So the inner hatch opens -- and these are the oldest type on the Lupine, slightly-modded 1950s submarine-style, with analog gauges and brute-force idiotproofing: if pressure's not equal on both sides, they won't open. So it's not like, say, a door in your apartment. Not even close. Opening (and closing) takes some effort and time, the four dogs scraping free, the bolts backed out by the gear and wheel (which is why there's a nice little "trip bar"to hook your toes under on at least the floor side of such hatches and a grab rail on the side opposite the hinge, too, or you wouldn't be able to open them in zero-g) and then a nasty "schluck" as the seals break.
Followed by Jonny Zedd, puffing like a steam engine but grinning nonetheless. "Helloo, Bobbi! Have you got it figured out?"
I gave him The Look, slightly modified on account I was glad for the help. "Well, maybe, but we're gonna have to hang a 'scope on a couple of spots on the Analog-Digital Interface board to find out -- hey, did you close up the outer hatch? -- and it's behind the Logic and Control board. It'll take three hands just to get in there..."
Jonny turns to check the hatch; over his shoulder, I can see the bolts are engaged but the dogs, not so much. So he sets them and makes a production of closing the inner hatch as well. I understand his casualness but we are at the end of a long corridor. While it is unlikely we'd end up trying to breathe vacuum, I'd still rather not have to try.
While he's finishing, I start in on the #2 'Drive Amplifier: unlatch the "door" that carries half the meters, the "Christmas tree" display and local control buttons. The door is about four inches thick and on the back of it, there's another set of latches that free up the back panel to swing away, revealing the tops of two circuit boards, each about two feet square: Logic And Control on the front-panel side and Analog-Digital Interface on the back. ADI has a nice collection of connections, fiber-optic, coax, data bus and such and up in one corner is the fiber lead for Back Heat OK and the IC that buffers it -- the hard-to-reach corner, naturally. Oh, and if you shut the door assembly with the back unlatched, it hangs up on the cabinet and takes careful finagling to open again. At least it's not surface-mount. Something to be said for conservative design! However, there would also have been something to be said for not having to have my head stuck in between two very critical PC boards, unable to see the local control panel.
Jon's done and peers over my shoulder. "Which one is bad?"
"I don't know if any of them are, yet. Should be an IC test clip stored on the 'scope cart, would you get it?"
He rummages and finds it, a gadget like a wide clothespin, studded with test points; I clip it over the IC, allowing me to hang the scope probe on the pin I want to look at. Jon takes up position on the control side and steadies the door without having to be asked, I get the ground and probe tip attached and we both look at the scope. Flatline. The signal asserts high and the amplifier's off, so that's right.
"Hit BACK HEAT, wouldja?" I ask. (Have I explained the "heater" is the glowing filament inside the phantasmajector tube? It is. Except we can't see it, since the tube is made of ceramic and metal and nestled inside the tuned cavities, so the "glowing" part is kind of a matter of faith, plus the tube would not work if it didn't).
"You got it."
No change. "Try HEATER ON."
" HEATER ON. See anything?"
For an answer I turn the 'scope a bit so Jonny can see it. Fat nil is what there is to see.
"Is the tube even gettin' heater voltage?"
Fair question, the answer to which is behind door #2, the tall door that closes the larger half of the amplifier cabinet, behind which is the phantasmajector, on a cart and wrapped in trick RF plumbing -- and behind that, there is a Plexiglas window through which can be glimpsed the various meters operating at the -36,000 Volt Beam supply, including Heater Volts. I pop the latches and we take a look: 6.4 Volts, normal for this tube. So why...?
Sometimes dumb wins. Why ask why when you can just turn it up? I have Jon drop us back to BACK HEAT, take the screwdriver from my back pocket, and run up the Back Heat voltage (counterclockwise, I have to remind myself, which is not the usual direction of "up"). I see the 'scope trace wiggle, turn into a series of square-wave pulses and then steady at about five volts, just as Jon calls out, "Back Heat light just went green!"
"Drop to STANDBY and we'll get unconnected."
He does, we do, and in a trice -- for a certain large value of "trice" -- the IC clip is off, the halves of the control door are back together, the door's shut and we're in BACK HEAT once more. I punch HEATER ON and half a minute later, the FULL HEAT led goes green, too. Hey-la! And then it flickers. I say a bad word and inch the Full Heat voltage up a bit, too. "Five minutes, Jon, then we can hit the go-button and find out. Could be loud."
"Loud? You don't know from loud. Why, I remember the time Vanderhooven cut through a 208 line up here and about threw his cutters through the bulkhead...."
It's an old, familiar story but we've got five minutes to waste while the beast warms up. Might as well hear it again.
(TO BE CONTINUED)
YANKEE RADIO TOOL KIT, #106
4 weeks ago