Advanced Repeater Systems

KB4ABE 6m Repeater

My brother Mike, KB4ABE, lives in Tampa, also an Electrical Engineer (with GE medical). He has built several 6m Repeaters over the years.  The one below is a 6m Repeater built with a GE Mastr II mobile.  It is on 51.64 MHz with the input -500 kHz on 51.14 MHz.  He bought a brand new, special order, Telewave 6m Duplexer.  The repeater is installed in a 6 foot Cabinet with the duplexer mounted in the rear.




The Controller is a S-Com 5K and the Power Supply is a GE Mastr II 30 amp Ferro-Resonant.  The large fan keeps the transmitter cool.

The repeater was once installed on the Channel 10 Television Transmitter Site in Holiday Florida with a four-bay Decibel Products Antenna, which Mike completely rebuilt and tuned for 51.5MHz, and installed at a 1200 ft. Unfortunately Broadband Noise Problems from a Television Station Remote Link power supply plagued the receiver.

He recovered the Antenna and moved the Repeater to its current location in Brandon Florida on a 200 foot free standing tower.


 
The Antenna is now a Decibel Products Ground Plane Antenna mounted on the west arm fed with 7/8" Heliax.  The Repeater transmits well, but sometimes
the receiver is again plagued with noise, this time from powerlines and power transformers.




However, on the day we made this measurement, the site was a quiet -134 dBm.  Typically a 50 watt mobile has about 50 mile range.

We added a remote receive site near the Tampa Airport, linked on UHF to improve talk-in It receives on the same frequency (51.14 MHz) but uses a 71.9 Hz tone.

The new main site transmitter is now a 330-watt Motorola Micor Paging Transmitter.


 
Here is the final repeater amplifier.  After getting it up and running on final frequency we discovered smoke coming from the high voltage power supply.  An inspection did not reveal the source of the smoke.  Maybe it was dust.

Continued testing the amplifier only to discover we had a bunch of bad tubes and the PA current meter wasn't working.  Mike found and purchased several used and new tubes from an Amateur in Arizona.  Troubleshooting the meter circuit we discovered a diode pack across the precision current resistor was shorted.  We rebuilt the pack with new 1N4001 diodes and the current meter came to life.  To blow the diode pack, this amplifier must have pulled a lot of current at one time.

Mike also discovered that the Motorola filament power supply didn't allow the tubes to reach the Eimac recommended filament voltage of 6.2 volts.  In fact, the early version of the power amplifier simply had the filaments wired in series.  We modified the amplifier to the later design that matched the power supply's resistive balancing network by adding a 6 volt return wire and feed through filter.  However, Mike insisted that the filaments must be operated at 6.2 volts and the maximum that the Motorola design would produce was 6 volts because the primary voltage wasn't high enough to overcome the drop across the regulator driver and pass transistor.  So we moved the filament regulator to the 18 volt ferro winding which greatly increased to output voltage.

We installed the one good tube we had and one of the "new" used tubes.  For testing we used an Alinco 6m mobile with a power pad as an exciter.  We set the filament voltage and balancing network to that each tube had exactly 6.2 volts.  We tuned the amplifier and finally made rated power! 

We modified a 100 watt, 42-50 MHz Micor mobile for use as the repeater radio.  The final power amplifier was bypassed to reduce the power to 8 watts required to drive the 330-watt PA.  The receiver was able to meet sensitivity without any modification, just a few slugs hanging out.  A 2N2222 was installed from the first PTT relay to key the HPA Screen circuit.

An old ACC RC-96 was interfaced to the Micor mobile.  On the link port we installed a Kenwood TM-401 mobile radio to link the repeater to Mike's 444.525 repeater in Tampa.

The repeater is usually linked to the KC1AR repeater system in Fort Myers and Naples via AllStar.

Mike's Repeaters are:

51.64, -500kHz, PL 141.3, Brandon
Remote receiver near the Tampa Airport uses PL 71.9

444.525, 141.3, Tampa (linked to 145.39, St. Petersburg)


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