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Old 20th May 2020, 10:18 pm   #21
Silicon
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Default Re: Marconi CR100 B28 Restoration Help

The metal cased bypass capacitors that are bolted to the chassis are paper insulated capacitors.

I measured the insulation at 250V of one of these in a CR100 just now. It was 200K.

A previous repairer had disconnected the old capacitor and left it in place.

They had soldered a replacement to pin 7 of the octal valve holder which is the heater ground/chassis. The other end was soldered to pin 4 which is the screen grid.
This is a very convenient method for attaching modern small ceramic or polypropylene capacitors.

The same could be done with the cathode bypass capacitor.

In theory this will introduce less inductance in series with the bypass capacitors than the original wiring arrangement. This may give a marginal improvement in performance.
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Old 21st May 2020, 6:29 pm   #22
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Default Re: Marconi CR100 B28 Restoration Help

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Originally Posted by Silicon View Post
The metal cased bypass capacitors that are bolted to the chassis are paper insulated capacitors.

I measured the insulation at 250V of one of these in a CR100 just now. It was 200K.

A previous repairer had disconnected the old capacitor and left it in place.

They had soldered a replacement to pin 7 of the octal valve holder which is the heater ground/chassis. The other end was soldered to pin 4 which is the screen grid.
This is a very convenient method for attaching modern small ceramic or polypropylene capacitors.

The same could be done with the cathode bypass capacitor.

In theory this will introduce less inductance in series with the bypass capacitors than the original wiring arrangement. This may give a marginal improvement in performance.
Thank you for pointing out that they are paper insulated capacitors.
I take it that they must be all replaced.

And the alternative way to replace and wire them sounds interesting too. Would you have photos for them how they are done? That would be great.
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Old 21st May 2020, 7:57 pm   #23
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Default Re: Marconi CR100 B28 Restoration Help

You may find when replacing bad wires that some of the ones which look good may have insulation which has gone hard and brittle, so the moment you disturb things to do the bad wires, the good ones fall apart.

Probably all your paper capacitors are in similar condition. They are known to not last this long.

Putting smaller modern capacitors right where they're needed is electrically superior. Restuffing the existing bolted cases is cosmetically superior.

You get to choose, based on your intent.

Me? I'd take the bolted ones right out, giving a lighter set with better access. I'd probably go for metal film resistors and see what a CR100 really was capable of. They're less noisy and more reliable than carbon composition resistors - better than carbon film as well.

What it won't bring back is the sunspot max and lack of domestic QRM of when I last drove a CR100 in anger.

David
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Old 21st May 2020, 8:10 pm   #24
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Default Re: Marconi CR100 B28 Restoration Help

Great advice David. Thank you.

Yes, I would go for easier and simpler option, as long as it works and reliably too.
I have got a box set of high quality metal film resistors and they cover most values. They will go into the CR100 replacing the old ones.
But for the capacitors, I must do some searching and shopping.
I might give the chassis and the parts on it, a good clean with old tooth brush and WD40. I think WD40 kills rust and dirts, and it evaporates fast, so it should be OK? The parts on the chassis top are mostly metal and steel, so it won't melt or expand like plastic.

I think the Ham Bands are pretty quiet and it has been like that for years now, and I am not sure when they will come back. But the SW BC bands are still active and gets very busy in the evenings and nights, so if you are into SWL, still the receivers can be good fun toy to play with?
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Old 22nd May 2020, 2:07 am   #25
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Default Re: Marconi CR100 B28 Restoration Help

For replacing the paper capacitors The common yellow plastic film ones ought to be fine. They're what I've used in a number of sets, as have lots of other people.

WD40 is a hydrocarbon mixture developed to chase water off of things in heavy industry. It'll loosen some sorts of muck for you, but it doesn't do anything about rust. Iso-propyl alcohol is another benign solvent for cleaning, and it doesn't leave a residue.

I think I'll nip down to the radio shack and put the main transceiver on for a quick listen to the broadcast bands. I haven't had a listen to them for some time. The housing area QRM ought to be low just now. 50MHz has been giving sporadic E openings in the afternoons for the past few days.

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Old 22nd May 2020, 10:21 am   #26
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Default Re: Marconi CR100 B28 Restoration Help

Some people have reported that the paxolin tag boards have become conductive.

Mine seem to be covered in dirt and mould.
When I restore my CR100 I will give the boards a good clean and then test the insulation at 250V.

If they fail I may buy some double row turret or tag boards made with glass fibre (FR4).

I will probable add a voltage stabilised supply for the oscillator and screen grids.
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Old 22nd May 2020, 10:40 am   #27
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Default Re: Marconi CR100 B28 Restoration

In the CR300 I'm slowly getting back to health the triple 0.1uF capacitor cans are all leaking a black gooey tar like substance. As they are almost impossible to remove and restuff I'm going to replace the caps externally and seal the ends of the old cans to stop them leaking.
One had already been done and its associated resistor replaced. I surmise that the cap went short and took out the resistor.
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Old 22nd May 2020, 2:11 pm   #28
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Default Re: Marconi CR100 B28 Restoration Help

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WD40 is a hydrocarbon mixture developed to chase water off of things in heavy industry. It'll loosen some sorts of muck for you, but it doesn't do anything about rust. Iso-propyl alcohol is another benign solvent for cleaning, and it doesn't leave a residue.

david
I once cleaned an old metal toolbox covered in rust with WD40 and wire wool, and the rust has cleared up quite well. WD40 itself alone will not clear rust, but with your elbow grease and wire wool, it seemed it had done the job.

Iso-propyl alcohol seem gone up the price since the COVID19, and is rather difficult to get hold of them these days, it seems. Where is the best place to buy them?

In this house, the Plasma TV is the worst source for HF reception QRM. It wipes out the 80m, but 40m and upwards is OK. I used to monitor the 50Mhz openings in the past when I was B class licensee, but now I don't have anything for the band. It is too unpredictable and short lived chance for QSOs.
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Old 22nd May 2020, 2:13 pm   #29
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Default Re: Marconi CR100 B28 Restoration Help

Quote:
Originally Posted by Silicon View Post
When I restore my CR100 I will give the boards a good clean and then test the insulation at 250V.

If they fail I may buy some double row turret or tag boards made with glass fibre (FR4).

I will probable add a voltage stabilised supply for the oscillator and screen grids.
From the insulation test, what ohms is regarded as fail? And what ohms for pass?
Would it depend on the input voltage?
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Old 22nd May 2020, 3:14 pm   #30
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Default Re: Marconi CR100 B28 Restoration

There is cheap IPA on Ebay But it is from China.Re WD40 I feel it will encourage more dust etc after you have cleaned.

Believe in the past on here ,Wizard Bang or Cillit Bang has been mentioned as very good.
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Old 22nd May 2020, 8:30 pm   #31
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Default Re: Marconi CR100 B28 Restoration Help

Quote:
Originally Posted by Silicon View Post
Some people have reported that the paxolin tag boards have become conductive.

Mine seem to be covered in dirt and mould.
When I restore my CR100 I will give the boards a good clean and then test the insulation at 250V.

If they fail I may buy some double row turret or tag boards made with glass fibre (FR4).

I will probable add a voltage stabilised supply for the oscillator and screen grids.
Long ago, and before I was really aware of the vintage restoration scene, I used double row tag boards in place of the original bespoke Bakelised linen component boards with their charming hand-painted Services component codes- this was when the electronics shops (remember them?) were doing their best to get rid of them at 50p for an 18" length, now I suspect they cost a bit more! I went a bit OTT with the overhaul, fitting feed-through capacitors to the brass RF boxes for HT and AGC to the RF stages, and a row on the IF divider screen for anode HT, screen grid HT, AGC and manual gain feeds- I had a bag-full, I though they looked neat and they seemed a convenient way to terminate wiring runs. Nowadays, I'd probably be a bit more sympathetic to originality on both counts.

As the LO uses series link-derived feedback on all bands, rather than shunted RC feed, I reasoned that I could reduce the hot and bulky 20k anode feed resistor next to the oscillator box fed from the 250-260V HT line with a small and cooler 4k7 fed from stabilised 108V courtesy of an 0B2. I tried various healthy and knackered KTW61/62s and 6K7Gs with either feed and found that weary 6K7Gs gave up oscillation at precisely the same point in both cases, thus an otherwise transparent mod.

Unfortunately, most CR100s are almost certain to be a "square one" overhaul to make them reliable, safe and effective with its rubber wiring, paper capacitors and composition resistors. I wouldn't be surprised if many had gone by the wayside because of the slightly daunting nature of complete overhaul. It does make for a good band-cruiser (as I believe they say over the other side of the Atlantic) with the ability to quickly span 4 to 30MHz with just one band-change click and that nice dual-ratio tuning.
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Old 14th Sep 2020, 9:33 am   #32
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Default Re: Marconi CR100 B28 Restoration

Thank you all for the very useful comments in this thread. I'm three weeks into restoration of my B28 (dated 1952) which came to me in untouched condition, complete with filth, persished/brittle rubber wiring and passive components that are mostly way out of tolerance. I hope I won't offend any purists by saying that I'll be using modern parts and connecting wire to complete the job.

I was overjoyed to find that the 3 x 8uF electrolytic reformed perfectly and resulted in residual leakage per section of 15uA.

As regards the thin coax, I was thinking of replacing it with BT3002 (as I have some to hand) and maybe extending its use in some areas. Would anyone see a problem with that please?

Thanks again. I'll report back when I've made significant progress

Bill Atkins
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