[TenTec] Need help understanding why two detectors in Ten-Tec 1210 transverter (and manual PDF link)

[wb0gaz at yahoo.com]

 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 at yahoo.com