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 4th Sep 2006, 11:24 am   #1
Radio_Dave
Nonode
 
Radio_Dave's Avatar
 
Join Date: Jun 2004
Location: Norwich, Norfolk, UK.
Posts: 2,543
Default Murphy B815 - thermal runaway.

Hi,

I got a dead Murphy B815 the other day. I removed and checked the AF117's, but could find nothing wrong with them .

My next step was to replace all of the electrolytics. Now when the radio is turned on it works fine, but for only a couple of minutes, then it goes quiet and a couple of resistors start to overheat badly.

I'm not clever enough to tell you the, exact, purpose of these resistors (something to do with the audio output) but they are R28 and R29 on the Trader sheet

Any advise on what to check next?

Thanks
David

Last edited by Darren-UK; 15th Jul 2011 at 2:02 pm.
Radio_Dave is offline  
Old 4th Sep 2006, 11:48 am   #2
mickjjo
Rest in Peace
 
mickjjo's Avatar
 
Join Date: Jul 2004
Location: Dartford, Kent, UK.
Posts: 1,661
Default Re: Murphy B815 problem

Hi David, This sounds like a bad case of thermal runaway in the output stage, The no signal current of this set at 18v should be 15mA. Check the 2 variable pots R17 and R26 for bad contact or open circuit, The set up procedure is given in R+TV servicing as follows. "Set the slider of R17 to centre position, then set R26 to minimum. Switch the set on with volume at minimum, The battery voltage should be 18 volts. The receiver consumption should now be 11mA, increase this by 4mA with R26, Now adjust R17 so that the voltage at the junction of R28, R29 and C40 is 8.7 volts" . Hope that helps .

Regards, Mick.
mickjjo is offline  
Old 4th Sep 2006, 1:19 pm   #3
Radio_Dave
Nonode
 
Radio_Dave's Avatar
 
Join Date: Jun 2004
Location: Norwich, Norfolk, UK.
Posts: 2,543
Default Re: Murphy B815 problem

Thanks Mick, I think you've fixed it

I thought I'd just check the current before I started and it was drawing 2 amps, no wonder the resistors were smoking!

Anyhow, followed the instructions the current is now correct but the best I can do is 9.5V at the junction of R28, R29 and C40... I guess this is because the batteries measure higher than required at 18.6V

Seems to be working fine though

Regards
David
Radio_Dave is offline  
Old 4th Sep 2006, 1:26 pm   #4
mickjjo
Rest in Peace
 
mickjjo's Avatar
 
Join Date: Jul 2004
Location: Dartford, Kent, UK.
Posts: 1,661
Default Re: Murphy B815 problem

Well done David , As long as the voltage is around half the battery voltage you should be fine. If only they were all that easy, at least the batteries will last longer now..... .

Regards, Mick.

Last edited by mickjjo; 4th Sep 2006 at 1:32 pm.
mickjjo is offline  
Old 4th Sep 2006, 6:15 pm   #5
Nickthedentist
Dekatron
 
Nickthedentist's Avatar
 
Join Date: Jan 2005
Location: Oxford, UK.
Posts: 17,820
Default Re: Murphy B815 problem

Quote:
Originally Posted by Radio_Dave View Post
Thanks Mick, I think you've fixed it

I thought I'd just check the current before I started and it was drawing 2 amps
You're lucky it survived that.

Nick.

Last edited by Darren-UK; 30th Jul 2007 at 9:16 pm. Reason: Emoticons condensed.
Nickthedentist is offline  
Old 4th Sep 2006, 7:07 pm   #6
Radio_Dave
Nonode
 
Radio_Dave's Avatar
 
Join Date: Jun 2004
Location: Norwich, Norfolk, UK.
Posts: 2,543
Default Re: Murphy B815 problem

Yes I know! The two resistors that were overheating are completely black with no traces of paint left on them. I can't belive it's working
Radio_Dave is offline  
Old 4th Sep 2006, 7:26 pm   #7
G0PKH - Pete
Retired Dormant Member
 
G0PKH - Pete's Avatar
 
Join Date: Aug 2003
Location: Stansted Essex
Posts: 263
Default Re: Murphy B815 problem

Hi David.

Just a little word of caution though, you said the best could do was 9.5V at the junction of R28 and 29. Presumably this is with the adjustment pot fully at one end of its travel ?.

If this is the case it might indicate that there is perhaps a fault still present , it would be rather unlikely that the pot was meant to be in this position when the set was first built.

Sorry to be a killjoy, but just a thought. I assume you have replaced the burned resistors.

Pete
G0PKH - Pete is offline  
Old 4th Sep 2006, 10:49 pm   #8
Radio_Dave
Nonode
 
Radio_Dave's Avatar
 
Join Date: Jun 2004
Location: Norwich, Norfolk, UK.
Posts: 2,543
Default Re: Murphy B815 problem

Hi Pete,

Actually, no, I didn't replace the burnt resistors... but I will tomorrow.

I'll also check the other, associated, ones and see If I can get that voltage setting better

Best Regards
David
Radio_Dave is offline  
Old 17th Sep 2006, 8:27 am   #9
howard
Nonode
 
howard's Avatar
 
Join Date: May 2005
Location: Godalming, Surrey, UK.
Posts: 2,593
Default Re: Murphy B815 problem

Hello Dave,

Ron is working on my B815, the one which appeared in Success Stories recently but which sadly deteriorated. There was nothing wrong with the transistors in this one either (mind you I replaced one of the AF117s earlier). He has now managed to attain a quiescent current of 14.5mA, the problem being C36 the 12uF capacitor from the volume control to the AC127 base was leaky and the 10k mid point adjustment pot (R17) was short circuit, caused by an original solder joint bridging to an adjacent track. There were many other problems as well, including nuts missing from variable caps etc !

By all accounts, not an easy radio to work on, so well done fixing yours. A complete write up of the repairs Ron has done to mine will follow soon.

Howard
howard is offline  
Old 17th Sep 2006, 10:22 am   #10
Radio_Dave
Nonode
 
Radio_Dave's Avatar
 
Join Date: Jun 2004
Location: Norwich, Norfolk, UK.
Posts: 2,543
Default Re: Murphy B815 problem

Quote:
Originally Posted by howard View Post
By all accounts, not an easy radio to work on, so well done fixing yours
Hi Howard, actually the story didn't end there . I had a real nightmare trying to get the voltage at the junction of R28 and R29 to the correct 8.7V.

I changed various resistors, caps and all the transistors without any success.

I only got the settings right by changing the value of R18 from 22K to 33K .

Anyhow it seems to be working well... sound good too

Regards
David
Radio_Dave is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 11:55 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.