For the want of a blue shop towel and some isopropanol

It’s done. It’s working. It’s……. perplexing as hell……

And I think it was just merely DIRTY!!!

So I was working on the Space Station Toilet again today. Over the past few days I had been gradually baby stepping it back up to normal output power from zero, and today after several crowbar dumps I almost entirely gave up on it before deciding…. hey, maybe that isn’t the tube, but something else. Something externally… arcy sparky? Anyway..

You’ll get your wattage…. IN HELL!

The first thing I decided to look at was the grid voltage setting. This is described in very short in the manual – you want to hook up a spectrum analyzer and adjust the grid voltage first so that you have less than 800mA cathode current with no drive, but then so you have the best possible shoulder attenuation without excessive cathode current once you have applied drive. In theory, it sounds like you should basically have a sweet spot in between the two. In practice…. I found that varying the grid voltage one step at a time allowed me to find a narrow peak. On one side I had excessively tall shoulders like some kinda wild vintage dress from the 80s, on the other side, the shoulders would abruptly pop back up accompanied by the amp faulting out on high collector current on collector #3 or #4, but without the cathode current being all that high compared to that on the other amps. Weird but, uh, ok.

Here’s a mostly passable reading

So once I had the grid peaked up nicely, I decided to try running more power. I got to 100% of normal output and it was stable! Then I walked away and walked past the cabinet again and *BANG* *thump* *gronk* *screech* everything crapped out. It crowbarred and took down the whole UPS and caused the other two cabinets to stop and restart. A real fucko boingo, as they say UwU.

I tried bringing it back with the drive off. It thumped twice at the end of the quiescent verify stage and oopsie poopsied everything again dumping out its beamy weamy.

After about three cycles of not even being able to get into beam on with drive inhibited, I got very frustrated thinking the tube might have been damaged and kicked the cabinet. Now I was able to get into beam on again. Hmmmm…. Time to look around a bit. I shut off and grounded the rig for a look around. Now, this isn’t a picture of the same exact cabinet so you won’t see what I found but I’ll describe it…

The big silver suitcase looking thing is the “ISO Power Supply”. It is mounted on big insulating rails that look like they’re made of FR4 material or something similar. I was inspecting the high voltage leads above that go to the tube when my coworker and I noticed big oily sticky patches on both sides of the ISO supply case.

There was no apparent source for any oily substance to have dripped down onto the supply, so I pulled it out and opened it just in case I was missing something weird like a leaky oil filled cap that managed to get junk everywhere. The black plastic latch releases and it opens like a briefcase… Here’s what it looks like on a spare unit we have.

Left side — Top left: grid bias supply (up to about -300vdc, up to 50mA or so). Top right: filament supply (variable to at least 6.5v, uh– 20 amps?). Right side — left: ion pump power supply (-3.5kv, few microamps). Right: microcontroller with optical canbus I/O.

Well, on this one, something jumped out at me right away, and with that, the light came on.


The arc tracked layer of filth came off with a little careful cleaning. Where it was located, it would have been arcing right near the logic side of the grid voltage supply, perhaps glitching it out. Now, one of the things that varying the grid voltage does is to cause the cathode current to change.

The way the transmitter detects a damaging arc inside the eev ESCIOT inductive output tube is by looking for sudden noisy jumps in the cathode current. The supply wire runs through this big toroidal transformer on the “Spark Gap Interface” card.

I took this weird closeup because I was amused at “TOROID BOAT”

The white object that looks like a big bottle stopper is a triggered spark gap tube. There are no power or signal connections to this card. All it does is if the current on the wire looped through the transformer at left, it rectifies the resulting ac current to DC, and uses the transistor hidden partially under the wire here to pulse the primary winding of the black trigger coil. This causes the spark gap unit to arc over, abruptly forcing the beam supply to ground and causing an ugly high current fault…. but protecting the tube from damage.

And what else could have been making the beam current dance and fire the spark gap? Yeah— a glitchy grid supply which would make the transconductance of the tube do the fandango.

And now the ridiculous beast is tame, running full power, big wattage, no whammies.

73’s and good night

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