Most of the older TenTec rigs I've seen have a sidetone oscillator which
produces a square wave type tone. For me, it was obnoxious. Here is how I
cleaned up the tone.
Look at the schematic for the "Audio Sidetone Board" on page 3-39 and note
that the output of the relaxation oscillator feeds down through R2 into a
filter comprised of C9, R5 and C7. This filter should act upon the square
wave and round off the flat tops. However, it can't do it's job due to R6,
the volume control which "detunes" the filter.
The cure is to replace R6 with a higher value control. I used 250k but
anything from 100k to 500k has the same effect. Now the scope pattern
indicates a modified sine wave with a sharp top peak and a rounded bottom
peak. The sound of the sidetone is still distorted but much easier on my
ears.
Not having the diagram for other TT rigs, I can't say for sure if the same
process will work but most likely it will. I recommend the use of the MFJ
ClearTone Speaker model 281 for cw. It will further enhance the sidetone
plus give a peaked response at 700hz for signals. Some of those anoying
clicks will be attenuated by this speaker too due to the narrow response.
Now for the ultimate sidetone modification here's what I did:
Remove all sidetone componets except the pitch and volume control and
rebuild it into a Bridge T type oscillator which produces a clean sine wave.
The Paragon and Omni V used this circuit and it sounds great. The process
only takes a couple of hours. Strangely, this type of osc. requires fewer
components and has been around since the tube days. Why TT used the
relaxation osc. beyond me.
Rebuilding the circuit required cutting some foil traces, drilling some
holes and mounting a few resistors on the bottom of the board for neatness.
Another approach would be to build the oscillator on a small board and tap
into the existing board but I wanted a more permanent look.
If anyone wants to attempt this, I can scan the Omni V's sidetone board
diagram and post it here on the reflector or is that forbidden?
Steve Ellington N4LQ
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