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  #31  
Old 05-21-2018, 11:47 AM
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maxhifi maxhifi is offline
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Quote:
Originally Posted by dieseljeep View Post
Paranoia runs deep, into your life it will creep!
The sets built in the 240 volt areas used valves (tubes) that had a higher heater voltage at lower current. Many times they added up to more than 120 volts, so they only had to drop less voltage from 240 volts. The mains dropper was a high wattage item and still a common failure item.
I've worked on TVs and radios for years just by identifying what prong is connected to the chassis, plugging it into the neutral side of the receptacle, and then lifting the ground on my test quipment to break the ground loop. Never once shocked myself, but I always checked voltage to ground before touching anything. There's no way I'm the only one to do it this way, it is obvious and easy, but only one mistake away from a shock. It's nice to get rid of all that nonsense.
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  #32  
Old 05-21-2018, 07:33 PM
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irext irext is offline
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Back in the days when I worked on live chassis equipment I always used an iso tranny as we didn't have earth leakage breakers then (70's). I saw a colleague being electrocuted and unable to let go. Fortunately he eventually threw himself backwards and ripped the mains plug out as he went. He was o/k but very shaken. Could have been much worse. I don't take any chances. Especially when you are working on your own.
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  #33  
Old 05-22-2018, 12:08 PM
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Originally Posted by irext View Post
Back in the days when I worked on live chassis equipment I always used an iso tranny as we didn't have earth leakage breakers then (70's). I saw a colleague being electrocuted and unable to let go. Fortunately he eventually threw himself backwards and ripped the mains plug out as he went. He was o/k but very shaken. Could have been much worse. I don't take any chances. Especially when you are working on your own.
I think out 120V is substantially less dangerous than your 240, but all the same, I agree about using an isolation transformer. Especially since most line operated devices are switched on the neutral side, so even if you have it hooked up right, the chassis becomes live when you turn it off.

I didn't get to do as much on the Marconi this weekend as I thought I would, but right now the priority is ordering new resistors. I am really surprised how bad the situation is, usually when I work on vintage audio equipment from the same era, the original resistors are mostly fine - but this thing is way out to lunch, some are double the original value, some are half.. it's just all over the place. It "works" as-is, but it's certainly not working as per original design. Considering that over 50% of the ones I have measured are way off, I'm tempted to just change all of them. This will be a lot of work, but I can't see another way to make it work like new.

What kind of resistors to buy as replacements? I'm thinking 1/2W 1% metal films for the majority, and then 2W and 5W metal film for the larger ones as needed. Unfortunately the schematic doesn't give power ratings, so I have to figure them out.
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  #34  
Old 05-23-2018, 12:52 AM
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With resistors in older gear I tend to go double the wattage since a 1/2 watt resistor nowadays is about half the physical size of the original 1/2 watt. Metal film resistors seem fine. I know some have concerns about breakdown with high voltages but I've not had any failures (yet).
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