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 9th May 2017, 9:04 pm   #61
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: MURPHY A272C Restoration.

And HT (approx. 200 volts) on the other side of R5?

Lawrence.
ms660 is offline  
Old 9th May 2017, 9:07 pm   #62
sobell1980
Octode
 
sobell1980's Avatar
 
Join Date: May 2011
Location: Bromyard, Herefordshire, UK.
Posts: 1,241
Default Re: MURPHY A272C Restoration.

Yes that's correct.
sobell1980 is offline  
Old 9th May 2017, 9:29 pm   #63
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: MURPHY A272C Restoration.

So to confirm, the anode voltage is ok, the screen grid voltage is 1.5 volts, R5 is ok and C22 is ok, when the valve is unplugged the screen grid voltage rises up to the HT voltage of 200 volts?

Lawrence.
ms660 is offline  
Old 9th May 2017, 9:31 pm   #64
sobell1980
Octode
 
sobell1980's Avatar
 
Join Date: May 2011
Location: Bromyard, Herefordshire, UK.
Posts: 1,241
Default Re: MURPHY A272C Restoration.

Yes. That is all correct except c22 is still not connected to ground. Many thanks.
Dave.
sobell1980 is offline  
Old 11th May 2017, 6:54 am   #65
sobell1980
Octode
 
sobell1980's Avatar
 
Join Date: May 2011
Location: Bromyard, Herefordshire, UK.
Posts: 1,241
Default Re: MURPHY A272C Restoration.

I can only think it is the valve? Replace this alongside the resistors that have gone high.
Dave.
sobell1980 is offline  
Old 11th May 2017, 9:27 am   #66
Phil G4SPZ
Dekatron
 
Phil G4SPZ's Avatar
 
Join Date: Apr 2005
Location: Bewdley, Worcestershire, UK.
Posts: 4,735
Default Re: MURPHY A272C Restoration.

It's unwise to run without C22 connected as its purpose is to prevent instability.
__________________
Phil

Optimist [n]: One who is not in possession of the full facts
Phil G4SPZ is offline  
Old 11th May 2017, 1:24 pm   #67
Boater Sam
Banned
 
Join Date: Dec 2013
Location: Middlewich, Cheshire, UK. & Winter in the Philippines.
Posts: 3,897
Default Re: MURPHY A272C Restoration.

G3 to G4 short? How is the cathode volts/current?
Boater Sam is offline  
Old 25th Jun 2017, 1:27 pm   #68
sobell1980
Octode
 
sobell1980's Avatar
 
Join Date: May 2011
Location: Bromyard, Herefordshire, UK.
Posts: 1,241
Default Re: MURPHY A272C Restoration.

Sorry it's been such a long time. I've just managed to sit in front of this set the first time since may. Life took over gor a while. Right, now where did I get to?
I remember thinking the 6c9 valve was causing the voltage issue. I will need to re read the thread i think.
sobell1980 is offline  
Old 27th Jun 2017, 1:25 pm   #69
sobell1980
Octode
 
sobell1980's Avatar
 
Join Date: May 2011
Location: Bromyard, Herefordshire, UK.
Posts: 1,241
Default Re: MURPHY A272C Restoration.

I have replaced the resistors R5 and R29 today. Not that I thought they were causing the screen grid problem on valve 2 but I'd removed them for checking and they had gone high especially R5. I think reading my previous posts and also checking c22 that it looks possible it is in fact 6C9 valve 2. Im out of ideas now. I've ordered a UCH42 new old stock valve which should be with me tomorrow or Thursday. If anyone would like to add anything they think I may have missed, please feel free.
Many thanks.
Dave.
sobell1980 is offline  
Old 27th Jun 2017, 2:33 pm   #70
sobell1980
Octode
 
sobell1980's Avatar
 
Join Date: May 2011
Location: Bromyard, Herefordshire, UK.
Posts: 1,241
Default Re: MURPHY A272C Restoration.

Apologies, that should be ECH42 and not UCH42.
Im told this is a near substitute for the 6c9.
Dave.
sobell1980 is offline  
Old 27th Jun 2017, 3:30 pm   #71
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: MURPHY A272C Restoration.

Yes, near but not exact, so far as I can make out the 6C9 is a triode heptode and the ECH42 is a triode hexode, It's not unusual to find different screen feed regimes for the two different types, due to the lack of a suppressor grid in the latter the hexode usually requires a more stable screen voltage and is often fed from a potential divider, the heptode usually has a sliding screen voltage due to it being fed from a single series resistance.

The different screen feed regimes is basically down to the effects (or not as the case may be) of secondary emission, which, in effect can damp the IFT's primary thus reducing selectivity, the damping will vary with the AGC control voltage, more AGC voltage = more damping.

Whether you will notice any difference is another matter.

Lawrence.
ms660 is offline  
Old 27th Jun 2017, 3:35 pm   #72
sobell1980
Octode
 
sobell1980's Avatar
 
Join Date: May 2011
Location: Bromyard, Herefordshire, UK.
Posts: 1,241
Default Re: MURPHY A272C Restoration.

Lawrence,
Do you feel my diagnosis is correct? I think I've covered everything.
sobell1980 is offline  
Old 27th Jun 2017, 3:47 pm   #73
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: MURPHY A272C Restoration.

Hard to say, it's been a while, is R5, C22 ok and is C22 connected in circuit.....anyways, you'll soon find out when you fit the new valve.

Lawrence.
ms660 is offline  
Old 27th Jun 2017, 3:53 pm   #74
sobell1980
Octode
 
sobell1980's Avatar
 
Join Date: May 2011
Location: Bromyard, Herefordshire, UK.
Posts: 1,241
Default Re: MURPHY A272C Restoration.

Yes it has been a while. Life took over for a month or too with family illness and I have a new job to start on Monday which has involved exams etc.
R5 is replaced. C22 is now in circuit and has also been replaced.
sobell1980 is offline  
Old 27th Jun 2017, 4:15 pm   #75
sobell1980
Octode
 
sobell1980's Avatar
 
Join Date: May 2011
Location: Bromyard, Herefordshire, UK.
Posts: 1,241
Default Re: MURPHY A272C Restoration.

Good news, I can confirm that valve 2, a Mazda 6C9 was indeed faulty. I was buzzing around tidying up my radio room when I thought to myself, Dave you idiot. You have another Murphy set under the bench. Indeed I had an A188 console version under the bench in the to do pile. I whipped the back off it and sure as damn it has a Mazda 6C9 in it. Swopped it into the set I'm working on and all is well. The sensitivity is tons better too. It was struggling to pick up even strong stations before and sounded shrill. It sounds much better and rich in sound now.
Dave.
sobell1980 is offline  
Old 27th Jun 2017, 4:17 pm   #76
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: MURPHY A272C Restoration.

Good result.

Lawrence.
ms660 is offline  
Old 27th Jun 2017, 4:45 pm   #77
sobell1980
Octode
 
sobell1980's Avatar
 
Join Date: May 2011
Location: Bromyard, Herefordshire, UK.
Posts: 1,241
Default Re: MURPHY A272C Restoration.

Thanks Lawrence. I must say I was starting to doubt myself. Especially when I heard that this valve is usually very reliable. But I guess on a set of this age and it's faults I've repaired who knows how it failed. Some one could have done what I have and swopped it for a dead valve from another set. We just don't know the sets history so anything could have happened over the decades.
Dave.
sobell1980 is offline  
Old 28th Jun 2017, 12:22 pm   #78
sobell1980
Octode
 
sobell1980's Avatar
 
Join Date: May 2011
Location: Bromyard, Herefordshire, UK.
Posts: 1,241
Default Re: MURPHY A272C Restoration.

For those that are interested and have kindly followed this, even after a long break I have tested the duff 6c9 valve this morning. The valve has a short between the cathode pin 7 and screen grid and grid 4 which is pin 5. I obviously have compared this against the known good one and there is no continuity between these pins on the good one. Jus wanted to understand myself what had occurred and it wasn't educated guess work. So I've once again learned from this great set.
Dave.
sobell1980 is offline  
Old 28th Jun 2017, 12:39 pm   #79
Nickthedentist
Dekatron
 
Nickthedentist's Avatar
 
Join Date: Jan 2005
Location: Oxford, UK.
Posts: 17,820
Default Re: MURPHY A272C Restoration.

Well done Dave, and hope the new job goes well
Nickthedentist is offline  
Old 28th Jun 2017, 1:03 pm   #80
Boater Sam
Banned
 
Join Date: Dec 2013
Location: Middlewich, Cheshire, UK. & Winter in the Philippines.
Posts: 3,897
Default Re: MURPHY A272C Restoration.

A rare fault but not unknown.
Be very pleased with yourself that you found the fault by solid deduction and that you were proven correct, proper pro repairer.

Sam.
Boater Sam is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 5:55 pm.


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.