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Forum 135

B&W TV Valve era vertical circuits by HMV in Oz

Black & White Television

Last Post by Till Eulenspiegel 1 year ago

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irob2345

(@irob2345)

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I've been following some UK TV designs, which seemed to change quite frequently over the years.

HMV in Australia was part of the EMI group. Their first TV, the E1-A1 chassis, although it performed well, was somewhat of a disaster for EMI (the UK-sourced CRTs and LOPTs all failed inside the 3 months warranty) so the E1-A1 became the E1-A2 very quickly, using locally-sourced CRTs and LOPTs.

The -A2 introduced an interesting vertical circuit that was to survive with only minor changes right through to the end of the valve era. It used a current-to-voltage transformer to implement yoke current sensing feedback, a technique that was to be used almost universally in solid state designs right up to the end of the CRT era. Other advantages were that performance was maintained with valve aging and, theoretically, there was no need for a linearity control (although HMV relented and put one in later versions).

Here are 3 example circuits - the order is reversed!

The first is from 1956 and the E1-A2. Note there is no vertical linearity control. 6BM8 = ECL82

The 2nd is HMV's first 110 degree chassis, circa 1959. Different valves, same circuit. The drive to the yoke is now balanced to eliminate horizontal crosstalk through the yoke that would degrade the interlace.

The 3rd circuit is from the last all-valve chassis, circa 1969. We now have the 6GV8 / ECL85 and some tweaks to the vertical oscillator, but still recognizably the same circuit.

HMV changed overall physical chassis designs 4 times in the early years. The final one was an engineer's dream that folded down flat

to provide easy access to everything. This same chassis pressing was to remain largely unchanged for nearly 15 years, except for big rectangular holes that appeared to accommodate the PCBs of the hybrid chassis.

Posted : 26/08/2023 10:10 am

irob2345

(@irob2345)

Posts: 625

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Topic starter

The mention of the hybrid chassis reminded me of how the circuit morphed into the solid state era.

Transistors have replaced the triode, the blocking oscillator transformer and the current-to-voltage transformer.

The by-then-quite-old 6CW5 makes a re-appearance!

Posted : 26/08/2023 10:43 am

Till Eulenspiegel

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The Kolster-Brandes model HV40 employs a feedback transformer in the frame timebase output stage. Result is excellent linearity and and ease of adjustment.