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Old 21st Apr 2015, 1:02 pm   #1
FERNSEH
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Default Marconi 702. Sound problems.

I've owned this set since the early nineties. The vision circuits have been always reliable but the sound section is another matter, it never seems to work properly. Let's see if can be sorted out once and for all.
The Marconi 702 is the vision and sound only receiver, no radio facilities in this set. The vision receiver employs a TRF circuit, however, the sound receiver is a superhet. The four valves in the sound receiver are typical radio types, X41 frequency changer. VMP4G IF amplifier. MHD4 detector, AVC and audio amplifier. N41 or KT41 output. Where the set differs from normal AM radio practise is the IF, it's 1.5Mc/s. The higher IF will give better bandwidth and tolerance to oscillator drift.
So my first task is to try a different freqency changer valve. An X41 or AC/TH1. Let's see what happens.

DFWB.
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Old 21st Apr 2015, 3:55 pm   #2
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Default Re: Marconi 702. Sound problems.

That was an easy fix. A wire had become disconnected from the grid coil of the X41 frequency changer valve. The sound was restored but was distorted and there was also patterning on the picture. The oscillator trimmer has two positions, one with the bad sound and picture and the correct one which is completely free of any defects.

DFWB.
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Old 21st Apr 2015, 10:15 pm   #3
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Default Re: Marconi 702. Sound problems.

All good now for this classic television?
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Old 22nd Apr 2015, 10:41 am   #4
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Default Re: Marconi 702. Sound problems.

All the upper cabinet parts have been refitted. There are two minor jobs to do to complete the restoration. Apply dark brown paint to the base of the cabinet. Also a piece of veneer has come away from the bottom right side of the front panel. All are easy fix tasks.

Coming up next: the HMV 901, a badge engineered version of the Marconi 701.

DFWB.
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Old 22nd Apr 2015, 11:11 am   #5
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Default Re: Marconi 702. Sound problems.

Quote:
Originally Posted by fernseh View Post
coming up next: The hmv 901, a badge engineered version of the marconi 701.
702 ?
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Old 22nd Apr 2015, 11:20 am   #6
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Default Re: Marconi 702. Sound problems.

Hi Peter,
I should have wrote 702. We all would like a Marconi 701 in our collections. That's the mirror view set with the front magnifying lens, a similar arrangement as the HMV 902.

DFWB.
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Old 22nd Apr 2015, 1:06 pm   #7
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Default Re: Marconi 702. Sound problems.

To think I was repairing things like that in the '50s.

Peter
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Old 22nd Apr 2015, 1:40 pm   #8
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Default Re: Marconi 702. Sound problems.

Quote:
To think I was repairing things like that in the '50s.
Unfortunately, a lot of these sets were being scrapped in the 50's

I would love to add a prewar set to my collection, but sadly my pockets are not deep enough!

Mark
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Old 22nd Apr 2015, 2:53 pm   #9
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Default Re: Marconi 702. Sound problems.

Quote:
Originally Posted by FERNSEH View Post
I should have wrote 702. We all would like a Marconi 701 in our collections. That's the mirror view set with the front magnifying lens, a similar arrangement as the HMV 902.
701 for restoration after the 901?

Peter
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Old 22nd Apr 2015, 3:16 pm   #10
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Default Re: Marconi 702. Sound problems.

Hi Peter,
after the 901 is sorted out the next EMI set to receive attention will be the HMV 902, the huge TV radiogram combination. A replacement EMI K3 record deck is to be installed.
The Marconi 701 is on the wish list, but it is unlikely one will ever be found.
It was manufactuctured in tiny numbers. I'd reckon no more than twenty.

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Old 24th Jun 2015, 4:09 pm   #11
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Default Re: Marconi 702. Sound problems.

The CRT is out so now I can gain access to the power supply unit in order I can take pictures of it for another forum member. The RF unit must also be removed because the sensitivity control needs replacing.

For safety's sake buy a visor so that your eyes and face can be protected from implosion. It's worth bearing in mind that these tubes are almost eighty years old.

DFWB.
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Old 24th Jun 2015, 8:53 pm   #12
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Default Re: Marconi 702. Sound problems.

And made of pyrex I think?
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Old 24th Jun 2015, 9:25 pm   #13
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Default Re: Marconi 702. Sound problems.

It is indeed Pyrex. Actually the CRT in it's metal cradle is surprisingly heavy. The metal is of a thick gauge.

Scary business removing the tube.

DFWB.
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Old 25th Jun 2015, 9:29 am   #14
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Default Re: Marconi 702. Sound problems.

Quote:
Originally Posted by mark pirate View Post
Unfortunately, a lot of these sets were being scrapped in the 50's

I would love to add a prewar set to my collection, but sadly my pockets are not deep enough!
Sadly we were scrapping them, great lumbering things when there were nice compact sets coming onto the market weighing less than half a hundredweight.

Peter
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Old 25th Jun 2015, 11:17 am   #15
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Default Re: Marconi 702. Sound problems.

It's understandable that they were being scrapped. Money was a little easier to get and lots of people wanted something light, bright and modern after the austerity years of the 40's and early 50's.

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Old 28th Jun 2015, 7:13 pm   #16
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Default Re: Marconi 702. Sound problems.

Pictures of the power supply unit as fitted in Marconi 702 and HMV 901 vision only TV receivers.
This unit must weight almost as much as a modern CRT mono TV set, I estimate it weights 15KG. It's not an easy unit to remove from the cabinet. There is a box assembly on the underside of the chassis which is positioned below the floor level of the cabinet so this heavy power unit has first to be lifted up to be clear the rear cabinet batten.
These sets might be fun for TV enthusiasts and collectors but as Peter says in the 1950s most service engineers would be glad to be shot of these monsters.

DFWB.
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Old 30th Oct 2015, 12:18 am   #17
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Default Re: Marconi 702. Sound problems.

The sound problems in the Marconi 701 and the electrically identical HMV 901 have been sorted out.
The Marconi 702 still has that low CRT video drive problem to sort out. It's only 10 volts from sync tip to peak white. It should be at least 15volts.

This evening I turned my attention to the HMV 901. The replacement vision sensitivity control was fitted some time ago. The new part is a high grade 5watt wire wound 25Kohm potentiometer. A slot had to be cut into the end of the control shaft to suit the type of control knobs employed by EMI in those times. See attachment.
No frame sync was traced to the anode load resistor of the frame sync separator, R5 350,000ohms. Separate line and frame sync valves are employed in these early EMI sets. The valves are Marconi MSP4 pentodes, V1 and V5.
After replacing the resistor the frame sync lock is solid and the interlace is perfect.
Not bad at all for a 1936 design. Actually it's possible that the design work on these sets was started sometime in 1935.

So that leaves the Marconi 701 to receive attention. The video drive has to increased somehow.

DFWB.
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Old 31st Oct 2015, 2:40 pm   #18
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Default Re: Marconi 702. Sound problems.

The only approach to solving the low video drive fault is to remove the receiver chassis for fault finding. It's a bit difficult to do any work on it when it is in the cabinet.

DFWB.
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Old 31st Oct 2015, 11:29 pm   #19
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Default Re: Marconi 702. Sound problems.

Gosh what fun! The CRT has to come out again in order that the RF chassis can be lifted out for service.

DFWB.
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Old 1st Nov 2015, 8:23 am   #20
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Default Re: Marconi 702. Sound problems.

Good morning David.

Yes, this has become a familiar sight recently.

Jac
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