Wireless Set No. 52 Canadian - Project

[David Dunlop]

I finished rechecking the third and final Sender Resistance test this morning. All is now quiet on the Low Pressure Front.

This set of tests involves testing resistance from selected pins on the seven Sender valve sockets to just three terminals on either of the two 8-pin connectors on the back of the Sender. Some of these tests might require specific settings for the Band Selector Switch, Mode of Operation Switch or Power Switch.

22 tests make up this set and in the initial work through, I had 7 suspect results. Of those, three now came out spot on and the remaining four read excessively high, but not as bad as the first round. Interestingly, these four readings all come from the V1J socket for the Voltage Amplifier.

So now that I have believable data, the next step is to look at the actual circuits on the schematics to see what components relate to each test in the three sets of results, and start looking at those parts on an individual basis.


David

[David Dunlop]

I had my fingers crossed there was logic behind the layout of the three sets of resistance tests (there are no guarantees with electronics design) and I was happy to discover the testing sequences in each test indeed follow the Sender Block Diagram, starting at the front end with the Master Oscillator Tuning Circuit built around V5A, a 6V6G. They then added in V6A (the VR 150 Voltage Regulator) following into the Amplifiers V5B and V5C and arriving at V7A (the impressive 813 Power Amplifier). Testing then moved to the Audio Input Circuit V1J (ARP-3) and V5D, which ultimately end up at V7A as well.

Starting with the first test set, the first thing I noticed was I had good results through the first four blocks of the Sender (V5A, V6A, V5B and V5C) with the exception of Pin 1 on V5A, V5B and V5C.

A quick look at results on the third test set showed good results across all these valves as well. The problem showing up with the results on Pin 1 was that each test to ground should have produced a reading of 1 meg worth of Ohms. My readings were 1.50 meg taken from the top of the sockets; a little too high for my liking. From the bottom side of the chassis, access to the V5A socket is completely blocked and parts of the other three are tricky to get at as well. The real annoyance, however, was that Pin 1 on all of these valves is an open pin, never used by the valves and the designers take advantage of this feature to use the Pin 1 terminal on the socket as a connection tag to solder leads and wires from other components to, in order to complete other, often unrelated circuits. The issue with this is that when you look at the circuit schematics for any one of these valves, connections to Pin 1 do not show up. They are blank. The only way to find out what is connected to them is to actually look very carefully at the actual terminals on the sockets and hope the connections can be easily traced to identifiable components.

So before being able to dig any deeper into the resistance test results, I needed to get a better look at what was going on underneath the V5A socket. The attached photo shows what this section of the chassis looks like from the bottom, covered with a heavy metal plate with the Modification Card Holder mounted to it.

David

[David Dunlop]

I tend to be very careful when taking on something new, that I know nothing about and gaining access to the metal box hiding this circuit is a prime example of that action. I had no idea at all what was inside, or where and how it was mounted. I could see an access hole in the lower rear chassis wall for purposes of tuning the coil, the V5A Holder (socket) on top of the lower chassis and that was about it There also had to be a switch lurking inside somewhere for purposes of selecting Crystals.

The first discovery was this piece of metal did not exist in the Illustrated Parts List at all, first or second Issue. It was not a cover, a plate or a shield. Not an encouraging start. I then thought about the existence of the Modification Card Holder fitted to the bottom and wondered if the installation instruction for the holder might shed some light on things. It did. It was called a ‘Plate’, four screws held it in place (2 top and bottom in the photo in the previous Post), and the Plate was to be removed carefully.

After looking closely at how the Plate was installed in the Sender chassis, I could see the outside end was slipped in behind the lower left rear side of the Sender chassis rail, in behind the heavy reinforced rail the Sender slides in and out of the Carriers No. 4 on. The inside end of the Plate just slipped in beside an adjoining chassis compartment wall. So, with the Sender resting on its right side, putting the heavy rail at the top, I removed the four retaining screws and lock washers. The plate was a snug fit. I had to very carefully insert a slot head screw driver blade just into the open slot either side of the Plate at each lower side and ease the Plate out enough to be able to grab the lower edge with my fingers and pull it free, but not out. While holding it in place, I then slid the screwdriver blade carefully up the slot to the top of the Plate to keep it flush with the chassis rail. If this end of the Plate slipped inwards, it hung up on something unknown. Once you can clear the inside end of the Plate from the partition wall it attaches to, you can move the lip of the Plate inwards enough to allow the upper end tucked behind the chassis rail to drop free, and remove the Plate.

The four photos here illustrate the location of the four screws the Plate is mounted to the Sender Chassis with, what the bottom of the Plate looks like once removed and what the inner (top) of the Plate looks like, full of dirt and dust bunnies.

The real surprise in this exercise was the discovery of the hole, roughly centered in the Plate, which is effectively blocked by the Modification Card Holder. My initial though was it being another access hole for tuning something. NOPE! Turns out it is a ‘finger hole’ thoughtfully designed into this Plate by Canadian Marconi to make removing and installing this Plate an easy task to accomplish. My guess is that any surviving Senders in the UK will still have an easy time working with this Plate. Canadian owners, not so much.

David

[David Dunlop]

Finally, my first look inside this section of the Sender chassis. What struck me was how clean everything was, with little or no dirt and most important, no sign if heat discolouration or burnt smells lingering about.

The most notable thing was the pristine looking L31A COILS, RF sitting front and centre, complete with its CMC Part Number stamped on the side of it – CMC 119-209. I am not sure what prompted me to do it, but I decided to run that number against the Parts List at the back of the Operators Manual. Did that ever turn up the excitement!

According to said Parts List, L31A had Part Number CMC 119-107. The number stamped on the assembly in my Sender was nowhere to be found. So next stop was the 1945 Illustrated Parts List. It showed L31A as being, COILS, RF, 1.75-4-mc, MO, No. C1. It had a 1-1/32 inch diameter with two riveted 6-32 mounting screws at one end and no sign of a central, adjustable core. The ZA-Number reference given was ZA/CAN 4355.

A pail of tea later, I decided to check the 1948 Issue of the Illustrated Parts List for a possible explanation. Bingo! The same reference for ZA/CAN 4355 was found, but this time, directly below it was a second item, ZA/CAN 4221. It was a stink smaller diameter and was fitted to its mounting Bracket, that also held the central, adjustable iron dust core. So this complete assembly bears the CMC 119-209 Part Number.

My thought is the original design, in theory, allowed for replacement of either the coil, or the core, but the amount of work required to do either, was more than making the Coil and Bracket a single item replacement. Interestingly, the Bracket itself does not show up at all in either the 1945 or 1948 Illustrated Parts Lists. Having confirmed at this point that all the components in the Master Oscillator Tuning Circuit were replaced during the Senders last overhaul when in service, I then traced the connections to Pin 1 on the V5A Holder (socket) and found nothing worth a red flag. To be on the safe side, I retested the resistance of this Pin circuit to ground after getting a good clean contact point on the pin and now found the reading to be a steady 1.25 meg. I liked that reading and took the time to recheck Pin 1 on V5B and V5C in the same way. They both now showed 1.25 meg as well. So I was glad in the end to have opened up the shielding plate for this circuit and had a closer look.

The surprises were not quite over, however.

David

[David Dunlop]

While rechecking the Pin 1 terminal on the underside of the V5A Socket, I had noticed the two rivets securing the retaining ring on the socket to the Sender chassis floor. There was something odd about that, so I decided to look up what the Illustrated Parts List had to say. It would be an easy item to find because the Holders (sockets) for the 813 and this particular 6V6G are the only Holders in the entire 52-Set that are made of porcelain (Steatite).

As expected, it was easily found but the one in my Sender was now clearly confirmed to be a replacement item as well during the 1966 Overhaul. The attached illustration from the 1948 Issue of the Parts List shows this Holder as being of the Ring Retained style, not the riveted retaining plate as per mine. So perhaps the originals were discontinued for some reason, or got too expensive by 1966.

The last photo in this Post is of the Plate after all the dirt and dust bunnies were cleaned off the inside face.

To reinstall this Plate, at least on a Canadian Sender with the Modification Card Holder fitted, the best way is to let gravity help you.

Turn the Sender onto its left side so the heavy side rail of the chassis is on your bench. Slip the end of the Plate with the two tabs carefully down behind the rail and tip the other end in towards the bottom of the chassis. Then press down gently on the long tab at this end until it slips past the partition it fastens to and push it home until you can see the screw holes in the partition holes. Use a small nail, or similar item to centre the two screw holes and install the two screws half way home. Flip the Sender to its right side to put the two chassis rail holed to the top, do the same centering trick and install these two screws carefully home. Make certain, however, that the left hand screw in Photo 1 of Post #854, just clears the chassis rail lip it near. if not, it will hang up on the lip and dig into your bench top when you least expect it. don't ask how I know. You can then tighten the other two all the way.

If you are lucky enough to have a UK Service 52-Set Sender, you will have much better control of the Plate with a finger in the hole intended for this purpose.

Next step will be a close look at what is going on with V7A, the 813, where some seriously high resistance results were showing up.

David

[David Dunlop]

I have been looking at the test results for Valve V7A (the 813) this morning. It is quite a comprehensive set of tests in its own right for this valve. Everything gets checked twice, in each of the Power Settings, LOW, MEDIUM and HIGH and the Mode of Operation Switch set to RT. Then you repeat the tests with the Mode of Operation Switch at any of its other settings, M.C.W., CW or BREAK IN.

What I finally noticed was that the vast majority of the extremely excessive readings I got took place in the RT Mode. By excessive, I mean results that should have been 10,000 Ohms, or between 1 and 2 Megs, were coming back 25, 200, 500 Megs or Infinity.

Something definitely is not right. The test indications of an excess load lurking somewhere match up with the observation of the Supply Unit being hit with a massive load with both Dynamotors running in SEND Mode in order to check proper operation of the Receiver Isolation Relay. That prompted a quick shut down of the set as soon as proper function of the relay was heard. A careful visual examination of the Sender shows no heat discolouration anywhere. Nor are there any burnt smells anywhere, and the Supply Unit continues to function nicely with the Sender out of the Carriers No. 4 circuit and on the bench alone.

I do wonder if the Mode of Operation Switch may have a buildup of oxidation on its terminal wafer connections that I missed, so to rule that out, I am going to give that switch a couple of cleanings with Dioxit today and then repeat the set of tests for V7A. Nothing may change, but at least I can rule the possibility out and know the Mode of Operation Switch will now be nice and clean.


David

[David Dunlop]

I was able to give the Mode of Operation switch a couple of good cleanings with Deoxit the other day, which produced some positive results regarding the resistance tests for the V7A (813) Socket Pins.

First, the suspect results on Pin 3 all dropped by nearly 250,000 Ohms and are all now in a quite acceptable range. There was also an improvement in a couple of the readings for Pin 4. One reading should be 40,000 Ohms and I was getting ‘Infinity’ initially. I now get 60,000 Ohms. The second reading is supposed to be 11,000 Ohms and it was also initially giving me ‘Infinity’. It now reads 700,000 Ohms. Neither is acceptable yet, but at least I have hard numbers to work with. Every little improvement helps.

In the course of doing all this work, I stumbled across a mystery on the Circuit Diagram for the Sender, as published by Canadian Marconi on Page 203 of the Operators Manual. Still have not got it all sorted out yet (maybe never will) but I will document it all shortly.

By the way, with regards to Circuit Diagrams for the 52-Set, should you ever need them, trust only those published by Canadian Marconi Company in the Operators Manual for the 52-Set. There is another set out there redrawn and included in an evaluation document for the 52-Set. These ‘copies’ have a lot of missing information and mistakes in them. Not at all reliable for serious repairs or servicing.


David