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Old 31st Jan 2015, 9:55 am   #1
mark pirate
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Default Marconiphone VRC74DA

I recently aquired this set and am trying to find out what year it was released.
It has the infamous 1807 chassis and 10" emiscope CRT, the radio chassis looks like a T18DA from 1949, so it could be from around this era.

There is no info on the interweb, so I am hoping someone on here has some info.

I have cleaned it externally and it has polished up well with T-cut, but the top will have to be refinished due to pot plant damage.
Internally, it is covered with decades of undisturbed dust and shows no sign of being messed with.

A quick check reveals both fuses blown on the TV chassis, the two for the radio are intact.

The CRT shows heater continuity, so it looks promising.
My task this weekend is to replace the fuses and gently wake it up via my variac, starting with the radio.

If anyone has any service info, please let me know.

Mark
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Old 31st Jan 2015, 10:42 am   #2
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Default Re: Marconiphone VRC74DA

The superhet version of the VRC54 series [1807] tuned to Sutton Coldfield. Nasty to work on due to radio position and wiring.
Blown fuses usually direct you to the mains filter, 14A86 metal rectifier and the radio/television switching. The CRT 10" Emiscope 3/16 is usually as flat as a pancake but you may be lucky. J.
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Old 31st Jan 2015, 11:20 am   #3
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Default Re: Marconiphone VRC74DA

If Sutton Coldfield then probably about 1950, although the design does look something more like 1946/47.
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Old 31st Jan 2015, 1:38 pm   #4
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Default Re: Marconiphone VRC74DA

On further inspection I noticed the chassis was stamped 'Midland', so 1950 would be about right, No doubt I will find a date somewhere.
My other 1807 chassis set, HMV 2811 (12" CRT) had a date on the EHT smoothing cap. It is also a Midlands set, strange how these were both found here on the south coast!

I am hoping there is some emission left in the CRT, my other set with the 12" Emiscope is fine, it probably had little use due to the unreliable chassis

I do like the look of it, I don't think their can be many of these left.

Mark
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Old 31st Jan 2015, 1:58 pm   #5
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Default Re: Marconiphone VRC74DA

HMV's first AC/DC receiver and built without Blumlein's input, hence the rubbish design. Yes 1950 and why did they use that rather reluctant to oscillate, B7G X78 frequency changer with the cathode INTERNALLY joined to the heater. Surely EMI were not put off from drilling a larger hole in the RF chassis. We will never know. John.
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Old 31st Jan 2015, 4:03 pm   #6
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Default Re: Marconiphone VRC74DA

I acquired my VRC74 in the late eighties. It had well and truly been butchered.
A rheostat device was fitted on the cabinet side and this was connected up to the heater chain, presumably it was adjusted to compensate for variations in the mains voltage. There was also some pretty adventurous wiring to the radio receiver which in this set is a completely separate unit. The radio chassis is actually made by Plessey.
After all the circuits had been tidied up the set does work surprisingly well. Very bright CRT.

DFWB.
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Old 31st Jan 2015, 9:54 pm   #7
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Default Re: Marconiphone VRC74DA

Hi Mark
I also have one of these sets and I`ve always thought it dates from 1950.
Your set has the later crt with the evacuation seal in the middle of the base connector so it may have had a replacement crt.
This is an easy chassis to restore but the three 25k slider pots can cause problems.
The LOPT is reliable though .
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Old 31st Jan 2015, 10:47 pm   #8
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Default Re: Marconiphone VRC74DA

Quote:
Your set has the later crt with the evacuation seal in the middle of the base connector so it may have had a replacement crt.
There is a rubber cover on the bowl covering the evacuation seal, so very likely the original tube. does your set work by the way?

I carried out the usual pre power up tests and could not find any reason for the blown fuses, but had a heart stopping moment when checking the CRT for heater continuity, luckily just a poor contact on the tube base!

I first switched it to radio and wound up the variac, all valves light but no audio at all. Possibly no HT or the output tx is open circuit.
Next I switched to TV and once again wound up the volts, firstly the audio came up, followed by the line stage.
I can draw a small spark from the boost diode, but no EHT at the CRT. Line hold alters the whistle, and the tube heaters look healthy.

So it looks promising, but it will have to wait it's turn for some workshop time.

Mark
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Old 31st Jan 2015, 11:26 pm   #9
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Default Re: Marconiphone VRC74DA

I have just remembered that I got EHT up on my HMV 2811 by setting all the presets to the left and turning the focus fully clockwise, so thought it worth a try on this set.
We now have first light
So looks like the CRT is a goodun, the brightness control has no effect.
I am getting just over 5Kv of EHT. So it looks to be a viable project.

Mark
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Old 31st Jan 2015, 11:42 pm   #10
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Default Re: Marconiphone VRC74DA

Looking good mark.
My set is still on the to do pile.
Hope we can see you bring this set back to working order.
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Old 1st Feb 2015, 12:04 am   #11
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Default Re: Marconiphone VRC74DA

I will get a set of caps ordered, now I know the CRT and LOPT are ok.
These were a really unreliable and poorly designed chassis, so it will certainly be satisfying to get it working again.
We all know how bad Hunts capacitors are, apparently EMI capacitors were ten times worse, so it may work well with decent modern caps installed.

Mark
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Old 1st Feb 2015, 12:07 am   #12
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Default Re: Marconiphone VRC74DA

Mark.

You lucky so 'n' so! That's too easy so far. There's a few caps to replace though, especially on the RF deck. That's bound to be singing the wrong song due to those 1000pF de-couplers that have gone leaky or open circuit!

I have 3 of these chassis here in varying states of undress. I look forward to your progress.

Good luck.

Simon.
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Old 1st Feb 2015, 7:52 am   #13
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Default Re: Marconiphone VRC74DA

Thanks Simon, Recapping this set will be a breeze after dealing with my TV24!
That had a lot more caps to deal with, All wound through the tagtsrips
My HMV also showed great promise, but I lost EHT after fitting a few replacement caps, so I will have to get back to that soon....

Mark
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Old 1st Feb 2015, 5:16 pm   #14
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Default Re: Marconiphone VRC74DA

Quote:
Originally Posted by mark pirate View Post
My other 1807 chassis set, HMV 2811 (12" CRT) had a date on the EHT smoothing cap. It is also a Midlands set, strange how these were both found here on the south coast!
There were couple of BBC relays in the South that used Channel 4 (same as Sutton Coldfield):

Folkestone
Hastings
and Les Platons (OK, Channel Islands - at a push on 405 this might have been received).
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Old 3rd Feb 2015, 11:01 am   #15
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Default Re: Marconiphone VRC74DA

These EMI models would have worked on the South Coast from Alexandra Palace. They were certainly working in Bognor Regis but it would have taken a very good dealer plus an excellent telly engineer aided with a pre-amp, to keep these operating in such a low signal area. to The relay stations came along many years after this EMI 1950 series. Being a superhet it may have been retuned to AP. Regards, John.
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Old 4th Feb 2015, 4:03 pm   #16
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Default Re: Marconiphone VRC74DA

I have ordered a full set of caps for the set, in the meantime I will whip out the radio chassis and make a start on it.
The CRT can also be removed, as there is loads of dust and muck on it's face!
And it reduces the chance of any accidents when removing the radio chassis.

I am hoping the output transformer is not open, although the total lack of sound does point to it.
I reckon the radio was still in use long after the TV was no longer used, THAT cap has probably taken it's toll!

Mark
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Old 4th Feb 2015, 11:52 pm   #17
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Default Re: Marconiphone VRC74DA

A set well before my time in the trade! It still surprises me how many sets from the 1950's survive.
What was so bad about this particular chassis? I would have thought most sets of this era would have required fairly regular repairs, what made this chassis so notorious?

Rich
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Old 5th Feb 2015, 10:21 am   #18
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Default Re: Marconiphone VRC74DA

For a good explanation of this chassis see here: http://www.thevalvepage.com/tv/marco...3da/vc73da.htm
This is the basically the same set as mine, but without the radio.

Sets such as these present a real challenge, after all if they were all easy we would soon get bored!
Having another set with the same chassis will be a great help, bits can be swapped out or compared. I wish I had more service info, but will have to make do with the Broadcaster sheet.

Mark
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Old 8th Feb 2015, 7:50 pm   #19
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Default Re: Marconiphone VRC74DA

I finally got round to removing the radio chassis today, a really fun job
It was held in with four long bolts from underneath, a job made easier with a socket on a ratchet drive, as access is a bit awkward.
The whole operation was carried out very carefully, as the CRT is still in situ.

The chassis then refused to come out until I noticed the tone control pot extension shaft needed removing first!
It was then possible to coax it out on it's back, but not before unsoldering both the mains input wires from the fuse holders and unsoldering the speaker leads from the TV/Off/Radio switch.

This is an odd marriage of two toggle switches for power, and a rotary wafer switch (see photo).

With the radio removed it became clear that the lack of operation may be due to a wirewound resistor undressing itself!
Under the layers of dust and dead spiders, I could see that I was correct in identifying it as the same chassis as the Marconi T18DA radio I restored recently. It has some mechanical differences, but trader sheet 920 should cover the circuit.

I intend to get this on the bench and crack on with it's restoration tomorrow, at least I will be able to listen to the radio whilst tackling the TV chassis.

Mark
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Old 8th Feb 2015, 8:34 pm   #20
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Default Re: Marconiphone VRC74DA

Quite an ingenious switch arrangement, bet they never went wrong.
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