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Old 27th Oct 2020, 8:32 pm   #39
dave walsh
Dekatron
 
Join Date: Jan 2006
Location: Ramsbottom (Nr Bury) Lancs or Bexhill (Nr Hastings) Sussex.
Posts: 5,814
Default Re: Adding a selectable line input to valve radiogram

Given that you have a professional background and a strong sense of responsibility Richard, I can see why you are concerned but this almost belt and braces" approach could be hazardous in itself. I can't give you a technical analysis but members often do start trying to add an earth and it's usually not a good idea. The really interesting aspect of live chassis design era is that there don't seem to be any examples of fatality or series injury that can be located [except in Repair Workshop's that is]. This applied to the more basic designs, where the valve heaters would be "daisy chained" at mains potential'. You seem have an Auto transformer included here. Historically, there were two reasons for the adoption of AC/DC techniques [a] Cheaper production costs [the mains transformer was a costly item] and [b] post war flexibility when many areas still had DC mains. Both types of power systems could be simply covered without a transformer in place.

It wouldn't look so good now but it was safe and it worked. People were careful and got the benefit of cheap consumer goods, using highly insulated casings and electrically separated links. Even the, perhaps higher risk, metal record deck or a valve mains/battery portable could be isolated, via a suitable capacitors, as already mentioned. Only Tape Recorders were a no go for obvious reasons perhaps. If your RG was returned to it's original standard I'm sure you would ensure that the negative lead went to chassis and not vice versa for example and be in sole charge. Often the two core lead had the same colours. That's easily updated without making a significant change I'd say but even a "live chassis is only a problem if it is live and can be accessed.

Of course I'm not making any recommendations here. It's up to you in the end as stated. I hope you ultimately get to safely enjoy your carefully refurbished Radiogram.

Dave W

Last edited by dave walsh; 27th Oct 2020 at 8:50 pm.
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