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A Marconi 702 Mirror Lid

 
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Re: A Marconi 702 Mirror Lid

Post by ianm » Thu Aug 25, 2016 10:07 am

Hi Brian,

Watching with great interest. The rate of progress is amazing!

 
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Re: A Marconi 702 Mirror Lid

Post by Briancuff » Thu Aug 25, 2016 10:25 am

That comes with not having anything else to do, Ian!
I'm off to the Hospital again this afternoon for some radio activity :ccf just to slow things down.

 
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Re: A Marconi 702 Mirror Lid

Post by Briancuff » Sat Aug 27, 2016 10:59 am

Yesterday was the last dose of radiation so it's back to the bench!!
I’ve now completed the restoration of the Marconiphone 702 Sound unit. The two pictures show the resulting chassis which has been tested as it is a superhet which accepts 41.5MHz and converts it to a 1.5MHz IF. The circuit is exactly like a normal superhet with FC, an IF amp, A DDT as detector & audio amp and an output valve to drive the speaker. No real problems were found using a bench PSU but fault finding on a rebuilt unit needs, sometimes, a different thought train: “Why would there be instability?” – “Have I left out a decoupler or a connection to a decoupling cap?”. During ordinary repair, one has to assume that the unit worked before it became faulty so no missing wires! I only had two missing – one was to a decoupling cap and the other earthed the cathode bias resistor of the triode audio amp. After installing those, I was able to tune in the Aurora. The DDT audio amp valve had very flakey metalisation with only about one third of it remaining so I scraped it all of and cleaned the envelope. It looks a lot better now and the chassis is stable.
I find it very difficult to make multiple connections to these typical EMI solder tags used on their tagboards. They always seen to look messy. I am also not too happy with the caps. and I have ordered some custom SRBP tube to try to make some like the original caps. with the leads coming out through tiny eyelets, radially, at the ends. I have found that eyelets are available but I will have to make a tool of some sort to install them in the tube walls. That will be three weeks away!

Completed-and-working---under.jpg
Completed-and-Working---Top-View.jpg
The next part of the project is to test the rebuilt Sync unit using the bench PSU remembering to load the line and frame output transformers to suitably match the coils. This is very important with the line scan transformer as it could be wrecked with huge voltage spikes – they’re large enough when loaded with the coils!

 
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Re: A Marconi 702 Mirror Lid

Post by Panrock » Sat Aug 27, 2016 7:23 pm

Neat and workmanlike. Well done.

When I first did this sound unit (about 30 years ago now!) a dicky IF transformer winding was causing an annoying rustling under the sound, necessitating rewind.

It used to drift too... the sound tuning control came in useful.

Steve

 
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Re: A Marconi 702 Mirror Lid

Post by Briancuff » Sat Aug 27, 2016 7:39 pm

That sort of fault is always a real problem to solve, Steve. EMI fitted a Negative Temperature Coefficient cap to the oscillator tuned circuit to combat this problem - did yours have one fitted?

The 702 sync chassis has been bench tested and as far as I can tell without the high level video drive to the sync separators for the two time bases. One thing which surprised me was that the line and frame hold controls were in the middle of their travel for the correct frequencies (my old Tek 2230 scope gives a readout on a seven segment display of the triggering frequency - a very useful function). This could change when the time bases are triggered but for a chassis that is nearly 80 years old, that's pretty good.

The next job is the PSU, a massive, extremely heavy unit which sits both on and through the base of the cabinet, has two huge mains transformers, three rectifiers and two large EHT type capacitors which will probably need re-stuffing. At least the EHT mains transformer high voltage secondary has continuity - they're mostly open circuit - so hopefully, that means no rewind. This will be a much more time consuming job as there are many connections which pass through the chassis so will have to be recorded and dismantled (I sound like an archeologist!). Here are a couple of pictures:
702-PSU-Top.jpg
The tubular electrolytics were fitted, by EMI, later in the life of the set. They replaced two boxes containing three capacitors. I will make a replica for the box on top of the chassis and one that was underneath.
702-PSU-Under.jpg
The box with the 12 holes in it is the bit which passes through the cabinet base.
The flat bar to the left of the unit is the safety interlock which shorts out the EHT when the back of the cabinet is removed.
In the first picture, The black connectors hanging up (I rotated the picture) are the two connectors required by the Emiscope 6/6 CRT which is a hexode with electrostatic focus and magnetic deflection. When the tube is in position, the base reaches down to just above the PSU chassis (hence the PSU being partially sunk into the cabinet base) as the deflection angle is so narrow.

 
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Re: A Marconi 702 Mirror Lid

Post by Briancuff » Mon Aug 29, 2016 9:19 am

The PSU chassis is now stripped and ready for re-finishing. It was not too difficult a job to strip but the proof of the pudding will be in the re-assembly. Most of the wiring is in Systoflex and it is not in bad condition - nothing as bad as the Sound Chassis was. This means that I will put the majority of the wiring back exactly as it was and replace the looms which use braided flex. I have some which I bought from Frosts Auto Restoration and apart from being a bit too thick, it looks the part - more lacing!

One large resistor (500k) on the EHT bleed panel was O/C so that has been replaced with a suitably large component - 5kV across 10 resistors = 500V across each one. This is more than the average 1W resistor can take.

I tried the large green EHT smoother on my 3kV PSU and it held out so I am hopeful that the cap. is OK at 5kV. I have to test the focus cap and maybe re-stuff it. I have suitable caps in stock.

EMI seem to have always used term panels onto which small open spade terminations are clamped using 6BA screws. These spades are VERY firmly clamped to the loom cables and are very difficult to get off. After ruining two, I finally got a system which, although tedious, results in a spade which can be re-used on the new looms.
A few more pictures:
Empty-Chassis---Top.jpg
Ready for a good clean-up
Empty-Chassis---Under.jpg
A very heavy 3/32" steel chassis
Components-Removed.jpg
The two mains transformers are huge and weigh a ton. The chassis has to be strong!

It's now onto a day of cleaning with Scotch pads, brushes and thinners before spraying. Luckily its a fine day so I can work outside.

 
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Re: A Marconi 702 Mirror Lid

Post by peter scott » Mon Aug 29, 2016 1:08 pm

I hope you don't mind Brian but I've taken the liberty of highjacking part of one of your photos to show the location of the power supply chassis number (I don't think my power supply has a number.) in the hope that other owners will send me their chassis numbers. :)

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

Peter

 
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Re: A Marconi 702 Mirror Lid

Post by Briancuff » Mon Aug 29, 2016 8:59 pm

Be my guest, Peter. It's important to get all the info possible on these old sets recorded (nearly 80 years old!).

 
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Re: A Marconi 702 Mirror Lid

Post by Briancuff » Tue Aug 30, 2016 7:27 pm

Here we go again! - I've made a bit of progress on the PSU chassis today - 7 hours working on it with rests in between. I'm a bit tired now but I'm so keen to get this 702 working that I seem to have become obsessive!!!
I was going to plate the chassis but I have found that the zinc plating is far too shiny for my liking so I looked into paint. I found some silver paint intended for re-finishing alloy car wheels which, by definition, must have to be hard wearing. The finish the aerosols give certainly looks good and is a cross between satin and matt finish so, in my opinion, is perfect. The pictures show the sprayed and partially complete chassis.
For interest, the vertical flat metal bar you can see at the LHS of the chassis is the EHT interlock which shorts out the EHT smoothing capacitor when the back is removed from the set - remember, these sets used to be called "Widow Makers".
The transformer mounted is the EHT one which is fed from the 230V tapping on the general mains transformer via Systoflex insulated wires and a heat coil (a la old telephone system) but I will wire it in Bn/Bl wires to conform with the specs of today. The next phase will be mounting the general mains transformer and rectifier valve holders and the Term Panel. I will then judge how much Systoflex needs to be replaced.
All three original electrolytic capacitor boxes were removed by EMI during the post war refurb of the set so I had to make some replacements. I'm not sure what colour to paint them - does anyone have an original? One of the boxes made is shown in the top picture painted light grey but it doesn't look right.
702-PSU-under-during-re-build.jpg
EHT transformer tag strip, EHT rectifier and smoothing caps mounted
702-PSU-Chassis-top-during-re-build.jpg
EHT "box" covering the EHT rectifier and mains transformer. New cap box in forground.

 
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Re: A Marconi 702 Mirror Lid

Post by peter scott » Tue Aug 30, 2016 9:33 pm

Hi Brian,

I have seen the HT caps both boxed and unboxed. Mike Barker painted a box with same green that is on the EHT cap but I'm not convinced that is as original.
The HMV901 that was presented to The National Museums of Scotland by EMI has the TCC caps interleaved with clear plastic and I have seen this on a few others. (Similar to overhead projector transparency film.)

Here's the Museum HMV901:
File0010 (Large).JPG


Here's Marconi 702 Y241:
Y241 BackInteriorOrig Crop.jpg


Here's the Science Museum 702: (Click on this one to see the lower parts)
Science Museum 702.jpg
Science Museum 702.jpg (32.84 KiB) Viewed 2212 times


Steve used naked cardboard (I think) on 702 Y224:
Steves rear.jpg


Kind regards,

Peter

 
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Re: A Marconi 702 Mirror Lid

Post by Briancuff » Tue Aug 30, 2016 10:37 pm

Thanks for that info Peter. The picture of the Science Museum is especially interesting as it shows a dark colour for the cap box and a different mounting. It seems that the little Top Hat brackets were fitted for the Paxolin panel which carries the three electrolytics retrofitted by EMI after the war. I also notice that the EHT smoothing cap is not the large green TCC type in most of the 702s. I will re-visit the construction of the re-made cans and their mounting.

 
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Re: A Marconi 702 Mirror Lid

Post by peter scott » Tue Aug 30, 2016 10:44 pm

Hi Brian,

I think the green EHT cap raised on legs is the correct item. I think the Science Museum EHT cap is unique.

Peter

 
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Re: A Marconi 702 Mirror Lid

Post by Refugee » Tue Aug 30, 2016 11:34 pm

In the second photo of the newly sprayed chassis what is the component at the front left above the interlock?
From here it looks like a tall fuse holder but surely it can't be.

 
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Re: A Marconi 702 Mirror Lid

Post by Cathovisor » Wed Aug 31, 2016 5:36 am

Refugee wrote:From here it looks like a tall fuse holder but surely it can't be.

It is. It holds the PO-type heat coil.

 
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Re: A Marconi 702 Mirror Lid

Post by Briancuff » Wed Aug 31, 2016 10:11 am

I've just noticed that the safety EHT interlock bar has been removed on the Science Museum specimen! I wonder how many of them were removed due to the possibility of burning the EHT transformer out when the cover was left off and the set switched on. There is a heat coil in the primary of the transformer but they are very slow to open (a small eureka element melting solder) - I would have thought that something a bit faster would have been better!

 
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Re: A Marconi 702 Mirror Lid

Post by Refugee » Wed Aug 31, 2016 11:15 am

So it is a hybrid sort of "fusug" part slug and part fuse.
I have got a Phillips AC bridge with a lovely bright EM34 that has a thermal fuse on the mains transformer.
It looks like a pair of pincers with a spring soldered between the jaws. It has live mains all over it :elc:

 
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Re: A Marconi 702 Mirror Lid

Post by peter scott » Wed Aug 31, 2016 12:08 pm

It's mounted in slight tension:

Heat Coil Green.jpg


Peter

 
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Re: A Marconi 702 Mirror Lid

Post by Briancuff » Wed Aug 31, 2016 12:33 pm

The service info says that the heat coil is coded blue. The ones I have are green - does anyone have a blue heat coil for sale/swap?

 
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Re: A Marconi 702 Mirror Lid

Post by Refugee » Wed Aug 31, 2016 1:54 pm

That is one seriously complicated thermal fuse.
Could one be re-stuffed with a modern thermal fuse?

 
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Re: A Marconi 702 Mirror Lid

Post by peter scott » Wed Aug 31, 2016 2:07 pm

You could leave it as is but make the real connection with a modern fuse on the chassis underside.

Peter

 
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Re: A Marconi 702 Mirror Lid

Post by Briancuff » Wed Aug 31, 2016 8:01 pm

A little bit of progress today as I had to re-felt some garden storage units which have seen better days but at least I had some good news. My hemoglobin level is up to 104 from 69 so I don't need the blood transfusion I was scheduled for tomorrow. This means that I can get on with the PSU! :aad

The general mains transformer and associated rectifiers etc. are back in and I've mounted the 3 chokes. In order to remove some of the Systoflex wiring, I have added a contemporary tag strip to the chokes as the leads from them were long and direct. I can now use different wire.

As far as the capacitor blocks are concerned, I am keeping the two under-chassis units as they are as these are relatively straightforward but the top mounted one I'm still not sure about! I have a feeling that the Science Museum picture shows an original box, painted a dark colour - I think that that is correct so I am now redrawing the assembly so that I can make one. Here's a couple of progress pictures:
702-PSU-Chassis-top-during-re-build-3.jpg
The Tywraps are precursors to lacing the harness.
702-PSU-Chassis-under-during-re-build-2.jpg
A couple of cap boxes, the bleeder network and odds and sods to install before completion of chassis and testing.

 
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Re: A Marconi 702 Mirror Lid

Post by peter scott » Wed Aug 31, 2016 8:12 pm

More great progress both in terms of blood and (702) guts.

Peter :aap

 
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Re: A Marconi 702 Mirror Lid

Post by Briancuff » Wed Aug 31, 2016 10:12 pm

:aaj :aaj :aaj :aaj

 
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Re: A Marconi 702 Mirror Lid

Post by Katie Bush » Wed Aug 31, 2016 10:16 pm

Hi Brian,

That paint job is pretty darned impressive! :aad Where did you get the aerosols? I have some car wheels that could benefit from a similar application! :bba

Marion

 
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Re: A Marconi 702 Mirror Lid

Post by Briancuff » Wed Aug 31, 2016 11:00 pm

From Amazon, Marion. £6.49! It's Hy-Cote Silver Alloy Wheel Paint. Excellent stuff.

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