The Sequencer Unit is responsible for a controlled power-up sequence of the LGP-30. It’s a rather clever design, using ‘Cramer Time Delay’ units for the delayed sequencing, and open frame relays for switching.
When you hit the Power On button, a contactor pulls in and electrically latches. The Blower Unit spins up and the tube filaments light at half power. The first time-delay relay, which is a modified telechron style motor with spring loaded arm, begins advancing. After 50 seconds, the arm makes contact with a microswitch that snaps in a second contactor. When this happens, power is cut to the Cramer unit, and the spring resets the arm. The contactor spins up the magnetic drum, full power is applied to the filaments and the machine switches from ‘Standby’ to ‘Standby to Operate’. A second time delay relay starts, and after another 50 seconds, a 4-pole relay pulls in which applies B+ to the logic units and switches the status to ‘Operate’. The machine is now ready to go!
Should you want to idle the machine, you can hit the Standby button which cuts B+ and takes the filaments to half power. To return to full power, you’d first hit ‘Manual Input’, then ‘Operate’. 50 seconds later you’re up and running.
The design is straightforward with little in the way of discrete components other than an electrolytic cap. One 6.3V transformer provides power to the control panel lamps and another actuates the Compute/Stop relay triggered by the machine’s logic. A Running Time meter is also wired to this relay and therein lies an interesting story. When I first pulled this chassis for cleaning, I found a lamp cord stuffed into the cabinet; one end loose, the other hacked into the wiring of the sequencer. If you look at some of the B&W photos from when the machine was installed at Underhill, you’ll see a darkroom timer on top of the cabinet used for charging computer time by the minute- This was the power cord! In order to power it they had hacked into the run-time circuit.
The sequencer had been installed for a week or two after a cleanup and recapping, and appeared functional, though I was getting a little erratic operation of the Stdby/Stdby to Op/Operate indicators, though with the machine not actually functional, it wasn’t clear if this issue was specific to the sequencer. With the mag drum installed and all chassis connected, I could now investigate the issue. After studying the schematic, I tracked down the issues to a single relay. This 4PDT unit is designed to electrically latch after the second time delay relate actuates, it controls the three status indicators on the control panel and it has contacts to switch the B+ power. Unfortunately, a couple of these contacts appear to be beyond saving. A little net searching the next day turned up a surplus NOS 3-pole ‘Advance’ brand unit that looks like it’s from the same family. A click of the mouse later and it’s on its way. The thought being I can swap contacts from one unit to the other, though soldering doesn’t look like a lot of fun given the location.
Update: The new relay arrived, and the leftmost set of contacts above were changed out; the others actually looked good once taken apart for inspection. With some tweaking and adjusting, we were ready to go. The Sequencer was reinstalled and all is well.