Hi
I found another fault.
The AM/FM internal modulation isn't working. I quickly traced the problem to the op-amp pair that converts the square wave digital signal to a sine wave.
There are two things that helped narrow down the fault.
1. Binary division of the signal chain.
2. Parallel operation of the 400Hz and 1000Hz signal chain.
There is a long chain of components that process the 400Hz and 1000Hz signals that modulate the RF output. Rather than following the signal path from one end to the other, I started in the middle of both. This just happened to be at the junction between the digital and analog signals. If I had found that the digital signal was faulty, I would have jumped half-way toward the input/origin of the modulation signals. I did find the signals were OK so I jumped half way towards the end of the signal path. Repetition of this binary division along the signal path quickly isolated the fault to IC107 (Fig 4.2), a dual 5534AN low noise op amp. I have already replaced another of the same op-amp type elsewhere.
I also compared the 400Hz and 1000Hz signal paths because they are the same except for frequency.
The signal path is spread across two circuit diagrams, not including the control signals originating from the 6805. Following the signals from end to end would have taken ages and been very inefficient. In contrast, binary division to isolate the fault was quick.
Comparing the 1000Hz and 400Hz signals confirmed the diagnosis beyond doubt.
Both filter inputs include 1uF tantalum Caps (C171 and C172). These are DC blockers and I checked these for shorts. The same component type has failed on the 18V rail, but none have failed on the 15V supply. They are OK.
I think this brings the fault count to about 6.
I am aware that AM/FM modulation isn't used much these days but it would annoy me if I didn't fix something that I can fix so easily. The Wavetek 2520A includes an external modulation input which opens the possibility of other modulation techniques being applied to the RF output.
Dazz