Dave - I have no knowledge of the R7 board and don't have the schematic for that, but did recently do some work on my TR7 digital control board and will tell you what I found.
Excuse me if I go over some stuff you already know. U201 is simply a BCD to Decimal decoder. On the TR7, two sections of the bandswitch produce a 4 bit BCD code by grounding various of those 4 bits. All four are tied up with 10K resistors on the digital control board. These 4 bits can be accessed on the TR7 at pins 1-4 of the second board position on the motherboard (that's where the digital control board is). There's absolutely nothing unique to Drake in this case - the BCD to Decimal converter (MC14028B in my case) converts the 4 bit input to a one-of-8 decimal value and pulls one of the output pins low for each unique value. The first two bits, identified as Q0 and Q1 on U201 are NOT used in this case as the possible BCD values don't translate to those decimal values. I had a need to be able to sense the bandswitch position, and produce a single bit output, so I needed a device to duplicate the U201 function on a new board using Pins 1-4 described above as input. While MC14028B is long obsolete, I found no shortage of BCD to decimal converters when I went looking at Mouser and Jameco. The problem was the "truth" table (published for the TR7 on page 2-7 of the service manual I have - mine does not show a version using a Prom). Some of the converters outputted the decimal value by holding a pin high, others by holding it low (ground). I found a TI IC that pulled the pin low, but then found conflicting info on another datasheet for the same device. I finally ended up calling Jameco and talking to one of their tech people (try that at Mouser!) and he steered me to the correct datasheet and the device did indeed pull the output pin low. The truth table in the datasheet matched the one in the TR7 manual perfectly, with the exception I already mentioned - the TR7 doesn't produce the decimal values 0 and 1. The cost of the IC was something like 27 cents. My new board, which uses the converter to select the proper PIN diode attenuator for the PA input, works perfectly using the scheme. I'm guessing that the outputs from both versions of the TR7 board (with and without PROM) are identical, but can't venture a guess on the R7 board. I don't know if the pinout for the MC14028B, Prom, and my converter IC are identical. I used a 7445 BCD to decimal converter IC and Jameco shows only the expensive TI version on their site, but the very inexpensive version from other vendors is available and the pinout is the same. If you think I may be able to help you further, don't hesitate to contact me. The digital converter board outputs are very easy to check with just a VOM, so if someone has an R7 they could open up, they could check those for you. 73, Floyd - K8AC
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