G3TXQ

Amateur Radio (G3TXQ) - Ten-Tec Corsair II display

The performance of the Ten-Tec Corsair II compares favourably with many more modern radios, and most of its components are still readily available, so it should remain a viable main-station radio well into the future. One possible area of concern is the lack of availability of the microprocessor unit which provides the frequency readout and keyer function. As an exercise in 'self training' I looked to see if the MPU board (part#81246) could be replaced with a couple of PICs - one performing the frequency display function and one the iambic keyer function.

PIC board

The prototype was hand-wired on a 4.7”x3” piece of matrix board. By drilling mounting holes on 4.3”x2.6” centres it fits the existing metalwork. Input and output connections were placed to line up with the existing Ten-Tec cable runs. Click on the photos for higher resolution versions

Although occupying just one board, the display and keyer functions are essentially independent of each other apart from sharing a +5v supply; a 16F84A PIC was used in each case.

PIC board

A double-sided PTH board was then designed and manufactured using the service provided by ExpressPCB. More details are available here:

Schematic
Display object code V1.1 (25/8/2011)
Keyer object code V1.1 (20/8/2011
Top copper layer
Bottom copper layer (viewed from top)
Silkscreen layer
PIC board

The display PIC is clocked via a 4MHz crystal, resulting in a 1uSec instruction cycle time. It counts how many cycles of the Corsair HFO occur in a 40mSec period, divides by 4 to reduce 'last-digit-jitter', adds/subtracts the appropriate IF frequency depending on the band and the sideband in use, and then converts the resulting 24-bit binary number to 6-digit BCD and outputs the data to the LED display driver board (81247). The prescaler in the 16F84A is used to handle the HFO frequency which can extend up to 27MHz, and a novel technique described in Microchip's Application Note AN592 is used to gain access to the residual count that remains in the prescaler at the end of the count period and which forms the least significant 8 bits of the 24-bit count.

The keyer PIC implements an iambic keyer function. Morse speed is determined by an external clock provided within the Corsair, so instruction cycle timing is not critical and a simple RC oscillator is used for the PIC clock. I did not implement the original Corsair keyer memory functionality, although the schematic shows wiring which would allow that to be added in the future.