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Old 9th Aug 2017, 8:40 pm   #1
hamid_1
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Default Murphy V2423 - A774 Chassis

Murphy V2423 - A774 Chassis

This 24 inch monochrome set was made in December 1972 and acquired from Mikey405's legendary vintage telly bash event in May this year. It came in a vanload of old tellies brought by fellow collector neil29 . The tellies were unloaded from the van and piled up on Mike's drive (see pic), then some of them were powered up, including the above mentioned Murphy.
Attachment 142567
Initially, the valve heaters glowed but nothing else happened. Replacement PL504 and PY88 valves were fitted, but during this operation the wire from the PL504 top cap broke off and was reattached. The set was powered up again. This time there was some sound, but no picture or EHT and after a few seconds, smoke came off a component attached to the line output transformer, then the sound went off. The set was quickly condemned.

These sets were notorious for faulty line output transformers and many other problems. Back in the late 1970s our family TV was a 20" Murphy A774. I've wanted to get hold of another one for nostalgic reasons. I remember all the great programmes I watched on it, but I also remember it was unreliable. Sometimes I missed my favourite show because it had broken down. Very frustrating. My memories of the A774 are bittersweet.

Nonetheless I was keen to take on this rather unloved A774, hoping it could be saved. There were some encouraging signs. First, the line output transformer appeared to be a later replacement, with a stick EHT rectifier instead of a DY802 valve. Secondly, there was some sound from the speaker, albeit briefly. The sound stage gets its power from the LOPT so I thought the LOPT must have a little bit of life left in it.

First of all I downloaded a copy of Television magazine July 1977 from http://www.americanradiohistory.com/...n_Magazine.htm

This has an article on servicing the A774 chassis including a circuit diagram and common faults. With the help of the schematic, I found the wire to the PL504 top cap had been reattached to the wrong tag on the LOPT. After fixing this, I powered up the set. No more smoke, instead EHT and brightness on the screen for the first time. But the raster looked squashed and didn't fill the screen. I then tried to apply a signal to the set. At that point, I discovered the tuner didn't work. It was stuck on UHF channel 31. The channel change buttons had no effect.

Ignoring the tuner fault for the time being, I set my RF modulator to channel 31 then connected my test card generator (actually a digital photo viewer with an image of the test card loaded onto it). The picture was squashed with bad vertical linearity. I tried to adjust the height and v.lin controls but this was the best I could get.
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Consult the TV magazine article. It suggests replacing the PCL805 valve and its cathode capacitor 220uf 25v (3C18). Tried that, but no fix. In the end, I found the fault - capacitor 3C17 0.015uf 1000v DC / 350v AC connected to the PCL805 anode. I used a 0.02uF 630v as that was the nearest value I had.
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Now I could set the height and linearity correctly.
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That's as far as I've got. I'm pleased to have obtained a reasonable picture on the set, but there are more faults. The tuner is still stuck. Another fault - the brightness suddenly increases after the set has been switched on for a couple of minutes. The brightness then remains stable. Any ideas on that one? Also, I can't see any arcing, but for a short time after switching on, there is a slight hissing sound and a smell of ozone - it stops once the set is warm. I wonder if the LOPT is about to go pop
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Old 9th Aug 2017, 9:46 pm   #2
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Default Re: Murphy V2423 - A774 Chassis

Hi Hamid, I'm glad to see its working so far provided the lopt holds out. and i never even powered the set up so wouldn't have been treated to the smoke or arcing that you experienced. good luck with it .

Cheers
Neil.
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Old 9th Aug 2017, 9:48 pm   #3
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Default Re: Murphy V2423 - A774 Chassis

If you can smell ozone, turn out the lights carefully look for the blue haze around the discharge. Very often the DC side of the EHT system.
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Old 9th Aug 2017, 10:53 pm   #4
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Default Re: Murphy V2423 - A774 Chassis

That's great news Hamid. Well done! It's a great looking set and the crt looks nice and bright. I always liked the A774 and used to buy them in the 80s from the 'reconditioned tv' shops. I always thought the pictures were really good compared to BRC1500s etc.

The 24" Indesit b&w which was also powered up on Mike's driveway is alive and well too. I'll have to start a thread on that in due course.

Lets hope your LOPT holds out!

Steve
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Old 10th Aug 2017, 7:17 am   #5
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Default Re: Murphy V2423 - A774 Chassis

Hello Hamid,
I'm really pleased you managed to get that A774 operational.

You may well find that the tuner 'nuts' have disintegrated. I have posted a picture of the offending part. I have quite a few spare if you need some replacements FOC. They have been sitting asleep in the RBM draw since 1971.

I had serious troubles with the A774. The first models delivered were the Bush TV181SS. All six had lost their line sync after 30 mins. This was due to CRT flash over destroying the line sync diodes. A 1k resistor in series with each one was the official mod.

Within a week of sale the LOPTs had all failed [overwinds] and the replacements followed a similar path. These were all replaced by RBM under guarantee but it was a very bad mark for a brand that had gained such a good reputation. It was a while before improved transformers were available and by this time the printed circuit 'lands' were beginning to take off.

The transformer was later modified replacing the valve EHT rectifier with a 'stick' but this did little to improve reliability.

The last production transformers did away with the overwind replacing it with a doubler arrangement. If I remember correctly this occasionally flashed over but was a big improvement on earlier attempts.

The picture quality was also very poor on unpacking from the factory. It was very 'milky' and a massive let down from all previous chassis in the RBM range. The TV181S the previous model was fitted with the single standard version of the A640 [A793?] so you can imagine the shock when you switched on the first delivery of the A774!

A big improvement could be obtained by advancing the video drive pot until sync crushing just became evident then backing it off slightly. This considerably improved picture quality but RBM very much frowned upon this action referring to the procedure published in the service manual.

The remaining sections of the chassis were reliable with a very stable line oscillator employing a single EF184, a very odd choice.

To be honest customers, as always, did not notice the rather tired pictures probably due to the fact that they viewed very few of them between LOPT failures!

The A816 all transistor big screen chassis was excellent but very few of these appeared in the London area. It was very expensive and colour was making big inroads by 1971/2 making it obsolete before it's time..

Let me know if you would like a set of new 'nuts'.

Regards, John.
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Old 10th Aug 2017, 9:43 am   #6
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Default Re: Murphy V2423 - A774 Chassis

We had loads of A774's out on rental in the mid 1970's. Not my favourite set and certainly not the most reliable of chassis. But we did see a few decent ones and they kept us busy.
Yours looks to have a very good CRT with a good cabinet so is well worth the effort.
As well as LOPT failures we changed a lot of CRT's which seemed to be driven hard. PCL805's seem to have a high mortality rate, on-off switches also seem to fail on a regular basis this was often down to the mains filter capacitor either being snipped out or going open circuit. Those plastic tuner nuts splitting were a common fault on this tuner also used in the RBM A823 colour chassis.
Finally make sure all the earthing lugs from the PCB to the chassis are well soldered otherwise you can end up with all sorts of strange faults from poor line sync to hum bars. If I remember correctly the official mod from RBM was to wire up all the lugs using lengths of insulated wire. Connecting the outer lugs together then the inner lugs.
As I said we did see a few of these that worked really well but on the whole we felt this set was a bit of a let down compared to the excellent RBM A640.
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Old 10th Aug 2017, 12:12 pm   #7
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Default Re: Murphy V2423 - A774 Chassis

The most popular version of the A774, at least in my shop was the Murphy V2027 which was a 20" CRT model offered in either white or teak cabinets.
To be honest I didn't like these sets. During the seventies I had loads of A774 sets out on rental in various cabinet presentations and although the sets seemed to be well liked by the customers the reliability was not good. Every day there would be at least one set needing repair and it was usually the line output transformer. The customers weren't bothered how unreliable the sets were, after all apart from paying the weekly rentals all the servicing was in with the deal, and the repair jobs were done in a matter of hours from receiving the request for service.
I doubt if any money was made from renting these sets.
The last of the A774 sets was cleared out in the early eighties.
In 1985 after the TV licence fee was increased there was small demand for black and white TVs and as I had no Bush and Murphy mono TVs in stock a number of Rediffusion Mark 13 sets were acquired and refurbished.

DFWB.
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Old 10th Aug 2017, 8:16 pm   #8
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Default Re: Murphy V2423 - A774 Chassis

Good job Hamid.

Brilliant write up. A very nice looking set!

Cheers

Matt
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Old 10th Aug 2017, 9:09 pm   #9
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Default Re: Murphy V2423 - A774 Chassis

A lovely job. I hope I too find a set to to restore.

Regards, Robin.
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Old 21st Oct 2017, 6:08 pm   #10
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Default Re: Murphy V2423 - A774 Chassis

I wish I had a quid for every set of tuner cams I changed in these units. When I was serving my 4years apprenticeship with the coop, this was one of my jobs - changing the cams, and boy did I change some. The A774 was one of our main mono sets and we still had a few of the COOPs own brand 'Defiant' (Bush) dual standards on the go - this was back in 1977. Then as I got better at what I did I ended up being the No1 repairer of the BRC 3000/3500 chopper boards - only because nobody else liked to do them happy days though
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Old 23rd Oct 2017, 11:19 pm   #11
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Default Re: Murphy V2423 - A774 Chassis

You've reminded me - I forgot to post the follow-up to this story.

I opened up the tuner mechanism and sure enough, the plastic 'nuts' had broken (see attached photo), causing the tuner to be stuck on one channel. John (Heatercathodeshort) very kindly sent me some spare parts. I was able to replace the broken plastic parts and make the tuner work again, though it was a bit of a fiddly job. I removed each of the threaded bars, removed the metal end stop (a bit like a circlip but not designed to be removed and replaced easily), then screwed the new plastic part onto the bar, replaced the end stop, repeat this for all 4 bars and finally reassemble.

There may have been a quicker method, but at least the tuner is fixed now. Thanks to John for sending me the replacement plastic parts as seen in post 5.
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Old 25th Oct 2017, 6:31 pm   #12
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Default Re: Murphy V2423 - A774 Chassis

Tip-top job Hamid. Well done getting that going. Regarding replacing those plastic thingamajigs on the tuner, a mate of mine used to just break the old ones off (if they weren't broken already) and then clip the new ones onto the shaft. It used to bend (and presumably stress) the plastic wotsits to a great extent but they didn't seem to break. I've never had the nerve to try it - and I normally do what you did. Very fiddly.

Kind regards.

From Mike.
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Old 25th Oct 2017, 7:23 pm   #13
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Default Re: Murphy V2423 - A774 Chassis

That is exactly how I used to do them Mike!

Our good friend Hamid is very careful with his work and would do the job far more professionally than my ham fisted methods.

Glad you got it sorted Hamid! Regards, John.
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