EEVblog #905 - REPAIR: HP85 Vintage Computer

hi in my previous video I did a teardown of this classic HP 85 and it was very interesting so cliquey if you haven't seen that but I put it back together afterwards and I went to pair it up and hmm not even the power LEDs coming on zippity-doo-dah oops so I'm gonna try and troubleshoot this thing so bear with me ah I did do something stupid though when I reassembled it I completely forgot to put this ground in earth grounding strap from the chassis here up to the main main power supply board up here now I don't think that's going to matter but technically I switched it on and it did nothing and then I had a quick look around inside and I noticed that was actually disconnected so I've reconnected that now and it still does exactly the same thing so yeah I don't know let's go through and troubleshoot this puppy should be fun anyway now granted this could be as easy as me just having left a cable out or bad connection or something like that so let's have a squeeze around I don't know I maybe you won't see this video it'll just go on my second channel I don't know but let's start first thing I'm gonna do is a very quick primary transformer test make sure I haven't blown the fuse cuz that ground could have caused an overload a lack of ground or something I don't know let's check that out nope 54 ohms that's alright for a your transformer primary so nothing wrong there fuse hasn't blown so it should be powering up now I really don't think that leaving off that earth strapped down in there would have caused any issue why because this is the pair this is the transformer power output here okay and look it's got an earth wire looks like well it does go down to earth if you actually follow that down so that middle thing there I what I'm going to do is I'm going to measure between that pin the middle pin and the earth strap which is right down in here and it is a dead short so it's actually connected on the board so it's just an alternate path for the earth there so it was you know it didn't make any difference so leaving that strap off would not have blowin anything so copy that okay so what I'm going to do now is measure the output of the transformer secondary to make sure we've got our voltage going over here it should do because we're measuring the primary so I'll go between one of the pins there and there bingo 14 volts ac the other one it should be like a center tap there we go yep that I don't know what the actual value should be but hey you know 15 volts ac aside sounds right to me then anyway what we need to check is that the power supply board here is actually getting power and it is so that's fine next step now at this point it would be wise to actually have the schematic and we do have the schematic available but I'm lazy and also you know really you shouldn't need it at this point I mean there's some obvious stuff here here's one of the main filter caps one of the big blue filter caps on the thing those big studs down in there so bingo I can actually get in there and measure that big capacitor stud are we getting a voltage on it mm-wha point one volts on a 25 volt cap there you go so that kept there 25 volt DC has got basically bugger all across it not zero but that could just be some residual charge or something so has something gone now at this point once you realize there's no voltage on the cap this is where you'd probably get out the schematic or I can take out the whole module and just have a visual check to make sure I don't know but yeah it's like I don't know what would have blowin on this thing or how I could have blown anything leaving that earth strap off I've already verified that that should not have done anything and nothing went pop didn't smell anything no magic smoke escaped so yeah it's just not powering up there's something wrong there's no voltage across that cab that damn well should be otherwise what they'll sit there for now we've got the schematic but hey we didn't have to go to the schematic at this point we still could have started you know and kept probing around not just randomly monkey bashing on keys hoping to get Shakespeare probing around but you know we could have basically trouble shot this thing without the schematic but we've got it to hand so let's have a look now we've established that we've got our AC on here by the way here is the here's the ground and there's that earth strap okay so that shows that's going down to earth and this is going down the circuit common you can see the ground or you know ground not earth like shezzy so that one's going down to show Z now if the cap we actually measured was this one over here forty-five hundred microfarads right over here okay so that's the main plus 12 volt rail okay so we're not getting that out so the first thing I'd check is I was going to do it anyway is measure across the other main cap which is our main output field a cap for our full wave bridge rectifier here so let's just measure that one quickly interesting I've got a crowbar circuit there that's nicer any over voltage and we'll clamp it down and you know like if you accidentally plugged in you know it's had it set to two hundred and ten volts you plugged in your 240 then the crowbar is going to act and hopefully save the rest of it they might blow the ice out of your crowbar but hey save the rest so anyway we've got ourselves a crowbar the transformer here is actually not a trance well it's being used as an inductor basically it's not like an isolated transformer anyway interestingly I see a fuse in there so uh-huh that's the one of the after we check a voltage across here I'd be checking the continuity of that fuse so the voltage across that cap it's way way down in there so you don't want to go have both probes in there because you don't want to short accidentally short out you would get maybe get the adapters which have the insulated you know the cat 4 adapters on there that just exposed the points like that but I don't need to because we've got that handy strap so down there with the earth strap and in we go and what do we get thirty six point six volts it's just fine so nothing wrong with our full wave bridge rectifier so to access that fuse now I had to take out the entire assembly but hey it was easy four screws and the entire assembly just swings out our five if you count the earth strap there as and we can still pair it up from the I say because it's just AC in here we don't have a load on it of course but hey it's good enough we can at least get in there access stuff and pair it up without pairing up any of the high voltage CRT stuff over there either so nice and safe and I've mentioned this before but you know we've got a lot of energy in that huge cap up in here so you just want to make sure it's drain so this is where your low impedance mode on one of these lows ed ranges can really come in handy actually I can just go from the earth strap there there we go and probe that there we go it's already discharged so no worries ID up yeah it's already gone they don't have a bleeder resistor on it but yet the rest of it just discharged it so if there's no energy left in that so it's safe to go probing around not from a high voltage point of view I mean 35 volts is a fine but just from an energy point of view and certainly a measuring ohms and things like that which we want to go do measure our fuse we don't want any voltages in there upset in any of our readings it's actually really annoying that there's no component designators on the board here no silkscreen overlay on the thing so it's a bit annoying so you can either trace things out or we can go to a component overlay which is in the service manual we don't need to do that here cuz we know it's on the around the transformer there there it is it's like there we go near Q to there but we don't know where q2 is but it's in Syria like it's near one of the taps of the transformer anyway that's all we need to know so you have a look around here well which one looks like a fuse bingo that one there so let's just measure that should be zero bingo gotcha yep fuse blowing so no wonder we're getting nothing on the output of that big 4500 mic cap or measuring the main 12 volt rail because bingo the fuse is blown Wow but why is it blowing does it have to do with that earth strap which I foolishly left off I I still don't think so I can't see a mechanism that would do it because the it already still had the connection there so via the cable instead of a strap so a coincidence I don't know is there a short somewhere else on the 12 volt rail throughout our circuit maybe on another board perhaps something like that so what we should actually do is although we just measured some voltage on there before so it's not gonna be a dead shot but just for kicks we'll just measure the resistance across our 12 volt rail here all right so let's just measure that cap oh hello Oh night-night there we go nope that was just and now you can see the cap charging up watch now because the caps charged up and we swap our probes around ha we get 0 in the other direction as well so just wait a bit wait a bit so we put the probes on backwards and I had some residual charge so it caused the meter to just display 0 truck for young players arm dipping different meters will vary depending on how they're you know the chipset and how they measure ohms and everything else so yeah not every meter is going to be perform the same when you force in a voltage onto here during the when you've got the resistance range let's take the venerable fluke 87 for example and let's probe this puppy 175 776 K wow wow it doesn't know I like the eevblog me too much better anyway if we swap it we get hep - 176 K there you go like completely behind it's gonna say completely opposite pun intended so it doesn't show zero so a lot of meters are going to perform differently here by the way one thing I am I suspect in when I took these ribbon cables out there were two of them in particular this one that went is actually the output of the power supply it was miss it was a bit misaligned on the pins apparently like those connectors actually let you put them in at like an offset angle and maybe it even wasn't connected or could have been shorting out two pins or something like that so that could easily have caused it so I I'm going to assume that that was the cause of the problem ie me idiot me actually didn't you know I just shoved him back in thinking it would like self a line on the edges and all the pins and um but no I think they were almost touching I didn't actually measure it but it just looked that way and then boom it popped out and you know but yeah I think that could have been the issue that would certainly explain a blown fuse on the power supply was one of these connectors which is mainly power coming over these ribbon cables onto the main board so let's hope that's all it was otherwise it doesn't really make sense i mean i was using this for quite some time and it worked just fine so you know yeah I think I did something really now before you jump in and replace that fuse and just do some visual inspection to make sure there's no burnt out tracers going over to the power connector here no burn out parts or anything like that give it a bit of a smell to make sure none of the magic smoke is escaped it's just got that 30-year old electronics smell that's the only thing there um but I wouldn't have expected because if I shorted out a power supply on here accidentally between two pins you know we've got that 375 milli amp fuse it's not much it's going to blow fairly quickly on a shortened hopefully we haven't damaged anything else that's the theory anyway ah 30 year old electronic smoke do that just a week that soldier out there couldn't have been bothered turning on my solder pup here we go fresh holes now I know this might look a bit how you're doing but this is all I had I couldn't find my axial fuses and I think I've only got one amp types anyway so I just watched in a M 205 fuse holder like this that'll get me out of trouble until I get a proper axial replacement one like that and then I'll walk it back in but this should get me up and running nice and safely so let's power this baby up outside of the unit and see what we get shall we bingo 12 volts winner winner chicken dinner all right so let's measure some other rails this'll do nicely for our negative that's our plus 12 which we measure before the good thing is they don't have silkscreen but they do have an issue in the copper this should be +5 yep 5.1 - that's fine and dandy that's within the usual 5% tolerance ie four point seven five - five point two five and usually on high current or high power 5 volt computers like you know old-school stuff like this was - a bit of current on the 5 volt rail for all the digital logic then you probably want to set it like slightly to the high side you don't want to set you know trim it to precisely 5 volts thinking you're really smart trimming it right like that and then getting drop over the cables and the connectors and the traces and everything else out to the large boards and things like that so it's common to actually set that reasonably high this is plus 6 yep six point one seven um I think that's it I'm happy with that winner now here's where I think the problem came about this allows me well let's see it allows me to put them at an angle that is not lined up hopefully you can see that that is not lined up with those so I've got to be very careful about putting that back in and lined up perfectly with the pins so I've got to do that for all four of these plus the other ones arm down the bottom going to the CRT display as well all right so let's try this again here we go hopefully I've got it okay oh yes I've put the earth strap back there but like I said I that should not matter cuz it's already connected through the earth of that one so anyway let's go hey pal LED power LED I'm liking the power LED that means we were five volts and a riot bingo error 23 self test oh wow wow something else has screwed up but the process is working so again in our five volt so let's see our T's working so that's interesting what self test has failed hmm Murphy yeah that wasn't that fuse just alone just wasn't gonna fix it no it was never gonna be my day now the interesting thing is look I have not got the memory expansion pack plugged in so this one is actually 16 K a ram built on to the main board it's supposed to be a K but maybe it was you know and maybe it was an optional extra at the factory maybe it's an aftermarket update just plug in extra chips I higher capacity chips I don't know but 16 K on board that's why we'll get in 32k total with the extra 16 K pack but look everything's working just fine I mean I can run programs and stuff in it what is there a 23 let's go to the manual well the manual doesn't tell me what the self-test error is but dull and I'm not having a good day I forgot to plug in the printer and the tape tech unbelievable let's try that again shall we you can do it yes winner list woohoo and we can actually run the self-test it's got a test key up here it's great and that would have got theirs the CRT so it's scrolling all the memory so that was two up nope nope don't have any paper in there nope but yep it passed the self-test no worries try that again I wish some paper this time scroll let's see our t memory love it boom there we go we're not and I might have to do another video maybe trying to fix this tape drive because it doesn't seem to work I just ran the catalog command cat and it's supposed to read from the tape if there's anything on it presumably takes a very reliable so they had something on there but yeah it just it doesn't stop when it rewinds or anything it just doesn't seem to yep there's something could be something physical wrong with the capstan in they're not you know pinching correctly or something like that I don't know what the deal is but the motors whirring and lights going but not doing much else so there you go I hope you enjoyed that little light repair video it was kind of it was good that I uh screwed that up actually it we got a nice little repair out of this as simple as it was a fuse but hey step-by-step tracking it down eventually found it and then we forgot to plug the cables back in dolt double dull and yeah I'm a twit but anyway we fixed it if you like that please give it a big thumbs up and as always I'm links to other videos are here and subscribe and you know all that sort of stuff and form and mmm whatever catch you next time hi welcome to tear down Tuesday yes we're going back to the future of computer technology today right back to 1977 where it all began one of the pioneering computers of the modern era and one of the biggest selling computers that a lot of people forget about everyone remembers the Apple 2 and computers like that but well this thing was actually the biggest selling computer of its day

Channel: EEVblog

Views: 110,591

Rating: 4.9483328 out of 5

Keywords: eevblog, video, hp, hp85, personal computer, repair, how to, teardown, power supply, fuse, crt, vintage, retro, soldering

Id: 14KncLx5frg

Length: 20min 16sec (1216 seconds)

Published: Thu Jul 21 2016