Marconi 2955A Radio Communications Test Set
Problem
The self tests for frequency generation failed semi-randomly, reporting either too high or too low frequency.
The reception self tests were consistently failing the FM and PM tests, reporting that the reception level was too low.
The FM and PM reception was not working at all when trying the Transmitter Test mode with an external signal generator providing a frequency or phase modulated signal input. AM functions worked well.
Diagnosis
Measuring the output of the signal generator portion of the 2955A revealed that it was working ok on most ranges, the frequency was close to accurate and the level was also correct.
A damaged transistor was preventing some filter switching from working correctly. Providing an external signal to the 2955A revealed that the counter was inconsistent at lower frequencies, but seemed to work fine with 100MHz and above.
A shorted capacitor was found in the demodulation circuits, stopping the FM/PM discriminator circuit from having any power due to shorting out a zener diode regulated 4.5V supply. A failed OM345 hybrid circuit was stopping the frequency counter working.
Solution
Replacing the shorted capacitor fully restored the function of FM and PM demodulation. Replacing the failed transistor in the RF signal generator circuits restored full range of RF output at correct levels.
The frequency counter needed an inventive solution to fixing the failed OM345 hybrid circuit, adding an external resistor to replace the failed transistor base biasing inside the hybrid module.
With these replacements and after adjusting the FM/PM demodulation level the unit passed self tests and all functions worked properly again.
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