Marconi 702 advice
 

Now that I have examined the set, I need advice. Several questions:

Does anybody recognize this very set?

There are obvious modifications, for instance the "heat coil" is now a fuse
(a 1 amp one ... vastly too high, what would be right?) Should not I add
another fuse in the main power lead.

I don't see any trace of the EHT shorting bar seen in the diagrams
on the "omnibus". Is it clearly gone? The first test I intend
is to see if the EGT transformer works by applying 5 volts to the primary
and an AC voltmeter on the secondary. Does the lid of the EHT box just lift
off if I remove the screw?

Many caps have been replaced. What vintage are they? Should they too be replaced? Should the HV caps be replaced? (Yes, I can test them but they always test somewhat bad.)

The RF unit looks unrestored, and see the clearly bad mica cap. Does
it need serious work, if so ... how does one get the bottom "cups" off?
Can the whole unit be safely removed with the CRT in place?


The first photo shows an American relative.

more photos in another post

Doug McDonald

Re: Marconi 702 advice
 

more Marconi 702 photos

Re: Marconi 702 advice
 

The EHT transformer is bright shiny new.

http://www.vintage-radio.net/forum/a...1&d=1494383560

Re: Marconi 702 advice
 

The pre war HMV I worked on the RF unit did come out without removing the CRT.

Re the cans I cannot remember that far back.

Re: Marconi 702 advice
 

Hi Doug,

I think the heat coil was rated at 0.6 amps.

The shorting bar is very bad news for the EHT transformer if it is ever allowed to short.

The components in your TRF look original and the mica caps may look suspect but in reality normally work well. They are also a rare type that you will find difficult to reproduce. I suspect you will not need to do anything to the TRF unit.

The components in your timebase are largely what I would regard as temporary replacements to get it working. I can send you photos of what they should look like.

The TRF unit cans both top and bottom are only retained by the bars over their tops. The bottom ones just need a little side force to get them off.

The EHT transformers were originally covered in black pitch. I think yours has been rewound using the original core.

I've attached a photo of your set working. The CRT looks very healthy.

Regards,

Peter

Re: Marconi 702 advice
 

I would say it is down to what you want to do with it. If you want to return it to as new condition then you would need to strip down the chassis and have them re-plated, find capacitors that match the originals so you can restuff them or make replicas (many of the components are the same as used in radios from that period), and carefully rebuild it. If you just want it working then judging by the working picture and the near new replacement parts there is little to do.

Re: Marconi 702 advice
 

I now recognise it on the basis of the picture that Peter has posted. I think I saw it at the website of a collector who owns (or owned) several pre-war sets. I do recall his name, but have never known him personally. The website no longer appears to exist.

Re: Marconi 702 advice
 

The set doesn't appear to be listed in this thread:-

http://www.vintage-radio.net/forum/s...ghlight=dating

Re: Marconi 702 advice
 

6th up from the bottom of this list:

http://www.nostalgiatech.co.uk/new_page_31.htm

Lawrence.

Re: Marconi 702 advice
 

That's the website of Peter Scott who posted up thread.

Re: Marconi 702 advice
 

As to my philosophy, if I do not believe in making one "look like new", unless it's hopelessly rusted out, in which case I probably would not buy it. The TRK-12/120
in the photo was built from pieces , one of which was the actual chassis used to test the modifications to the RF section to make them run post-war channels (great shape!) and one a mouse-house radio, which simply had its rust removed and the mouse eaten coils either roughly rewound (oscillator coil) or replaced with a generic (antenna coil). The cabinet was in good shape except it had had the chassis replaced with a postwar one with controls on the front, and had a partial re-veneer job done.

Otherwise for a set like this, were it unrestored, I would restuff the restuffable and try to get replacements for bad micas. For unrestuffables I would just replace with modern (Japanese) parts. That's what I'll do to the bad mica in the photo.

But with this one, the highly unperiod replacements will be left as-is if good. They are now part of history. A future owner could of course try to make it look like new.

Oh yes, as to history, the previous owner was John Estep (his name was on the ETF owner list) and before that I do know the name, but I'm not sure I'm supposed to say it.

My question about its history actually is, did someone on this list do the restoration work?

How much power does this set draw? What is the correct voltage to feed it, as I have to get the right transformer. (For the moment I will use an isolation transformer to add 105/115/125 volts on top of a variac.)

Re: Marconi 702 advice
 

Mine is set to the 231-250 volt tapping and I'm feeding it from 248 volts.
It draws 220 watts.

Peter

Re: Marconi 702 advice
 

What does one do for standards conversion? I tried to buy an Aurora but apparently they are not available any more. I can make a static picture with my HackRF, but ....

Re: Marconi 702 advice
 

Have you tried contacting tubesrule on this or various other fora?
http://www.vintage-radio.net/forum/member.php?u=1100


Peter

Re: Marconi 702 advice
 

Yes, of course. He's Mr. Aurora. He was also the former owner of the TRK12 chassis in the TV in my picture. Building that set was an exciting journey
starting with his offer of it.

Re: Marconi 702 advice
 

If you can't lay hands on an Aurora then you might find this thread interesting.
http://www.vintage-radio.net/forum/s...hlight=foth+tv

Peter

Re: Marconi 702 advice
 

Had I known the set worked that well I might have tried to fire it up.

Re: Marconi 702 advice
 

I'm still in the examining phase.

For information for that list, the timebase is probably 3677, 10% chance it really is 3577.

The power supply is 3764.

The audio is 3246.

There is no number in the shown location on the RF chassis, and
the board near it is a bit different (it has only two contacts, labelled
S and GR.

Re: Marconi 702 advice
 

Hi Doug,

Thanks very much for your chassis numbers. I've updated the table. My photo of the TRF chassis just happens to be from an HMV900 which has the three connections rather than two but the chassis number is normally in the same location. If you do spot it in another location please let me know.

Thanks,

Peter

Re: Marconi 702 advice
 

There was a time when it was considered OK to fit modern components into very early receivers and in fact many of my sets were serviced in that manner.
Not any more, now I'm a avid capacitor restuffer.
The attachment shows some of the capacitors taken from the 1804, these were restuffed today.
I think we have to agree those yellow hi-viz capacitors look awful in vintage sets.
Actually I don't mind seeing Philips polyester capacitors fitted in old equipment.

DFWB.