Wireless Set No. 52 Canadian - Project

[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