Only DEC could make a PDP-11 PSU this insane…

[@mitchwright1558]

Hi David, Mitch here, you have the schematics, I'm certain I loaded them in your truck, they're also up on bitsavers. The 11/44 PSU was notorious for failing, the DEC guys joked that 11/44 PSU repairs was a very profitable business. It's most likely the oscillator, for that voltage rail convert, is not starting up. Great to see you working on it, will make an awesome system paired with that drive. Ping me if you need ideas.

[@Katchi_]

@UsagiElectric Tagging for visibility. Please read OP. You have the schematics.

[@mitchwright1558]

bitsavers.org/pdf/dec/pdp11/1144/MP00897_11X44sys_Dec79.pdf page 203ish

[@RWBHere]

I hope that David reads this.

[@neilbarnett3046]

Many DEC PSUs had an overvoltage crowbar circuit, if the 5V rail went above 5.2V, it would fire a relay that could short out and disconnect the 5V rail. Then, of course, it wasn't overvoltage, so it allowed the rail to rise again and then back to start. I had an instructor at Systime who would deliberately overdo the 5V rails and let us chase that fault. The ticking of the relay was a dead giveaway. 120 amps was fairly normal for TTL in those days. The VAX750 had a 5V/120A PSU and a 2.5V/240A PSU, separate little 600W units.

[@isettech]

I remember a PSU failure in 1978, in a PDP 11/35. The DC fuse is after the main bridge rectifier. A rectifier failure resulted in enough heat to vaporise the epoxy under the bridge rectifier. When working on it and found the fuse after the bridge, gave me a what were the engineers thinking moment.

[@gryffuscze]

Lol, this is exactly that moment: kzbin.info/www/bejne/apfdg2eka9SSi6c Unfortunatelly only for Czech and Slovak listeners :(

[@bobdinitto]

I was a PDP-11/40 technician at Digital's Marlborough facility where we made DECSystem 10/20 computers. It was the front-end of the Dec 10/20. The PDP-11 was one of the first computers I ever worked on. It has my favorite machine instruction set. The unibus architecture was novel and ahead of its time. My most hated component was the TU-56 tape drive. So many problems! People would come to me and say "Will you fix my TU-56?" They just couldn't do it!

[@TrimeshSZ]

I was once told there were a bunch of TU-56s in the mill pond at Maynard that were thrown there by various people who became frustrated at not being able to fix them.

[@АндрейМилованов-у9у]

instruction set is amazing (I wrote big parts of code on it instead of C/Pascal/..), but excessive to built "price effective" chips in 1980-90. Architecture of PDP-11 is the same - nice ideology, but not the best efficiency.

[@andljoy]

DEC design meeting: " guys should we use off the shelf 120v AC fans" " no way that is exactly what people would expect us to do, we have to do better" " I know lets build a load of logic to run some oddball 35v AC fans that is way more DEC" " yeh good call lets do that "

[@Zeem4]

Obligatory Airplane reference: kzbin.info/www/bejne/r2G7YY2Pl9N7rqM

[@scottlarson1548]

Like back when Textronix made their own screws and bolts in house.

[@mitchbertone3809]

" Off the shelf" wasn't a thing in 1980. Practically everything comput related was custom contract built.

[@andygozzo72]

probably got a load of them cheap 😉but 3 in series is pretty close to 120v 😁 (105) that 75hz could result in faster fans, compared to 50 or 60hz supply

[@No-mq5lw]

​@@mitchbertone3809 Then wouldn't making a 12 or 24VAC fan be better anyways? Nothing is standard about a transformer or circuitry that generates 35VAC, and going for one that outputs 24V means you can reuse doorbell transformers or the tooling from doorbell transformers. Even going for a custom 240V 10:1 transformer with a center tap means you can always have a 24V source from any 2 terminals even if it's being fed 120V.

[@rhodaborrocks1654]

I worked for an outfit that used a lot of DEC gear, one centre was still using a pool of 11/35s and 11/45s with RK05 disks, other areas had 11/34s, a couple of 11/24s with RL01/02 disks, and there where 11/03s dotted all over the place. Over the years I probably had to repair every type of card and disk drive installed in them but I don't recall ever having to repair a DEC PSU, they seemed to be very well engineered. I was mighty pleased about that because switcher power supplies make me nervous ever since I had one blow up on the bench after a failed repair attempt, it was so violent that I was still pulling bits of transistor etc from my hair when I got home that evening. Needless to say I toggled the switch with a broom stick after the 2nd and thankfully successful repair attempt !!

[@baxtermullins1842]

I worked with 11/45’s, 11/55’s, 11/60’s and VAX machines. I own an LSI-11. I only programmed in assembly language for radar, missile, AAA, aircraft real-time. Everything was tied together by busses with up to 128 processors. One of the best machines I ever worked with, tied to hardware and human interfaces. We wrote our own OS, editors etc. Beware of the Master Interrupt card and location on the buss. It must be in the correct slot or another board could try to take control and lock the machine until a reboot!

[@АндрейМилованов-у9у]

it was inpossible (high cost and falling performace) to isolate devices (like PCI/PCI-ex now) in 1980-90 .

[@topgazza]

I was a mobile engineer with Xerox in the UK in the early to mid 80s on their successful and brilliant 9700 large laser printers. They were made and used in marketing companies and in companies mainframe setups as high speed double sided laser printers. At 7 double sided prints on A4 they were rapid. At £250,000 plus, a fortune in 1982, they were not cheap but companies loved them. They had reel to reel tapes but were powered by the PDP 11/34 and on later models the PDP 11/44 Proper computing days

[@jeffl4810]

Those cables at about 6 min are called FFC, or Flat Flex Cables. They are basically a PCB made with Mylar, with the copper etched, plated and sandwiched with more Mylar. More modern variants can have parts mounted on the FFC, They are a great way to simplify and make robust connections, while greatly reducing size and cost.

[@wesleymays1931]

Sometimes known as FPC, for Flexible Printed Circuit, since they're often just flexible PCBs [kapton core instead of fiberglass] with connections on two or more ends

[@mistermac56]

The use of FFC goes way back to NASA's Apollo program, where they were used to connect various scientific instruments on the surface of the Moon, as part of the ALSEP (Apollo Lunar Surface Experiments Package), to the ALSEP's central transmitting station.

[@wtmayhew]

I have worked on 1970s era switcher PSU, but not this specific DEC unit. The designs are quite different from modern, where they typically switched an intermediate voltage at a much lower frequency of maybe 10 KHz or so. You still had the huge magnetics and soup can size capacitors on top of the complexity of the switcher. I did FORTRAN programming on an LSI 11/03 in a two-drawer cabinet with a walnut Formica top. That system had a very loud switcher PSU. So loud in fact that I used ear plugs even though it was in a carpeted office. I was mostly trying to have the whine not drive me crazy, but I believe it was actually loud enough that hearing protection was probably called for.

[@bborkzilla]

My first computer job was in the 1980's looking after a bunch of Sun 3 computers. The servers were Sun 3/280 each with two Fujitsu Eagles. If nothing else those computers instilled in me an undying love for the 68000 architecture.

[@bobdinitto]

The 68000 was a very nice machine. It had a nice instruction set and none of the memory segmentation problems of the Intel architecture.

[@peterfireflylund]

@@bobdinittoit had really dumb and complicated addressing modes that made it really hard to make a fast implementation. It also had the infamous “microcode spew” on the stack when the COU took an exception/fault + lots of completely unnecessary incompatibilities between models. The x86 did all of these better. The segments stopped mattering much with the 386 (from 1985), which also improved the addressing modes to something nice and orthogonal without the madness of, say, the 68020.

[@tonyfremont]

3:29 that wire wrapping looks incredible.

[@kerryedavis]

Take a look at a PDP-12 sometime!

[@timradde4328]

@@kerryedavis Or the KA-10 backplane.

[@smrp1984]

23:12 - Most small bunny ever!!!

[@clienttablet3821]

Cuddly snuggly little bunny 🐇🐇🐇🐇🐇🐇🐰🐰🐰🐰❤❤❤❤❤❤

[@NullReference119]

So... it might be time to consider picking up or making a few pieces of test equipment for working on analog things. 1. a non-contact signal tracer, when working with things that oscillate or have RF this allows you to quickly trace how far the signal is getting. Given that this PSU uses oscillators... that could be useful in figuring out if the signals are getting where they are supposed to go. 2. A low voltage curve tracer. Just because the multimeter says the part is good doesn't mean it is. A curve tracer lets you see how it responds over a range really quickly, and if it's being suspect. You'd be surprised how many 'working' parts look really funny when you check their curves. You may want to get the type called a 'step' tracer or make your own, there are kits. Or you can just head to your nearest Ham radio swap meet and see if someone has one or can help you make one. These can be much better than oscilloscopes in some cases at finding faults.

[@TheHylianBatman]

For me personally, I say throw the priority list away! I don't care what you work on, or even if you work on anything at all; getting to share some of the time you spend on these computers is joy enough for me!

[@actually_it_is_rocket_science]

Now this makes a father's day all the better

[@robertlewis4216]

New verb 'dioding'. I like it.

[@awesomecronk7183]

The TA in my electronics class said that once, forever a personal favorite!

[@f.k.b.16]

My dad was an electronic technician at Schlumberger back in the early 80's. This thing reminds me of things I saw on his work bench. No clue what he worked on but brings back quick flashbacks of memories!

[@musiqtee]

Wow, this PSU alone seems to be as complex as CuriousMarc’s Apollo units… Time is your friend, not “the enemy” - take all you need, as being a mere passenger on your journey is a wonderful experience… 👍

[@geoffpool7476]

The Fujitsu Eagle! 300 Meg that sounds like a airplane taking off. Awesome to see!

[@RobSchofield]

@ 14:17 - if you look at your close-up @ 6:50, there's a couple of unsoldered pins on those supply voltage pins.

[@Orzorn]

Playing Lunar Lander on the Centurion at VCFSW was awesome! It was great to talk to you and hear all the trials you went through, as well as hear the gentlemen nearby share their knowledge about all the arcane workings on the machine. It was a blast, and I'm just in awe of how you got such a huge and complex machine there and kept it running.

[@Oli1974]

6:23 DANGER - Remove rings and watches WHEN servicing - HIGH CURRENTS :o omg

[@rocketman221projects]

You really don't want your ring or watch to weld to the terminals and heat up red hot while you're wearing it.

[@danl6634]

​@@rocketman221projects absolutely. Less shock hazard, more burn hazard. Recall hearing a shop mechanic that had a wrench accidentally complete a circuit between car battery terminals & a wedding ring. It didn't go well.

[@KeritechElectronics]

Looks like it's at least trying to give a fanspin, haha! Keri on fighting, you'll do it. I believe in you. Flex PCB cables as busbars? Very interesting and modern - that stuff was certainly high tech and expensive, but of all companies, DEC definitely had the resources to pull that off. I wouldn't be surprised if these were multi-layer, at least on the high current busses.

[@bzuidgeest]

You see that on a lot of failing power supplies, it comes from the fraction of a second it takes for the power supplies protections to kick in.

[@cathrynm]

That title is no click-bait. This power supply is indeed insane.

[@mitchbertone3809]

battery supply is possible issue as MEM BATT IN goes to MEN VOLT GEN and then output to FAN VOLT GEN.

[@itguydave2164]

I'd be checking those logic inputs mentioned upstream to the memory supply on that block diagram, making sure those at the least are good. The fact the fans briefly spin suggests the MOSFETs are at least working along with the switching driver, but one of the protections is going off as is suggested by the flashing DC LED. This could be in error if a component in the protection itself has failed, or it could be that part of the switching circuit is running out of spec. The fact it begins suggests at least the switching driver & bootstrap is working though, so you shouldn't need to mess with the high voltage side. Perhaps find the IC or circuit driving the switchers and identify the protection lines that could initiate a shutdown, trace them back to the source. I'm a little suspicious of that flex too - if a hairline crack is in it and one of the protections is on the other end of it, that could be a cause.

[@DAVIDGREGORYKERR]

That is very similar to the DEC VAX 750S which I have used but is no longer in the building that I went to for 5 years for my Pre-Apprentice Craft Course.

[@chuegen]

When I worked at Pyramid Technology, we launched an old Eagle off the roof into the parking lot, where it created a small visible indentation. Those drives were crazy heavy.

[@horusfalcon]

Wow... what a beast! That PDP-11/44 is eventually going to be a source of pride in accomplishment for you, though, so do keep on pluggin' away at it. Cute bun-bun, too! Hungry little critter, huh?

[@stevesether]

And if you look closely at the fan, it's not only an odd voltage, but an odd frequency. 75hz. I couldn't find any country that uses 75 hz. My only explanation is that DEC designed this so they were the only ones in the world that could sell you a replacement fan.

[@KameraShy]

Before Louis Rossmann was born.

[@Thorsten369]

Did work for Digital in the 90's, did repair a lot and seen a lot. Even then the old DEC VAX servers where still maintained. Later on Digital became Compaq which i also worked for, sad part was the nice blue Digital letters became red and even after that Compaq became HP also worked for it but everything what Digital was? Was completely destroyed so i move on to another job back then. Anyway glad to see some DEC equipment ;)

[@nufe]

5:40 - That makes so much sense though, given the limitations of the technology and component inefficiency at the time.

[@georgegonzalez2476]

Reminds me of when I bid on a pair of 11/44's at a surplus equipment auction. I won the pair for like $120 and I was prematurely elated. I got the computers home, took off the covers, and found that there were no PC boards at all. Just the backplane and power supplies. Very sad. I threw the stuff away.

[@ipcamper9940]

That brings back memories. In 1983 I had a class called microcomputer interfacing, offered by the chemistry department at VT. We had a PDP-11 (?) bare bone system, a teletype for IO, an oscilloscope, a large bread board, and a massive collection of discrete components. With the help of a xeroxed copy of the lab manual, we had to hook it all together and get it to actually do something! I have long forgotten the details, other than we had to program some of the code in assembler in octal.

[@douro20]

That's a massive drive for an 11/44. It is something I would expect on a later machine like an 11/73 or even a VAX.

[@therealchayd]

I think you're right, we had one on the old computer club VAX-11/730 at university. On the other hand, I have a (dead) LSI-11/24 with a more reasonably sized 8" Fujitsu stashed in storage.

[@levieux1137]

35 years ago I could salvage a few capacitors from an IBM36 PSU. The machine had been literally cut into pieces that could be carried. The PSU was not too much damaged, it was made of a lot of huge capacitors. Their volume was around 1 liter each to give an idea. I saved a few 100000 uF 15V and even 220000 uF 10V ones. And looking closer to the PSU, it was pretty simple. A huge transformer with many outputs, several rectifiers bridges, a bunch of huge transistors for the "low" current outputs, and the capacitors. Nothing else. The 5V output (well more like 5.3) was not even regulated, it was directly taken from the 220k capacitors after the rectifier. By then I was told "do you imagine how many transistors it would require to regulate that?". So I guess the voltage was not smoothed very well, but the huge capacitors would limit the ripple on the output. Probably that they had strong requirements for the stability of the voltage input.

[@TheStefanskoglund1]

That PSU isnt switch mode, it VERY probable a linear supply ie it has a very large transformer.

[@CommodoreGreg]

Thank you for bringing us all along on your journey through all this interesting computer history.

[@jazzdirt]

12:25 In case of thermal shutdown, quickly turn off the fans... That idd seems to be a bit counter intuitive.

[@woldemunster9244]

Maybe it's set to "thermal limit = flames"? Don't want to fan the flames you know. :D

[@highpath4776]

If the Fans spin, then stop, that indicates to me shutdown circuit kicking in ?

[@splitprissm9339]

Always tempted to install some kind of conspicious lamp into the kind of chassis that has live mains in reachable spots :) It's never when you are doing measurements in stuff where you know it is dangerous - it's always when you "just need that stuff out of the way for now", don't notice it is still plugged in....

[@tony359]

"This is not my priority today... but I don't care" Love that :) Looking forward to the next step!

[@erickvond6825]

Believe it or not there's a channel dedicated specifically to DEC computers. This guy could possibly be a lot of help if you ask the right questions.

[@RikkiCattermole]

Always good to hear the introduction of minicomputer-era hardware into the New Zealand retro scene! We haven't got much, I've never seen a PDP come up for sale, and only one teleprinter (not a TeleType). I'd love a TeleType but shipping alone would be well outside my budget, oh well. Amazing machines.

[@matthewday7565]

My ancient memory has Eagle drives associated with an ICL system, and a recollection that they needed an end to end seek task run at intervals to avoid them getting sticky if they idle on the same track. I was on the DEC side, not the ICL, but we were on VAX by then, a 3x 8820 cluster and some MicroVAX, RA81, RA82 drives in 3-up cabinets with start sequencing (cable between the drives so that the spin up power demand is staggered)

[@andygozzo72]

use a filament bulb in series with mains with 'unknown state' power supplies, it could save a lot of destruction 😉

[@orinokonx01]

Those Eagles are a fun beast. Incredibly heavy, too! Can't wait to see you get everything all up and running!

[@rty1955]

In our museum we have the only PDP-9 in existence that is fully operational. We have a "desktop" PDP-8 as well. We probably jabe the largest collection of PDP equipment around. We also have a PDP guru who keeps it all running. If you ever need expert PDP help, let me know

[@rlgrlg-oh6cc]

Did you check the loads on the +12 and -12 to see if there are shorts? Maybe the supply is shutting down from overcurrent.

[@shawnerz98]

As a fellow Teac owner, it was really cool seeing you at VCF SW yesterday. :) Great work on the power supply!

[@timradde4328]

I had 4 of those Eagle drive back in 2008 to 2014. All would spin up and come ready but one. Never did get to see what was on them. Before my last long distance move I donated them to Dave McGuire in or near Pittsburgh. He now has the Large Scale Systems Museum there. Don't know if he ever got them working or not. Supposedly they were drives used by the Highlights magazine for kids. So I hope he was able to get the drives connected.

[@RudysRetroIntel]

More amazing big iron!! Thanks for sharing. I have been looking for some mid range computer, but here in Canada, it's super hard to find for a reasonable price. It's easier to find gold!! Thanks for sharing the experience!

[@tronmcconnell4465]

When you finally get a console terminal connected to the CPU, it will be interesting to determine which version of the console firmware you are actually running. As a former departmental computer system manger of, among other things, an -11/44 running Unix, and a 2-node MicroVax-3800 cluster running OpenVMS, we had discovered bugs in the -11/44 console firmware and worked with DEC to provide a test-bed to evaluate the bug fixes they developed. Unfortunately, that was over 25 years ago now, so I don't remember the details.

[@josephkarl2061]

Even though it was free, I wince a little bit at the cost of shipping something like that to New Zealand. Fellow Kiwi here. It's a bit depressing when the cost of shipping something is almost as much as the cost of the item itself 😒 However, it's great to see some of that stuff down here!

[@andygozzo72]

screechy switching psus can be caused by a bad chopper transistor base drive cap, if it uses one, this was a known 'thing' in many crt tvs , if left can result in blown(shorted) transistor

[@BrunoRegno]

I think I saw insulation punch through on one of the green bodge wires. Check them for continuity maybe? Also maybe check that replacement capacitor.

[@theelmonk]

I do that, but the result is always that the instant project can only be half-done and has to be put away again or left in pieces. Making one more thing that's now on the urgent list.

[@oidpolar6302]

Don't worry much, everything you are working on is interesting. Even mixed/interlaced style

[@MeriaDuck]

3:59 half expected a sound effect of shattering glass there for good measure 😂

[@gowdsake7103]

That fan cct utterly insane over complicated and not needed. I see a Tant bead shall we guess its shorted

[@davidmbolden]

Yup. You're right! This might be one of your more sketchy decisions - but we're all here because we do it too!

[@rw-xf4cb]

I got caught out with an old VAX and it had a cable from the compute shelf down to the base of the unit with a power distribution box - and a switch there probably either turn everything on in one go or some old battery system. DEC Power supplies are pretty bullet proof - possibly around bus terminators given you talking about memory power generation circuits. You would get some very odd things happening without out the correct continuity grant boards or terminators at the end of the bus as well which could cause the system to hang and fans to not start. Even back then fans were variable speed and wouldn't start up if the CPU wasn't happy.

[@johnvanwinkle4351]

Another great project! I enjoy watching you bring these back to life……😂

[@baronvonschnellenstein2811]

Well, I'd be one of those that are glad you broke out a PDP-11 to work on in a pique of jet-lagged inspiration :D

[@tekvax01]

You know who also makes switch mode power supply’s extremely complicated… tektronix! Some of their old SMPS on their old 70s and 80s oscilloscopes are incredibly complicated to understand, and even more difficult to repair and service!

[@halfsourlizard9319]

23:12 That lil guy is so so so cute!

[@johnstevens6223]

I agree totally 💯💯

[@bzuidgeest]

I don't care about priority's. Whatever is fun has a priority, nothing in the room has a "done by" date attached. If you feel burned out on something or interested in something then that's what it's going to be.

[@c1ph3rpunk]

The YT algo REALLY pushes for regular, constant and consistent publishing. It places an inordinate amount of pressure on creators. If you’re not publishing regularly, ideally weekly, preferably on the same day, you won’t gain views, new or existing.

[@jnharton]

People are completely capable of telling others to check things out. And as long as the search function returns the available videos...

[@monad_tcp]

@@jnharton "and as long as the search function returns the available videos" that's not very reliable, but this is youtube after all, nothing is reliable

[@jnharton]

@@monad_tcp It's as reliably as anything Google's made, so I expect it will work to locate the channel even if it puts the video you want at the bottom of the results.

[@scrambledmandible]

Don't care about priority's what?

[@Davide0033]

even if i've been following for a while (still remember when the centurion was in a garage next to some classic car) i complitly missed the giveaway i've never been so mad at myself for not paying attention

[@ArthurBugorski]

4:14 almost gave me a heart attack. Whatever is swinging in that bottom rack should have been secured before moving because that seems like it would be a needless disaster if it tipped out.

[@ejcrashed]

Man you sure look jet-lagged 😅, thanks for giving us our weekly dose of usagi shenanigans even so, get some more sleep and everything will be allright, btw, love the datsmo shirt.

[@tohaason]

Sitting on a shelf in my hobby room I have a power supply for a type of minicomputer I used to work with - the 5V rail can provide 200A. And it's not even the biggest PSU of that type, but I didn't manage to catch a bigger one at the time. I also have an actual mini sitting around, just a smaller one so its own PSU isn't as big as this one I'm talking about. But at least it's not home-made by the mini maker - these were produced by Philips, though I've never seen them elsewhere. That DEC PSU seems unnecessary complex, what with the special fan voltage generation etc.

[@radarmusen]

Scary with so many amps in 5volts. I have working on a simulator there has a IG station there was 3 pioneer magnetic, when running they give 200amp out in 5 volt, you can feel the cables are hotter even they are a big AWG.

[@VEC7ORlt]

What a cutesy multimeter you have there!

[@idio-syncrasy]

Do you have an electrical fire extinguisher available in your room?

[@Hunter-is-gay-af]

hopefully

[@WagonLoads]

19:09 The unit bottom right only has 3 input lines. Can you toggle each line high/low and see if the output changes? also try the same procedure with the non colored lines going into the red block.

[@redgek]

God what a beautiful looking machine. And that HDD is on another level hahaha!

[@Ranger_Kevin]

Sounds like you can also use this as a welder in a pinch (if you need to fix the chassis) 😛

[@classicaudioadventures]

Those blue sprague 500D caps seem to have a somewhat high failure rate in my experience. It might be worth checking them out.

[@MCW1955]

Beard is looking good!!

[@bzuidgeest]

This my just have been lost in editing, but before taking out components like the diode on the power supply main board, why not first check the voltages on those points. If their good, no need to unsolder anything. I think you are seeing the fans twitching because it takes a fraction of a second for the power supply protections(like overvolt or short) to kick in . A schematic would be incredibly useful at that point, but the block diagram does tell a lot. If you know which module is which block ( if it's that convient) that you can check the signals between them and determine which block and which protection is kicking in. That way you can focus on the stuff on that board. Though the problem could be in how boards interact. Curious Marc did several power supply diagnostics like what you have in recent months, maybe his videos can give you some tips. And you also seem to have his personal line😮 It's going to be a lot of methodical work maybe even going master Ken and drawing a partial schematic from the boards. A lot of work.

[@jtveg]

Thanks for sharing. 😉👌🏻

[@leifjansson8074]

I see your cute little bunny.... i tried to help a tiny lovely one also... but she was hurt by evil magpies... 😥

[@grumpyoldman5368]

Does it need the memory backup battery voltage?

[@garthhowe297]

It would be interesting to hear from the DEC engineer involved in that 35v fan design.

[@Snowsea-gs4wu]

That might be one of the reasons why DEC eventually did not win in the marketplace.

[@jagmarc]

40 yrs I had a 35Vdc fan and wondered what used them

[@TomassAfastass]

omg i handled those hard drives in a warehouse i worked at. i thought i would never see one again.

[@danboid]

BEASTS! I love unwieldy disks bigger than most quadrupeds.

[@ahbushnell1]

I had a teac tape drive just like yours.

[@jj74qformerlyjailbreak3]

Speaking of the J11 DEC/Harris PDP 11 on a chip. I'd really like to build something around that 60 pin beauty I have looking good all alone by itself. It needs some coupling in its life. Any suggestions are welcome.

[@c1ph3rpunk]

35V just another tap off the transformer, oh, dude, this is DEC… DEC had some of the best docs in the business, even their doc docs (documentation guides) were excellent. I always found them more accessible, and easy to read, compared to IBM. DEC systems were also some of the most easily field serviceable systems around, they were just a pleasure to wrench on. They really seemed to think about us on the other end, in their design, make it easy to work on. Another one of the day like they were AT&T PBX’s, the System 75 as an example. Manufacturers of today should be taking notes from companies like DEC.

[@baronvonschnellenstein2811]

I think manufacturers of today _do_ take notes from companies like DEC ... then strive to do the complete opposite!

[@liquidmandotcom]

Battery should be first place to start . . .?

[@asbestomolesto]

This is totally crazy! I'm scared because we have to fix the PSU of our PDP11/34 and I hope it's not so insane as yours :) We smelled RIFA magic smoke, so we hope is just that :)

[@gammarayflash1170]

Why did you try to test it with the missing 12V? I didn't get it. If a voltage rail does not have the correct voltage, first thing to do is to troubleshoot it. Imo. 🤷‍♂

[@jnharton]

Might be one of those dumb decisions he mentioned... I have to say that my first thought when he mentioned the fans ran on 35 V power was that 3 x 12 V = 36 V which is awfully close to 35 V. So the 12V supply could be involved in powering the fans... Would always be handy to have a bench power supply so that you test the individual fans and get them spinning smoothly if possible beforehand.

[@Drew-Dastardly]

@@jnharton My first thought was the 3 fans being 35VAC they may be wired to the 110VAC in series, maybe with a dropper resistor. Not this crazy over-engineered system.

[@pgqneto]

I have a question regarding commercial airplanes. Do you know what operating systems are used in their avionics system, or if it's not an operating system, what language they use. Thanks

[@kevinmerrell9952]

Always follow your muse.

[@DECcomputers]

DEC PSUs can be weird sometimes. It took me quite a while to bring my H7874 back to life for my VAX 4000/300. Fortunately, your PSU seems to be well documented :)

[@highpath4776]

only sometimes ?

[@DECcomputers]

@@highpath4776 I have some PSUs from smaller MicroVAXs with a pretty standard design, so I would assume not all of them are weird, but the bigger ones are ;)

[@Lilithe]

23:00 awww

[@gryffuscze]

I can see the Elevator Music slowly starting playing back im my head... Marc, are you watching? :D

[@awesomepuppy404]

Schematics should be available on Bitsavers

[@rivimey]

Could you do an intro to the cpu boards please to satisfy the curious?

[@wirebrushofenlightenment1545]

"Remove Rings and Watches" - Oh yes, I think I would!

[@kencarlile1212]

Yay shotgun troubleshooting!