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Radio Broadcast Technical Consulting and Sales
 
10032 Island Drive, Brainerd, MN  56401
218-829-1326

 


Technical Tips from Mark W. Persons
LPB 2-20 AM Transmitter Modifications


Some might recognize this as an LPB 20 watt AM transmitter.  It is my understanding that LPB is out of business so it it up to people like me to keep this equipment running.  A ruler across the bottom shows the unit is about 11-1/2 inches wide. 

If you look closely, the RF oscillator crystal has been removed along with two integrated circuits in the oscillator circuit.  The original design started with a crystal at 6 times the transmitter's operating frequency and divided it down.  The unit you see here must have been an early version as there was no adjustment to set the crystal to within +/- 20 Hz of the required frequency to satisfy FCC rules.  This one was about 200 Hz off frequency when received in our shop.

Since the transmitter needed to go to a new frequency, we used a fundamental on-channel crystal.  In this case it was 1560 KHz.  The new oscillator board is to the right of the meter and is orange/brown in color.  There is an FET RF oscillator stage followed by a transistor buffer amplifier to drive the modulator stage in the transmitter.  It all worked out very well.

In some cases, the RF output low-pass filter network will need to have components changed when moving to a new frequency.  


One known problem with these units is the 28 VDC regulated power supply.  The original Darlington regulator transistor was a 2N6043, which was later replaced with a 2N6388. 

My answer was to replace it with an MJ11028 or an MJ11032.  These have TO-3 cases for greater heat dissipation.  In this case, I also added a heat sink to further cool the regulator.

No matter how you look at it, this transmitter needs plenty of cooling.  It draws 135 watts from the 120 VAC power line to produce 20 watts to the antenna output.  The unit needs to be wall mounted with the PA heat sink vertical so air can flow through it.  Air cooling also applies to the voltage regulator.  You don't need a cooling fan for the unit, but it does need plenty of room for convection air cooling.  An air conditioned building helps too.

 


The stories go on and on.  Stop in again sometime.  I'll leave the soldering iron on for you. 
Mark W. Persons   Ham WMH      

Questions?  Email Mark Persons:  teki@mwpersons.com        

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page last edited 02/21/2016