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Old 4th Mar 2012, 1:19 pm   #1
Bob_Moss
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Default Murphy A122 low anode volts

I have just repaired the above set, very good sound and sensitive.
Before returning it to its case, I gave the voltages a check in accordance with Trader sheet 962.
V1 anode is at 110V instead of 155V. Main HT is spot on. I'm using a DVM instead of an AVO 7 but this should read high, not low.
Cathode voltage is OK at 2.5V. Substituted V1 to little effect. Removed grid connection - no effect. Substituted R3 - no effect.
What am I doing wrong, and does it matter (set sounds good)?
Over to you, Forum Gurus.

Bob.
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Old 4th Mar 2012, 1:57 pm   #2
Steve_P
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Default Re: Murphy A122 low anode volts

If it sounds good, leave it. HT isn't as exact as LT, Don't go looking for faults.

However, if you are really unhappy about it, disconnect the tone control, as this can go to earth, Check R31 and C46. See if it's going to earth somewhere else. V6 may be a bit past it's best, but if it's working, don't play with it.

Cheers,

Steve P.
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Old 4th Mar 2012, 2:00 pm   #3
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Default Re: Murphy A122 low anode volts

The schematic on the diagram dungeon says V1 hexode anode volts is 100V. The anode voltage is dependent on the test conditions. If you are receiving a signal then the AGC will nearly cut-off V1 and the anode voltage will rise. Never trust Trader sheets!

PS: Don't seem to be able to post links anymore?
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Old 4th Mar 2012, 3:59 pm   #4
Bob_Moss
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Default Re: Murphy A122 low anode volts

Thanks chaps,

I had already checked out the AGC part, removed the aerial, and tuned to the highest wavelength on MW.
I've always trusted the "Trader" service sheets implicitly -until now.
At least I've had a few extra hours tinkering with it and gained some more experience.
Back in the case it goes, then to write up the success story.

Thanks again,

Bob.
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Old 4th Mar 2012, 9:15 pm   #5
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Default Re: Murphy A122 low anode volts

Many original service manuals had mistakes. These were usually corrected by add on sheets that never got added on to the actual manual..Your readings are probably correct. Service manuals can be a bit like the old 'Home Doctors Book'. Symptom: Pain in leg. Cure: Amputate leg. Put the back on and enjoy it. Regards, John.
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