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Old 17th Mar 2021, 11:41 am   #61
FERNSEH
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Default Re: Marconi VC76DA

Lookout for an Advance E2 signal generator, an ideal instrument for the alignment of vintage radio and TV receivers.
https://www.vintage-radio.net/forum/...d.php?p=998629
From the Valvepage website: https://www.thevalvepage.com/testeq/advance/e2/e2.htm

The IFs in the VC73DA are 34.5Mc/s vision and 38Mc/s sound. Align the sound first.

DFWB.
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Old 17th Mar 2021, 12:43 pm   #62
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Default Re: Marconi VC76DA

Thanks for the recommendation, I'll keep a look out for one and post a wanted, I feel its much needed as I'll just be going round in circles attempting to get it right with just the Aurora. I have the full Marconiphone service data which includes the alignment procedure so I should be good to go once I sort a sig gen out.

Cheers
Bren
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Old 17th Mar 2021, 12:56 pm   #63
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Default Re: Marconi VC76DA

Circuit diagram of the line output stage of the HMV 1807 series.
Apart from the inclusion of the focus control in the grid circuit of the KT36 line output valve the rest of the circuit is similar to line output stages in other makes such as the GEC BT5144/5. In other sets the boosted HT is negative and supplied to the cathode of the line output valve.
The focus control arrangement isn't very nice but varying the EHT voltage was one method used in early sets to provide fine adjustment of the focusing, an example is the pre-war Cossor 1210.
Along with the grid to cathode capacitance of the KT36 the focus control can vary the shape of the flyback portion of the waveform and thus the rate of change of the flyback pulse without effecting the forward part of the scanning waveform.
Note the small value of the capacitor across the width control, only 0.01mfd, I guess the idea of this is to maintain the amplitude of the flyback pulse irrespective of the width control setting.

DFWB.
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Old 17th Mar 2021, 5:00 pm   #64
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Default Re: Marconi VC76DA

I do often think that these sets look somewhat pre-war in their design. Even this one, the screen mask somehow makes the CRT face look really bulbous and the cabinet for some reason just looks like it has much more depth than it should. You'd think EMI would have been at the forefront of TV upon its return but I'm sure I read even their first post war sets were actually pre-war Blumlein designs.

This type of focus setup is not something I've seen on a set before, the closest I've come across with electrical adjustment is the setup on the Bush TV12. I do like this particular set, not so much because of its performance but more so because it's quite an odd-ball, I'm sure with less penny pinching and a little more R&D they could have been quite good sets really.

I believe the EHT to be a little low on mine as I said before, I've currently got the focus control set to max and the magnet adjusted to focus the picture. When I back off the control the picture dims and balloons very rapidly but I'm still to change some resistors and I'm going to re-check the line stage valves.
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Old 17th Mar 2021, 9:34 pm   #65
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Default Re: Marconi VC76DA

Quote:
I'm sure with less penny pinching and a little more R&D they could have been quite good sets really.
There are some good quality components in these sets, the cabinets are as well made as their pre war sets.
The mains dropper & LOPT never fail, the worst items are the poor quality EMI capacitors and the Emiscope CRT that makes a Mazda look desirable
Couple that with an awful circuit design, and you can clearly see how these sets deserve their terrible reputation.

I am amazed just how many survive, especially the console sets.
I must admit that I do like my sets for all their faults....


Mark
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Old 18th Mar 2021, 10:44 am   #66
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Default Re: Marconi VC76DA

They are quite bizarre in that sense, I do like these old EMI products for their cabinets as they look nice and are very well made. It's obvious that they had the ability to make really good components but I suppose this was a company that previously never had any experience in budget products, whereas its quite obvious with companies such as Bush that they often tried to keep cost and component count down with most of their products and probably had the better hand at budget sets.
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Old 18th Mar 2021, 11:12 am   #67
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Default Re: Marconi VC76DA

In 1950 a strange deal between EMI and Michael Sobell's company was implemented on a short term basis in which an exchange of shares between the two companies took place.
Products sold under the Columbia brand name were made by Sobell.

Attachments show the similarities between the line output stages of the Marconi VT73A and GEC BT5144.

DFWB.
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Old 18th Mar 2021, 5:46 pm   #68
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Default Re: Marconi VC76DA

Yes David, Sobell knew how to make television receivers! John.
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Old 18th Mar 2021, 5:54 pm   #69
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Default Re: Marconi VC76DA

Quote:
In 1950 a strange deal between EMI and Michael Sobell's company was implemented on a short term basis in which an exchange of shares between the two companies took place.
Products sold under the Columbia brand name were made by Sobell.
EMI also got Plessey to make radios, the T18DA (1949) for example, my Marconiphone VRC74DA is also fitted with a Plessey made radio.


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Old 18th Mar 2021, 7:19 pm   #70
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Default Re: Marconi VC76DA

Mark Pirate wrote: "EMI also got Plessey to make radios, the T18DA (1949) for example, my Marconiphone VRC74DA is also fitted with a Plessey made radio."
In the 1920s Plessey made radios for the Marconiphone company.
Many Marconi TVs made about 1954/5 were fitted with a Plessey chassis. 14" and 17" models with that Band 1 and 3 tuner which had a preset position for BBC and continuous tuning on Band 3.
Getting back to the VT73A, was the X78 triode-hexode valve made specially for this superhet receiver? The more common X77/6BE6 is unsuitable for use above 30mhz.

DFWB.
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Old 18th Mar 2021, 7:27 pm   #71
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Default Re: Marconi VC76DA

In a word David I think yes! The RF deck is a TRF for London and superhet for channels 2 3 4 and 5. The chassis is fitted with B7G valve holders making it necessary to fit a B9A holder for a conventional frequency changer. This would have made the receiver unit even more cramped and added cost. I don't think MOV had a suitable mixer/osc in 1949.

The X78 is an odd little beast with the cathode internally connected to one side of the heater so that it could be accommodated on a B7G base. [Especially for the 2/3/4/5/ versions of the 1807 series.

Was the X78 ever put to use in a radio? Regards, John.
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Old 18th Mar 2021, 8:35 pm   #72
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Default Re: Marconi VC76DA

According to the national valve museum, GEC also made the X78.
Although to quote "The GEC label suggests that this valve was sold to the professional market rather than the consumer one"

Did GEC ever use this valve in any of their televisions, or was it only used in EMI sets?


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Old 18th Mar 2021, 9:19 pm   #73
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Default Re: Marconi VC76DA

Hi Mark,

The GEC I've just bought and posted about on the other thread uses Z77 valves, so certainly this chassis which was used in several other GEC sets utilised them.
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Old 18th Mar 2021, 10:27 pm   #74
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Default Re: Marconi VC76DA

There has been many discussions about the origins of the Z77 and equivalents EF91, 6AM6 and Mazda 6F12. Some suggest that the developer of this miniature RF pentode was Cossor with their SP6 whist others believe it was Mazda which designated the valve as 6F12. Valve also known as the CV138.
First application was in the Murphy V114 of 1947? Marconi-Osram Z77 appeared in TV sets in 1948.

DFWB.
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Old 19th Mar 2021, 9:08 am   #75
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Default Re: Marconi VC76DA

Quote:
The GEC I've just bought and posted about on the other thread uses Z77 valves, so certainly this chassis which was used in several other GEC sets utilised them.
I have just realised that my two GEC BT1252's from 1955 use 4x Z77's, I wonder what set was the last to use them?
I don't recall having any problems with them in these sets, maybe it was just the Marconi version that suffered internal shorts?


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Old 19th Mar 2021, 11:25 am   #76
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Default Re: Marconi VC76DA

The last GEC TVs to use the Z77 and other MO sourced valves was in 1956.
The following year the valves and CRTs were sourced from Mazda and were given MO type codes, for example the 30F5 is the Z329.
By 1952 TV sets made by EMI under the HMV and Marconi brand names used valves made by Mullard which like GEC were given MO type codes, for example the Z152 is the EF80 and the LN152 is the ECL80.
Likewise, Cossor switched over to Mullard valves which were given RETMA codes, EF80 = 6BX6.

DFWB.
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Old 24th Apr 2021, 6:25 pm   #77
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Default Re: Marconi VC76DA

Hi all,

I've spent a little more time on this set today and aligned the RF chassis following the sequence in the Marconi service manual. Upon connecting the aurora I had absolutely nothing on CH4 and again a picture on CH3. I decided to try again and followed the sequence setting all of the trimmers at the right frequencies and again had the same results.

I'm wondering if this fault lies deeper somewhere in the set such as problematic coils. Now I'm working out whether it is best to repair the Cyldon tuner and connect this back into the set and see if this sorts my problem. Only problem is the cyldon tuner is completely valve-less and I need to know where it connects in the chassis.

I know that the X78 is disconnected as I remember this missing from the set but I don't recall which the other valve was or which leads from the cyldon tuner go to which valve socket as they were disconnected when I bought the set and I have no data for this to hand.

Another alternative is whether somebody has a spare midland RF chassis which they'd be willing to give up and I can replace the current one fitted and try again.

Cheers
Bren
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Old 3rd May 2021, 2:33 pm   #78
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Default Re: Marconi VC76DA

Hi all,

I've continued to persevere with this set and after sourcing a missing valve for the Cyldon tuner, cleaning all of the contacts and tuning some of the IF coils I now have a good picture and sound on channel 4.

I'm still tracing a low EHT effect which I'm getting a bit stumped by and I hope that the CRT isn't low, being a Mullard I'd expect it to be okay. I have fairly healthy line whistle but sometimes I do think it is a bit quiet, I don't know if this is due to the cabinet hiding the sound well or if it is indeed a lack of EHT.

The focus control still has to be at the very upper end otherwise the picture balloons and becomes unfocused very sharpish. All voltages seem to be as the service info suggests but around 10v over as I've noticed the HT line is a bit higher than it should be, but I can't see this making a big difference.

I've also noticed the sound of discharge coming from somewhere when the focus control is towards the upper end. I can't actually see where it is coming from but it wouldn't surprise me if it's the U35 base, I replaced this as the old one had burning from one pin to a chassis screw.

I might borrow a HV probe and check the actual EHT voltage but in the mean time is there anything else to check?
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Old 6th May 2021, 9:42 pm   #79
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Default Re: Marconi VC76DA

Well I think I've confirmed today that the CRT is probably low emission. I've borrowed a HV measuring probe and I have 6.4kV on the anode of the CRT. As I stated before a watchable picture with fair contrast is really only seen in almost complete darkness and the picture quickly balloons, goes out of focus and dims at about 5.2kv.

Luckily with it being a Mullard I should be able to source a spare easily enough but it may mean waiting for a poor condition set to come along and donate its CRT. Hopefully if the Retrotech UK (prev. NVCF) goes ahead there is usually something there in such a suitable state.

Funnily enough this doesn't look like the fault that ended the sets life so presumably whoever was using it last must have been watching it in almost complete darkness!
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Old 23rd May 2021, 12:21 pm   #80
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Default Re: Marconi VC76DA

Hi all

I had another look at the Marconi last night. I began to doubt my earlier conclusion that the screen was flat as I've never came across a Mullard that was so low as this seemed to be and from looking at the getter and overall condition of the neck it doesn't look as though its done a massive amount of hours.

I decided to check the heater voltage and it only measured 4v. For now I've just fitted a seperate transformer to run the heaters at the correct 6.3v. I just wondered if anyone knows what the heater voltage was for the EMI 3/18 CRT that was also fitted to this set? I ask because according to the service data all sets fitted with a mullard CRT had a /1 suffix to indicate this, my set does not and it may be possible that it originally had a 3/18 and was later substituted, this would make sense if the heater for the 3/18 is 4v.

The set is now working much better with a bright and fairly sharp picture. I just need to give further attention to the linearity and there seems to be some cramping horizontally as can be seen on the test card circle.
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