Re: Marconi 702. Sound problems.
The sound problems in the Marconi 701 and the electrically identical HMV 901 have been sorted out.
The Marconi 702 still has that low CRT video drive problem to sort out. It's only 10 volts from sync tip to peak white. It should be at least 15volts.
This evening I turned my attention to the HMV 901. The replacement vision sensitivity control was fitted some time ago. The new part is a high grade 5watt wire wound 25Kohm potentiometer. A slot had to be cut into the end of the control shaft to suit the type of control knobs employed by EMI in those times. See attachment.
No frame sync was traced to the anode load resistor of the frame sync separator, R5 350,000ohms. Separate line and frame sync valves are employed in these early EMI sets. The valves are Marconi MSP4 pentodes, V1 and V5.
After replacing the resistor the frame sync lock is solid and the interlace is perfect.
Not bad at all for a 1936 design. Actually it's possible that the design work on these sets was started sometime in 1935.
So that leaves the Marconi 701 to receive attention. The video drive has to increased somehow.
DFWB.
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