On Fri, Sep 1, 2023 at 4:51 PM dwight via cctalk <cctalk@classiccmp.org> wrote: > > My first computer was a Poly-88. I had no monitor and no keyboard. > I read and understood the instructions about finding a TV that used a > transformer power supply. Many newer TV's of that day were not using a > transformer for the main supply. I went to several secondhand stores and > found one that would work.
The original TRS-80 model 1 monitor was based on a live-chassis (hot chassis?) RCA TV set. The USA version had an optoisolator circuit on the video input, the LED driver circuit was powered from the +5V line in the computer (hence the +5V on one pin of the video DIN socket). As the TV was designed for 115V mains only, the European version has a step-down transformer on the AC input. They used an isolating transformer, meaning there was no need to isolate the video input in the European models. Philips made a viewdata termal set where the colour monitor was a modified KT3 television. The power supply in that starts by bridge rectifying the mains input, meaning the chassis is dangerously live no matter which way round the mains is connected. The solution to that was a 240V 300mA (or so) secondary winding on the mains transformer in the the viewdate terminal unit. This provided an isolated AC supply to the monitor, so the chassis of the latter could be earthed. > The keyboard was from a surplus Singer data entry machine ( I thought they > only made sewing machines ). Didn't Singer own Friden (or at least the name) at one point? I am sure I've seen calculators batched 'Singer Friden'). And I have a telecoms test tone genrator that's badged 'Singer' (I assume the same company) -tony
