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Old 8th Feb 2015, 9:02 pm   #21
mark pirate
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Default Re: Marconiphone VRC74DA

Hi Stephen, Yes it is quite clever. Both toggle switches are four pole and are activated by a cam activating a slot in the toggle, the rotary wafer switch controls the switching to the speaker.

I found it easier to remove the toggle switch to the radio, rather than unsolder it.
The radio should be fairly straightforward (famous last words!) to restore, I have got my order of caps in, so it is full steam ahead

Mark
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Old 10th Feb 2015, 12:21 am   #22
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Default Re: Marconiphone VRC74DA

The radio chassis is now out of surgery
Working from the trader sheet for the Marconi T18DA, which is electrically identical, it was quite straightforward.

The radio has never been touched before, it has all it's original Marconi branded valves still in place, just as it came out of the factory.
I thoroughly cleaned it, replaced all the waxies and the 100 ohm surge resistor.
All valve sockets, switches and pots were cleaned. The valve pins were also cleaned.
I soldered on a test speaker and temporary mains lead and powered it up, it rewarded my efforts by working perfectly!
It has been happily playing all evening, it should sound even better when connected to the nice big speaker in the set.

I need to remove the dial for cleaning, and lubricate the tuning pulleys.
Unlike the T18DA this does not have a scale light, so I may fit one as wired in the T18DA circuit.
Or I could use one of the 8 watt 240v bulbs I have in the workshop, these are physically small and could be easily mounted in the cabinet and wired to the radio's switch.

I need to paint the aperture for the tuning dial before I fit it back in the cabinet.

The next job will be to remove the CRT assembly for cleaning, then on to the TV chassis itself, which is certainly not going to be as easy as the radio....

Mark
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Old 10th Feb 2015, 7:49 am   #23
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Default Re: Marconiphone VRC74DA

Looking good so far Mark.

Interesting to note that those waxies are Dubiliar rather than EMI's own manufacture.


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Old 10th Feb 2015, 10:08 am   #24
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Default Re: Marconiphone VRC74DA

Apparently the radio chassis was manufactured by Plessey, although the valves are of Marconi manufacture.
All of the electrolitics test fine, the radio is pumping out Smooth Radio as I type.
I will decide how to retrofit a dial lamp. I suppose it was decided not to include one, as it would be very difficult to gain access for replacement. It also shaved a few pennies off the manufacturing cost!

The real challenge lies with the TV chassis. I will probably start with the RF deck, as it is full of those nasty little brown Hunts caps that crumble away on touch!

The Z77 valves were notorious for shorts that blew the set's fuses, these can be replaced by the Mullard EF91 or 6AM6.

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Old 11th Feb 2015, 1:10 am   #25
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Default Re: Marconiphone VRC74DA

Today I removed the CRT to give the mask and safety glass a proper clean.
Removing it was pretty easy, as it was all glued together by the mask. In fact it took a fair bit of gentle teasing to free the mask from the CRT, luckily it came free with no damage and cleaned up well.

Fitting it all back into the cabinet was another matter
I found the easiest way was to lay the set face down, drop in the safety glass followed by the mask. then push the CRT into the mask, it is only held in place by a canvas strap and a couple of springs.

The mask has squashed down over the years, so it was difficult to get enough tension to hold it in place securely. I managed to get it to fit pretty well, only to discover some muck had got between the CRT face and the glass, so it had to come out again!

All in all this took me three hours, but it looks so much better for it

Next up will be to remove the TV chassis and clean it up, then tackle the capacitors and test the valves.

Mark
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Old 11th Feb 2015, 9:46 am   #26
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Default Re: Marconiphone VRC74DA

Makes it look so much better with the face and CRT clean, looking forward to the chassis restoration write up.
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Old 11th Feb 2015, 10:09 am   #27
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Default Re: Marconiphone VRC74DA

It does look so much better for cleaning, I love the rear view of these Emiscope CRT's. Lovely 'Chrome' finish you can see your face in
I am pleased that this job has been completed, I am not too keen on handling CRT's. This one is a lot less scary that the 12" version in my HMV set!

It was a wise move to do this with the radio chassis out of the set, as access would have been very difficult and fraught with danger.

Seeing as the weather looks dire on Friday, I will dedicate the whole day to getting the chassis restored, and hopefully working to some extent (i hope).

Mark
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Old 12th Feb 2015, 10:08 pm   #28
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Default Re: Marconiphone VRC74DA

A bit more progress today, I have removed the TV chassis and given it a close inspection.
There is a few signs of some repairs done long ago, three of the valves are replacements. I think the audio output transformer is a replacement, but all the caps appear to be original.

I have noticed a broken resistor on the RF deck, the top cap on the EHT rectifier has come away from the valve and five of the valve screening cans are missing.

Tomorrow I will start replacing those nasty little Hunts mouldseals, I can see at least 18 of the crumbly little blighters!
I will completely recap the RF deck, test all the valves and check/replace the resistors.

I will then do a power up test to note any improvement. Then to tackle the main deck. I want to do this one cap at a time and log the results, as it will be of help when I get back to my HMV 2811.

Here are some photos to show what I will be dealing with!

Mark
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Old 14th Feb 2015, 12:01 am   #29
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Default Re: Marconiphone VRC74DA

The RF chassis restoration has taken up most of the day. I decided it would be easier to remove it from the main chassis, as it only means unsoldering half a dozen wires.

Apart from recapping and cleaning as I went, there were quite a few unforeseen problems to deal with.
The new capacitors are physically larger than the grotty little Hunts, so it made the job a bit more difficult.
Great care was taken when desoldering, as the valve holders are small and fragile.

Under chassis work went without a hitch, but was a bit tedious as I went through nearly a whole box of cotton buds cleaning away greasy black grime and lumps of gooey fluff

The topside of the chassis was even worse, not helped by catching a valve with my sleeve, which merrily rolled off the bench and smashed!
I managed to find a replacement resistor for the broken dogbone, but was further miffed to find the odd half moon wirewound contrast pot was open circuit, due to the mounting panel being loose the windings had got damaged by lugs on the chassis.

Luckily I had a replacement in the 'stores'. Fitting it was a little tricky, as I had to use a small file to elongate the mounting plate.

I ended up replacing all the caps, bar one electrolytic which tested fine.

With the RF deck now finished, it was refitted and wired in to the main chassis. It will require a couple of Z77 valves replaced, as they read low on the tester, I think I have a couple of EF91's in the workshop somewhere.

The set was powered up, and after a fiddle with the channel switch on the Aurora, I now could hear the test tone.
Nothing on the screen though, but shorting grid to cathode on the CRT proved that the screen still lights.

I will recap the main chassis over the weekend, so will hopefully get it closer to showing a picture.

Mark
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Old 14th Feb 2015, 10:27 am   #30
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Default Re: Marconiphone VRC74DA

Coming along well Mark, that dogbone resistor looks well burnt.
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Old 14th Feb 2015, 2:26 pm   #31
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Default Re: Marconiphone VRC74DA

Hi Mark,

So far so good
It's certainly inspiring me to knock the dust off my VCR74D and get it restored although mine has already been "got at" in the past so it's going to take a little more sussing out than it should really

Keep up the good work

Marc.
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Old 14th Feb 2015, 6:51 pm   #32
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Default Re: Marconiphone VRC74DA

I have just completed the recapping, the only originals are the main smoother and three smaller electrolitics.

There has been no improvement though, in fact the EHT has dropped from 6.5 to 4.5kv! I just tacked in a new EHT smoothing cap, but it has made no difference.

Sound is working, but I still get nothing on the screen.
Shorting grid to cathode still give the same half inch line I have had from the start

HT voltage is fine, so I now need to go fault finding.
Any thoughts?

Mark
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Old 14th Feb 2015, 7:46 pm   #33
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Default Re: Marconiphone VRC74DA

Thats slightly disappointing after all the hard work.

How does the frame timebase respond to the height & vertical hold wirewound sliders on the back? I had problems with these on mine, both tracks being open circuit.

Also check the double triode - if its an original 'Marconiphone valve' (B36) it is certainly worth swapping with another. If it's not original, make sure that it is a suitable equivalent eg. 12SN7GT.


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Old 14th Feb 2015, 8:14 pm   #34
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Default Re: Marconiphone VRC74DA

Hi Simon, I am getting a feeling of deja vu with this set, I had similar problems with my HMV 2811, although the EHT all but disappeared

Quote:
How does the frame timebase respond to the height & vertical hold wirewound sliders on the back? I had problems with these on mine, both tracks being open circuit.
I will check these, as they are notorious for wearing out. They do seem to be operational though, as the EHT varies when moving them. The focus pot also varies the EHT, as this is how focus is achieved!

The B36 had already been replaced by a 12SN7GT. I will test the valves after dinner, I can also try those from my HMV.

I have already replaced a couple of low Z77's with EF91's, but it has made no difference.
At least I can get sound from the Aurora now, it was totally dead before I recapped the RF deck.

Looks like I have a way to go before I see test card C.

Mark
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Old 14th Feb 2015, 9:04 pm   #35
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Default Re: Marconiphone VRC74DA

Whats the cathode voltage on the tube ?
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Old 14th Feb 2015, 9:18 pm   #36
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Default Re: Marconiphone VRC74DA

With my DMM it is measuring 108v.

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Old 14th Feb 2015, 9:48 pm   #37
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Default Re: Marconiphone VRC74DA

Capacitors getting leaky aren't the only problem with aging components. High-value resistors have a tendency to go even-higher value. You can usually test resistors in situ in a valve circuit, as a cold valve is effectively open-circuit between any pair of electrodes. Replace any that are out of tolerance (gold = ±5%, silver = ±10%, no tolerance indicating band = ±20%); and if you encounter any dry joints using the test probes, re-solder them.
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Old 14th Feb 2015, 10:17 pm   #38
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Default Re: Marconiphone VRC74DA

There are a couple of resistors that have gone up in value, but not high enough to stop the set working, although they will be replaced.

I have just tested the valves, the 12SN7GT (B36) tests low on one side, the KT33C is also reading low. both the Z63 and U31 test good, as does the KT36.

I will borrow the valves from my HMV and try these in the tester tomorrow.

Mark
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Old 16th Feb 2015, 10:21 am   #39
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Default Re: Marconiphone VRC74DA

After testing and set testing the valves from my other 1807 chassis set, I still have got no further with getting anything on screen.
I have removed the rear preset control panel to find the horizontal hold slider is shot, the width slider is intermittent at one point in it's travel.
There is no way they can be repaired, and they were c**p when new.

So I have decided to replace all four with decent wire wound pots, I will see what I have in stock and just lash them in for now.
I can make up a paxolin panel to fit them properly later.

I have gone through all the resistors on the main deck, there are a couple that have risen in value, but not high enough to stop the set working, although I will be replacing them as a matter of course.

If anyone has the trader sheet for the 1807A, or the manufacturers service info, please let me know.

I am determined to get this set working one way or another!

Mark
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Old 19th Feb 2015, 5:05 pm   #40
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Default Re: Marconiphone VRC74DA

A bit more progress to report, I found an ideal mounting plate in the junk box.
With the old sliders removed, I fitted the plate over the paxolin panel and drilled some fixings for the plate, once secure I drilled out the paxolin behind and fitted the pots.

I have only fitted three 25K pots, as I don't have a replacement for the 250 ohm width control. The pot's I fitted are carbon type, not wirewound. Not sure if this really matters?

The EHT is now variable by the line hold pot, the vertical hold pot has no effect on what is on screen, neither does the height pot.
The brightness pot is now working though!

Feeding a live signal via the Aurora gives sound, but does not appear to alter what is on screen, there also seems to be vision on sound, this can be reduced by the contrast control.

I am now trying to sort out the lack of frame output, it does not look like total collapse though.

Mark
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