| Electronics > Projects, Designs, and Technical Stuff |
| Adding composite input to an old B&W TV |
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| SiliconWizard:
Definitely be careful whatever you do. There are some dangerously high voltages in those CRT TV sets. ::) |
| vk6zgo:
--- Quote from: james_s on April 24, 2020, 05:30:42 am --- --- Quote from: vk6zgo on April 24, 2020, 01:46:57 am ---The date of manufacture of the TV set also casts doubt upon the idea of it being a simple "transformerless" unit, with the "chassis" connected to Mains Neutral. The dubious advantages of such architecture to tube TV receivers are pretty much non-existent for solid state designs, so it is more likely to be a SMPS design. --- End quote --- Sorry but this is just wrong, and dangerous advice. I worked on a LOT of TVs back in the day and I can say with certainty that a great many solid state TV sets up into at least the late 90s used a hot chassis design. The Flyback/LOPT doubled as the transformer in the SMPS, with the whole rest of the circuit being completely non-isolated. Arcade monitors were similarly designed and virtually all required the use of an external isolation transformer in the cabinet. Only small portable sets with a DC power option and large fancier TVs had isolated power supplies. I don't think I've ever seen a CRT TV other than a small portable AC/DC set that had a removable power cord. --- End quote --- Maybe this was the case with US made TVs, It is strange, indeed, if TVs made in Asian countries would follow that design. I also have worked on a lot of TVs from that era, & I have not seen one which used the type of SMPS you describe, especially, small BW sets. I believe such designs were used in Europe & the UK for larger Colour TVs. The only reason, apart from showing how "clever" they can be, as far as I can see, is to make any interfering signals from the SMPS in sync with the horizontal frequency. In the same time period, Sony, Sanyo & others used free running SMPS in the models sold in this country, with no interference problems. In Australia, "hot chassis" design was never popular, as in earlier times, power transformers were neither expensive, nor prohibitively heavy, & later, normal SMPS were exclusively used, even in imported TVs. Considering the TV under discussion. Its origin in Taiwan makes it more likely to follow standard SMPS design, with an isolated chassis. Of course, it may be that RCA, for some arcane reason, insisted upon a "hot chassis" design. "Figure 8" in Australian vernacular does not imply a removable power connector, but refers to the appearance of common twin flex, and has done, back at least into the late 1950s. In any case, I wasn't offering "generic" advice, just suggesting a bit more research by the OP. If they have a DMM, they can, using the ohms range measure between each pin in turn of the plug on the power cord to see if the chassis is indeed, connected to one side of the Mains. To the OP, & any "noobs" who are "playing along at home":- "The plug on the power cord" implies that the TV is unplugged from the Mains, & the measurement is made between each, in turn, of the pins on the end of the power cord which normally plug into the power point & the chassis. |
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