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.