The front panel of an FT-757GXII with a working display!
One clue was that when power cycled, the display would occasionally flash very briefly, a possible indication that something was almost doing what it was supposed to. Via email (he lives across the country from me) I had him do some preliminary troubleshooting such as the checking of voltages - but based on the rather sparse information available in the service manual and the difficulty in accessing some of the test points: Even a couple key capacitors in circuits that often cause problems with displays in some radios - namely the switching supplies that provide the odd voltages for the vacuum fluorescent display - were swapped out, but the display remained blank.
I offered to look at it, so he packaged it up and sent it to me. When I put it on my workbench I started probing the various lines on the display processor with an oscilloscope: I could see many of the signals that I was expecting - namely the 500 kHz signal from the display processor's clock, the data coming from the radio's main processor that changed as I pushed buttons and turned the main tuning knob and another signal that appeared to be an acknowledgment pulse from the display processor to the main processor. What I seemed to be missing were half of the multiplexing signals that drove the display: It appeared that I was seeing the "common" signal lines for the display, but the signals on the pins that appeared to carry information as to which display segment was to be illuminated were missing as if the display was supposed to be blank. Without both sets of signals activated appropriately, a multiplexed display will remain forever dark.
I'd already consulted the internet and determined, based on postings in various forums, that at least for its predecessor, the 757GX, the failure of the display processor wasn't terribly uncommon - but not surprisingly this part was long gone from the spare parts inventories of Yaesu and other means of repair/replacement such as getting displays from scrapped radios or even the construction of an "alternate" display unit using a different processor and driver transistors was discussed. What was interesting was that the "important" signals - namely those for data, acknowledgement, scanning and synchronization - seemed to be present, so the display processor clearly wasn't completely dead.
At about that point the old adage drilled into me from the early days of computers and Windows came back to me - although it probably should have been one of the first steps to be taken when the display went blank: "When in doubt, reboot!" Perusing the user's manual I determined that a complete "memory reset" was done on the FT-757GXII by setting both the "Linear" and "Marker" switches on the back panel (see Figure 2) to the "in" position at the same time and turning off the radio for 30 seconds - and then turning it back on and restoring the two rear switches to their normal position: It would appear that these two switches have a second, "non-intuitive" function that when used together, disconnects the internal battery.
The result? The display came back to life!
What had apparently happened was that somehow, the data stream between the display and main processor wasn't what it should be and the main processor was apparently sending some sort of garbage that the display processor didn't understand - probably due to something in the main processor's static RAM. It would appear that in the absence of sensible data, the display processor remains blank, relying on the main processor to send the various bits and bytes that display frequency, mode, etc. rather than reverting to some sort of static display. Clearing the battery-backed RAM of the main processor and resetting it apparently cleared whatever junk had gotten into the memory that had caused it to work improperly.
I checked the back-up battery - an innocuous-looking 2-cell NiCd pack that was near the rear of the main synthesizer board - and it read 2.8 volts with the radio having been disconnected from power for over 24 hours indicating about 1.4 volts/cell, which was appropriate for a properly-charged NiCd. Visually, this small battery pack looked OK in that there were no signs of corrosion, so it is probably OK, despite its age - longevity being one of the virtues of a properly cared-for, high-quality NiCd cell.
How did the main processor's memory get scrambled? Who knows - it could have been an entirely random event, due to static from a finger touching the front panel, the back-up battery's voltage having sagged below the point of memory retention while the radio was turned off or the results of some sort of spike - perhaps lightning - intercepted by the antenna that found its way into other circuits. This sort of "display failure" - apparently caused by the processor's memory being scrambled - doesn't seem to be too common, so my friend considers himself very lucky!
After restoring the radio's operation I did a few tests and found that everything seemed to be working as it should, so it will be packed up and returned to its (very fortunate!) owner very soon.
This page stolen from ka7oei.blogspot.com