Hi to all,
Looking for some historical information on Drake TR-7 boards. I have before me
two DR-7 boards, one being green, and labeled C-3849 R. The second board is
beige in color and looks to be a bit better quality. It is labeled C-3849 W.
I also see that the circuitry is slightly different. Is the W a newer board?
Next up, the power supply boards. Also have two before me, both green, one
labeled B-3970 L and the other is B-3970 W. Again, the circuitry is slightly
different. The later has a 0.05 ceramic cap bridgeing the center taps of the
two onboard transistors. The L board has the same cap, but , its on the
circuit board. Both have additional resistors on the bottom across R2129 and
R2130. On the W board these appear to be 66 ohm each (if my eyes are seeing
correct). The L board has resistors in the same position and these are 68 ohm
resistors (ok, close). By the schematic, (and I have both the original 78
manual and the 80 revised editions), these two "on board" resistors should be
15 ohms. Checking both boards, (I did'nt pull the resistors to check their
value), they are in fact marked 15 ohm and are carbon comp. Simple
calculations net out just a shade over 12 ohms equivalent. I realize its not a
big difference, but why was it done? Increase the current to the T-2101
Transformer? Which in turn would raise the voltage on the 25v line?
So, the questions are, 1) Whats the history in the labeling, and 2) Are the
bridging resistors on the underside of both power supply boards factory wiring?
All this started because during an alignment, I could'nt get the required
voltage on the VCO board, (shy by around 1-1.5v) and found I only had 22 volts
going in instead of 24-25v.
Thanks in advance for your input.
David
Wd9cmd
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