Marconi C100 B28 vs. Eddystones

[Julesomega]

Thanks for the reminder, Dave. Just as well the pandemic is keeping us grounded or I'd be out looking for these, disregarding that I've no space left for any more boatanchors

[G3VKM_Roger]

Originally Posted by ORAWA01

Thank you. When I start working on it, will start a new thread for working on the CR100. It has no power lead and 4 x valves are missing just by quick look around it. I am not sure what will need for replacement and what more. Will need your advice and info.

Be interested to see how your renovations go, I acquired two CR-100s last year with a view to making one decent set plus spares and then fit the little 5 watt transmitter inside the lid, I found that on eBay in unissued condition.

I was looking for a power plug and a member of this forum sent me a pair of them, along with a shoe box of other unused spares, like the band-switch and an output transformer. If you find you need a part, send me a P/M and I'll see what I have. The person who sent me the spares wouldn't take any payment for any of them, so I'll pass the favour on.

Cheers

Roger

[dave walsh]

Well that's the best of the Forum for you Roger and why we participate
Glad to see you get it! When I was an impoverished young student in a village pub near Bolton it was hard to buy a drink. Two local residents [a Plumber and a Merchant Banker] had the same attitude ie pass it on to someone else when you can afford it! That taught me a lot.

Dave W

[ORAWA01]

Originally Posted by G3VKM_Roger

Be interested to see how your renovations go, I acquired two CR-100s last year with a view to making one decent set plus spares and then fit the little 5 watt transmitter inside the lid, I found that on eBay in unissued condition.

I was looking for a power plug and a member of this forum sent me a pair of them, along with a shoe box of other unused spares, like the band-switch and an output transformer. If you find you need a part, send me a P/M and I'll see what I have. The person who sent me the spares wouldn't take any payment for any of them, so I'll pass the favour on.

Will update with the progress, and the final result of the restoration work here.
That is great. Thank you so much Roger. Will PM you if / when the CR100 needs parts. I was thinking of making up a power lead with wire and soldering them into something, but it would be nice to have a proper power lead.

Thank you 73

Jay

[G6Tanuki]

If you can get hold of a copy of "Short Wave Wireless Communication" by A.W.Ladner and C.R.Stoner [Jarrold and sons, third-edition, 1936] it has a good technical-design and performance-analysis of the CR100.

Because of the coronavirus-thing I can't do proper hi-res scans of the relevant pages - but if you'd be happy I could do some phonecam shots of the appropriate pages and email them to you?

[turretslug]

Did they come up with any notable points of praise/detraction? I got the feeling that it was a commendable attempt to come up with what, for the time, was a well-specified general coverage receiver that ticked the professional expectation boxes of the time (4-gang tuning with 2x RF stages for good image suppression, separate LO and mixer stages for improved stability, 3x IF amps for good passband and AGC performance, and a good selection of IF bandwidths plus a narrow AF filter) and yet used simple consumer radio production techniques and components so that lots could be churned out quickly and cheaply at a time of duress. The baked paper coil formers would have been a minimal-cost option but probably don't age well, especially in humid conditions, and the CR100 seems to have carried a reputation as prone to drift- which is a shame, as otherwise it would be a very good radio. I particularly liked the dual-speed coaxial tuning drive which on mine was very smooth and precise after overhaul, but I have heard it can be prone to wear and damage. The commercial-quality mains transformer also seems to have been a weak point.

[G6Tanuki]

Here we go: apologies for the crap phonecam resolution...

Remember that this analysis was done from the perspective of commercial point-to-point HF circuits, not ad-hoc ham-radio operation.

They don't mention the instability and wobbliness in the face of mains-voltage fluctuations, or the LO drift over several hours as the receiver warms-up after a cold-start. I guess 'commercial' applications had stable mains and involved receivers that were not subject to turning-on-and-offable thermal-cycling.

[Radio Wrangler]

The other radios had their own failings.

THe AR88 was supposed to have been made as expense no object, but it's got lots of 'Micamold' capacitors which are fake small value mica capacitors, which are actually paper ones packaged like mica ones of the period.

David

[turretslug]

Thanks for posting those pages- perfectly legible here. I'm sure I heard of a case at an outlying coastguard station where a CR100 sat on 2182 for several decades! I expect that even before war's end lots of CR100s were looking for something to do as the tsunami of AR88s etc. from across the Atlantic took on much demanding work.

I was always puzzled that Marconi's subsequent marine receiver duo, Mercury/Electra introduced in 1953, (and still featuring CR100-style rolling frequency-scale display) continued to use the same grid-cap IO valves as the CR100 (KTW62/X66 etc. series) when two subsequent valve series (Loktal, Rimlock) had come and gone and the B7g/B9a miniature era was well underway. Maybe M-OV had crate-loads left over, or the shipping world was conservative in outlook and trusted what it was used to.

[ORAWA01]

Thank you for posting the documents. Very useful and interesting.Yes, they are readable to me too.

I think CR100 is very interesting radio in that it is now more than half a century old, and there are plenty of them still going strong.
I personally find myself getting enthusiastic about the radio because it has lot of room to work on, and it feels that it will turn out to be very interesting restoration project.

For its performance, I cannot say much simply because I have never operated one by myself. My feelings are from the reviews, posts and a few Youtube videos, and I thought it could be a fun receiver to run in SWLing or listening to CW / SSB transmissions on HF amateur radio bands regardless of its shortcoming on the paper.

I might even try using the CR100 as a accompanying receiver pairing with one of my HF transmitter on HF, when it is fully operational.

Yes, there are more sophisticated and better performing receivers out there, but this CR100 is one for me for restoration project to learn and use in daily reception on HF.

The weight and size of CR100 is not too overwhelming, and that is also very plus point for me.

There seem a lot of interest from radio community as well, which is also great. This means that help, advice and info will be plenty available from the forum members, and also it would be relatively easy to source missing parts and worn out components, if / when they are not available from the commercial Electronics parts suppliers.

[CarbonMike]

"If you can get hold of a copy of "Short Wave Wireless Communication" by A.W.Ladner and C.R.Stoner [Jarrold and sons, third-edition, 1936] it has a good technical-design and performance-analysis of the CR100."

I don't want to be pedantic but 1936 seems very early for the CR100. My copy of that book is the fourth edition (1942) and has the CR100 material on the same pages.
Does anyone know when the CR100 entered service? Apart from the merchant marine, who else used it?

[Andrew Sinclair]

The CR100 was introduced in about 1940, as the existing equipment could not be mass-produced. (There are surviving examples in the HMS Collingwood heritage collection, I don't think they would have had a high performance either.)
I restored a B28 from a very poor example a few years ago. I found that the paxolin insulation on the tag boards was leaky. I also had to replace the system switch wafer as it added to the problem. The main leakage path is from the HT rail to the AGC line. Mine had +12V on the AGC with no signal rather than 0V.

The Marconi CR100 manual is very helpful and repays close study.

I have attached a page from "Low Medium and High Frequency Communication to and from HM Ships" by Anderson and Grainger (1947). This paper explains much of the thinking at the time.

[Radio Wrangler]

Ah, a memory just stirred. The epic G2DAF receiver got mentioned a bit back in this thread. The last time I was over at a Stirling club openday on the industrial estate at Throsk, there was a G2DAF rx sitting on a top shelf in one of the shacks.

David

(Throsk sounds like the vernacular name of a malady of cattle)

[turretslug]

Originally Posted by Andrew Sinclair

The CR100 was introduced in about 1940, as the existing equipment could not be mass-produced. (There are surviving examples in the HMS Collingwood heritage collection, I don't think they would have had a high performance either.)

I have attached a page from "Low Medium and High Frequency Communication to and from HM Ships" by Anderson and Grainger (1947). This paper explains much of the thinking at the time.

Thanks for posting that, it made for an interesting read. Reading between the lines, peace-time military equipment can have a tendency to over-engineering without necessarily resulting in performance bonus. If only a relatively small number of examples of a piece of equipment are needed, both the cost and production effort won't raise too many eyebrows but once quantities required increase by orders of magnitude, price and urgency become very significant indeed.

It's hardly surprising that the Eddystone 358 came to be regarded as an oscillator radiation liability- the coil-pack-in-a-pluggable-metal-case recipe will cause circulating current coupling between sections without specific effort to isolate them electrically, it's noteworthy that the other receiver famous for adopting this format, the HRO, also had this reputation. The Scott Laboratories shipboard receivers took care to isolate front-end sections with insulating bushes and strips, plus the use of insulating switch-shafts and gang-capacitor couplings. Post-war, many manufacturers adopted these measures- one thing that marked the Eddystone 680's evolution into the 730 was the use of sectioned screening plates secured by insulating washers to minimise stray coupling.

The CR100's metalwork is simple to the point of looking almost home-made, but understandable under the circumstances. Part of the greatness of the AR88 is that it combines advanced techniques such as the stable, low-loss polystyrene coil formers, bandspreading by switching gang-capacitor sections and oscillator voltage regulation with simple steel foldings and pressings that were inexpensive yet precisely made. It certainly followed the US mass-production philosophy of investing to save, i.e. the machine that stamps precise parts out of standard mild steel sheet might cost lots of money but if it can repeat that process thousands of times with inexpensive feedstock, then it's worth it.

[ORAWA01]

There is a very interesting thread on CR100 and AR88.

https://www.vintage-radio.net/forum/...ad.php?t=75600

[Peter F4VSA]

I used to have three B40's, lovely receivers. I'm down to only one now and I can't lift it!! I do like my R210's though

[trh01uk]

Originally Posted by G6Tanuki

Here we go: apologies for the crap phonecam resolution...

Remember that this analysis was done from the perspective of commercial point-to-point HF circuits, not ad-hoc ham-radio operation.

They don't mention the instability and wobbliness in the face of mains-voltage fluctuations, or the LO drift over several hours as the receiver warms-up after a cold-start. I guess 'commercial' applications had stable mains and involved receivers that were not subject to turning-on-and-offable thermal-cycling.

And what they do note is virtually all about the sensitivity of the receiver compared to its thermal noise floor. Which is odd - because virtually everyone knows these days that the noise on HF is such that thermal noise is simply an irrelevance. And with 2uV sensitivity I would have thought it would have been the same in 1936. Unless of course, the HF bands were genuinely much quieter back then?

[CarbonMike]

"......the HF bands were genuinely much quieter back then? "

There is anecdotal evidence that the HF bands were quieter prewar- the old timers in our radio club used to reminisce about it. One of them had been a Marconi Marine operator and reckoned he could regularly work UK coast stations from Australia with just a few watts. However I think your point about thermal noise is still valid.