Just had a look at the circuit for the HMV M1-B7 Ch= M1 and although it uses a transformer in the PSU, the set uses a voltage doubler for HT. Was there a reason for using a doubler when the transformer could have been wound for the correct HT and used full wave rectifier?
Frank
Yep. A good reason.
High voltage silicon diodes were still rare in 1959. The OA210 diode is rated at 400 volts. You'd need an 800v part otherwise.
The full wave voltage doubler was widely used in Oz valve TVs. It uses a less expensive transformer and gave better regulation than a CT full wave, plus it allowed the use of just two lower spec diodes.
HMV engineers were chary of these new fangled crystal diodes so they included a thermistor. It was this part that failed, and frequently! Standard repair procedure was just to short them out. The diodes themselves never failed. After 10 or 20 years use the 100uF 200v can caps would sometimes fail O/C.
Well there you are, I was always led to believe voltage doubler circuits had poorer regulation than FW circuits, never needed to prove it
Frank
It does depend on the transformer and the ESR of the caps.
But you've got me wondering now. What you say makes sense. I'll have to run a sim and check it out.
It's just what I was told years ago and never questioned. In any case, HT regulation on a valve TV isn't a major design consideration.
I think it comes down to the diodes and the cost of the transformer.
When the set is switched off a brilliant spot appears on the screen and takes a long time to disappear. Something is needed to rapidly discharge the EHT capacitor.
About the time the Stella ST1007U was made a simple circuit modification was employed to do this, the earthy end of the brightness control was returned to the un-switched end of the mains neutral.
It is desirable that the mains neutral is connected to the chassis which nowadays is no problem because we use non-reversible main plugs.
The spot suppression is discussed on page 28 in the 1960/61 Radio and Television book.
Till Eulenspiegel.
Well there you are, I was always led to believe voltage doubler circuits had poorer regulation than FW circuits, never needed to prove it
For a brief while it was very popular in radios c. 1932-1934 that used the new-fangled Westinghouse metal rectifiers. Pye, Bush and McMichael all used that technique where a metal rectifier was present. I assume it kept the rectifier to a manageable size as regards the PIV of the diode stacks.
The simple spot suppression proved popular in some sets. Later sets used a VDR in the brightness control circuit.
Frank
Hi Frank, earlier this evening I implimented the spot suppression modification. It works perfectly, when the set is now switched off the collapsing raster brightens up and discharges the EHT capacitor.
Till Eulenspiegel.
What with the nice beam current limiter circuit and now the spot suppression mod, I now have two good reasons to re-visit my ST8617U.
To understand the black art of electronics is to understand witchcraft. Andrew.
Hi Andrew, the spot suppression modification will be easy to do in the ST8617U as the on-off switch, volume and brightness are on the same control assembly. In the ST1007 the brightness control pot. had to be relocated to the rear of the chassis.
Till Eulenspiegel.
It was extremely simple and now saves the CRT having that very bright spot upon switch-off.
To understand the black art of electronics is to understand witchcraft. Andrew.
