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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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18th Oct 2020, 10:02 pm | #1 |
Nonode
Join Date: May 2006
Location: St Albans, Herts, UK.
Posts: 2,193
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Murphy B818 Mixer/oscillator problems
Hello all.
An enforced clear out once again was interrupted by my amazingly easy to be side-tracked mind! There were several transistor radios in varying states of disrepair that had been in my to do pile for God knows how many years and this Murphy B818 was one of them. I seem to remember that I paid about a fiver for it yonks ago and I purchased it as it was the same as one that my nanna used to have which passed onto my mum after my nanna passed away. I digress. I connected it to my bench supply and it burst into pretty much no life drawing a fair bit of current into the bargain. Now most of you seasoned restorers will know that in the RF and IF stages lurk the dreaded AF117 transistors which have the habit of growing internal "tin whiskers" causing shorts, generally screen to collector. A quick fix but occasionally affecting stability is to snip the screen leads which is what I did. Power on again and there was some fairly poor reception of strong stations and the sound was very "thin" and lacking in bass. A check on the voltages in the RF/IF sections showed nothing too far out and a check on the decoupling electrolytics showed that they were all kaput. Reception was now better but the sound quality was still very poor. This was traced to being the coupling capacitor to VT4, the audio amplifier being all but open circuit. A new 10uF electrolytic in place brought forth tons of nice bassy volume. At this point I hadn't tried Long Wave and discovered that at the 1100m end, it was just picking up signals but advancing the tuning further along the dial caused the mixer/oscillator to stop oscillating. I swapped the transistor with the 1st IF amp but there was no real positive difference. I checked the bias resistors and replaced them as they were both high but again, no improvement. The collector voltage was spot on at 6.65V, base slightly up at 1.2V (should be 1.1) and the emitter also slightly up at 1.0V (should be 0.95) I found that by increasing the collector voltage, the oscillator would, well, oscillate such that more of the LW band was available. I tried swapping the AF117 with the second IF amp but again, no improvement. Is it likely that all three AF117s are faulty and should I be thinking of ordering replacements or could it be something else that is stopping the mixer/osc at lower frequencies? It does sound terrific on MW with plenty of volume and good sensitivity too. It's just the LW problem that remains. I did give it a really good clean too and I think I'm going to hold onto it after all, it's not exactly big, is it? It does sound big though! Thanks for you help.
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All the very best, Tas |
18th Oct 2020, 11:28 pm | #2 |
Octode
Join Date: Jan 2003
Location: Guildford, Surrey, UK.
Posts: 1,958
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Re: Murphy B818 Mixer/oscillator problems
Is the mixer emitter decoupler C9 0.022uF in good condition?
Ron |
19th Oct 2020, 9:20 pm | #3 |
Nonode
Join Date: May 2006
Location: St Albans, Herts, UK.
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Re: Murphy B818 Mixer/oscillator problems
Hello Ron and everyone.
A quick correction. It was the BASE bias voltage that I tried to increase not the collector. C9 was spot on but I did try replacing it just in case. No change unfortunately. Is it likely that the very low value caps C11 and C12 could be faulty? I'm clutching at the oft mentioned straws now!
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All the very best, Tas |
19th Oct 2020, 11:06 pm | #4 |
Octode
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Re: Murphy B818 Mixer/oscillator problems
The Luxembourg oscillator coil L8 is tuned by C11 and C12 in parallel. These components are shorted to ground by LUX switch SC a and b contacts when LW is selected, so C11 and C12 and L8 should have no effect on LW.
You could check emitter resistor R4 is 1k5 to make sure the AF117 is drawing enough current to oscillate and maybe C17 310pF, the LW fixed tuning capacitor could be faulty or lossy, causing the oscillator to go at the wrong frequency or run out of steam. Does the LW oscillator trimmer C16 have any effect on LW calibration? Ron Last edited by ronbryan; 19th Oct 2020 at 11:21 pm. |
26th Oct 2020, 7:37 pm | #5 |
Nonode
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Re: Murphy B818 Mixer/oscillator problems
Hello Ron.
Sorry about the long gap in replying. I've been on split shifts all week until Saturday so today's the first chance of getting back at it. I'll check the emitter resistor and get back to you regarding the other bits.
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All the very best, Tas |
30th Oct 2020, 12:20 pm | #6 |
Nonode
Join Date: May 2006
Location: St Albans, Herts, UK.
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Re: Murphy B818 Mixer/oscillator problems
Hello Ron.
I checked the emitter resistor as suggested and it was indeed high at 1.8k instead of 1.5k. With some renewed confidence, I replaced it and...the set JUST managed to reach Radio 4 but no further The C17 was checked and was found to be fine and C16 did vary the calibration of LW. All very curious. There was a clue now though in that whenever I moved the set, the tuning would vary wildly. I also discovered that the calibration on was miles out too. I hadn't noticed before as LW hadn't been working. I discovered, eventually, that the core of L11/12 was as loose as a I don't know what! I set it up correctly and the oscillator would now oscillate to almost all of the LW band and the stations were now in the correct place on the dial. It still cut off at around 1700m though! Regarding the emitter current of VT1, I tried and experiment and reduced the value of the emitter resistor to 1.2k and now we had the whole of the band available! I'm not 100% over the moon about this slight bodge and I still think that the AF117 may be faulty - it certainly is minus its screen connection - but seeing as the radio now works beautifully, and I haven't got any other AF117s or indeed any equivalents, I've decided to leave it at that. It really is a pretty little set and sounds an awful lot bigger than it looks and is a set close to my heart as it is the same as one that my Nanna had when I was a kid (along with her kitchen radio, a HMV 1131). Any thoughts as to the "faultiness" of the transistor would be welcome. Thanks again
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All the very best, Tas |
30th Oct 2020, 3:41 pm | #7 | |
Octode
Join Date: Sep 2017
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Re: Murphy B818 Mixer/oscillator problems
Quote:
Tas Have you seen this thread on AF117s? https://www.vintage-radio.com/repair...or-faults.html Mike |
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31st Oct 2020, 11:28 am | #8 |
Octode
Join Date: Jan 2003
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Re: Murphy B818 Mixer/oscillator problems
Hi Tas
Well done for fixing the set. The only other items I had in mind for checking were C8 the 0.1uf collector supply decoupler, trying an AF127 as a replacement transistor or replacing the MW/LW oscillator coil with one from scrapper. Faced with that list the 1k2 emitter resistor seems a sensible, cost effective short-cut. The set looks very smart and presentable with the bright chrome surround. A lot of the similar Bush TR130s used to be found covered in paint, as they were favoured by the decorating trade because of the 1 Watt audio power, so Howard has told me. Ron |
5th Nov 2020, 6:37 pm | #9 | |
Nonode
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Re: Murphy B818 Mixer/oscillator problems
Quote:
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All the very best, Tas |
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5th Nov 2020, 6:42 pm | #10 |
Nonode
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Re: Murphy B818 Mixer/oscillator problems
Thanks for the comments and help Ron. I have a feeling that C8 is a Philips "mustard" and although not completely above suspicion, didn't trigger me! If the radio was still in bits, I'd check it but seeing as my Ron approved bodge is working very well, I'll leave it alone!
Tas PS I had, at one time, a modern reproduction Bush TR130. It didn't sound nearly as good as the original! PPS The radio was filthy to begin with with crud and muck in all the grooves of the thumbwheel controls and a very mucky scale. As you say, the chrome work polished up vey well.
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All the very best, Tas |
6th Nov 2020, 10:45 am | #11 |
Nonode
Join Date: Jan 2004
Location: 1966-1976 Coverack in Cornwall and Helston Cornwall. 1976-present Bristol/Bath area.
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Re: Murphy B818 Mixer/oscillator problems
I bought my Bush TR130 about 25 years ago.
Got it going with a replacement AF117 and full alignment as someone had a go. I did a full write up which I have attached here. http://www.vintage-radio.com/recent-...yy1ZJiPVV0umvQ Just like all those that I saw when I was working in the trade back in the 1970's I was amazed by its performance with its gain and AF power and quality its a pure joy to listen to whether music on Absolute or speech on BBC R4 or R5L AM at its best. Despite its powerful performance battery life is also good. Over the years I have noticed that the Long Wave calibration has drifted slightly and the gain is now slightly down when compared to Medium wave but its not bad enough to investigate at the moment but I have my suspects. The Murphy version like yours are not as common as the Bush models but I think they have better and stronger cases than the Bush TR130. Forget and avoid the retro version of the TR130 they sound terrible in comparison to a decent original model and that useless tuning scale drives me up the wall.
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Simon BVWS member |
6th Nov 2020, 1:21 pm | #12 |
Octode
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Re: Murphy B818 Mixer/oscillator problems
One worthwhile TR130 mod is to change the AF coupling electrolytic capacitor which is connected to the volume control wiper, to a non polarised type. This is to minimise the possibility of the volume control becoming very noisy which occurs as a result of leakage in the coupling capacitor putting DC between the track and wiper.
A Mullard 344 series 100V polyester will fit the board exactly and just clear the metalwork. I forget the precise details, but the capacitance value I chose was slightly less than the original - with no obvious loss of bass response. My very noisy volume control was completely silenced by this component change. As others have pointed out, these sets perform unexpectedly well - and are probably worth a set of AF126s in the front end. Leon. Just looked - I fitted 3.3 uF. Last edited by Leon Crampin; 6th Nov 2020 at 1:24 pm. Reason: More info. |
10th Nov 2020, 7:03 pm | #13 |
Nonode
Join Date: May 2006
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Re: Murphy B818 Mixer/oscillator problems
Thanks for the replies Simon and Leon.
I did have one of those reproduction TR130s that you mentioned Simon. It sounded very poor compared to the original and the sensitivity although reasonable on AM was awful on FM. Of course the original was AM only. I did change the coupling cap from the wiper of the volume control as it was all but open circuit causing very weak and thin sound quality. I did use a standard electrolytic though Leon. It's back together again and sounding terrific but if I have any further problems with those AF117's tin whiskers growths, I will, as you say treat it to as set of AF126 replacements.
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All the very best, Tas |