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Your goop ruined my vias!

A project log for today's assorted project ramble "grab-bag"

Assorted project-ideas/brainstorms/achievements, etc. Likely to contain thoughts that'd be better-organized into other project-pages

esot.ericesot.eric 05/21/2020 at 08:003 Comments

This was sitting as a draft, a bit outdated; the printer now works [except for a few dead nozzles], but I'll write about that adventure later....

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been trying to get an old printer running... it's a portable doodad which probably cost some executive a small fortune back in '91. But it's got a sticker on it for $3.50, I'm guessing an old buddy grabbed it from a thriftstore back before he decided guitars were better at picking up chicks than robots, and gave everything to me... I held onto the things that could plausibly one day be useful to <i>someone</i>. 

This printer I never planned to find a use for, and actually put almost immediately in a box marked "To Donate" [to a thriftstore]. I'd done quite a bit of research buying my desktop photo-quality printer, I'd planned to hold onto for decadeS, and invested nearly $300 to do-so. I didn't even have a drivers' license for another fifteen years after, so I really saw no need for a portable inkjet... Sufficed to say, this printer's been in *my* storage for nearing 20 years... but these days I actually have a use [if not a need] for a *portable* printer... 

so, cool-beans!

This thing's actually EXTREMELY well-documented... right down to schematics *and* functional descriptions of the subcircuits. And the service manual's even kinda funny in some places. Its ink cartridges are well-documented, and even still available nearly 30 years later; which actually isn't a huge surprise, because at one time they actually made multiple printers, over several years, even from different brands, which used the same cartridges.

[This thing shoots out droplets at <b>12 Meters Per Second!!!</b> I wonder how/if that could be used?]

So... this thing might be a useful tool for some time, if not, frankly, a piece of test equipment which again might be useful for quite some time.

Here're some thoughts, in the "hacker" realm... old printers like these were *designed* to be compatible with even older printers... plausibly all the way back to the days when one might literally log on to a system with only a keyboard and [dot-matrix? Teletype?] printer... no monitor!

Simple commands can be used to print one line [maybe even one character?] at a time. E.G. in linux: 'echo "hello" > /dev/lp0' And maybe even one /character/ at a time E.G.2 'echo -n "a" >/dev/lp0'. Nevermind the fact they'll work with *countless* machines of the era, without special drivers!

Now... imagine how friggin' useful such a thing could be in *this* era... e.g. attached to an arduino... true, it's a little more difficult than using printf [or cout?] statements, because it has a parallel, rather'n serial, interface. But only slightly. in fact, serial to parallel adapters existed for exactly that purpose, and even modifying printf to output parallel isn't particularly difficult. Oh, sure... debug-info may be best-suited to a console/terminal window, but having a physical printout of whatever you want at your fingertips?! How is it this ain't "a thing"?! 

I dunno what newer printers do, but older ones like this were actually capable of *both* "text-only" <i>and</i> graphics... and that was no small feat on their part. In fact, it was so difficult, printers of the time had processors *on par* with the computers' CPUs of the time, just to handle e.g. rendering inbuilt fonts... Similarly; modems, and even some [e.g. Commodore] floppy drives were full-on computers in themselves, again, on-par with the systems that used them... because those seemingly-simple tasks--many of which we've exiled to "obsolete," such as converting a single byte of ASCII input into Hundreds of pixels on a piece of paper--were once handled by the devices themselves, rather than by the computer. In a way, many older peripherals are *way* smarter than the newer, seemingly more sophisticated ones, which require the *computer* and drivers to do all the work. [See also "winmodems," wavetable MIDI, and "framebuffer console"]...

So, yahknow... as a 1337 Haxor, I kinda like making use of such things' abilities [hey, there's a "Print Screen" button on most keyboards that did/does!]

...

Anyways...

This printer is non-functional, presently... Its dysfunction looks to be mostly power-related. It takes a 6V-9.5V input and breaks that out to a 5V output, a *switched* 5V, 14.4V, and  ~20-22.5V. Yeah, two boost converters from the era when switching supplies used BJT rather'n MOSFETs. Apparently this means using BJTs which were *barely* realized, *just* before MOSFETs took over. The 22.5V boost converter's BJT has a saturation-voltage [VCE-sat] less than half a volt [was it 0.03V?] at several amps... and... well... apparently that's uncommon... to even be specified in datasheets! [I later found one locally that claimed "low saturation voltage" of 1.2V! At fewer amps!]

Anyhow, that *uncommon* BJT let out smoke... and somehow I mistook it for another on the board, which happens to be far more common and a *mostly-compatible* cross-reference replacement available at my nearby store... but. $7.50?! I wouldn't feel right--rather, I'd never live it down--spending that on a lowly transistor, unless it was *exactly* what I needed, especially in an era when the stock is probably as old as the store and still sitting there [read: decent supply, no demand... sad, really... but not at all surprised, what with so many past-buddies switching from electronics to guitars, or XBoxes]. So I checked the relevant specs and found a suitable replacement equally as old for ~$3, which is admittedly still a lot for *one* BJT, *BUT* *BUT* available immediately, and *NUMEROUS* other justifications. [Thank you, Fry's... PLEASE stick around!].

But, it turned out the one I thought blew smoke was *not* the one that did.., and the one that did... well, frankly, is darn-near a freak of nature... so at 5 minutes to closing time I ran in and grabbed darn near every seemingly-plausibly suitable replacement under $3bux [I'da spent $7.50 for a direct replacement, at this point, but this was rare; VCE-SAT=0.03V@4A?! B J T ?!]... 

So, I fought this for a while... threw in the closest-match.... again, sat-v=1.2 vs. 0.03[?] is a *huge* difference... but I figured it *might* work, since the switching supply relies on a feedback loop...

....

So, several *several* hours later...

It seems the switcher-chip is resetting due to an undervoltage/brownout/"short-circuit" condition... yet nothing, even the new transistor, seems at fault....

But get this: the feedback voltage at the switcher-chip is 0V. What?!

Even if the switcher-chip was dead, the feedback voltage should've been some fraction of the supply voltage [6-9.5V], a fraction giving ~1.2V... yet it was 0V-0.03V.

WAIT, WHAT?

So, I start probing around... and, mind-you, this is *hours* of troubleshooting later... and I discover that the feedback pin is *not* connected to the feedback voltage!

So, I probe around some more... and the feedback *voltage* is at a topside-via, and the feedback *input* [~0V] is at the bottom-side of that same via. Yet, the two are not connected!

It seems a big-ass electrolytic capacitor, which spilled its guts [and I replaced], leaked goop *through* this via, and ate up the through-hole plating!

Well, shit. I bypassed that via, and suddenly I have stable [boosted] voltages! No more "undervoltage"/"brownouts," steady boosted voltages... yet, get this... the printer is acting *exactly* the same!

So... I guess that electrolyte goop eats copper?! *Numerous* vias filled with that same black sludge...

WEE!

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Again, days [weeks?] after writing this, the printer is running! I'll get around to that write-up later. Oh, and... yes, single-character printing is a thing! And so is reverse linefeed! I've got big plans, and a simplistic proof-of-concept that's fun to watch. More later...

Discussions

Dr. Cockroach wrote 05/30/2020 at 08:52 point

12M per sec? Wow, a lot more force than I would have thought :-)

Funny when you mention single character and reverse LF as I had made sure that my modified typewriter for IO would do that. Dont you just love that goop, Heh 

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esot.eric wrote 05/30/2020 at 18:39 point

ah hah! Perfect test-tool!

here I was thinking it'd be useful for arduinos-n-such, hadn't even pondered homebrew CPUs!

It's a little funky, 'cause it prints a character then reseats the nozzles on the cleaner/sucker, and cleans itself somewhat randomly while printing, got a bit of a buffer... and sometimes waits to move/print 'till several characters later... but does the job!

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Dr. Cockroach wrote 05/30/2020 at 23:36 point

As long as it gets it done :-)

I only had a Daisy Wheel to work with and had to hard wire all the functions, a bit of a pain but a great learning experience :-)

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