Think I finally figured this out - D6 is exposed to a high RF voltage during
transmit, as the IF attenuator is really just R25 (the 50 ohm load clamped to
the housing) and the action of the ALC circuit (1SV80 PIN diode controlled by
op amp.) D4 and D5 produce a negative voltage as a function of the IF radio's
transmit power, which reverse biases D6/D7/D25 to avoid any of them coupling IF
transmit power into Q13 (receive post-mixer amplifier.) Useful as originally
designed, and why they had a separate negative-output detector when D1/D2 do
the same thing is unknown, but I think the reverse bias would not be necessary
with low-level transmit IF (at least up to a few dBm, which is probably more
than the mixer wants anyway.)
On Saturday, April 10, 2021, 01:01:28 PM MDT, <wb0gaz@yahoo.com> wrote:
I have a ten-tec 1210 transverter first obtained as a swapmeet basket case
some years ago, which I have repaired and is now working normally as it was
originally designed.
I wish to modify it to accept separate (low-level) transmit, receive and
PTT/Key signals from a radio which is equipped with this interface.
If you would be able take a look at the schematic (near the end of the PDF
linked below, or as a separate supplement page if you still have an original
manual), could you help me understand why there are evidently two separate
circuits that detect transmit RF from the IF radio and are involved in putting
the transverter into transmit mode?
These two detectors/circuits are cited first in reference page 8 which has a
paragraph about power control, and mentions both circuits without explaining
why the second one (D4/D5) is necessary. One circuit is D1/D2 (which switches
the multi-output regulator into transmit mode, and is party to the path (D3
cathode, etc.) that you manually ground to put the transverter into transmit
state if needed for testing), and the other is D4/D5 (which serves to
remove/isolate the post-mixer receive amplifier circuit from the signal path
during transmit.) I plan to use D3 cathode to ground to put the transverter in
transmit at behest of the IF radio, but I still need to dig out usable
low-level transmit/receive RF interface points.
It appears to me that the D4/D5 detector is redundant - when D1/D2 puts the
transverter into transmit mode, the OUT+ output (which supplies 8V for receive
circuits) from the regulator goes to 0, removing DC from Q2, which would then
remove bias from D6/D7/D25, removing power from the post-mixer receive
amplifier Q13, and cutting off RF that would otherwise flow through D6 and D7.
My question is this - why then does D4/D5 appear to do the same thing - remove
bias from D6/D7/D25, when the regulator OUT+ output is being turned off by
D1/D2 and the CONT input of the regulator?
Here is a recently scanned copy of the manual:
https://groups.io/g/TenTec/files/ten-tec-1210-transverter-manual-schematic.pdf
Very grateful for any comments/observations!
73 Dave WB0GAZ wb0gaz@yahoo.com
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