Help! I'm at my wits end trying to troubleshoot my old faithful. I am the original owner, and happy since about 1985 until just a few weeks ago. It won't get past the usual 1 second display of all segments on the display lit. Attached are a couple of photos that prove all the characters are being scanned with all segments in each character turned on.
I still have the complete user manual which includes all the schematics, calibration procedures, troubleshooting guide, and detailed theory of operation. So I have been working on this for a couple of solid days already.
It passed all the usual suspects: All the power supplies, isolated and non-isolated are present and are at specified values. Address bus signals on the microprocessors are all active as are the data lines. All the processors have the proper clocks going to them.
I know it's an old instrument, and there are a lot of IC's in DIP sockets in the front panel display and keyboard, so I re-seated every IC on the front panel display and keyboard board that had a socket. All the processor are in DIP sockets too so I carefully removed, cleaned pins, and replaced each one. All the IC's in sockets have been removed and replaced now to no avail.
The only things worth mentioning that I have noticed are these four things: (1) There is only a single pulse, likely a start bit, output by the front panel display and keyboard board from it's serial output line when the unit is started up and there is no activity at all on the one serial input line from the frequency board to the panel display and keyboard in return. (2) The watch dog timer circuit that watches the processor that controls the frequency board is constantly reseting that processor because without any serial response from it's own output the watch dog timer circuit times out and resets the processor it is watching. (3) There are no keyboard scanning pulses just as there is no activity to the display segment decoder IC. The processor is apparently neither reading the keyboard nor making an effort to use the display. (4) The beep that normally sounds is missing, although the unit appears to make a faint effort when it is first turned on.
It's not clear if the front panel display and keyboard board is not completing messages required before the frequency board processor would reply or if the frequency board processor is not able to startup correctly.
My hope is that someone has some experience with this problem can share their experiences and offer a suggestion. A little information about the behavior of the system, for example, when one of the serial signals is interrupted, and how that front panel display should react to that might be helpful.
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