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Old 12th Jul 2020, 6:54 am   #1
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Default Murphy A816 / Acoustic Deluxe

This thread, https://www.vintage-radio.net/forum/...highlight=A816 is closed. I was wondering if there was a solution to the drop resistor overheating problem?

I have one of these in white, in pristine shape, in NYC. I only ever turned it on 2-3 times because, while it works, a few minutes in, smoke and smells arise from the resistor and I immediately turn it off.

While I wait — hopefully — for a response and solution, here is how I managed to get it. I was actually looking for the color version of the same design but, after a few years, only the A816 turned up on UK eBay. Problem was it was at a girls’ school, not far from Gatwick, and they would not ship it. As it happened, a few weeks later, I had a business meeting in Prague. On my way back to New York, I arranged for a short stopover in Gatwick. I only had a few hours to rent a car, go pick up the telly, find materials, bubble wrap it in a double-box package and check it into my flight. Well it worked out, I even had time to explain to the sisters at the school that I was not really insane (a small donation helped). In fact the hardest part was sneaking this, my — perhaps — 20th TV into our apartment without my wife noticing it.
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Old 12th Jul 2020, 7:26 am   #2
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Default Re: Murphy A816 / Acoustic Deluxe

You were really determined to get the set.

If this is the large wire wound resistor at the top of the chassis they do get very hot. Only way to be sure there is not a fault is to do some testing. Dust and a little dampness are usually the cause of the smell and smoke if there isn’t a fault and burns off after a little while.
At the age of the set there probably will be some items that could overload that resistor and require replacement.

UK sets used the ”Live” chassis technology to drop voltages around the set to suitable levels, unlike many USA sets that used transformers to do a similar job, that’s what the big resistor is doing,

Unless you feel capable to do the testing it may be worthwhile getting it verified by someone who can. Techniques used between USA and UK sets differ but there are many on this forum who can give advice.
This type of chassis requires you understand the dangers of a “Live” chassis.
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Old 12th Jul 2020, 7:43 am   #3
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Default Re: Murphy A816 / Acoustic Deluxe

Quote:
Originally Posted by Nuvistor View Post
Unless you feel capable to do the testing it may be worthwhile getting it verified by someone who can. Techniques used between USA and UK sets differ but there are many on this forum who can give advice.
This type of chassis requires you understand the dangers of a “Live” chassis.
Thank you, I am generally familiar with this matter and in fact have an isolation transformer for the purpose.

I just found the combination of smell and smoke alarming. I did have a few sets — in particular two beautiful French Continental Edisons — which shortly after such smell / smoke went dead. Now they are just decoration, at least until I find schematics which, for French TVs, seems to be very difficult.
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Old 12th Jul 2020, 8:18 am   #4
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Default Re: Murphy A816 / Acoustic Deluxe

I haven’t a circuit to hand but some items that cause overload are the large reservoir and smoothing electrolytic capacitors in the power supply, shorts on the HT line, other PSU components.
HT rectifier diodes but those usually blow the fuses or cause other faults that stop the set working.
Standard fault finding techniques should sort out the problems. I must have worked on this chassis we were RBM dealers but I can’t remember, too long out of the trade.

Others will be along with more useful advice.
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Old 12th Jul 2020, 10:41 am   #5
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Default Re: Murphy A816 / Acoustic Deluxe

When you say that the set works is the picture normal size and stable? Any picture faults like hum bars? If all is OK there most probably isn't an overload.
It would be an odd fault if the set is not showing any other symptoms but is cooking the resistor.
Run it for a while and then check for any warm capacitors check the LOPT temperature etc..
If no other obvious problems I would run it while watching the dropper, obviously if it shows definite signs of overheating or any other signs of distress switch off. You may find a few runs extending the run time a bit each time will dry out the dropper or burn off the dust/ coating?
It goes without saying to keep a constant watch on it while switched on!

I recently replaced a dropper with a new old stock one in a GEC the new dropper smoked and smelt for quite a while. enough for me to open a window and watch it constantly but after around 10 minutes all was well.

The Rank monochrome sets after the A640 were not the best IMHO, enough has been said about the A774. This chassis did work better but wasn't much more reliable I didn't see that many because they weren't that common around here, the Bush dealer was fairly small and I am not sure the Co-op used this chassis in their own brand mono sets. I remember changing a few LOPTs in them but other repairs are long (and probably best) forgotten...
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Old 12th Jul 2020, 4:15 pm   #6
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Default Re: Murphy A816 / Acoustic Deluxe

The original Design Aim Specification for the A816 called for a thyristor based power supply and several were built in the lab. Sadly, the bean counters said it was too expensive, so it was replaced by that big dropper. None of the engineers were happy about it, but we were all overridden.
To this day I still find it sad.
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Old 12th Jul 2020, 10:52 pm   #7
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Default Re: Murphy A816 / Acoustic Deluxe

Thank you for the responses. I had to look up LOPT, huh, it means flyback transformer as they call it here.

Frankly, I have not dared to keep the TV on long enough to check what happens with the picture — until recently, I had to first readjust the oscillators on my European sets to get a decent picture with 60Hz line frequency (and use a US-standard signal over a UK or continental modulator) but now I have a good 50Hz source, so I will not need that trick.

So the dropper works on the DC side? I have a partial schematic and have not yet figured out what voltages come out of it. Maybe eventually I can fashion a better power supply.
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Old 13th Jul 2020, 9:16 am   #8
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Default Re: Murphy A816 / Acoustic Deluxe

The A816 is a high performance all transistor model that was quite expensive in the UK and must have been expensive to manufacture.
When you switch on you should hear the rushing sound from the loudspeaker and the tube heaters should be visible. If this is the case, let it warm up and see if you have screen illumination. The flyback transformer is reliable unlike the earlier A640 chassis.
They do give very high quality pictures but did not sell in huge numbers due to the colour TV explosion around this time. John.
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Old 13th Jul 2020, 5:07 pm   #9
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Default Re: Murphy A816 / Acoustic Deluxe

I didn't see many but in my case the ones I did see had faulty flyback transformers. It's an unusual circuit, being a little like the colour Z718 with the HT regulation being carried out 'under' the line output transistor, if you see what I mean.
Probably the only set with a customer switchable filter to stop chroma patterning.
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Old 14th Jul 2020, 10:24 am   #10
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Default Re: Murphy A816 / Acoustic Deluxe

It's strange how certain receivers give trouble in certain areas. Apparently the A816 was very popular in the North East. I didn't see many of these in South London and I can't remember why they were booked in for service but it wasn't the LOPT. It sounds as if the LOPT was as bad as the 640. John.
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Old 14th Jul 2020, 10:32 am   #11
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Default Re: Murphy A816 / Acoustic Deluxe

I remember on the first one I saw going round in circles until finally realising it was the LOPT. Like John, we didn't see many and customers didn't really want to spend as a colour TV was their priority. Good pictures, though.
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Old 14th Jul 2020, 8:59 pm   #12
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Default Re: Murphy A816 / Acoustic Deluxe

Hi,

That original thread was mine, and yes, at first I was alarmed at the temperature of the dropper on my example. However, after measuring the voltage drop and doing the sums it became apparent that this is quite normal operation. The smoke will almost certainly be dust burning off and will eventually cease.

That's some story regarding the aquistion of your set! I thought I'd been adventurous going to Scotland!

Best Regards,

Stu
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