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Old 29-01-23, 03:50
David Dunlop David Dunlop is offline
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Join Date: Feb 2006
Location: Winnipeg, MB
Posts: 3,384
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It is interesting how simple little things can bring a big project to a halt.

The valves from the Sender are all neatly packed away on a corner of my work bench and the Sender sits quietly covered up to one side, while I try and sort out finding replacement Carbon Composite Resistors that I can actually work with in the 52-Set restoration.

The problem is with the modern overall length of new resistors (lead, body, lead). Turns out wartime manufactured resistors of this type had an overall length of about 3.4-Inches. With the two main sizes of Tag Panels used in the 52-Set having tag spacings of 2.25-Inches and 2.50-Inches, this component length was ideal. What I have been finding in the modern market are most overall lengths coming in at 2.38-Inches, so you can quickly see the road block.

On the bright side, some international inquiries have resulted in leads trickling in (yes, there is a pun there) of a few suppliers about with Carbon Composite new made resistors with overall lengths close to 3.0-Inches. I may have to go to higher wattage ratings than the original resistors to get ones that will fit, but that will not affect performance at all. It is just all very time consuming.

And there is yet another wrinkle in the T-shirt. To replace all the resistors currently identified as needing it, two of the Tag Panels must be removed from the sides of the Sender chassis to better expose the tags for working on and these panels hold nearly 10 capacitors that need to be tested. I cannot reach the back terminals on any of them for testing at the moment and I know a few of them will be toast. Adding to the excitement, a few of the capacitors will have to be removed to expose resistors tucked behind them that need replacing. So a whole bunch of adventures are currently hurtling down the side roads towards the main highway and I still have not finished reading the road map!

There is an interesting curious side to all this, however.

Since I had carefully disassembled the spare Receiver I had on hand, I decided to check if any of the resistors I needed to replace in the Sender, might also have been used in the Receiver and test them for possible use. There were a few, but all were installed in locations where, if carefully removed, their leads would have been far too short for reuse in the Sender in any event. But something interesting in the way the Canadian Marconi Company arranged the parts lists in the Working Instructions Manual for the 52-Set unexpectedly popped up when you looked at long lists of the same component.

It is common practice for companies to list specific parts in ascending order of their electronic specifications. For example with resistors start with 1/4-Watt and within that group list from lowest Ohms value to highest. Then go to 1/2-Watt and repeat. And so on. Same process for capacitors. CMC took this organization one step further. They have a column in the parts lists that identifies which component the parts are used in and I discovered when looking at the larger lists of identical parts, CMC always listed the components in the following sequence of first to last:

R: Receiver
CC: Crystal Calibrator
S: Sender
SU: Supply Unit

So if you look at capacitors in the C3- series ( .1 uf, 500 Volts, 20%), there is a lot of them. C3A to C3Z are all found in the Receiver. C3AA to C3AF are found in the Sender and C3AG to C3AK are in the Supply Unit.

At the other end of the spectrum, C32A ( .01uf, 500 Volts, 20%) is in the Sender and C32B in the Supply Unit…end of list for item.

I do not know why CMC adopted this convention in the parts list found in the 52-Set Working Instructions, or even if they did it in other products as well, but at least I now am aware that any future components in the Sender I find with a Circuit Reference Number ending in ‘A’ will definitely not have a spare lurking in the Receiver or Crystal Calibrator.


David
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