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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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4th Sep 2006, 11:24 am | #1 |
Nonode
Join Date: Jun 2004
Location: Norwich, Norfolk, UK.
Posts: 2,543
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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. |
4th Sep 2006, 11:48 am | #2 |
Rest in Peace
Join Date: Jul 2004
Location: Dartford, Kent, UK.
Posts: 1,661
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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. |
4th Sep 2006, 1:19 pm | #3 |
Nonode
Join Date: Jun 2004
Location: Norwich, Norfolk, UK.
Posts: 2,543
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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 |
4th Sep 2006, 1:26 pm | #4 |
Rest in Peace
Join Date: Jul 2004
Location: Dartford, Kent, UK.
Posts: 1,661
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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. |
4th Sep 2006, 6:15 pm | #5 | |
Dekatron
Join Date: Jan 2005
Location: Oxford, UK.
Posts: 17,820
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Re: Murphy B815 problem
Quote:
Nick. Last edited by Darren-UK; 30th Jul 2007 at 9:16 pm. Reason: Emoticons condensed. |
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4th Sep 2006, 7:07 pm | #6 |
Nonode
Join Date: Jun 2004
Location: Norwich, Norfolk, UK.
Posts: 2,543
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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
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4th Sep 2006, 7:26 pm | #7 |
Retired Dormant Member
Join Date: Aug 2003
Location: Stansted Essex
Posts: 263
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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 |
4th Sep 2006, 10:49 pm | #8 |
Nonode
Join Date: Jun 2004
Location: Norwich, Norfolk, UK.
Posts: 2,543
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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 |
17th Sep 2006, 8:27 am | #9 |
Nonode
Join Date: May 2005
Location: Godalming, Surrey, UK.
Posts: 2,593
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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 |
17th Sep 2006, 10:22 am | #10 | |
Nonode
Join Date: Jun 2004
Location: Norwich, Norfolk, UK.
Posts: 2,543
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Re: Murphy B815 problem
Quote:
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 |
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