My apologies for taking a while to post more information on this project. I am at the point where I have a Remote Receiver that is functioning very well, but not completely checked for electrical basics, and I have no idea when it was last aligned and calibrated.
By comparison, the Main Set Receiver cannot detect a signal to save its soul and even its own Crystal Calibration Signals drop off the tuning dial above 4.40 MC. and, when it was last aligned and calibrated is also unknown.
So to start, I took the Main Set Receiver and went through all the valves to test them and did a complete Meter Reading of all its valves internally to see what that told me, compared to the manufacturers specs in the Operators Manual. Since the same Grid Circuit is used to obtain the meter readings from each valve, I also had to identify and find the Bias Resistors used in each of these valve circuits to obtain the meter readings, to check the resistors for the ability to still be providing valid readings.
The meter readings were the easy part. Warm the receiver up for 50 minutes and then turn the meter switch through each position according to the manual and write the readings down. The valve testing went really well also; thanks to the British Valve Adapter I built last year. All tested in a tight cluster in the upper 70’s on my Tube Tester. NOS valves, brand new, test consistently in the upper 80’s by comparison.
Finding all the Bias Resistors to check was another matter. Canadian Marconi Company went the route of consolidating nearly all small components related to the valves on Terminal Boards remotely located from their respective valves to make access easier, sort of.
As you will see in the first two photos, Capacitors and resistors frequently share common terminal sets and then additional capacitors get fitted next door, so access for replacement gets limited. Components on adjoining boards can also overlap, further challenging access. Then the next BIG wrinkle surfaces.
75 plus years ago, electronic components were large and lots of space was needed in a chassis to mount them. The spread between terminals on these boards is exactly 2.0 inches. That means the total length of a new resistor, or capacitor, tip to tip on the leads and the component body included was very likely 3 inches. The component could be tied to the terminal posts, soldered in place and the leads trimmed easily. To replace one of these components today, you are lucky to find any with total length greater than 2.5 inches and sometimes only 2.25 inches. Next to no wiggle room. One has to be very careful planning to work out.
What I did discover, however, is that CMC mounted these Terminal Boards of small hollow steel tube standoffs about one quarter to three eighths inches long. The base end is essentially a hollow tube rivet that gets fastened to the chassis and the other end is threaded to take a slotted, round head machine screw and external toothed lock washer to mount the board. Removing the screws, hopefully, should allow the boards to be carefully manoeuvred a bit to gain a little more access to components needing replacement.
The last wrinkle was I could not find the R20D Bias Resistor anywhere for valve V1B. Checking Allan Isaacs work in England on these receivers, I discovered he could not find it either. I put a call out for help and Jacques Fortin eventually replied, complete with photos to show me this resistor and a related Grid Leak Resistor were actually mounted on a tiny little terminal strip directly underneath V1B at the front of the receiver. The Grid Leak Resistor can just be seen, left centre in the last photos with the Green, Black and Orange bands painted on it. The all-important R20D, Bias Resistor is hiding directly behind this one, completely inaccessible without performing a 2nd to 4th Echelon Repair.
So now I have all the initial data for the Main Set Receiver and have crafted a spreadsheet to log it all in. Next step is to repeat the process with the Remote Receiver, so I can compare notes between them.
David
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