Re: HMV 1804 Television.
Effected an improvement to the picture quality and stability by replacing the final IF amplifier valve, another Z66. There was a problem with sync crushing, a very unusual condition with positive modulation TV signals. Normally when the gain controls are turned up it's the video part of the waveform that's becomes crushed. Replacing that valve solved the problem. The Z66 is not a good valve and as a reliable replacement it might be a good idea fitting an international octal base on a Mazda SP61.
Despite the reintroduction of the original EMI diode interlace filter circuit the interlace is not good. Careful adjustment of the frame hold control is required for optimum results.
To improve matters I will employ the Pye double-diode interlace filter circuit as was used by the engineer who did the modifications but using the existing D63/6H6 valve rather than the separate subassembly.
DFWB.
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