Archive for December, 2012

Crystal saver for tube type transmitters

December 23, 2012

I recently had an email from a builder of a Pastime Projects “Remember When” slat board transmitter kit.  The circuit is from a 1940’s ARRL Article and used a 6V6 tube as a crystal oscillator on either 40 or 80 meters depending on the coils installed.  The chassis used orange crate wood.  Our retro kit uses Amish prepared oak and brass screws in the design.  If you are not familiar with our kit, go to www.pastimeprojects.com and look at the home page as well as the ‘transmitter’ page.

The problem was crystals used in these transmitters can become damaged and ruined by excessive current. 

Crystal Current Limiter

Crystal Current Limiter

An old trick to prevent overheating and burnout of the crystals used in these transmitters is to put a 60 ma. bulb in series with the crystal.  We have experimented with this, and find it really works.  For this reason, Pastime Projects now offers this idea for crystal protection. We cannot guarantee it but its a lot cheaper than a new crystal!  It sells for $8.50 delivered to your US home..  Why not order a couple?  It come wired, (no socket) with pig tail wires for connection to your crystal.  See the web site to order.  Yes, there is instructions with each crystal protector.  Best regards  Glenn 

pastimeprojects(at)yahoo(dot)com

Dummy Loads – What use are they?

December 14, 2012

A dummy load (in amateur radio) is a resistive termination load for a radio frequency source.  It usually consists of resistors made of carbon or similar materials that can absorb energy.

The dummy load is used to absorb the radiation from the transmitter so that the power output can be tuned,  or the frequency or audio can be checked and adjusted,  or so you can monitor your signal without interfering with other stations by broadcasting “on the air”.

If you are operating a hand held radio on two meters and its antenna is a little rubber duckie or flex antenna on the top of the radio, you probably don’t need a dummy load.  However, once you transmit, the chances are you are going to interfere with other stations.  How do you adjust frequency,  listen to your signal, check audio or peak power out, without interfering with others?  You disconnect the antenna and connect the transmitter/transceiver to a dummy load.

It doesn’t matter what ham band or power level you operate at,  the problem of interfering with other stations is resolved by testing with a dummy load attached instead of an antenna.

The ideal dummy load should measure 50 ohms (or 70 ohms) resistance regardless of frequency.  Carbon resistors meet this need.  A light bulb will work , but its internal resistance changes as the filament heats up.  In the “old days” we soon discovered that light bulbs do radiate over short distances!

How do we get rid of energy?  Energy can be turned into light, heat and/or sound. Today’s dummy loads absorb energy by heat.  If you have a dummy load,  you must know its power limits!  A five watt rated dummy load can handle 5 watts for a short time.  Always watch your dummy load for excessive heat!  Larger rated dummy loads depend on large cooling fins or are immersed in oil for heat dissipation.  Any of these can be overheated.  Always allow a cooling down period between ” transmitter on”  intervals. 

The Pastime Projects Dummy Load Kit uses 4 internal resistors of 2 watts rating each.  This means they should handle 2 times 4 or 8 watts power.  So they will handle 5 watts pretty easily.  Again, just watch the case for heat, and pause your “transmitter on”  time when necessary to allow the dummy load to cool down a bit.  This dummy load kit can be connected to the output of our 6V6 “Remember When” kit to prevent radiation while testing the transmitter.  As long as the power output is within ratings,  the transmitter can be tuned for maximum output and other tests made using the dummy load.  Another use is to check for the second harmonic by watching the “S” meter of a nearby receiver while the receiver monitors the signal on the primary and secondary (harmonic) frequencies.  The harmonic frequency – for instance the 40 meter signal when operating on 80 meters – should be much weaker that the fundamental frequency output. 

The dummy load resistance is chosen to be same as your antenna impedance.  That means, your transmitter will be happy if it is “looking at” a 50 ohm dummy load or a 50 ohm antenna.  In other words,  you can switch from dummy load to on-the-air antenna without adjustment so long as they both are resonant at 50 ohms.. More later…. 73 Glenn  

pastimeprojects(at)yahoo(dot)com.

 

 

 

 

 

Pastime Projects assembled kits available

December 8, 2012

Pastime Projects can assemble and test any of our kits here for our customers.   

So, if for some reason you are not interested in the construction, assembly and soldering of the project,  please contact us.

We will quote you a price for your favorite kit fully assembled and tested here and mailed to your home.  The package will include the full manual with each item.  It is important to understand the technical and safety aspects of the project. 

Shipping schedule and lead time will be part of the acknowledgement when you purchase.  

E-mail your interest to pastimeprojects(at)yahoo(dot)com.  73 Glenn