External Vacuum Coaxial Relay for the M2 6M1000 Amplifier

During the use of my 6M1000 during the TX5K Clipperton Island DXpedition, I noticed that the internal mechanical relays failed to reliably return cleanly to receive.  After returning home, I inspected the relays but found it difficult to open them to clean them.   The relay manufacturer advised me that they were not meant to be opened and cleaned - they were meant to be replaced if the contacts were worn.   It was decided that the relays needed to be replaced with sealed vacuum units before the next DXpedition.   Note that M2 has since installed vacuum relays in its newer solid state amplifiers.

Since removal of the relays was not easily done without dismantling the entire amplifier, and there was no space inside the amp for mounting of new relays, I decided that the easiest solution was to build a set of vacuum antenna relays in an external case that could be mounted on the rear of the amplifier. Photo #1 to the right shows the rear of the 6M1000 amplifier with (2) RFU-538 Teflon male-to-male UHF coaxial adapters attached to the SO-239 UHF chassis connectors on the rear of the amplifier.   I labeled the coaxial connectors upside down so I could properly connect cables to the amplifier when viewed from the front of the amp. 

The female RCA chassis connector is the standard PTT connection, and the center pin is grounded for transmit.  On the small terminal strip, there are four connections (from left to right):

1)  +50 VDC to run the fans (available whenever the amp and power supply are turned on).
2)  Negative lead to the cooling fans, which is automatically grounded to turn them on when required.
3)  Negative lead to turn on an external coaxial relay, which is grounded during transmit.
4)  13 VDC to run an external relay (available whenever the amp and power supply are turned on).

It was decided to use a pair of vacuum relays that I had here on the shelf (Jennings RJ1A-26N969, or equivalent  26 VDC vacuum relays), and wire them in series between pins 1 and 3.   The  relays would be mounted in a box with connectors spaced so they would simply screw into the connectors shown in Photo #1 to the right.  The enclosure for the relays was a Bud AN-1319 box (external dimensions of 3.13" L x 2.93" W x 2.05" deep).  Although the exterior size worked out pretty well, it would have been more convenient to find a box without the mounting holes intruding on the interior space, and I don't think the waterproof design was really needed for this project.   I had one available, though, so that is what I used. 

Initially, I just planned to directly connect the Normally Closed connections of the two relays, so the antenna would be connected to rig on receive, simply bypassing the amplifier.  However, I decided that I might as well provide for the option of inserting an isolation relay and preamplifier in the receive line if I ever decided it was necessary.   For that reason, I installed BNC connectors out of the top of the box.  Because the box I used had limited space available for single hole connectors, I had to use four hole mounting types.  Also, because I did not have enough space between the two connectors to use a pair of female chassis connectors with standard right angle adapters and a male to male BNC to join them, I selected a pair of  male and female four hole chassis mount connectors that could be joined together directly by a pair of right angle BNC connectors.  Because of slight differences in the connectors, some experimentation had to be done to find the right combination that would fit together properly.   It would be much simpler to use connectors as I first described, if the box had provided enough interior space.  Photo #2 to the right shows how the box was laid out and the BNC connectors spaced to match the dimensions of the combined right angle BNC connectors.

The main coaxial connectors were (4) RFU-521-ST Teflon 4 hole female chassis mount UHF coaxial connectors that were purchased, along with the (2) RFU-538 adapters, from RF PARTS.  

#1 - Rear view of 6M1000 amplifier with Male to Male UHF adapters installed on the amplifier's chassis connectors

#2 - Inside view of the relay enclosure and lid with partition

#3 - Relay enclosure showing interface with the amplifier

This page most recently revised 10 May, 2013