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Old 12th Feb 2019, 10:38 pm   #29
dave walsh
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Join Date: Jan 2006
Location: Ramsbottom (Nr Bury) Lancs or Bexhill (Nr Hastings) Sussex.
Posts: 5,814
Default Re: [Restoration AND Light Modification] Murphy 146

We are concerned about safety generally of course Michael. Some people can be reckless and worrying these days! The prospect of a live chassis raises the game but it's not a major drawback in itself. My comment about posting relates to someone who asks a lot of questions [fine] but perhaps doesn't seem to be able to research and slowly build up knowledge-just like you are doing now or maybe changes subject/emphasis all the time! That makes it hard to assist them. You are doing fine and I'm not technically proficient. I just know the basics gained from reading and experiment. Do the same. Don't worry whether you should remove components to measure their voltage. You measure the voltage before/after or across them but first you need a voltage

This may be wrong but it seems that-
You have the 146C [Live chassis No Mains Tr at all]. This an economy measure as a Mains Tr would often be the most expensive component so the set is made more affordable. Those two wires in the plug are just badly fitted as I see it. A simple thing you can do [without applying any power to anything] is to trace which of the leads goes to the chassis-visually with the circuit diagram and later using a meter. The chassis should be connected to the "negative" pin on the plug. The lead from the "positive" pin goes through the on/off switch and into a daisy chain of valves etc using up full voltage available [Roughly speaking-No Tr] Think of Christmas Tree lights. Non of them gets the full 240v just a percentage of that. If the leads in the plug are reversed though, the chassis then has the full voltage on it, which you don't want to find out the hard way

The CM [M for modified] 146 version would seem to be using the Auto Transformer idea [same as not having a Tr really]. Three leads with earth [as Nanozeugama and Trevor say P25+26]. This puzzled me a bit Normally you do NOT earth a live chassis as that will just blow the fuse. It seems that some sets eg the U124 as well [a sort of Murphy half baffle board design-curious] had a mains filter that needed earthing but this was done via an "isolating" [that word again] capacitor to solve the fuse issue!
If you've just got a no Tr 146 then the Isolated Earth question doesn't apply-one thing less to worry about!

Dave

Last edited by dave walsh; 12th Feb 2019 at 10:45 pm.
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