Lafayette KT-135 EXPLOR-AIR radio kit

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More KT-135 stuff.
 
 
 
REPAIR NOTES AND TIPS CONCERNING THE KT-135
 
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.

By the way, when the chassis is "hot" it's sitting at 94 VAC in reference to earth ground. The current is limited by a 100K resistor, so it won't kill you.
 
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 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.
 
I found this picture somewhere on the world wide web. Someone has replaced the capacitor with a standard trimmer. Unless that person used nylon washers and screws to mount it, we can assume it is not 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?]
 
The least damage the builder did with that trimmer cap was to short out all the other tuning capacitors by connecting both sides 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.
 

 
Rebuilding The Filter Capacitor.
 
 
The filter capacitor 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.
 
Replace the guts with three 33uF caps rated at 160 volts. They may fit perfectly inside the casing and the voltage rating is higher than the original 150 volts. 250 volt caps won't fit.
 

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

 
Use cardboard disks at either end, push everything into the empty casing, then seal the ends. I used pieces of beeswax, then heated the wax gently with the end of a hot glue gun to melt it. I've also used 5 minute epoxy, which does a very nice job and makes the capacitor look like it's right from the factory.

All the capacitors and resistors needed to restore your radio can be found at justradios.com.
If you need tube sockets as well as 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.  They show the 2.2 meg grid leak resistor and the 390 pF capacitor. The leads are very short but somehow survived the heat from the soldering iron. As the tube heats the socket, 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 this type of 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.
 
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.
 
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.
 
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.
 
 
 
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. I found that 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?

I hope you found this entertaining and informative. Check out Carol Maher's KT-135 site  here.

Epilogue: 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.
 
We now return you to your regularly scheduled program.
 
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