UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Radio (domestic)

Notices

Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

Closed Thread
 
Thread Tools
Old 16th Jan 2017, 4:51 pm   #1
colebrooker
Retired Dormant Member
 
Join Date: Feb 2016
Location: Swindon, Wiltshire, UK.
Posts: 51
Default 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.
colebrooker is offline  
Old 16th Jan 2017, 5:04 pm   #2
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default 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.
ms660 is offline  
Old 16th Jan 2017, 5:16 pm   #3
ukcol
Rest in Peace
 
ukcol's Avatar
 
Join Date: May 2003
Location: Harlaxton, Lincolnshire, UK.
Posts: 3,944
Default 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.
ukcol is offline  
Old 16th Jan 2017, 6:08 pm   #4
colebrooker
Retired Dormant Member
 
Join Date: Feb 2016
Location: Swindon, Wiltshire, UK.
Posts: 51
Default Re: murphy u502 voltage readings

Lawrence, From notes I have made that measures 200v ac.
David
colebrooker is offline  
Old 16th Jan 2017, 6:13 pm   #5
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default 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.
ms660 is offline  
Old 16th Jan 2017, 6:14 pm   #6
colebrooker
Retired Dormant Member
 
Join Date: Feb 2016
Location: Swindon, Wiltshire, UK.
Posts: 51
Default Re: murphy u502 voltage readings

Colin, Thanks for your ideas and I will check it out tomorrow.
David
colebrooker is offline  
Old 16th Jan 2017, 9:32 pm   #7
stitch1
Heptode
 
stitch1's Avatar
 
Join Date: Jan 2008
Location: Durham, County Durham, UK.
Posts: 825
Default Re: murphy u502 voltage readings

Quote:
Originally Posted by colebrooker View Post
... Now I have replaced the Hunts and the 2 electrolytics c49 & c50....
Hello David
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
stitch1 is offline  
Old 16th Jan 2017, 10:22 pm   #8
colebrooker
Retired Dormant Member
 
Join Date: Feb 2016
Location: Swindon, Wiltshire, UK.
Posts: 51
Default 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
colebrooker is offline  
Old 17th Jan 2017, 2:14 pm   #9
colebrooker
Retired Dormant Member
 
Join Date: Feb 2016
Location: Swindon, Wiltshire, UK.
Posts: 51
Default 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
colebrooker is offline  
Old 17th Jan 2017, 2:30 pm   #10
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default 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.
ms660 is offline  
Old 17th Jan 2017, 3:00 pm   #11
Nuvistor
Dekatron
 
Nuvistor's Avatar
 
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 9,427
Default 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
Nuvistor is online now  
Old 17th Jan 2017, 3:06 pm   #12
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default 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.
ms660 is offline  
Old 17th Jan 2017, 3:13 pm   #13
Nuvistor
Dekatron
 
Nuvistor's Avatar
 
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 9,427
Default 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
Nuvistor is online now  
Old 17th Jan 2017, 3:18 pm   #14
ukcol
Rest in Peace
 
ukcol's Avatar
 
Join Date: May 2003
Location: Harlaxton, Lincolnshire, UK.
Posts: 3,944
Default 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.
ukcol is offline  
Old 17th Jan 2017, 3:27 pm   #15
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default 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.
ms660 is offline  
Old 17th Jan 2017, 4:46 pm   #16
ukcol
Rest in Peace
 
ukcol's Avatar
 
Join Date: May 2003
Location: Harlaxton, Lincolnshire, UK.
Posts: 3,944
Default Re: Murphy U502 voltage readings

Yes, and high anode and screen voltages with a low cathode voltage as the OP stated.
ukcol is offline  
Old 18th Jan 2017, 11:57 am   #17
colebrooker
Retired Dormant Member
 
Join Date: Feb 2016
Location: Swindon, Wiltshire, UK.
Posts: 51
Default 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
colebrooker is offline  
Old 18th Jan 2017, 12:01 pm   #18
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Murphy U502 voltage readings

Measure the voltage across R9.

Lawrence.
ms660 is offline  
Old 18th Jan 2017, 4:39 pm   #19
colebrooker
Retired Dormant Member
 
Join Date: Feb 2016
Location: Swindon, Wiltshire, UK.
Posts: 51
Default Re: Murphy U502 voltage readings

Lawrence, the voltage across r9 is 65v dc negative to chassis.
David
colebrooker is offline  
Old 18th Jan 2017, 4:43 pm   #20
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Murphy U502 voltage readings

Geeze.....

That would confirm Colins diagnosis.

Lawrence.
ms660 is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 10:38 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.