UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Television and Video

Notices

Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

Closed Thread
 
Thread Tools
Old 29th May 2019, 11:35 pm   #1
dtvmcdonald
Pentode
 
Join Date: Mar 2016
Location: Champaign, Illinois, USA.
Posts: 227
Default Marconi 702 "restoration"?

I was examining my Marconi 702 this week. I was worried about the focus.
That problem was nonexistant, thankfully: it turns out that the cap in that
circuit is large enough to cause about a 1 sec time constant, so I was turning the knob far too fast. It actually focuses right in the middle
of the range. It works just fine.

But I noticed something about the "restoration" that was inflicted on the poor thing. All the caps in the power supply and sync chassis were replaced, as
was the EHT transformer, rewound. A few resistors were too. There are no caps in the RF unit to replace, and the audio ones were not replaced.

BUT ... I noticed that three of the valves in the Sync unit,
as well as one in the audio (already known about) were SOLDERED IN.

Its true that the sockets are in very bad shape ... but still, there must be
enough old "parts" radios to supply workable sockets.

I wonder who did this? It clearly was back in England, as the former
US owner never even took off the side panels. The only number
on it that could be a serial number is 4027.
dtvmcdonald is offline  
Old 30th May 2019, 3:59 am   #2
peter_scott
Dekatron
 
peter_scott's Avatar
 
Join Date: May 2003
Location: Edinburgh, UK.
Posts: 3,273
Default Re: Marconi 702 "restoration"?

I think your set had previously been owned by Ian Watson but I don't have contact details for him.
Perhaps you can verify this from the veneer pattern below. Oops, I don't appear to have a small enough copy and I'm struggling with a smartphone away from home. Email me if you want the photo.

Peter

Last edited by peter_scott; 30th May 2019 at 4:06 am.
peter_scott is offline  
Old 30th May 2019, 11:58 am   #3
beery
Heptode
 
beery's Avatar
 
Join Date: Jun 2006
Location: Ware, Hertfordshire, UK.
Posts: 988
Default Re: Marconi 702 "restoration"?

Hi dtvmcdonald,
This is indeed the same set that Peter mentions.
Ian offered it to me many years ago (wish I had bought it!). Anyway, at that time he had left the original capacitors bagged up inside the set. My intention was to re-stuff these original capacitors.
Anyway this is much of muchness now as I no longer have contact details for Ian.
I would not like to criticise a former owner too much, after all you can do what you like with what you own yourself. Regarding the EHT transformer, these always fail in pre-war EMI sets, so don't worry about that.

I do have quite a lot of photos of how these sets should look, both as originally built and after the post war EMI overhauls. I also have a few photos of the inside of your set from when it was offered to me. For instance the tag board on the top of the PSU chassis was fitted by EMI after the war to replace an original multi capacitor box and so I myself would keep it, but with the correct 1940s capacitors.

The post war electrolytics were by TCC. The wax capacitors pre and post war were made by EMI as were the valve sockets. The pre-war tag strips were of German origin and were not available after the war...
The best way to get parts is to break up pre-war EMI radios. To restore my HMV 905 I used two radios and the remains of a 907 chassis. New solder tags were specially made for the missing tag strips in my set.
I have quite a few HMV capacitors left over, but I don't think I have any more viable valve sockets.

Let me know if there is any way I can help you.

Cheers
Andy
beery is offline  
Old 30th May 2019, 2:02 pm   #4
dtvmcdonald
Pentode
 
Join Date: Mar 2016
Location: Champaign, Illinois, USA.
Posts: 227
Default Re: Marconi 702 "restoration"?

Thanks very much for the info.

I do not intend to do anything to the set, or use it much. If one of the four known problem valves does die, the new one will just be soldered in like the present ones ... the joints are reachable, and adding wire won't hurt the valves.

Yesterday the EHT died again, the "heat coil" had been replaced by a fuse and I am using grossly underrated ones and they die rarely but regularly. Also on the EHT supply: I just recapped another US postwar electrostatic deflection set last week and it uses 0.047 uF 6000 V caps to feed the vertical plates. These as well as 0.1 uF 6000 V mylar film ones are readily available here (at $14 per!) and amazingly small. If the postwar replacement ones there now were replaced by these (under the chassis!) the power supply would safely slide out without removing the CRT.

YES. I would indeed like all the old photos you folks have. Just email via the forums. There was no bag of old caps inside the set, just one of the low voltage rectifiers rolling around loose inside it. The valve survived. As I said before, the original USA owner never tried to use it.
dtvmcdonald is offline  
Old 30th May 2019, 2:05 pm   #5
Argus25
No Longer a Member
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Marconi 702 "restoration"?

Quote:
Originally Posted by dtvmcdonald View Post
Its true that the sockets are in very bad shape ... but still, there must be
enough old "parts" radios to supply workable sockets.
You certainly do not have to put up with bad sockets when you restore a set of this vintage. The original wafer sockets are now a bit of a disaster with excessive corrosion and loss of spring force in their contacts (Not improved by re-tensioning them). however I found little difficulty obtaining new ceramic sockets on ebay UK and the octal ones from ebay USA. You just have to be prepared to hunt them out.(photo of a 904 chassis with new sockets fitted attached)
Attached Thumbnails
Click image for larger version

Name:	sock.jpg
Views:	233
Size:	64.2 KB
ID:	184181  
Argus25 is offline  
Old 30th May 2019, 5:12 pm   #6
dtvmcdonald
Pentode
 
Join Date: Mar 2016
Location: Champaign, Illinois, USA.
Posts: 227
Default Re: Marconi 702 "restoration"?

Thanks for the note on availibility of new sockets for old valves.


Somebody or some bot removed my email from my previous post. I will monitor my
local messages and send it to whoever has useful photographs. However,
it is very very non-secret as I am the DNA Project administrator for the Clan Donald.
dtvmcdonald is offline  
Old 30th May 2019, 5:41 pm   #7
peter_scott
Dekatron
 
peter_scott's Avatar
 
Join Date: May 2003
Location: Edinburgh, UK.
Posts: 3,273
Default Re: Marconi 702 "restoration"?

Sorry about the lack of a photo in my previous post. I 'm away from home at present and am not clever enough to reduce photo sizes on my phone.

Peter
peter_scott is offline  
Old 30th May 2019, 7:31 pm   #8
akuram1
Pentode
 
Join Date: Sep 2009
Location: Dundee, UK.
Posts: 158
Default Re: Marconi 702 "restoration"?

Those ceramic sockets really look a big improvement and your chassis is amazing. You have done a wonderful job.
akuram1 is offline  
Old 31st May 2019, 12:07 am   #9
Argus25
No Longer a Member
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Marconi 702 "restoration"?

Thank you for that kind & supportive remark.

There are better quality images to look at in the restoration article:

http://www.worldphaco.com/uploads/HMV__904_ARTICLE.pdf

Over the years I have come to document my restorations and put them on a website as pdf's, as it is easy to forget over time, all the things that were done to complete it. It then becomes easy to look at them again or reference them from any net connected computer.
Argus25 is offline  
Old 31st May 2019, 2:13 am   #10
Catkins
Pentode
 
Catkins's Avatar
 
Join Date: Feb 2015
Location: Chepstow, Monmouthshire, UK.
Posts: 234
Default Re: Marconi 702 "restoration"?

Quote:
Originally Posted by beery View Post
The best way to get parts is to break up pre-war EMI radios. To restore my HMV 905 I used two radios and the remains of a 907 chassis. New solder tags were specially made for the missing tag strips in my set
I concur. Even though it may sound "sacrilege" to break up a set that has survived 80 years+, there are so may undistinguished EMI sets from the later 30s, that poor condition examples are worth more for spares, than anything else.

Breaking up two old EMI radios is also how I replaced some of the missing pieces/unrestorable pieces for my HMV 904 restoration. I was lucky enough to find a radio that had the same multi-capacitor block that the HMV 904 used, and was missing, removed at the 1946 repairs. But, even little things like matching screws and washers are hard to find unless you strip down an old radio.

I hope this doesn't sound like a plug, but, Steve Harris has lots of other-wise unsalable sets from the 30s, which he'll sell for spares. This is where I got some of my sets which I stripped.
Catkins is offline  
Old 31st May 2019, 2:42 am   #11
Catkins
Pentode
 
Catkins's Avatar
 
Join Date: Feb 2015
Location: Chepstow, Monmouthshire, UK.
Posts: 234
Default Re: Marconi 702 "restoration"?

Quote:
Originally Posted by dtvmcdonald View Post
I do not intend to do anything to the set, or use it much. If one of the four known problem valves does die, the new one will just be soldered in like the present ones ... the joints are reachable, and adding wire won't hurt the valves.
As Argus25 said, I don't think you want to put up with such poor quality sockets. If you do go this way, then you should be aware that most soldering irons available today simply don't have the wattage to ensure a good solder joint with all of that metal acting as a heat-sink.
Catkins is offline  
Old 31st May 2019, 10:02 am   #12
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,526
Default Re: Marconi 702 "restoration"?

Quote:
Originally Posted by peter_scott View Post
Sorry about the lack of a photo in my previous post. I 'm away from home at present and am not clever enough to reduce photo sizes on my phone.
The forum operational software automatically resizes photos to fit within its limits AFAIK.

Most of my uploads, straight from my camera card are 2M+ to start with.
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is offline  
Old 31st May 2019, 3:35 pm   #13
dtvmcdonald
Pentode
 
Join Date: Mar 2016
Location: Champaign, Illinois, USA.
Posts: 227
Default Re: Marconi 702 "restoration"?

Quote:
Originally Posted by Catkins View Post
As Argus25 said, I don't think you want to put up with such poor quality sockets. If you do go this way, then you should be aware that most soldering irons available today simply don't have the wattage to ensure a good solder joint with all of that metal acting as a heat-sink.
I have many soldering tools. My regular one will solder to tube pins no problem, and surely even to sockets as ratty as these.

As a tip (yes, a pun), I have made a very useful tool I used in working on the audio chassis while in place. This is a solder gun thing (125 or 250 watts nominal) with custom tips made of various size wire (#6 to #14). I can make these as long as 8 inches. I file down the wire near the tip a bit to concentrate the resistance and the heat there. Before attaching to the gun itself I put Teflon tubing, at least 0.4 mm wall thickness, over the wire up to the tip. I can then bend the wire to fit way down into cramped areas, snaking around obstacles, and the Teflon prevents burning things it might accidentally touch. This type thing has proven to be a panacea in many cases, including said audio chassis and Hallicrafters SX25 and Racal RA17c12 radios.
dtvmcdonald is offline  
Old 31st May 2019, 5:22 pm   #14
usradcoll1
Heptode
 
Join Date: Mar 2017
Location: Cedar Grove, Wisconsin, USA.
Posts: 823
Default Re: Marconi 702 "restoration"?

Quote:
Originally Posted by dtvmcdonald View Post
I was examining my Marconi 702 this week. I was worried about the focus.
That problem was nonexistant, thankfully: it turns out that the cap in that
circuit is large enough to cause about a 1 sec time constant, so I was turning the knob far too fast. It actually focuses right in the middle
of the range. It works just fine.

But I noticed something about the "restoration" that was inflicted on the poor thing. All the caps in the power supply and sync chassis were replaced, as
was the EHT transformer, rewound. A few resistors were too. There are no caps in the RF unit to replace, and the audio ones were not replaced.

BUT ... I noticed that three of the valves in the Sync unit,
as well as one in the audio (already known about) were SOLDERED IN.

Its true that the sockets are in very bad shape ... but still, there must be
enough old "parts" radios to supply workable sockets.

I wonder who did this? It clearly was back in England, as the former
US owner never even took off the side panels. The only number
on it that could be a serial number is 4027.
Is that the same set that the Texas collector and restorer had?
I wonder who brought it to the US!
Dave, USradcoll1 or as known on VK, dieseljeep.
usradcoll1 is offline  
Old 31st May 2019, 7:35 pm   #15
dtvmcdonald
Pentode
 
Join Date: Mar 2016
Location: Champaign, Illinois, USA.
Posts: 227
Default Re: Marconi 702 "restoration"?

Quote:
Originally Posted by usradcoll1 View Post
Is that the same set that the Texas collector and restorer had?
I wonder who brought it to the US!
As far as I know it was never associated with Texas.
dtvmcdonald is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 5:23 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.