Copyright © 2021 KG7TR. Technical information on this site may be shared in the interest of promoting the hobby of amateur radio. I do ask that you give proper credit to KG7TR for my equipment designs.
Rear view shows most of the custom cabinet construction. .062 inch perforated aluminum sheet panels are fastened to a frame made from .75 by .062 inch aluminum angle stock. Braces at the rear make the installed cabinet very sturdy. The complete radio weighs 16.3 pounds. The AC power inlet module includes a 2.5 amp fuse. I put a knob on the S meter zero pot. This makes it much easier to reach in back and adjust when the radio is installed in its operating position.
Octalmania 80 Receiver
Top view of the 10 x 12 x 2 inch chassis. The VFO variable capacitor is from an ARC-5 Command set transmitter. For this radio I had to use the front capacitor from the Command set, so I fashioned a custom preload for the worm gear at the rear of the variable. The ARC-5 VFO coil is mounted inside the cage in front of the variable. The Arduino module is mounted vertically behind the panel meter. A modern toroid power transformer from Antek sources all voltages required. The VFO components are mounted to a tempered aluminum plate for extra structural rigidity. Coils and IF transformers are mounted in repurposed cans from an ARC-5 receiver. Note that metal versions of octal tubes are used in eight of ten locations. These have the metal envelope connected to pin 1, which is grounded to provide a shielding effect. While these tubes may be available in glass envelopes (GT suffix) that glow in the dark, I figured the integral shield feature was more useful than the aura of a lit filament.
Front panel with the radio tuned to the Boatanchors net frequency. The panel measures 12 x 7.5 inches. A six digit, seven segment LED display shows the exact frequency. It is driven by an Arduino Uno R3 module using the built-in frequency counter function to measure the analog VFO frequency. A front panel switch selects the resolution of the display to 10 Hz or 100 Hz. A three inch speaker is built into the front panel. An external speaker can be plugged into a phone jack on the rear of the chassis. The S meter uses a custom scale as in most of my other radios. The knobs are from old HP test equipment, with a spinner added to the main frequency control.
Bottom view shows a vertical aluminum shield running sideways across the chassis that supports the VFO mounting plate topside. It is mounted to a strip of .5 x .062 inch angle stock. A choke input supply is used for the B+, with the choke itself mounted to the chassis side under the power transformer. A small filament transformer also mounted to the side of the chassis is reverse connected to provide bias voltage. A dual section variable capacitor is used as a preselector control to tune the input and output circuits of the RF amplifier. All tube sockets except the VFO are snap ring types salvaged from old Command set radios.
The button at the right will take you to the schematic.
The button at the right will take you to the Arduino sketch (i.e., program). It is a .txt file that can be opened in Notepad, then copied and pasted into the Arduino IDE application and compiled. In the opening comments you will also find a link to the display libraries (.cpp and .h files) needed in addition to the sketch provided here to make it work.
The block diagram is shown below. The radio uses pretty much the same architecture as many of my other creations.
Completed in July, 2021 is another Octalmania creation. This is a receiver for 80 meters LSB that uses all WWII octal tubes - 10 of them. Like the HB-75 transceiver, it uses a Kokusai 455 KHz mechanical filter with 2.1 KHz bandwidth. But this time it is the USB version of the filter. Because of the mixing scheme, the VFO is 455 KHz above the incoming signal. So sideband inversion occurs, which allows use of the USB filter.
This radio took about four calendar months, and an estimated 400 hours of effort, to get the radio to the state shown in the pictures below. Another month and 75 hours or so went into drawing up the schematic and writing the technical description, both available at the buttons below.
This radio is now owned by K5LYN, who has mated it up with his 80 meter conversion of the Cheap and Easy transmitter. He regularly uses this homebrew setup as net control for the Boat Anchors net on Wednesday nights.
The button at the right will take you to the Technical Description