GB3NF   

 November 1981 - August 1984

The MKI repeater installation consisted of:-

• An extensively modified Pye T461 transmitter

• An extensively modified Pye R460 receiver

• a Jaybeam Co-linear C8/70  8.2dbi

• a Jaybeam dipole

• A hybrid CMOS/TTL logic control system based on a design by G3RTL

• A four cavity diplexer using ' Y ' or  'rats tails'  (ref G8CKN)

• A backup battery pack and charger

• A remote shutdown system

The transmitter produced about 10 watts to the cavities with the receiver managing 0.5uV for 12db SINAD measured at the diplexer antenna input.  Receiver squelch threshold was 0.3uV.

Operation was simple.  A 1750 Hz tone for 300 ms would wake the repeater up and off you went.  No timeout but, after the receiver signal was removed, the logic would send two pips 1.5 seconds apart.  This was the psuedo forbidden gap left for new stations to call in and this resulted in excellent operational procedures being used on the repeater.

An emergency logic mode was incorporated into the control logic system to allow for RAYNET use.   This had to be manually selected on site and consisted of a simple talk through facility without the need for tone bursts.  The transmitter was carrier operated and the logic could be either left running to provide beacon facility or switched off completely.

In the event of a mains supply failure the repeater automatically switched to battery backup.  This state was indicated to users by a 'B' being sent in morse code  after the second pip.  The repeater could run without mains power for 24 hours in beacon mode or 4 hours continuous use.

By the time the repeater became operational the engineering team, based on their experience building the MKI,  had already decided to build a MKII repeater.   

The days prior to the official switch on were not without worry.  Upon moving the repeater to site and connecting it to mains power, control circuits and antennas we were presented with a lot of RF flying around making the repeater unusable.   We thought we had cured all of this as it had been soak tested for weeks in my shed correctly connected up to the cavity arrangement and an antenna.  Initially we did not know if it was the repeater itself or the close proximity to the refinery RF installations that was causing the problems.

Clive and I spent several very late evenings sorting it out.  Early one morning (02:30) we even had a policeman knock the door to find out what we were up to.  After many hours, a lot of head scratching  and a fair amount of grounding cable all was cured and ready for switch on.

The MKI repeater remained in service until August 1984 when the MKII repeater was commissioned.  It remained in my garage for many years taking up bench space but in the end, mainly due to it suffering from the effects of rust, I was forced to consign it to the dump.

 

GB3NF MKI cabinet, TX and RX prior to modification	Initial bench testing of the MKI logic circuits	Advanced testing of the MKI logic circuits. The FT221R is used as the repeater receiver and a starphone as the transmitter.  This lash up was used to test the tone access circuit, the  through audio switching circuits, time out and beacon modes.	Bulbarrow Hill, Dorset- It's not there anymore. This is where the original feeder for GB3NF originated.  I think the person on the bottom platform is either Clive, G4KCM or Clive, G8BOU