This morning while thinking about testing the tubes in our 026 keypunches at the Computer History Museum, I remembered one limitation of the circuit board I built. It can supply filament voltage only up to the supply voltage of the power brick - just over 19V - which is insufficient to test the 25L6 tubes or any others that have high heater voltage requirements.
The solution for measuring is to hook up external DC for the filament. At that moment, I was faced with new design choices.
I could add in two more positions on each rotary switch to connect external heater power to the tube socket pins, in addition to the two positions I was about to wire to connect the screen and anode voltage reservoirs for continuous measurements.
Alternatively, I could use two positions on the switches as a generalized set of special sources, with switches to route the reservoir volts and connectors to route external filament power to the same positions.
I liked the new method and set about adding that. I means that I have to find a suitable toggle switch and mount it, to connect or disconnect the reservoir voltages to these special loops. I also need to find and mount some kind of connector for external heater power.
While I sought these parts and mulled over locations on the panel, I did some remaining tasks - hooked in the top cap connectors to grid and anode voltage loops plus moved the 'power on' and 'high voltage present' LEDs off the board and onto the faceplate.
I will have a number of connections and a switch to mount, which I decided to place on the side of the cabinet instead of the faceplate due to restricted space up on top. The items to mount on the side are:
- RS232 DB9 connector for PC communication
- Switch to connect reservoir capacitors to special rotary switch positions
- Terminal block to connect external heaters to special rotary switch positions
The circuit board is now firmly mounted inside the case. Once I have the connectors added to the side, and the power brick mounted inside somehow, there are some ferrite beads and fuses to add to the filament lines before I hook it all together and close it up, plus a fuse and large choke to install on the line from the power brick to the board.
When I first mounted the DB9 connector, it stopped communication with the PC. Likely this was caused by a short from some of the handshaking bridge wires that are often soldered onto the back pins of DB9 connectors. After inspection and rearrangement, the connector is back in the case and fully functional.
I now have the ferrite beads on the filament lines, the fuses for filament and main power, the main 300 uH choke and the wires coming from the screen and anode reservoir capacitors. I needed to find and mount the switch, connected to the reservoir lines and the new loops I soldered for switch positions 10 and 11 to deliver either reservoir or external heater power to tube socket pins.
The reservoir lines can carry 300 or so volts each, which is above the voltage rating of most switches, complicating the sourcing. I needed the switch to be on-off-on, stay in any of those three positions, and have twin poles and double contacts for each pole.
Having found an acceptable switch, I have it wired in place, although it is only an on-on, no center off position. This is okay because the terminal block that will connect to external heater power can be connected to the rotary switch positions 11 and 10.
The only risk with this on-on switch would be if I switched a tube pin to 10 or 11 and inside the tube that pin was connected to another pin with screen or anode potential. Even there, the high voltage will only be present for a millisecond at a time during testing.
The power brick is installed, although I have to recalibrate slightly to accommodate the slightly different voltage supplied to the board. Mounting and insulating of a few components and connections, e.g. choke and fuses, will wrap up the work.