Re: HMV 1804 Television.
Been using a Mullard GZ34 for the HT rectifier. The reason for using this valve instead of the original U52/5U4 was because the GZ34 has an indirect heated cathode which means there is no HT surge so it is kinder to all those aged components. Also, it has a much more compact glass bulb.
This evening I decided to replace the GZ34 with an U52 or 5U4G. The 5U4G has a direct heated cathode which results the HT being present seconds after switch on. Until all the valves have warmed up the HT voltage can rise to >450 volts dropping back to the correct 380volts when all the valves are at full emission. Well, that HT surge took out two components in the video amplifier, the 0.047mfd decoupling capacitor C29 and HT feed resistor R29.
With the GZ34 the HT voltage was a bit on the high side at about 400 volts.
The 5U4 delivers the correct HT voltage which is 380V. The result of fitting the older valve is a reduction of width and height, no problem because the timebases have plenty spare power even with the reduced HT.
The second attachment shows snow on the screen when no signal are supplied to the receiver.
DFWB.
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