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HB-75 Transceiver
Bottom view of chassis. As in my Octal Tribander, separate circuits are used for the receiver preselector and transmitter driver. Each has its own dual section variable capacitor. A total of four relays are used to accomplish T/R and tune functions. The main T/R relay is mounted sideways in the VFO compartment. Towards the chassis rear can be seen a small module that contains two plug-in relays in DIP packages, as well as various other miscellaneous components and circuits that would have been onerous to mount on terminal strips. The fourth relay is a SPDT type in the PA cage used to switch the antenna between the transmitter and receiver.
A piece of angle stock runs sideways across the mid-chassis area. In conjunction with the vertical shields, this adds a lot of torsional strength to the LMB chassis, which is only .050" soft aluminum.
I used old fashioned carbon composition resistors almost everywhere to get that vintage look. Plus I have thousands of them. All were checked prior to installation to make sure they were within tolerance. But the simple fact is that in most of these tube circuits, exact values are not all that critical. I did use metal film resistors in the metering circuits, where stability is more important.
The radio is housed in a LMB Heeger cabinet that is similar in size and appearance to a Collins KWM-2 or 30L-1. The frequency display is the same as that used in my Real McCoy transmitter, and is available from mpja.com. The custom meter scale was a three day project in itself.
Top view of chassis. The VFO uses the variable capacitor and coil from an ARC-5 transmitter. The chassis is beefed up under the VFO components, and vertical supports run from the front panel to the chassis to prevent flexing. The VFO is rock stable mechanically, electrically and thermally.
The coil cans are from countless junked TV sets that were once in my dad's basement. A three inch computer speaker is mounted to the front panel. To the right of the VFO variable is a 7360 tube used as a balanced mixer for the transmitter. This arrangement prevents the VFO signal from appearing in the RF output, since there is only 455 KHz separation between the two.
The button at the right will take you to the technical description.
The button at the right will take you to the schematic diagram.
Completed in November, 2020 is a transceiver I have named the HB-75. It covers 75 meters (3.75 to 4.00 MHz) lower sideband (LSB). There are several vintage SSB radio nets in this frequency range. The radio uses 16 tubes, including a pair of 6146s that deliver 100 watts peak envelope power (PEP) output. At the heart of the transceiver is a 455 KHz mechanical filter, made in the mid sixties by a Japanese company called Kokusai. This filter has a 2.1 KHz bandwidth.
It took about seven calendar months, and an estimated 750 hours of effort, to get the radio to the state shown in the pictures below. Another month and a 100 hours or so went into drawing up the schematic and writing the technical description, both available at the buttons below.
Inside of PA cage. PA coil is wound on a piece of one inch PVC pipe.
Copyright © 2023 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.
Below is the block diagram. All power comes from an external Heathkit HP-23B power supply, which was modified to add a +12 VDC regulated source. This addition powers the relays and display. It also powers the VFO tube's filaments through a +6 VDC IC regulator mounted in the transceiver.