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Old 9th Aug 2017, 8:24 pm   #1
longneck90
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Default Murphy A252 R14 burning up on 'Gram' and 'VHF'.

Hello to everyone. This radio has taken a considerable amount of my attention, off and on, for months now. I had to fit new valve bases for V3, V4 and V5. I removed all the IF transformers, repaired broken wires and broken connecting pins. When I first examined it, there were issues with the power output. I have forgot what these are now. Presently, the radio is going, with a wonderful audio output. The problem. When I switch to either 'V' (VHF) or G (Gram) R14 starts burning up. There is also sparking from wave change switches. If I disconnect wire from Point 68 on wave change switch, the burning/stress on R14 disappears, but then, of course, the radio doesn't work. I have removed L23/24 and repaired broken wires, but can't see anything that would cause R14 to suffer. Any help/advice would be greatly appreciated. All the best. John.
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Old 9th Aug 2017, 8:35 pm   #2
Nuvistor
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Default Re: Murphy A252 R14 burning up on 'Gram' and 'VHF'.

If you bypass the switch does R14 still burn up, just thinking about the switch having a leak to chassis.

Frank
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Old 9th Aug 2017, 8:56 pm   #3
ukcol
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Default Re: Murphy A252 R14 burning up on 'Gram' and 'VHF'.

There are 2 possibilities that occur to me.

1] During your extensive work you have made a wiring error.

2] Section f of S1 has carbonized and is track to chassis (but only in position V and G for some reason).

If it is number 2 then you can rewire the circuit as shown in the second picture and it should work fine.

EDIT. I have made the same point in 2] as Frank.
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Last edited by ukcol; 9th Aug 2017 at 8:57 pm. Reason: crossed posts
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Old 10th Aug 2017, 12:39 am   #4
longneck90
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Default Re: Murphy A252 R14 burning up on 'Gram' and 'VHF'.

Hello to all, and thanks to Frank and Colin for advice. I have bypassed R14 as suggested, and all is well when I now switch to'V' or 'G'. However, the radio has stopped working for some reason, which I will investigate tomorrow. What I forgot to mention in my original post, was, when I switched to either 'V' or 'G', there was a voltage drop of 230 Volts across R14. Thanks again, and all the best. John.
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Old 10th Aug 2017, 5:11 am   #5
TonyDuell
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Default Re: Murphy A252 R14 burning up on 'Gram' and 'VHF'.

I don't think anyone suggested shorting (bypassing) R14. If you have done this and there is a short to earth somewhere around section f of S1 then you will short the HT+ line to earth and that would stop things working (and may do other damage).

What I find curious is that in the original circuit diagram (shown a few posts ago) the 'G' contact of S1f is linked to those for S,M,L. So (assuming the are no wiring errors) the set should behave the same way in all 4 of those positions. Either S1f is faulty and shorting to earth in some positions only or it's not wired as the diagram shows.
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Old 10th Aug 2017, 2:30 pm   #6
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Default Re: Murphy A252 R14 burning up on 'Gram' and 'VHF'.

Bypassing R14 wasn't suggested as Tony states, it was bypass the switch. The other point is probably more valid with a wiring error.

Frank
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Old 10th Aug 2017, 6:36 pm   #7
longneck90
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Default Re: Murphy A252 R14 burning up on 'Gram' and 'VHF'.

Hello to all, Sorry Tony. I didn't make clear about re-wiring as you suggested. I didn't bypass (in reality) R14. What I done was bypass wave change switch, as your enhanced circuit diagram shows. When I said the radio had stopped working, I didn't mean that the reason was anything to do with the re-wiring/bypasssing wave switch. I'll investigate tonight, and see what happens. All the best. John.
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Old 10th Aug 2017, 11:43 pm   #8
longneck90
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Default Re: Murphy A252 R14 burning up on 'Gram' and 'VHF'.

Hello to all. Just an update. I have found a broken wire in L26/L27. I haven't removed the L26/L27 container yet. I will be amazed if I can repair/reinstate wire, as I am barely able to see it. All the best. John.
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