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  #31  
Old 05-21-2018, 11:47 AM
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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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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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  #35  
Old 06-16-2018, 01:45 PM
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Originally Posted by irext View Post
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).
I'm changing them out one by one with metal films. I never realized how tedious it is to change resistors and maintain original lead dress... There's just so many of them! Very few are within tolerance, the good ones are 10% off, but some are double in value and even worse.
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  #36  
Old 06-16-2018, 04:58 PM
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By saying the resistors are far off, how far off are they? The resistor values are generally not too critical except for some circuits. You may find changing all the resistors a wasted effort if it doesn't make any noticeable difference.

Note most are likely +/-20%. Even outside that it often Doesn't matter. It would be better to focus on the capacitors and only attack the resistors if there is a specific problem.
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  #37  
Old 06-16-2018, 05:22 PM
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Originally Posted by Penthode View Post
By saying the resistors are far off, how far off are they? The resistor values are generally not too critical except for some circuits. You may find changing all the resistors a wasted effort if it doesn't make any noticeable difference.

Note most are likely +/-20%. Even outside that it often Doesn't matter. It would be better to focus on the capacitors and only attack the resistors if there is a specific problem.
Most are between 20%-100% off. On average 30-50% high, with cathode resistors reading up to 250% of marked value. This specific TV has the most off resistors I've ever encountered in on TV during a restoration, so I decided to change them all, usually I only replace parts which cause a specific malfunction, as you say.
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  #38  
Old 06-16-2018, 08:50 PM
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I usually find the higher value resistors to be the worst offenders. At least you will have peace of mind if you change them all. That chassis came up very clean. It must have been stored in a dry smoke free environment. In Aus back in the seventies a lot of houses had oil heaters. The chassis would be covered in a layer of soot/slime. Very hard to clean.
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  #39  
Old 06-16-2018, 11:26 PM
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Originally Posted by irext View Post
I usually find the higher value resistors to be the worst offenders. At least you will have peace of mind if you change them all. That chassis came up very clean. It must have been stored in a dry smoke free environment. In Aus back in the seventies a lot of houses had oil heaters. The chassis would be covered in a layer of soot/slime. Very hard to clean.
Yeah this one was full of dust, dead insects and spiders, and strangely sawdust (my guess is it was stored in a garage workshop). I blew it out really well with compressed air, and cleaned all the filthy internal wood surfaces with a rag and a bucket of warm water soapy water. Thank goodness it didn't live in a smoker's house or worse yet a kitchen. Kitchen radios are the worst, impossible to clean grease worked into everything.
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