Thursday, April 20, 2017

Construction continues on vacuum tube curve tracer


I grabbed some lockwashers and other hardware and attempted to install the three remaining rotary switches. Alas, the chassis opening for a 9 pin mini tube socket is slightly wider than the lockwasher. While I can use flat washers to cover the gap, they don't have any grip to keep the switch from rotating when the knob is twisted. Back to the drawing board (hardware store). 

Armed with new wider star lockwashers and a combination wrench, I tightened on the three new switches and began wiring them up. By midafternoon I had seven switches wired. The next two are easier, because both skip the 7 pin mini socket and one of them also skips the two 9 pin mini sockets. 

In the meantime, I started the lines for each voltage, bridging across the various rotary switches on the same position of each, ultimately leading down to the curve tracer circuit board itself. To do this correctly, I had to decide where the board would be mounted. 

The choice is on small standoffs on the bottom inside the case. This complicates things a bit because wire runs have to be somewhat longer to allow the top to be opened for servicing. I want the wire runs to the rotary switches to be as short as I can, in a loop themselves, to minimize inductance. 

I will also run wires out from the PCB to remotely mount the two LEDs under the faceplate, so show the 'power on' and 'HV present' states to the operator. For this I had to desolder the existing LEDs, put on the wire and place my own LEDs on the faceplate. I don't need to worry about wire length for these thus it won't cause problems opening the unit for service.

This will be powered by a 19+ V DC power brick, also installed inside the case. I had one from an old laptop I used to own, which worked out great for this purpose. It will be fixed inside and the AC cable routed out in the case slot that contained the original AC cord. 

By dinnertime, I had completed wiring all nine rotary switches to the associated pins of the five tube sockets on my tester. I had also wired up the two filament loops - wiring looped through switch positions 2 and 3 respectively of every rotary switch. This worked well and lit heaters on real tubes.

I will wire the cathode, grid, screen and anode voltage loops to their relevant positions 5, 6, 7 and 8 on the rotary switches. By the evening, I was done with the screen and anode loops. A terminal snapped off the old rotary switch that I was leveraging from the original Century circuitry, which required me to remove it, drill out the hole to fit my new type of switch, and wire the new one into place. 

After these are seen to work properly, I can bring the two reservoir capacitor lines in a non-loop to positions 10 and 11 of the switches. I will finish up the cathode and grid lines tomorrow, test out everything carefully with a continuity checker, before hooking it all up to the circuit board and testing with a real tube. 

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