMA) said triode 1 shall be on pins 6, 7 and 8. What? WHY? Now I'll never get to sleep!

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. Make sure the meter is set to "AC."

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 plugged in correctly, you'll get no voltage while the set is on, but you WILL get a voltage while the set is OFF. However, this voltage won't be directly from the AC outlet, it has to go through several capacitors before it gets to the chassis.

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 can just follow the instruction in the manual to mark the "hot" side of the plug. This side goes into the narrow slot of the AC outlet. (If the outlet is non-polarized, you will have to mark it as well.) I used red paint because a year later I'd forget and think, "Dolp, I wonder what that dot means." Now I can just think "red hot."

Basically, the plug will be wired as above. If it's wired the other way, the hot side of the AC outlet will be connected to the chassis via the 100K resistor, and you will feel a mild electric shock if you touch it while grounded.

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.

Why isn't the on/off switch on the hot side? I believe it's because if the non-polarized plug is plugged in backwards, and the switch is on the "hot" side, Switch K, a lug on each coil, and the stators of the variable capacitors will have a direct connection to the hot side of the AC outlet even if the set is off. Like this.

Since the cabinet was sold separately, some KT-135s never had one. If a rotor plate on the variable capacitor was bent and shorted, the whole chassis would be hot. This sounds a little far-fetched, but it's the only thing I can think of. In any case, it's probably not a good idea to switch the wires to make the radio "safer."

Lawr Salo modified his KT-135 this way to isolate the radio from the electric mains. Since the condition of the radio was poor he had no qualms about drilling some holes in the chassis and adding the isolation transformer. "Hot" and "Neutral" are shown in case you're using a polarized plug, otherwise it makes no difference. You can also use a cord with a grounded plug, and ground the chassis if using the transformer. Without the transformer, grounding the chassis is an extremely bad idea.

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.