WG2E Ameritron AL-82 Modifications
Vacuum T/R Relays

After operating my Ameritron AL-82 for a bit over four years, I began to experience received signal attenuation when returning to receive from transmit. Cycling the T/R relay would return received signal strength to normal. I suspected dirty or arced contacts in the T/R relay. The amplifier was taken from service for inspection. Although the schematic that came with the amplifier shows a 3PDT 10A T/R relay, it was not to be found. Instead, a small circuit board near the antenna jacks contains two small PCB mounted relays for T/R and bias switching. These relays are certainly undersized for a legal limit amplifier. Also the input and output to and from the SO-239s are twisted wires rather than coax.

 

Original TR Board

 

Asking for advice on The World Famous Toad Harbor Net, suggested improvements included coaxial relays and vacuum relays. I decided on vacuum relays due to their small size and the limited space available inside the amplifier. A call to Ameritron found that the schematic for this relay board is in an older AL-811 manual available for download. The Ameritron part number is 50-0453-1. I purchased a new board from Ameritron for study rather than removing the original board.

 

50-0453-1

 

The two PCB relays are both RTE24012, with 12V coils. As I would be transferring all the RF wiring from the PCB to the vacuum relays, I thought I could remove the unused relay and use its 12V to power the vacuum relays. The other would remain to switch the grid bias. I began my search for vacuum relays and decided to use a pair purchased from Max-Gain Systems. (https://mgs4u.com/product/vhc-1/). They are both SPDT. I considered one DPDT but it was larger and coax dress would have been more difficult. They arrived and I began circuit testing.

Vacuum Relays

 

Where the PCB relay was removed I ran its 12V power to both vacuum relays. In transmit I was unable to drive the additional relays. I considered another option. There is unregulated 29.7VDC at W8 on the PCB. Could I use this to power the two 12V vacuum relays wired in series? A test found that the 29.7V will drive both relays wired in series. The current draw was 150 mA, with the voltage across each energized relay coil being 13.3 V and 13.8 V. I could use the relay no longer carrying RF to switch the 29.7V to the series vacuum relay coils. I jumpered the 29.7V to the PCB relay COM and wired NO to feed the series relay coils. All relays closed properly in transmit. Both PCB relays were replaced with new ones.
The series vacuum relay coil configuration may actually be safer than a parallel configuration. If either of the series coils were to fail open, neither would work. In parallel, one coil failing open could find the amplifier in an unstable condition.

50-0453-1 Modified Schematic

 

The vacuum relays were installed behind the plate transformer above the time delay relay for the step start circuit. A mounting bracket was fabricated from an aluminum project box and attached at two existing holes in the chassis. Solder eyelets were included at each mounting screw. A barrier strip was mounted below the relays. A 1N4007 suppression diode is wired across each relay coil.

Vacuum Relay Mounting

 

For the RF connections, RG-400 coax was used except for the original coax to the tuned input board (small coax at left hand relay). The original RG-58 coax from the amplifier output had a solid center conductor and was very stiff. This was replaced with RG-400 at the right hand relay. The coax to the SO-239s was dressed behind the original relay board. I found that if gently heating the RG-400 with 180°F from my hot air station and holding until cool, the coax would retain its position for easier soldering. The coax shields are grounded at each end of all coax connections. The red and black wires from the vacuum relays go to the ALC board. An 18 pF mica capacitor to ground, mounted on the left hand relay, replaces C1 on the original relay PCB schematic. The red wire at the left terminal of the barrier strip carries the 29.7 V coming from the PCB relay. Ground is on the right at the mounting screw.

 

 

Vacuum Relays 2

 

Vacuum Relays 3

 

Since the modifications, the amplifier has been in daily operation for several weeks. No issues have been noted. Thanks to the members of The World Famous Toad Harbor Net for their ideas and feedback.