Marconi VC76DA

[EF80TVVALVE]

Today I collected this marconi set, as it was local I couldn't resist it. As soon as I got home the panel was off and I had a look inside. It looks to have been untouched for years. A nice layer of dust and plenty of dead spiders. As can be seen the set is fitted with a Cyldon 13 channel tuner and also a Mullard MW31-16 which I have no idea if it is original or not (I doubt it is). By the looks of it the chassis has been modified in one way or another, fist thing visible is the radial electrolytic cap that sits in the centre of the chassis has dissapeard leaving behind its mount, the transformers to one end look to be different too. At the moment I'm not sure if this was a different batch to this common EMI chassis or if it has been modded to accept the Mullard tube and Cyldon tuner.

If anyone has the exact info for the 76DA then that would be a great starting point, also if anyone has one of these sets and could clear up these mysteries then we may start getting somewhere with it.

[EF80TVVALVE]

Here is a photo of the other end of the chassis, normally where the speaker and controls sit. The cyldon tuner has been unplugged so I will need to work out where the two plugs go. Also there is a valve missing on the tuner, the other is a PCF80 so if anyone knows what type should be there let me know, I don't think it was the EZ80 that was rolling around in the cabinet, suggesting this may have been fitted in place.

[FERNSEH]

The Marconi VC76DA is a development of the HMV 1807 and was designed to receive the Birmingham transmitter. It differs from the 1807 in as much it employs superhet vision and sound circuits and are close to the BREMA frequencies being 34Mc/s vision and 37.5Mc/s sound.
Some sets were fitted with the Mullard MW31-16 CRT. The first anode voltage for this tetrode tube was derived from rectifying the high voltage pulse present on the frame blocking oscillator transformer.
the Clydon tuner uses the usual PCC84 and PCF80 valves.
The IF output cable from the tuner will plug in to the socket used for the X78 frequency changer valve, that valve is at the bottom of the heater chain and pin 3 the combined heater and cathode connection goes to chassis.
The HT supply for the tuner will be connected to the RF amplifier socket.
Info about the X78 triode-hexode: http://www.r-type.org/exhib/aaa0881.htm

DFWB.

[EF80TVVALVE]

Hi David,

Thanks for the help, nice to know that the Mullard CRT is an original part. I have noticed on the Cyldon tuner that the B3 aerial socket is still covered up, only allowing the input and contrast control of band I (photographed). I have just found an old ERT manual for the Cyldon tuner in my service sheets so that should be a great help too.

Regards

[EF80TVVALVE]

Another quick one, on the topic of the rectifier. I noticed that some rectifiers inputs start in the middle, presumably using either side of the rectifier in parallel. I am going to replace this with a 1N007+resistor but wondered would I need to wire this differently to the usual method (where the input and output is at either end of the rectifier) or is this just a case of snipping the input and output and wiring it up in the usual way?

Attached is a photo.

Regards

[FERNSEH]

You might find the metal rectifier is OK. The HT rectifier in my VRC54 is still supplying the correct HT voltage. If it does need replacement the 1N4007 will be a good replacement. A 25ohm 7watt resistor should be wired in series with the diode. The forward voltage drop is much less than the original metal rectifier. Without the additional resistor the HT voltage will be too high. Just snip the AC supply to the metal rectifier, the blue lead? According to the information in the Radio and Television book the HT voltage measured at the anode of the U31 boost diode is 230V. U31, pin 5 anode, pin 8 cathode.

Is the turret tuner mounted on the side of the cabinet?

The attachment shows the power supply arrangements.

DFWB.

[EF80TVVALVE]

Thank you David, I think the original rectifier is faulty, at least the HT fuse has blown and the smoothing can has a very leaky connection where the rectifier goes to, this may be the reason why the set was retired as it looks to have had a fairly long service life. I do have a spare rectifier but I think it is safer to use the 1N4007 + resistor.

The cyldon tuner is indeed on the side panel, the original aerial socket in the rear is also still connected up.

[Halfabee]

Hi EF50,
You will find 'Trader' service sheet 974/T7 contains most of the info need to tackle this set. Despite their reputation, I think they are actually quite good 'fun' (if you like that sort of thing) to work on. A 1807 was my first 'restoration' so I have a soft spot for them.
You've lucked out finding one with a Mullard tube, the 1807 is actually capable of OK performance with a MW31-16. I guess these were fitted due to supply problems with the intended Emiscopes, of which I've yet to see a good example.
The missing capacitor on your set is the field output cathode bypass electrolytic (50uf 25v).
I would start by replacing the main smoothers (64uf+120uf), I think that one is beyond reforming! it can always be re-stuffed at a later date. Fortunately AC/DC EMIs do not have a mains filter capacitor, so no surprise bangs on first power up.
The main thing with these EMIs is not to loose heart, plod on methodically fault by fault and you will get it working.
Look forward to reading about your progress.

[Mikey405]

Hi Bren.

I'm glad you got that set. I'd been looking at it for a good while (if it's the one I think it is) but decided I didn't have the room (or the money it truth be told).

Anyway, some service info that might help can be found at the link below. (Numbers 19 and 20).

http://oldtechnology.net/documentation.html

Thanks Bren.

Kind regards.

From Mike.

[mark pirate]

Despite their reputation, I think they are actually quite good 'fun' (if you like that sort of thing) to work on. A 1807 was my first 'restoration' so I have a soft spot for them.

I have restored both my 1807 chassis sets, they actually work well once the awful waxies have been evicted (apart from the flat EMI CRT's).

I hope your wirewound sliders are OK, most of mine were O/C.
Both my sets are still running the original metal rectifiers & smoothing caps.
One set needed a replacement frame TX though.


You are lucky to have a MW31-16 fitted to your set, in fact I have a scrap TV24 with a good tube. I wonder if it would be an easy transplant for my HMV 2811?

Mark

[FERNSEH]

You are lucky to have a MW31-16 fitted to your set, in fact I have a scrap TV24 with a good tube. I wonder if it would be an easy transplant for my HMV 2811?

Hi Mark,
the HMV 2811 is electrically similar to the Marconi VC76DA. The CRT supplied with the HMV was the Emiscope 3/18, a 12" triode.
It should be possible to fit the MW31-16. Provision will have to be made for the first anode of the tetrode tube and this can be done in the same manner as the VT76DA. The service notes for sets that were modified to use the Mullard tube should be studied. It will be noted that the first anode voltage for the CRT is derived from the flyback pulse present on the frame blocking oscillator transformer windings.

DFWB.

[mark pirate]

Thanks David, as it stands the Emiscope 3/18 is low but gives a reasonable picture, it would be interesting to see how well it performs with the MW31-16 fitted.

Sadly the 10" 3/16 in my Marconi VRC74DA is the one that really needs replacing, but sadly the chances of finding a usable example is extremely unlikely

Although these sets were notoriously unreliable back in the day, with multiple design issues, both my sets can run for hours without any problem.
Neither of my sets have been converted for band III, maybe they were retired early due to low emission CRT's?

I would love to get a table model to restore, I actually have got quite fond of working on the 1807 chassis!

Mark

[FERNSEH]

Extract from the circuit diagram of /1 versions of the VC76DA.
The first anode voltage for the MW31-16 CRT is derived from rectifying the high amplitude pulse present at the anode of the line oscillator valve.
The additional components are C58 (0.015mfd) and the selenium rectifier diode W4. The CRT first anode is connected to the junction of the new components. An UF4007 fast silicon diode would be a suitable replacement for W4. Note that the symbol for W4 follows the convention for diodes at the time the circuit diagram was drawn. (1950)

DFWB.

[Halfabee]

I actually have got quite fond of working on the 1807 chassis!

Hi Mark,
Yes they do have a strange allure about them.
Now you've restored a couple you will doubtless be tempted to collect the complete 'set': 1807, 1814, 1824, 1840 and, if your lucky, the rare (I've never actually seen one) printed circuit chassis.
Unfortunately they all suffer from the same fatal flaw... those pesky Emiscope tubes, which must have condemned many to a premature scrapping. Also, the cabinets on later models are a bit naff compared to the 'classic' EMI design.

David,
Interesting point about the diode symbol. Conventional vs electron flow used to confuse me no end as a lad!

Cheers
Eric.

[EF80TVVALVE]

Hi everyone,

Apoligies for not replying to everyone sooner but I haven't been around for the last few days. Mike it is indeed the one you are thinking of, the second hand shop is local and I have met the chap before so after some conversing a deal was struck, truth be told I have always wanted one of these sets to work on but the trouble with the CRTs means I hadn't came across one in good enough shape to warrant restoration due to duff tubes.

Mark, I am hoping those wirewounds are good too, although I do have two scrap chassis so I'm sure a full set can be made up if not.

I have started changing some caps, mainly the crumbly hunts ones as they were in such bad shape that I didn't even want to power the set up without changing them first. I have found a nice 50uF 50v cap from a scrap chassis I have, I have also restuffed this and this will go back onto the top of the chassis where it belongs.

The next thing is to sort out the smoothing electrolytic and then we shall see if the set responds to a slow waking up and with the replacement of all those tiny .001 caps the majority of the tedious work is out of the way.

[mark pirate]

Looking good, both my sets still have the original EHT smoothing cap in place, they seem to be a good quality component.

Looking forward to seeing it brought back to life.

Now you've restored a couple you will doubtless be tempted to collect the complete 'set': 1807, 1814, 1824, 1840 and, if your lucky, the rare (I've never actually seen one) printed circuit chassis.

I did spot another Marconi VC76DA at a reasonable price recently, but as usual too far away

I am running low on space, so the weenie table top set would be ideal....


Mark

[EF80TVVALVE]

Hi everyone,

Been a while since I posted about this set. It has now found its way on the bench and I spent most of the weekend on it. I have given it power and found a fault, the output transformer seems to have a short, it is getting very hot and oozing pitch. Now, I have a spare RF chassis from an earlier model, this one has an output transformer at the end which is mounted in the usual upright position, it is stamped with the numbers 22628BM, the VC76DA has a sidwards mounted type as pictured, the numbers are now indistinguishable. Would this be suitable for replacing the type on the VC76DA? It looks to be from a VT53A or similar so would have been powering a mains energised speaker, unlike the VC76DA.

[Ed_Dinning]

Hi Brennen, if the output transformers are from similar valves you may get away with it, if not a re-wind would be a fairly straightforward job.

Ed

[EF80TVVALVE]

Cheers Ed,

I'll take a look, as far as I know it is the same valve, if not then a rewind sounds like a good option.

Cheers
Bren

[Halfabee]

Hi Brennan,

The replacement output transformer will be just fine, both sets use an EF91 audio output valve.
Before changing the Tx please check that there are no shorts to deck causing it to overheat, either a shorted C35 (0.0022uf) tone correction capacitor, or an internal anode to suppressor grid short in the output valve itself.

Best of luck,
Eric.