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Old 10th Apr 2020, 12:19 pm   #1
ORAWA01
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Default Marconi CR100 B28 Restoration

Hi All

This set looks like it has not been serviced at all since out the factory. It looks it has all the old original parts (nothing replaced underneath = original caps and resistors), therefore will need full restoration. Even all the wires inside the set seems original and are falling apart. So, it will need new wiring as well.

And there are some missing parts, which are,
1. AC Power Lead
2. Voltage selector on the Mains Transformer
3. Bottom panel for the chassis
4. 4x valves

I will be trying to replace all the capacitors first, and then perhaps the resistors too? This looks like it will be a long term project of a few months if not more than a year due to the situation I could only work on it randomly whenever time allows.

There are about 10 spare valves supplied with the set, so I will see, if some of them are for the missing ones in the set. If not, then will need to source them as well.

But where do you get hold of the mains voltage selector? The manual says, it must be a 2W fuse for the voltage selector. I am not optimistic in finding these original parts for sale now, but if that is the case, then could it be made up with some fuse wire? The missing parts are not immediate problems in restoration, because it will be a while until the set will be powered on (after the recapping and replacing all the parts), but just wondering, if there is a good idea for sourcing the missing mains transformer voltage selector.

And any advice or ideas for restoring this CR100 in poor condition would be welcome and grateful.

Thanks 73.
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Old 10th Apr 2020, 4:08 pm   #2
g4aaw pete
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Default Re: Marconi CR100 B28 Restoration

Hello Jay

the attached shows an original fuse/jumper. However, because they’re made from some sort of brittle plastic, they get hot & split. This results in ‘no power’.

I’ve resolved this by soldering ‘legs’ onto a 20mm fuse holder, fitting a 2A fuse & spacing the legs such that they fit into the two jumper holes on the transformer.
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Old 10th Apr 2020, 4:56 pm   #3
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Default Re: Marconi CR100 B28 Restoration

Hi Pete

Talking of 2A fuses, I have a number of little boxes of 5 x 20mm fuses
all labelled Schaltvermogen/ Breaking Capacity 35 A followed by the AC symbol,
on one side and on the other side either Trage/time lag or Flink/quick acting
followed by various typed on low current values. I get the time lag or quick acting bit,
but what is the breaking capacity?

Incidentally I have got 2A ones if Jay hasn't got any.

Kind regards
Dave
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Old 10th Apr 2020, 4:58 pm   #4
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Default Re: Marconi CR100 B28 Restoration

You may know this but the bottom panel is often removed to align the set, and as it affects the alignment may not get put back. Apparently when aligned in the factories a special plate was used with holes to allow access to the coils, so that when the base was finally put back there was no change. Possibly an aluminium plate would not ruin the alignment?
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Old 11th Apr 2020, 9:58 am   #5
ORAWA01
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Default Re: Marconi CR100 B28 Restoration

Quote:
Originally Posted by g4aaw pete View Post
Hello Jay

the attached shows an original fuse/jumper. However, because they’re made from some sort of brittle plastic, they get hot & split. This results in ‘no power’.

I’ve resolved this by soldering ‘legs’ onto a 20mm fuse holder, fitting a 2A fuse & spacing the legs such that they fit into the two jumper holes on the transformer.
Hello Pete

I was also going to look into a couple of junk part cardboard boxes with old radio and electronics parts in the shed, looking for something similar which could be used for the selector.

But from your info, now I realise that it is possible to make up one using a fuse and soldering iron. Great idea and advice. Thank you.

Jay
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Old 11th Apr 2020, 10:00 am   #6
ORAWA01
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Default Re: Marconi CR100 B28 Restoration

Quote:
Originally Posted by Dave757 View Post
Hi Pete

Talking of 2A fuses, I have a number of little boxes of 5 x 20mm fuses
all labelled Schaltvermogen/ Breaking Capacity 35 A followed by the AC symbol,
on one side and on the other side either Trage/time lag or Flink/quick acting
followed by various typed on low current values. I get the time lag or quick acting bit,
but what is the breaking capacity?

Incidentally I have got 2A ones if Jay hasn't got any.

Kind regards
Dave
That's very kind of you. Will PM you if I cannot find one, and if the set needs one. Thank you so much, Dave.

Kind regards
Jay
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Old 11th Apr 2020, 10:07 am   #7
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Default Re: Marconi CR100 B28 Restoration

Quote:
Originally Posted by greenstar View Post
You may know this but the bottom panel is often removed to align the set, and as it affects the alignment may not get put back. Apparently when aligned in the factories a special plate was used with holes to allow access to the coils, so that when the base was finally put back there was no change. Possibly an aluminium plate would not ruin the alignment?
I was thinking of cutting a wood panel and screw it onto the bottom of the chassis as well. But shiny aluminium panel would be very nice and much better come to think of it. I have a mitre saw which can cut metal, so it is possible to to cut into exact measure, drill the screw holes to the chassis, and yes it is possible.

It will be the final stage of the restoration. It will be the exciting moment then.
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Old 11th Apr 2020, 11:35 am   #8
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Default Re: Marconi CR100 B28 Restoration

The most difficult part is likely to be accessing the metal-cased decoupling capacitors on grid 2 of the RF and IF valves. These are connected directly to the valve socket pins that are very hard to get at because of the depth of the metal shielding underneath. You will need a lot of dexterity unless you are willing to completely dismantle the shielding boxes which would be quite a job. Don't be tempted to skip them because they will almost certainly be leaky. Cheers, Jerry
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Old 11th Apr 2020, 10:08 pm   #9
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Default Re: Marconi CR100 B28 Restoration

Quote:
Originally Posted by cathoderay57 View Post
The most difficult part is likely to be accessing the metal-cased decoupling capacitors on grid 2 of the RF and IF valves. These are connected directly to the valve socket pins that are very hard to get at because of the depth of the metal shielding underneath. You will need a lot of dexterity unless you are willing to completely dismantle the shielding boxes which would be quite a job. Don't be tempted to skip them because they will almost certainly be leaky. Cheers, Jerry
Yes, I will be replacing them even if it had to be dismantled completely. Hopefully it will go relatively easy. Thank you for your advice. Much appreciated. cheers, Jay
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Old 11th Apr 2020, 11:19 pm   #10
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Default Re: Marconi CR100 B28 Restoration

Once the long bandchange shaft has been released and carefully withdrawn through the hole in the rear of the chassis, the four coilboxes can be individually or collectively removed- you'll need a fairly butch soldering iron to release the earth straps to the gang capacitor frame. This leaves the centre area of the chassis nicely clear and open for capacitor/wiring etc. replacement, the long IF strip screen can also be unscrewed to greatly ease overhaul down that side, too. The coilboxes' surrounding screen also unscrews easily once they are out of the chassis/off their two long mounting rails to make their innards far more accessible. In this respect, the CR100 is more straightforward to comprehensively overhaul than, say, the AR88 RF deck.

Good luck with it, there are plenty here who know this radio, Colin.
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Old 13th Apr 2020, 10:06 am   #11
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Default Re: Marconi CR100 B28 Restoration

Great info and advice. Much appreciated.
Yes, I might have to dismantle it fully, clean the boards and connections, replace the parts, and rewire as well. It looks like it will be a good long project, but will be fun and worth it.

If I encounter any problems, or for info and advice, will keep getting back here for help. Thank you 73

Jay
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Old 13th Apr 2020, 3:56 pm   #12
g4aaw pete
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Default Re: Marconi CR100 B28 Restoration

Quote:
Originally Posted by cathoderay57 View Post
The most difficult part is likely to be accessing the metal-cased decoupling capacitors on grid 2 of the RF and IF valves. These are connected directly to the valve socket pins that are very hard to get at because of the depth of the metal shielding underneath. You will need a lot of dexterity unless you are willing to completely dismantle the shielding boxes which would be quite a job. Don't be tempted to skip them because they will almost certainly be leaky. Cheers, Jerry
Because I’m very lazy, (I’ve won awards for it), I didn’t bother replacing the anode decoupling components in the HF and mixer boxes.
Yesterday, the anode decoupler in the HF box went short and there was much smoke.

The originals (C54 & C57), can simply have their earth leads snipped rather than complete removal.
Using a soldering iron with a fairly small diameter element barrel, it’s possible to replace the anode decouplers in each of the two boxes without removing the shields.
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Old 14th Apr 2020, 8:34 pm   #13
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Default Re: Marconi CR100 B28 Restoration

Those fuse-and-voltage-selector thingies were used on car battery-chargers well into the 1970s, as a combined fuse and 6/12-volt selector.

Once car-boot-dales become a thing again it would possibly be worth your time seeing if you could find an old, battered battery-charger and offer the seller 50p for it.
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Old 14th Apr 2020, 9:12 pm   #14
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Default Re: Marconi CR100 B28 Restoration

That's a thought. With everyone stuck at home for a month or two and nothing to do, eventually we'll get desperate enough to have a big clear-out. But bin collections are iffy and the tips are all closed...

When this pandemic is over, think of the car boot sales!

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Old 15th Apr 2020, 12:01 pm   #15
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Default Re: Marconi CR100 B28 Restoration

Going to car boot sales looks like events which will take place in some distant future. But will look out for it when we are there. Thanks for the idea.

Green bins are still collected and emptied as usual, but the recycle bins are not getting emptied at all. We are trying to cut down on packagings and cardboard boxed items.

The CR100 restoration will take place in very slow motion here. I moved it to the outside workshop next to the garden shed. It was not heavy as the R107, and reckon it is half the weight of AR88, but it was still very heavy. It would have costed quite expensive if it was posted. And there is no guarantee that it would have arrived in good shape.

The weather will get dry and bright for most of the days and well into the evening, so will spend lot of time working on this CR100, dismantling and cleaning, soldering replacing the parts, and rewiring.

A few more items apart from the CR100 to repair / restore, so plenty to do in this year.
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Old 20th May 2020, 6:44 pm   #16
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Default Marconi CR100 B28 Restoration Help

Hi All

I have done some work on the CR100 today, as the weather was fantastic warm dry and sunny. I could work in the shed worktop, and there was this CR100 waiting for me.

I took off the cover, and had a look. The inside of the set was really dirty with dust and dirts. Needing total clean with tooth brush and some detergent?.

My questions arose,

The wires in the set looks pretty old and worn, should they be replaced with new wire?

For replacing the caps, would it better taking off the long tubes which cross from the front panel to almost to the back panel, as they are blocking access to the 2x caps on the top part of photo? Would it be easy job taking these long tubes off?

What detergent / cleaning agent is OK to clean the inside of the rig = the parts and chassis?

Thanks
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Old 20th May 2020, 7:08 pm   #17
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Default Re: Marconi CR100 B28 Restoration Help

I will have a go at just one of your questions. You say the wires look "pretty old and worn".

I can't tell from your general photos exactly what problem you are seeing with the wires. Wires that definitely need replacing are usually the rubber insulated ones, where the rubber has gone hard, and now cracks and falls off when the wire has the slightest disturbance. Sets in this state will have bits of "rubber debris" fall out on to the bench every time you move it. Clearly this is a disastrous state for any wire that needs to maintain good insulation for its function. Any short to earth (or anything else) is likely to stop the set working, and possibly cause burn out of other components if there is excessive current flow.

That sort of damaged wire MUST be replaced. The modern synthetic rubber covered wire looks quite close to the original, and is available in a number of colours. It has a dull appearance (unlike PVC) which is pleasingly similar to the old wires.

Anything else which has reasonable insulation, which is not cracked or loose, I would leave well alone. "Old age" is a not a reason to replace it! After all, "old age" is presumably one of the reasons you acquired the set in the first place!


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Old 20th May 2020, 7:43 pm   #18
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Default Re: Marconi CR100 B28 Restoration Help

It looks clean - but the yellowish wiring is rubber-covered and is really a crumbly-nasty 1940s liability, which needs replacing throughout before you try to power it up.

There will also be loads of leaky paper decoupling capacitors, and a few equally-flaky electrolytics - that after 70 years wlll need replacing.

Then - any resistors which are of the "ceramic bodied with colour-bands on the ceramic" and a sort-of flowerpot-coloured 'stuffing' at the ends - they're well-known to going way-off-spec after a few decades, and should be replace-on-sight.

A CR100 with the aged capacitors/resistors replaced by modern within-spec parts can be 'intriguing'.

Trying to use one with loads of drifted, past-their-time parts is a frustrating folly.
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Old 20th May 2020, 8:21 pm   #19
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Default Re: Marconi CR100 B28 Restoration Help

Choosing replacement wire with heat-resisting insolation can make life a lot more pleasant when you're replacing it. there will be less strain on your vocabulary.

Silicone or Kynar would be my choice. PTFE is a devil to strip, so if you come across any cheaply, smile knowingly and walk past.

As tanuki says, lots and lots of paper capacitors of an age and reputation where you may wonder about whether to bother testing them. Some are in interesting cans which bolt to the chassis at one end. These may be considered restuffing candidates. It gives you somewhere to put the new things. While you have larger roadblocks out, exploit the opportunity to measure and change any resistors while the access is easier.

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Old 20th May 2020, 8:44 pm   #20
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Default Re: Marconi CR100 B28 Restoration Help

Yes, great advice thank you. I will leave good enough wires alone, but the ones with the plastic cover peeling = I will try to replace them.

For the paper capacitors, I couldn't spot a lot in the set, if any. Mostly they were metal covered cap, it looked. I will look into them again next time.
There are small resistors that look very worn and dirty. I have bought a box of resistors covering most values, so they will replace the drifted ones.

Would you recommend to dismantle the set further for the recapping? Or just leave it as is, and carry on with recapping?

What cleaning agent is good for the parts above and under the chassis and also the chassis itself?
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