Slow blow fuses sometimes fail due to the solder joint being dry look closely has it melted (a fault in the device) or just pulled out (a duff fuse) replace and power up Dave Caroline On 03/04/2014, supervinx <webmaster at supervinx.com> wrote:

Nice machine! Deend on what you mean mby 'familiar'. I can see one from where I am sitting. I worte s eries of articleso n how to fix them. Will that do :-) The fact that you say '1A' fuse means you're using 220 or 240V mains, right? Do not just repalce the fuse and power up. A fuse failure may mean a problem in th power supply area and it's best nto to risk the rest of the machine in this case. While the810 was built using mostly stnadard ICs, the ROMs are custom. They're actualyl an HP custom design, no EPROM will drop in palce of them (yes, there are ways to do it, but...). The rest of the machien was standrd chips _at the time it was designed_. Some are getting hard ot find now, like some of the 74H TTL parts. And the 1103 DRAMs. Start by goign to the Australian museum site and downloading the documentation. There is an HP service manuals there, which is mostly a boardwswapper guide, but which does cover the PSU to component level. And if you can stick my handwriting, you can get a complete schematic. OK. Beginby takign it apart. The top coer is held on by the 4 screws under the printer paper cover, but remvoe any ROM moduls (behind the display) first. Then take off the keybaord (4 screws under the front of the machine, I find it best to take ou the front 2 first, then the rear 2, not in the unsual diagnonal pattern).With the keyboard loose, firt it to unhook it fro the printer and unplug the logic cavbe under the front of the printer. Rmevoe all the PCBS next. They are colour-coded as to where they go. Start with the display biard. Then the complee memory box 4 screws, ejector handles o nthe backplance). The 4 processor boards next to that. Then the card reader controller (to the right of the printer), unpl;ug the edge conenctor at the front before removign ti. Then take out the printer (4 screws Pull out the I/O backplane (at the rear). There are 3 boards left, the pwoer supply regulators. thgere is a metal cover over these held on with 7 screws,. Take out the 4 pan head ones, then the 3 coutnersunk ones and take off the cover. Remveo the boards. Now time for soem tests. You left the keyobard mains switch conencted (if not, reconnect it). The mains input cirucit is basically : Mains input -> 6A fuse -> Mains outlets -> filter -> switch -> 1A fuse -> voltage selector -> transformer. The cooling fan is connected to a 120V section of the primary widnign which runs as an autotransformer. Repalce the 1A fuse. If you have a megger, do the obvious insulation test. Now, if posisble, connect a 100W or so mains light bulb (filamanet lamp, not one of these low-eneegy things) i series wit hthe amins input. Apply power. The bulb should be dim/out and the fan should run. If the bulb glowes brightly, suspect problems with the mains transformer :-( Check the secondary voltages. In the section where hte power supply bords go thereare 2 edge conenctors screwed to the shcassis, not part of the backplane. They are wierd the same, it doesn't matter which you use. Conenct one lead of an AC votlmeter to either for the front 2 contacts (doesn't matte rwhich). Ignroe the rear 2 contcts (not conencted), the remaining 6, startign from the rear should give about 26V, 26V, 20V, 20V, 17V, 17V. So Far so good. Power down, fit the PSU boards. Keep the series bulb in palce.This supply doens't eem to need a dummy load (or at least none of the ones I've worked on does, even thoug hte 5V regualtor is a switcher). Pwerr up again. If the bulb glows brigltly, syuspect a short on one of the PSU boards. If you don't ahve the series bulb, see if the fuse blows :-). If there is a problem in the PSU, most f othe time fuse blowing means a short in a rectifier diode or smoothing capacitor. If you can't find it, it might be a short in a regulator transsitor that is cuasing the outptu to go sky-high so the crowbar fires, btu of course the regululator cannot shut fown. The PSU is somewhat unconvnetional is that all outputs are frerneced to the +12V line, so this must be right before anythign else will work. And the 12V board gets its AC via the +16/+20/+24V board. So you can't jsut run the 12V board on its own. What I suggest is remove the +5V board ( the one with the iron-core inductors on it) and the +12V board. Power up with a series light bulb with only the +16/+20/+24V board in place. if the bulb glows brightly, the fautl is o nthat board, weiterh a rectifier, cpaacitor, or pass transistor, I think. If necesary, desoder rectifiers in pairs ot fidn the faulty cirucit, then test a coponent at a time. Now fit the 12V board. If the bulb is bright now, suspect either a problem on that board (most likely) or a regualtor problem which measn a crowbar is firing on eithr of the boards). Again, you may have to deoslder rectifiers to isolate particular supplies to find it. When that's OK, check the output voltages from those boards on the testpoints. the machien chassis is the common 0V rail. Now fit the 5V board. Power up again. If the series bulb is bright now, as before it;s likey to be the rectifiers, smoothing capsaitor or chopper transistor. There is no testpoin for the 5V supply, but it's avaialble on every edge conencotr -- look at the schematics to find where. Only once the PSU is working fine should you put the logic boards in. -tony