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Service Sheets for the HMV 901

TRF LayoutSound Layout

The Timebase with the System Selector Switch (S1) is shown below.

The additional triode in the line timbase was apparently to compensate for the lack of line synch pulses during the frame synch.
This only applied to the Baird system. EMI had a patent (425220) on the placing of line syncs in the frame sync. See the LF Impulses

Power Supply Layout

Synch Layout

EHT Transformer Design

Apart from the usual leaky capacitors and high resistors the most challenging repair for me

was the re-wind of the burnt out mains derived EHT transformer. Initially I tried to simply copy

the original structure but ran into insulation problems so I redesigned it a little

by separating the primary and high voltage windings with the 4 volt tube heater winding.

For both the tube heater and the EHT rectifier heater windings I used PTFE insulated wire

and this (touch wood) has now survived for nearly 40 years.