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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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16th Jan 2017, 4:51 pm | #1 |
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Murphy U502 voltage readings
Hello all,
I have had this set for many years. It did work for me before it started fading away after 10mins or so. It was then put away until I had time for it. Now I have replaced the Hunts and the 2 electrolytics C49 & C50. Also new panel lamps, 19v at 120ma, and the R33 mod. The only sound I can get is a crackle when tuned to what I suspect is a station frequency. This sound changes when I probe on the grids all the way through. Voltage readings are reasonable except for V3 where I have p7 anode 145v, p8 screen 90v and p3 grid 0.5v. The 2 valves V2 & V3 have been swapped with no change so not a faulty valve. Now is the time for me to have a bit of guidance as I only have an analog meter to hand. I have followed a number of other threads and find it all interesting. My radio experience was back in early 60's working on RAF aircraft radios. They were valve sets but it was long ago. I have 2 more project sets if I can manage this one. Regards, David. |
16th Jan 2017, 5:04 pm | #2 |
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Re: murphy u502 voltage readings
Check with respect to chassis the AC voltage at the junction of R31 and R32, it should be 197 volts AC approx.
Lawrence. |
16th Jan 2017, 5:16 pm | #3 |
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Re: murphy u502 voltage readings
Hello David
The voltages you give for V3 suggest, at first sight, that the valve is low emission. I assume you meant cathode and not grid when you gave the pin 3 voltage. Next you say that swapping V3 and V2 (both 10F18s) gives no change. If the voltages (as implied) on V2s electrodes are correct and after the swap the voltages on V3 are still wrong then it is not a low emission valve. If the V3 stage is going into instability (oscillation) that whould explain the voltage readings you are getting. Sorry about all the "ifs" but I hope you can follow what I am saying. Check that you haven't made a wiring mistake when you changed the paper capacitors, particularly the decoupling around V3. Also check you have fitted capacitors with the correct values of capacitance. |
16th Jan 2017, 6:08 pm | #4 |
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Re: murphy u502 voltage readings
Lawrence, From notes I have made that measures 200v ac.
David |
16th Jan 2017, 6:13 pm | #5 |
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Re: murphy u502 voltage readings
That sounds ok, if there's approx 13 volts AC across pins 4 and 5 of V3 then the voltage measurements you made on that valve suggests that the valve is low emission and possibly V2 is a well.
Lawrence. |
16th Jan 2017, 6:14 pm | #6 |
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Re: murphy u502 voltage readings
Colin, Thanks for your ideas and I will check it out tomorrow.
David |
16th Jan 2017, 9:32 pm | #7 | |
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Re: murphy u502 voltage readings
Quote:
Have you changed C41? also an electrolytic (FM discriminator I think?), I've just finished a U512 (same chassis) and that was causing problems. I was also educated about valve radio in the RAF, but late 70s and I've forgotten more than I learnt John |
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16th Jan 2017, 10:22 pm | #8 |
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Re: Murphy U502 voltage readings
John, yes I have changed c41 but I haven't changed c44 as it doesn't seem to be
an electrolytic. Lawrence, yes there is 13v AC across pins 4 & 5 V3. I still have to double check my previous work on the cap. changes. Perhaps I should purchase a couple of spare valves also. David |
17th Jan 2017, 2:14 pm | #9 |
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Re: Murphy U502 voltage readings
The c41 that I have put in is a 4.7 electrolytic, new, and measures -30v dc across it.
Relative to the chassis. Does this sound ok? David |
17th Jan 2017, 2:30 pm | #10 |
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Re: Murphy U502 voltage readings
If that's with a radio station tuned in it looks to high to me, if no station is tuned in then something's wrong.
Lawrence. |
17th Jan 2017, 3:00 pm | #11 |
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Re: Murphy U502 voltage readings
If that's -30 volts is on a strong station that is ok. My FM radios with a strong station and a dipole in the attic have that voltage on the AM suppression capacitor.
Indeed many of the original capacitors were 100 to 150 Vw rating, I have 63vw caps in my radios. If there is no station tuned in then has Lawrence states you have a fault. Frank |
17th Jan 2017, 3:06 pm | #12 |
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Re: Murphy U502 voltage readings
Just wondering what the fault symptoms are now, according to post#8 the grid coupling capacitor (C44) hadn't been replaced at that point.
Lawrence. |
17th Jan 2017, 3:13 pm | #13 |
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Re: Murphy U502 voltage readings
If the last IF amp is unstable that could put a large voltage on the AM suppression capacitor but you would probably not be able to tune a station in.
Frank |
17th Jan 2017, 3:18 pm | #14 |
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Re: Murphy U502 voltage readings
30 volts across C41 without a station supports my view, expressed in post # 3, that there may be instability in the IF amplifier.
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17th Jan 2017, 3:27 pm | #15 |
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Re: Murphy U502 voltage readings
If the last IF (V3) is unstable and producing a relatively high oscillation voltage then there should be some -ve grid volts on V3's control grid.
Lawrence. |
17th Jan 2017, 4:46 pm | #16 |
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Re: Murphy U502 voltage readings
Yes, and high anode and screen voltages with a low cathode voltage as the OP stated.
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18th Jan 2017, 11:57 am | #17 |
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Re: Murphy U502 voltage readings
The new c41 is rated at 63v, the original is 50v. I have tried the original back in the set
and no change. Still 30v across it. This is off station as I turned the tuning a random amount. David |
18th Jan 2017, 12:01 pm | #18 |
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Re: Murphy U502 voltage readings
Measure the voltage across R9.
Lawrence. |
18th Jan 2017, 4:39 pm | #19 |
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Re: Murphy U502 voltage readings
Lawrence, the voltage across r9 is 65v dc negative to chassis.
David |
18th Jan 2017, 4:43 pm | #20 |
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Re: Murphy U502 voltage readings
Geeze.....
That would confirm Colins diagnosis. Lawrence. |