Wanted to bring this solution to the community as it took several weeks of research and hair pulling to find why and what was causing it. This radio was new to me so its history is not known. Initial checkout found the 13.8v supply pin on Mainboard has a old tired cracked solder joint, J7-1 as I remember, resoldered, bam, fixed. Then the radio looked and preformed faboulous....except every band had receive censorship. Everything below 1600 dead, then parts of each band open and parts dead. Ok its PPL alignment time. Oscillators looked but did the alignment by the SM. No help. Schematic time with scopes, DVMs, SGens, Freq Counters. I could not find that was killing the rec. Short story here, finally set up receive at 4.0000 AM, where the select relays clicked and tuning 1HZ low the rec went dead, back to 4.0000 works fine. Tried another good Mainboard, same story. Had a 737 two board PLL, unfortunately the 737A in a single board.
Backing out and rethink. Mainboard known good, probable PLL. Examined both 1LO and 2LO no amplitude or unexpected freq change when dial freq changed that 1 Hz. Took DVM
and went pin by pin of every connector from PLL card looking for a signal that followed the rec loss. Bam the DNB went from 7v to 0 exactly in line with the 1Hz tuning causing rec loss. After a intimate night this PLL DNB, which just a quick muting during band switching, all the circuitry was not toggling the DNB line. Two other sources toggle that line, the PDD and ULN from the DSS chip. Wasnt PDD but was UNL, pin 71. Another night of research and the PLL Band Option Diodes were correct between the 737 and 737A. No info on the DSS chip anywhere, its an ICOM only. No input to the chip was found associated with the UNL toggle. What appears is the root is a software country option installed but no model number or ROM part number different as with a Japanese IC732 (737A). the test... I opened the UNL trace and problem gone, rec all bands perfectly. Trans LED lights on all bands too. Not sure if actual transmission is occurring since PA pallet is removed.
This is very strange finding. The UNL pin has no mention and serves no purpose but to "lock out" a specific band of frequencies. I have found nothing except a vague reference to a "France" frequency option via the option diodes but those diodes are correctly configured for standard US bands.....go figure.
Wanted to get this out in case anyone belse has or is having this issue.
Jack KG6INX