Hi
I think I am making some progress.
I have not found a fault in the DDS. Re-checks showed all the right wave forms where expected.
The Operator Manual includes info on the Auto Calibration feature.
This goes through a process of finding the upper/lower voltage ranges to control each of the 4x VCOs. Within each voltage range, the WaveTek finds the control voltage necessary to set a course frequency.
The AutoCal fails on the first VCO. When set to operating mode, the display indicates that stored settings are not being used.
I did think this could be due to a failed battery in the Dallas 1225 NVR (32 years old) but the user input settings are remembered between power cycles. The NVR still seems to be working, but probably not for too long.
There is no serial port available for loading the NVR but there is an option to set factory defaults plus AutoCal. I think both options were used in the factory to calibrate the Sig Gen (along with some other specified test equipment). I think the user AutoCal is the same as the initial factory calibration. If so, it means that I can replace the NVR with a DS1225AD, the modern version of the DS1225Y. The NVR would then be loaded with the factory defaults and AutoCal.
The VCO control voltage is set by a DAC. The DAC output is static during AutoCal. I would expect the DAC output to be hunting up and down as a search is made for each coarse tuning point (35 points for the first oscillator).
The data side of the DAC is also static during AutoCal.
Both the DAC input and output are also static when the frequency is changed by the operator.
The input to the DAC is a 74HC595 serial to parallel register on the main board.
The serial data comes from the 6502 microprocessor.
So the next line of investigation is down the data side of the VCO control DAC. This is where the Logic Analyzer I don't have would be ideal. I do have some Raspis and Arduinos so I may end up writing some simple code so they can simulate serial to parallel converters. They could read the data and I could then compare the simulated output to the hardware output. Any difference would reveal a fault.
I look with envy at the range of high quality logic analyzers available on e-bay, but the cost and risk of shipping them here would be more than SWMBO would ever approve.
So the hunt continues for the next fault without the right tools to do the job. I will just have to make do with what I have.
Dazz