Identifying The Drake TR-7 Differences and Versions
by: Ronald Baker / WB4HFN
The NB-7 Noise Blanker
The NB-7 series noise blanker came in three versions. There was the original version which is used in the early TR-7 transceiver production, referred to the picture as the early version. The NB-7 later version has a totally redesigned circuit board layout, but uses basically the same circuit design, with two exceptions. First, the later version incorporated an additional stage of amplification, transistor Q816, at the input to the noise blanker to make up for some signal loss needed over the earlier version. Second, the circuit board layout was redesigned to incorporate a crystal filter, assumedly for better noise selectivity. However, the later version of the NB-7 installed in the TR-7 transceiver never used the filter, a jumper wire was installed in place of the crystal filter. Notice on the NB-7 later version the open circuit board area on the lower left side of the circuit board.
In the picture you will notice the NB-7 later version and the NB-7A circuit board layout are the same. The 2nd generation noise blanker, the NB-7A, did incorporated the crystal filter. Notice in the NB-7A picture the previously discussed open circuit board area, lower left side now has the crystal filter installed. The best I can tell is the NB-7A was used in the companion R-7 and R-7A receivers. The noise blanker, NB-7A, with the crystal filter was standard equipment in the R-7A receiver and available as an installable option in the R-7 receiver.
All three versions of the NB-7 noise blanker are interchangeable between all models of the TR-7(A) transceivers and the R-7(A) receivers. However, I have noticed when using the NB-7A with the crystal filter, when the noise blanker is switched into the circuit you will notice on the "S" meter a small drop in signal strength.