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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
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7th Sep 2015, 6:07 pm | #21 |
Dekatron
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Re: Murphy V136C (1948)
Looking at the circuit and reading the notes it makes you wonder who designed the V134? I guess he was locked away in the basement at Broadwater Road Welwyn Garden City for at least ten years.
It was Murphy's only TRF receiver having produced very successful superhets even before the war. Two expensive 6P28 line output valves were employed to scan a 9" tube when every other manufacturer managed to do it with ONE easily. The high impedance expensively produced frame scan coils provided a feedback winding to allow a single valve RF pentode, self oscillating frame stage to be employed, the only one I believe ever to do so. CRAZY! but it worked! Murphy never produced a model that performed poorly but at what cost? MURPHY MADNESS but great for our interest in vintage television technology. I want one and I will scream till I get it! John. |
7th Sep 2015, 10:52 pm | #22 | |
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Re: Murphy V136C (1948)
Quote:
I would like to know what the designer was smoking when he came up with this design, it certainly was not Woodbines! Mark |
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8th Sep 2015, 4:50 am | #23 |
Pentode
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Re: Murphy V136C (1948)
Hi Mark,
I'm new here, but I just wanted to say that I've just started reading your thread and I'm hooked! I hope you are able to continue making progress, and I can't wait to read more! You are definitely braver than me, that's for sure! The only thing I've ever repaired on antique electronics was to just replace filter capacitors on a couple of my old vacuum tube radios. This looks like a huge project, having to rewire everything, check each part individually to test values and check for shorts, etc. It would be a massive job. But it looks like you're making progress already. Keep up the good work! Hopefully eventually you will be able to write a post saying that you have an image on the tube! By the way, where in the world did you find that tv? I come across old tube radios pretty regularly, but I don't think I have ever seen a television this early! It seems like it would be extremely rare. And it must weigh a ton! |
8th Sep 2015, 8:11 am | #24 | |
Dekatron
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Re: Murphy V136C (1948)
Quote:
Have a look at some of our monumental restoration threads. The Murphy V134C is just a 3 valve 'American Midget' compared to some of them. Great fun restoring these. Enjoy the Forum. Regards, John. |
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8th Sep 2015, 8:48 am | #25 |
Dekatron
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Re: Murphy V136C (1948)
I must admit restoring this set is quite a daunting task, it is certainly the most ambitious project I have tackled to date. I decided that due to the condition of the rotten rubber cables, it needed be fully rewired.
Taking it one step at a time, and taking lots of photos as I go is the best approach. With the main rewiring done, each of the three subchassis will be removed and rebuilt one at a time. The worst chassis is the receiver, this has been really hacked around in the past. The best option here is to return it to the original circuit and remove any mods and replace all suspect components. This really goes against how I would normally restore a set, I would just give the chassis a visual check over for obvious problems, and slowly power it up over a period of time with a variac with my meter monitoring the HT voltage. I like to get something on the screen to prove the major components are serviceable before undertaking restoration. Mark |
8th Sep 2015, 5:38 pm | #26 |
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Re: Murphy V136C (1948)
For a long time my interpretation of the self oscillating line timebase was that one 6P28 performed the actual function as the oscillator and the other 6P28 accepted the line sync pulse to initiate the flyback. Today JohnHKS informed me that in fact the two beam tetrodes are in parallel and closer examination of the circuit diagram confirms this. Feedback to maintain oscillation is supplied to the screen grids of both valves. The control grids are for all intents and purposes returned to ground through resistors.
Somehow they managed to produce a flyback pulse at the anodes of the two 6P28s which is about the right figure for the CRM92 CRT, i.e. 5KV. Murphy Radio did suggest employing the Mullard EL38 and Mazda 6P28. It is well known that these to valves can be used as substitutes for each other but it must be borne in mind the Mullard valve is a true pentode and the Mazda valve is a beam tetrode so it is likely the valves have quite different screen grid characteristics. DFWB. |
8th Sep 2015, 6:24 pm | #27 |
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Re: Murphy V136C (1948)
I think the design problem here may have been down to the fact that Murphy Radio were signed up to Mazda leaving the much more efficient EL38 out of reach. It must have been frustrating for the design team to see much more efficient and smaller valves employed by the competitors.
1948 was the change year from mains derived EHT to flyback. The 134C was released in August and the famous Pye B18T, September. Ekco and Ultra managed well with a single 6P28 so it certainly was a bit odd. Maybe if they had gone for a separate line oscillator there would have been enough drive for a single 6P28 and would probably have cost less. All water under the bridge of course to say nothing of the bizarre cabinet! J. PS The receiver unit is TRF. It is possible that this may have been converted to a superhet IF unit to aid Band 3 conversion. This may explain the hacked about appearance. Murphy front ends were well constructed and reliable. Thinking about it the decoupling capacitors may have gone O/C allowing the entry of the core twiddler....Two possibilites J. Last edited by Heatercathodeshort; 8th Sep 2015 at 6:33 pm. Reason: Added thoughts. |
9th Sep 2015, 10:15 am | #28 | |
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Re: Murphy V136C (1948)
Quote:
To quote the Murphy manual 'A new type of "Hard" timebase has replaced the more conventional "Soft" type'. The difference between the Pye B18T and the V136C are vast, the Pye just looks so much more neat and modern. the Murphy looks as if it was designed before the war! I have a rather special B18T to restore this winter, but I doubt it will be as interesting as the Murphy A big thank you to John for a copy of the Murphy manual, it is very detailed and will greatly assist in bringing the set back to life. Mark |
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9th Sep 2015, 11:01 am | #29 |
Rest in Peace
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Re: Murphy V136C (1948)
I am always very interested in the circuits of vintage radio and vintage TV and enjoy studying them to see how different circuit functions were achieved by different designers at different times.
This particular self oscillating line timebase using two 6P28s took my interest and I looked at my pdf copy of the Trader 1035 sheet to study it. Unfortunately when it was scanned there was a discontinuity in the circuit at the vital place (see below). I wonder if I can prevail on one of you gentlemen to post a complete snippet of the line timebase circuit of the Murphy 136C ? |
9th Sep 2015, 12:24 pm | #30 | |
Octode
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Re: Murphy V136C (1948)
Quote:
The thyratrons previously used are sometimes called "soft" valves as they are gas filled, in contrast the 6P28s have a proper vacuum and are referred to as "hard". This must have been Murphy's first non-thyratron line timebase then, they really were quite a bit behind Pye (as were most of them to be fair). |
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9th Sep 2015, 6:31 pm | #31 |
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Re: Murphy V136C (1948)
Hi Colin
Here's the complete line scan circuit. |
9th Sep 2015, 7:05 pm | #32 |
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Re: Murphy V136C (1948)
Thanks very much for the CD John. We can't ask for more detailed information.
Excellent. |
9th Sep 2015, 7:10 pm | #33 |
Dekatron
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Re: Murphy V136C (1948)
If you think the line timebase is odd, take a look at the frame circuit. It uses a single RF pentode as osc/output. The frame scanning coil has four windings, one acting as feedback. The coils contain many thousands of turns of very thin wire. There must have been a fairly high rejection rate during manufacture. J.
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9th Sep 2015, 8:27 pm | #34 |
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Re: Murphy V136C (1948)
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9th Sep 2015, 10:24 pm | #35 |
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Re: Murphy V136C (1948)
We've established that the V134 and V136 were introduced in 1948 so it is most likely these models were first sets made by Murphy to have line flyback EHT.
The V120C is a development of the V114/6/8 models has flyback EHT and was first marketed in 1949. The attachment shows my Murphy V120C. DFWB. |
12th Sep 2015, 9:49 pm | #36 |
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Re: Murphy V136C (1948)
I have spent the day working on the Murphy. I firstly removed the screws holding the timebase chassis to the framework, I labeled each wire twice then cut them to enable me to fully remove it. Wiring by numbers is tedious, but avoids mistakes (hopefully!).
Every wire had to be replaced, this included removing the LOPT from it's box. Even the grommet on the LOPT box was rotten and needed replacement With the LOPT rewired and refitted, I replaced all the chassis wiring one by one. I then set about replacing all the old capacitors, a couple of resistors were also replaced. The chassis was cleaned and the new wiring loomed up neatly. Tomorrows job will be to rewire the scan coils, rear controls and terminate the new wiring. the chassis can then be refitted. This set is certainly going to take a fair few hours of my time! Mark |
12th Sep 2015, 9:53 pm | #37 |
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Re: Murphy V136C (1948)
Some more photos.
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12th Sep 2015, 9:56 pm | #38 |
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Re: Murphy V136C (1948)
The finished chassis.
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12th Sep 2015, 10:20 pm | #39 |
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Re: Murphy V136C (1948)
That's looking Great, Mark. very neat and the small tywraps do not look out of place.
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12th Sep 2015, 10:53 pm | #40 |
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Re: Murphy V136C (1948)
Hi Brian,
I am glad you approve, seeing as it is being rebuilt with bright yellow caps, it might as well have a few tywraps as well I used to work as a wireman in a previous job, funny how those skills never leave you! Removing the each individual chassis is the only way to go, the old rubber insulation just crumbles away at the slightest movement. I have replaced rotten wires before, but never had to rewire an entire set! Mark |