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Old 11th Dec 2011, 4:52 pm   #1
2E1CIH Mike
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Default Marconi CR300

Regarding the restoration of a Marconi CR300, I'd be very grateful for advice on the following:

1) I've read that the use of petrol (definitely avoiding components) is OK for chassis cleaning. Is this likely to affect the paint/coating on the chassis please?

2) A couple of the valves are in tight positions - very close to IFTs etc. I can't get my fingers to the valve bases, and don't want to tug at the glass envelopes. Is there an easy answer to this removal problem please?

3) I intend replacing all fixed value capacitors. What's the best thing to do about the chassis mounted electrolytics please?

4) There's a creamy white substance around the coil cores. I'm not sure if it is some kind of sealer. What is the best way to clean this, and the coils generally please?

5) The wiring looks to be in a reasonable state. Is it "the done thing" to replace all the wiring when restoring such a beast please? (....hoping the answer is "no").

Many thanks in advance for any time spent in answering the above questions. The forum community certainly makes the learning curve a happier place to be.

Regards.....Mike
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Old 11th Dec 2011, 9:52 pm   #2
Sean Williams
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Default Re: Marconi CR300

Mike,

Replacing everything will certainly end up with a dead receiver, and probably an abandoned project.

You will learn little from changing every component.

Rewiring a comms RX is not to be taken lightly, performance will suffer if cableforms are not as original.

Petrol as a cleaning material is dangerous, and not to be reccomended - it attacks rubber, does not evaporate fully, and is explosive......

Leave the coils, and cores well alone - they have worked well for 60+ years, no need to ruin them by "cleaning"

Learn the circuit, look for faults as they present themselves, and only change what you need to make it work.

Once it is working, you can look at components that affect reliability, and change those.

Start at the power supply, assuming you have smooth DC for the HT, and heater voltage, move to the AF output stage, and get that working, next stop is the AF preamp, and detector, IF stages, and RF stages - once all of this works properly, the AGC will need looking at - once this works correctly the RX is done.

HTH
Sean
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Old 11th Dec 2011, 11:32 pm   #3
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Default Re: Marconi CR300

Quote:
Originally Posted by 2E1CIH Mike View Post
Regarding the restoration of a Marconi CR300, I'd be very grateful for advice on the following:

1) I've read that the use of petrol (definitely avoiding components) is OK for chassis cleaning. Is this likely to affect the paint/coating on the chassis please?

2) A couple of the valves are in tight positions - very close to IFTs etc. I can't get my fingers to the valve bases, and don't want to tug at the glass envelopes. Is there an easy answer to this removal problem please?

3) I intend replacing all fixed value capacitors. What's the best thing to do about the chassis mounted electrolytics please?

4) There's a creamy white substance around the coil cores. I'm not sure if it is some kind of sealer. What is the best way to clean this, and the coils generally please?

5) The wiring looks to be in a reasonable state. Is it "the done thing" to replace all the wiring when restoring such a beast please? (....hoping the answer is "no").

Many thanks in advance for any time spent in answering the above questions. The forum community certainly makes the learning curve a happier place to be.

Regards.....Mike
Hello Mike,

1) you can use it as you describe but I never used it on a chassis, it shouldn't affect any chassis plating so long as you don't grind it in, if the chassis is painted and it's old hard paint then I would say the same applies, I use a foaming cleaner and tooth brush...carfefully.

2) Yep they certainly are! grip valvle with one set of fingers and use other set of fingers to apply a firmish pressure on the bottom of the valve base spigot from the underside of chassis, use the blunt end of a biro if you can get one in there or a nice blunt end of a knitting needle (not to small....point loading etc) wiggle pull and push at the same time should come out ok, once out you can file a very small notch (if you want to) on the inner opposite sides of the top of the aluminium valve base shrouds, cut a piece of thin strong nylon fishing line, drape across and plug valve back in, tie loose ends into a knot and let it drape anywhere (not drive mech or tuning gang) this can greatly assist future removal without exerting to much tension on the glass/base interface.

3) If it's anything like mine then change/restuff the lot, same goes for most of the mica mould looking types (brown domino looking) especially any that are in circuit that are used as coupling capacitors etc be aware that some are used as fixed padders on some of the band ranges and should be replaced by capacitor of the same value (might have to parallel 2 or 3 up to obtain correct value or even stick a small variable trimmer type across) the ones on the RIS filter don't really matter, if they are leaky the set will still work fine. Mine only has 1 or 2 electrolytics in it all the rest are oil filled paper in the other cans.

4)creamy white stuff is like a putty stops the slugs rattling round otherwise they are to loose in the former threads. doing slugs: they can be a pig or they can go well, if you are of the opinion that the coils need realigning then take your time and go carefully, 1st job is to remove any sealing wax (brown sticky) from the open end of the former down to the slug, the slug is slotted, you need a screwdriver blade shaped to fit the slot, the slot can be filled up with old hard shellac, if it is, heat up tip of driver and melt/burn the shellac out untill the blade fits snug, try and remove or loosen any further shellac that might be around the top of the slug where it touches the former, then while applying (not to much) pressure to the screwdriver try and screw the core in further by a couple of turns at the most, you might have to do some short back and too's to get it going, don't force anything, it should go if the slug has not been previously damaged, once in a turn or so scrape out what dross you can from the former and former/original slug interface without damaging the threads, on mine I used a standard 8 mm bolt to help clean the thread recesses down to where the slug is now positioned as the bolt is a loose fit to the original bare former, remove bolt, unscew slug by about four turns or whatever and the coil should be ready for alignment and it should peak when screwing the slug back in (use plastic tool, shape if required) if there is a need to remove the slug then it can be turned though an 8 mm nut to help clean it up once removed. Leave a trace of the white stuff or the cores will rattle around, after re alignment you can re seal with whatever, small drop of wax etc. Tip for IFT's etc: there are two seperate formers in there, screw the slug in to far and it will run out of thread and you won't get it out without dismantling unless the other slug can be fully removed and a tool inserted to help push other slug back up.

PS always have a rough idea where the original slug positions are before altering.

5) no you don't have to, on mine there are only a few lengths of rubber coated, these have gone brittle and will be replaced, the rest are either Systoflex or early plastic, if they appear in good condition I would leave them providing there is no obvious fraying damage that might induce a short.

Tip: if the cathode resistors in RF/IF are much more that 30% high then you will probably get reduced gain, when replacing capacitors/resistors in the RF and in particular the Osc make sure they are connected to the same chassis points etc as the originals, extend the wires of new caps etc if you have to, other wise you might end up with a Class C sqegging transmitter with birdies!

Hope this is of some use and good luck with your project.

Lawrence.

(addition) whoops i missed you post Sean.

Last edited by ms660; 11th Dec 2011 at 11:34 pm. Reason: addition
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Old 12th Dec 2011, 3:39 pm   #4
2E1CIH Mike
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Default Re: Marconi CR300

Hello Sean, Lawrence,

Many thanks for all your advice. It's much appreciated. I'm not sure I could manage this without you chaps. It would certainly take infinately longer at least.

On a very generalistic note; It's difficult for me to gauge just how far to dismantle. I know quite a lot about cars, having restored a couple of MGAs,a Midget and a Singer Gazelle etc, and I know that doing a full "nut and bolt" restoration means dismantling completely.

I have to balance this with my level of knowledge/experience in rx restoration....ie this is my first.

With the CR300, I have removed the case, removed the dial assy (the cord was broken - I can see replacement being a challenge) and taken all but two valves out. I was going to remove the variable cap next, but wondering if that's necessary now.

Sean, I take on board your advice regarding the order of checking the various stages, but additional to this would you say a thorough clean and cap/resistor replacement is sufficient.....Then to power up and check as described.

I'm trying to figure out a plan.

Thanks again......Mike
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Old 12th Dec 2011, 6:34 pm   #5
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Default Re: Marconi CR300

Hello Mike, normally the tuning capacitors (both of them) are pretty reliable and do not need removing, lubricating the shaft bearings and clean/lube rotor earthing springs can normally be done in situ.
If this is your first time with this type of receiver then my advice would be to take everything one step at a time and make notes/diagrams of any connections etc that you might have to cut or unsolder. Below is a brief but not fully comprehensive list of things that I normally do. It reads as though I am talking to you.

Know/learn saftey rules when dealing with electricity.

Try and find a local "Buddy" to guide you if poss.

1) Before attempting to power the unit up give it a close visual inpection, look for anything that is untoward...has the set been "got at" or modified/altered in anyway from its original configuration, if so where and what...check against the circuit diagram/manual etc. this receiver was designed to run off approx 230v DC HT feed and 24 - 28v DC or AC for the heaters, you will need a power source to supply those voltages at a current of approx 50/60 Ma for the HT and approx 1 Amp for the heaters, don't forget the heaters were originally wired for series parallel operation ie: two parallel branches each having four valves in series in each branch along with the two dial lamps and the current equalizing resistor for the sound output valve heater in a sub parallel arangement. It is not uncommon to find these types of receivers with their heater circuits modified for parallel 6.3v feed. In restoration it is usual to reverse any unofficial modifications if it is practical to do so.

Make sure the power supply you intend to use is fully functioning, suitable and safe.

2) On the receiver check the HT line resistance for shorts/leakage etc to chassis, normally best done with an analogue test meter, Avo etc. Any resistance reading across that line should be above 20k ohm allowing for any capacitor charging times, if not investigate and rectify.
Using the diagram locate any capacitors that are connected directly across the HT line paper/can/electrolytic etc if any are found then temp. disconnect and temp wire in new suitable replacements making sure you observe any polarity markings. Locate and replace the sound output valve control grid coupling capacitor with one of sufficient rating and value. Temp fitting is fine as this helps reduce any self inflicted faults!! etc. can be tided up later after ascertaining receiver is basically working and when doing section overhauls.

3) Set all controls for normally operation, RF gain full AF gain halfway, desensitising control fully clockwise, selector switch to phone, bandchange to band 4 (MW) AGC on, Filter wide, check speaker continuity with a test meter, it should make some kind of noise as you place the meter probes across it. Connect aerial of some sought, make sure receiver chassis (and any power supply chassis) have a common earth connection to your electricity suppliers earth. Try and arrange the receiver on its side so you have access to topside and bottomside

4) Connect test meter set to 250 v or suitable range between the HT line in the receiver and the chassis, connect power supplies and switch everything on, you can use a variac to bring up the power if you have access to one. Keep an eye on the HT voltage it should be arround 230v once the valve heaters have warmed up (check the heaters are glowing) Eyes, ears and nose alert...for smoke, overheating sizzling noise etc.(switch off isolate supply and investigate) If the HT hangs low then ditto, if/when all is well see if you can get any response out of the receiver ie: any kind of signal when tuning, crackling when band changing etc. or is there a hum/crackle when touching the test meter probe on the top caps of the detector/IF valves etc these will give you a basic idea if the set is basically functioning. Carry out voltage checks on all the valves anodes and screen grids and cathodes and write them down anything 20/30% out indicates a problem and will need investigating.

5) Try and get it up and running in its basic form without doing to much to it, replace componets in one section at a time ie: get AF up and running done and dusted then IF, Mixer/Osc, RF, AGC. BFO, Xtal Marker, first componets to replace will be the capacitors in cans etc, and any out of spec resistors.
When all this has been done and the receiver is working reasonably (Voltages correct etc no excessive heating) then try and determine if any realignment is required.
This is not a comprehensive guide, it is sometimes difficult to give directions and converse via electronic means.
Other members might have a different slant or other tips/advice etc.
As you are aware, I have one of these receivers around the bench at the moment so no worries for comparison etc.

Lawrence.
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Old 12th Dec 2011, 9:16 pm   #6
2E1CIH Mike
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Default Re: Marconi CR300

Hi Lawrence,

Many thanks for taking the time to explain all of this. There's a lot to take in, but as you say, one step at a time is best. I really don't mind if it takes me a year to finish the project. My primary aim is to learn by the experience.

I'm going to copy what you've said into a Word doc and try to follow it once I have cleaned it using a foam cleaner as you have suggested.

Before I try connecting up (I might buy a variac off ebay) I'll replace the caps and check resistors I think. What's the best thing to do about the "canned" caps though? I'm assuming these are now unobtainable new.

Thanks for the comparison offer too.

Thanks again....Mike
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Old 12th Dec 2011, 9:33 pm   #7
Sean Williams
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Default Re: Marconi CR300

The "best thing" is to leave them alone, untill you prove they need changing - you may well end up being suprised.

The last CR300 I worked on was pulled from a pile of sets that hadnt been used in 25+ years - I did some basic safety checkes, and powered it up - I think I replaced no more than half a dozen resistors, and 4 caps from memory - the set, as far as I know is still in use....

Others will have differing opinions I am sure.
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Old 12th Dec 2011, 9:43 pm   #8
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Default Re: Marconi CR300

Hello Mike a variac would be a good purchase for you, a 240 v 2 amp job would be fine.
The canned caps sometimes turn up as new old stock, I personaly would not use them though.
Re stuffing those multi cap cans is relatively straight foward, note connections, snip, undo retaining nut and remove from receiver, with care you can unfold the aluminium from underneath, remove the bottom disc and pull the guts out.
Warning Hazard: there are varying opinions as to what is in the liquid inside them, there is a possibilty it contains PCB's (not good) or other nasties (not good either) so take precautions in extracting/disposing etc.
There a various suppliers of suitable caps for restuffing, you can use 450v Dc working types if they are decoupling low voltage heater lines, cathodes, AGC lines etc, I personally use 630v DC working, the yellow Vishay ones from Farnell and others will fit in no problem, once all in and suitably wired then you can bend the aluminium back over the bottom disc, this does not have to be perfect as when refitted to the chassis tightening up the retaining nut will crimp everything up tight to chassis topside.

By the way you have not mentioned the type of power supply you intend to use? it would be handy to know in case you run into problems and need to refer back to the forum.

Lawrence.

Other members might want to add to restuffing techniques hints ideas etc.

Last edited by ms660; 12th Dec 2011 at 10:04 pm. Reason: spelling
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Old 12th Dec 2011, 10:01 pm   #9
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Default Re: Marconi CR300

Quote:
Originally Posted by Sean Williams View Post
The "best thing" is to leave them alone, untill you prove they need changing - you may well end up being suprised.

The last CR300 I worked on was pulled from a pile of sets that hadnt been used in 25+ years - I did some basic safety checkes, and powered it up - I think I replaced no more than half a dozen resistors, and 4 caps from memory - the set, as far as I know is still in use....

Others will have differing opinions I am sure.
Hello Sean, not necceserily differing but I wish I had your luck, just about every cap in mine ended leaking physically and otherwise, but then it's the luck of the draw, when I first powered mine up after a very long abcence of power it worked fine then more or less on the hour every hour they were breaking down or going O/C including what I call the micamoulds. I run some sets on 100% originals no problem, voltage checks and two weeks use after power up usually tells you what's what. If they are in LT low impedance circuits as decouplers they usually work even if leaking, its just the leaking oil you have to contend with.


Lawrence.
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Old 13th Dec 2011, 7:21 pm   #10
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Default Re: Marconi CR300

Hi
Was wondering how the CR300 is doing?

Best to check it all out after you have run it up on a variac for some time.
If you have fired it up then the power supply caps may have suffered.

No need to source original types, just use new ones which are smaller and will fit under the chassis, leave the old ones in place for appearance.

Not worth at all just renewing components, the resistors are all 20% tolerance so usually fine , perhaps some of the electrolytic caps and the paper, alloy decoupling caps will be gone.
if you find DC on valve grids look for the cap attached to that grid and replace it.very common fault.

Use correct switch cleaner and keep wd40 away from the set!!

Do not mess with coils cores if‘s etc unless you really know what you are doing and have test gear to assist you.
Great old sets, just as good as many modern units. sadly so many have been trashed by people restoring and getting into problems.

Take your time, do one thing at a time and check the result, It is an easy set to get working correctly, take care good luck and enjoy.

All the best
James
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Old 13th Dec 2011, 9:28 pm   #11
2E1CIH Mike
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Default Re: Marconi CR300

Gentlemen,

Again, many thanks to you all for your advice. I value these tips so much that I'm compiling an expanding Word doc for reference. I wish you were all my neighbours, but then again, you are all probably glad this isn't the case!

I feel suitably equipped with knowledge now to get on with a thorough clean (I bought some foam type cleaner today) and look at replacing/stuffing caps (thanks for the tips Lawrence).

I haven't powered it up since buying it. The PSU which came with it is homebrew, and the power socket has been bypassed with some ropey looking flex/wires. I need to check to see if the heater wiring has been altered to work at 6.3V (thanks for the alert re this Lawrence).

I hope to make some progress over the Xmas break (family obligations permitting).

Best wishes.....Mike
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