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Old 28th Dec 2020, 2:01 am   #101
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Default Re: Marconi CR100 - £30 bargain

Yes it was the first time I’d ever soldered wires Rambo, it was just a practice. I know you don’t carry it across now lol, I’m going to get some tin lead in the future as you suggest.

What would be suitable for the tags, I’m really stuck with what to use that won’t rust or corrode anytime soon.
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Old 28th Dec 2020, 2:34 am   #102
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Default Re: Marconi CR100 - £30 bargain

Well, if you look at what the guy did on that chavfreezone link I provided in #79, all he did was bring the three positive wire ends through holes he drilled in the new base, and then formed them into pig-tails to act as solder tags.

Also note that the common negative leads of the three capacitors exit through a fourth hole and is connected to chassis somewhere convenient.
The original item used the can as the negative electrode, but it would be very difficult to make a good internal connection to the can, you can't solder to aluminium remember.
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Old 28th Dec 2020, 2:12 pm   #103
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Default Re: Marconi CR100 - £30 bargain

Yeah I had a look on that website, ****** good job he did of that.
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Old 28th Dec 2020, 5:16 pm   #104
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Default Re: Marconi CR100 - £30 bargain

The insulation that is burnt, I wonder whether it contains a resistor, or is that a wirewound one that is nearby?

I like Colins idea of one stage at a time, makes a lot of sense.
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Old 28th Dec 2020, 8:17 pm   #105
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Default Re: Marconi CR100 - £30 bargain

Guess it does somewhere. I’ll continue ripping it out and let you know.
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Old 28th Dec 2020, 11:14 pm   #106
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Default Re: Marconi CR100 - £30 bargain

Can anyone point me in the direction of how to refurbish the 3 mains transformers, the two black ones and the one tall eggshell one. Thanks.
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Old 28th Dec 2020, 11:27 pm   #107
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Default Re: Marconi CR100 - £30 bargain

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Originally Posted by AmateurRepairer View Post
... to be replaced with modern pvc single core copper stuff that won’t rot.
Hello AR. My vote would be stay clear of PVC covered wire for this job.

Point-to-point wiring particularly has a major impact on aesthetics so if you're wanting to improve things for posterity, I'd use Teflon-covered wire, which comes in a decent array of colours and wire diameters. It strips cleanly and can look wonderful.

Also, if you're not yet very experienced in soldering, you will much more easily make horrible melting and stinky blobs of PVC whereas with Teflon, happy days!

As a sidebar, I would say you've gone on a highly ambitious detour from your initial stated intention , which was to get this thing going.

Do carefully document and endlessly photograph any component arrays that you can and find a methodical way of recording things so you can re-build it without unnecessary headaches.

You clearly have plenty of support here but you won't have a continuous handrail on tap so some self-organisation will help everyone and lead to the happiest outcome.
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Old 28th Dec 2020, 11:52 pm   #108
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Default Re: Marconi CR100 - £30 bargain

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Can anyone point me in the direction of how to refurbish the 3 mains transformers, the two black ones and the one tall eggshell one. Thanks.
You will find only one is a transformer, the other two are HT chokes with a single winding.

What aspect of them needs refurbishment? If it's just the cosmetic appearance, perhaps a squirt from one of the rattle-cans you mentioned earlier.
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Old 29th Dec 2020, 12:10 am   #109
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Default Re: Marconi CR100 - £30 bargain

Rambo - just wondering if they are needing doing, they must have worked ok previously as the thing powered up,
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Old 29th Dec 2020, 2:20 am   #110
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Default Re: Marconi CR100 - £30 bargain

Just had it wired up to a 9v battery (the transformer) and it gives a hell of s lot of humming. I’m guessing it’s had it. Shame.
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Old 29th Dec 2020, 2:27 am   #111
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Default Re: Marconi CR100 - £30 bargain

Hi

Those capacitors that are bolted to the chassis look exactly the same as used in the R1155 receiver, I have restuffed a lot of those very easily.

first remove the capacitor from the set and remove the brown or black seal, this will expose the capacitor inside, put some heat on the capacitor, I use a heat gun, it does not take a lot of heat to start melting the wax insides, when the insides have been removed drill a 1.5 mm hole as close to the screw thread as possible, feed the end wire of the replacement capacitor down through the hole and wrap it around the base of the threaded stud, this will make the earth when you replace the can fixing nut, do not wrap the wire more than one turn , if anything 3/4 of a turn is better, otherwise the wire makes the can sit uneven when it is bolted down.
You can fill the can with some wax or insulating tape to stop the new capacitor from rattling around.
What I do with the "live" end is to connect a suitable length of PVC covered wire and to make it really neat I make a new top seal from punched out leather and make a hole to feed the wire through.

I have not followed the article all the way, but, does your set have multiple capacitors in what I call about the size of a cigar can, that is approx. 20mm across and approx. 100mm tall, if so these I deal with in a similar manner and when refitted in to set, it is difficult to see that they have been restuffed.

Some care has to be taken when restuffing these, but the receiver benefits from looking as though nothing has been changed.

I used capacitors that have a voltage working of 630 v DC and I have never, as yet, had one fail on me.

Regards

Bill

Last edited by bill knox; 29th Dec 2020 at 2:49 am.
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Old 29th Dec 2020, 11:50 am   #112
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Default Re: Marconi CR100 - £30 bargain

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Just had it wired up to a 9v battery (the transformer) and it gives a hell of s lot of humming. I’m guessing it’s had it. Shame.
OK you are going to have to explain what you have tried.

A transformer won't work on DC it's an AC device.

Be careful connecting batteries to transformers it can create high voltages that may damage the insulation, and the batteries can see near short circuits resulting in possible exploding batteries.

I would not expect it to hum as there is no AC content in the battery to make it hum.

To test the transformers your going to need a current limited source of AC.

I think a good place to start is here:

https://www.vintage-radio.com/repair...ion/index.html

Information on how to build a lamp limiter here :

https://www.vintage-radio.com/projec...p-limiter.html

Paul put a lot of effort into these tutorials and inexperienced restorers will benefit a lot from reading them.

Cheers

Mike T
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Old 29th Dec 2020, 11:54 am   #113
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Default Re: Marconi CR100 - £30 bargain

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Just had it wired up to a 9v battery (the transformer) and it gives a hell of s lot of humming. I’m guessing it’s had it. Shame.
Putting some DC into a transformer winding or into a choke should create a current which ramps up from zero to an amount set by the battery voltage and the resistance of the copper wire. Inductance doesn't affect the final current value.

Where the inductance comes into play is that the build-up of that current has wound up a firly large amount of magnetic field in the core. When you come to disconnect your battery, the field collapses quickly, and in doing so creates a high voltage spike.

If you were holding the connections on with your fingers on the bare terminals, you can get the kind of shock you would decide you don't want to repeat. You can get a noticeable zap off good inductors even from trying them with an AVO on its Ohms range.

THere may be a small audible click from the core as you make and break your battery connection.

But it shouldn't buzz with nothing more than the DC from a battery.

There is the possibility that a loose connection, shaken by the magnetic flux has made you an unintentional buzzer, but what you've described sounds very wrong.

You didn't have mains applied at the same time, did you?

If so, the DC from the battery can drive the transformer to enough extra flux in one direction that the core will go into saturation when the mains is driving it in the same direction. When the core saturates, its magnetic properties effectively disappear, and it is the inductance which they created, when not in saturation, which limits the mains current. So you lose the effect in the transformer which stops a lot of mains current flowing, and the mains thinks it sees only a length of copper wire across it. This will create a loud 'growl' as it's called in olden times. There was a test-tool used to apply mains mag field to the armature of a motor suspected of having shorted turns - it was called a 'growler' from the buzzing it made when you hit the bad winding.

Transformers don't like DC when they're trying to work.

David

EDIT: Crossposted with Mike...... good thing we're in agreement
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Old 29th Dec 2020, 3:29 pm   #114
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Default Re: Marconi CR100 - £30 bargain

Best bet AR, would be to study some basic circuitry testing techniques. I've heard that there are heaps on YouTube. As David says - "back EMF" can be an issue with applying DC to a transformer or choke. The only "High" "DC" testing of transformers should be with an "Insulation Tester" - i.e. a "Megger" - which most folk use. For continuity of windings - a simple Multi-meter on the "Ohm's" resistance range.
For mains transformers which are old or suspect, many folk use a "Variac" for powering them up from the mains, for safety. Also, talking of "mains" - modern 3 pin plugs can be fused from 1A up to 13A. Just cos they're called 13A Plugs doesn't mean that's the only fuse to use. A domestic radio mostly only draws less than half an amp. Larger commercial/military Rx's might only need a 2 or 3A fuse.

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Old 29th Dec 2020, 9:21 pm   #115
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Default Re: Marconi CR100 - £30 bargain

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Can anyone point me in the direction of how to refurbish the 3 mains transformers, the two black ones and the one tall eggshell one. Thanks.
There's not a great deal to do pre-emptively beyond cosmetics and keeping your fingers crossed, really. The wound components (transformer, LF chokes, RF coils) are the most obviously cost-cut components in the set, reflecting the stringency of the design situation and the need to produce large numbers in a hurry and are comparable with consumer sets, rather than much miltary equipment. I thought that the two chokes (above and below the chassis corner) seemed a bit marginal with their flaking cardboard bobbins and aged insulation and left them in one of my dad's part-used tins of International yacht varnish for a couple of days, fishing them out and leaving them to dry for several days before use. Whether this was necessary or even wise in retrospect I don't know but they've held up and at least haven't deteriorated in appearance since.
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Old 29th Dec 2020, 9:35 pm   #116
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Default Re: Marconi CR100 - £30 bargain

I think you're actually going too fast in too many directions for us at present AR

Most beginners spend a lot of time, initially, on both the theory and some practical skills [eg soldering]. The advice you are receiving now [not just from me] is to get into some of this before dismantling and re-building in all different directions, otherwise it's going to be quite difficult for you. Knowing something about the difference between AC and DC electricity is really quite basic but essential. I think it's possible you might be taking the "current" You Tube "watch and learn" route like many others with different motivations [of all sorts]. That's a bit different to a [largely] self taught hobby, with maybe a little help-occasionally. Whether YT is even really watch and learn, I might doubt!

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Old 29th Dec 2020, 10:06 pm   #117
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Default Re: Marconi CR100 - £30 bargain

As a rather successful professional golfer put it:

"The more I practice, the luckier I get"

Just throwing yourself into some repairs and restorations gives you an uneven sort of progress. If, however, you divert some of your energy into looking into theory and into practicing manual skills like soldering, you can find that your rate of progress is a good deal faster and more comfortable. Each aspect feeds and enables the others to an extent. Concentrate on one alone, and the others will hold you back.

Theory gives you a map, when otherwise all you would see is trees.
Manual skills give you quality and reliability when otherwise problems would lead you up the wrong creek.
Using these things and doing something with them cements it all in place.

There have been many threads of people asking for book suggestions, there's also a lot on the forum held in stickies, or findable with searches. Go exploring.

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Old 29th Dec 2020, 11:07 pm   #118
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Default Re: Marconi CR100 - £30 bargain

I will do as radio wrangler suggests. Thanks all.
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