HP9815A repair and OVP modification  - Page 1
Forum section: electronics / repair

author: Mysion
author_rank: Regular Contributor
timestamp: August 30, 2025, 09:28:55 pm
content: Recently I acquired an HP9815a. I'd like to restore this machine to it's full glory. Maybe even get the tape drive working by following in the foot steps of curious mark. The machine is in poor cosmetic shape but I am hoping it cleans up nicely.I have not plugged it in yet since I am worried about the health of the power supply. The power supply board is the big board at the bottom of the case. I've tested the caps and they are good. My worry is that a bad resistor or transistor could let the fully power supply voltage of 36V onto the 5V rail and fry every thing.Ideally I would add a set of OVP protection circuits to the calculator. However there is not much empty room in the machine.Getting a bit creative I found a location for a new OVP board. I plan to clamp it onto the metal bracket directly behind it. There's not much space here but it should be enough. The red part is a 3D printed board outline. There's a small part bolted too it that clamps on to the bracket. There will be another part on top too.This is the circuit I settled on. It's a simple crow bar using the TL431. I'm going to need to make 5 of them for the +5V, +12V, +15V, -12V, -5V rails.A 2nd TL431 acts as a comparator for a power good signal.I still need to look at the power supply board and figure out the best place to intercept the power rails. It's going to be a messy affair but hopefully it'll be worth it.As a bonus this will make it easy to test the power supply with external loads first.I'm curious if any one else has experience working on these old machines or retrofitting in new circuitry.

author: artag
author_rank: Super Contributor
timestamp: August 31, 2025, 04:58:15 pm
content: I have one but it's been in bits for a while as it has an intermittent fault that stops it booting. It has the 'S' upgrade board with only 8 1kx4 ram chips and two roms. I think you have the earlier one with less (or equal ?) ram but more chips.Interested in what you discover though. I've seen Marc's video on the 9825 and considered adding a crowbar but not done it yet.One job that's been on my list for a long while is dumping the ROM in order to help make a re-assemblable version of the ROM listing (by correcting an OCR of it) and get a copy of the maths routines which aren't in the listing.

author: Mysion
author_rank: Regular Contributor
timestamp: August 31, 2025, 07:39:04 pm
content: I'm lucky to have a model A with option 1 that takes it from 472 program steps up to 2,008 steps. The extra ram is on the on the PCB Infront of the black transformer box. The s version that you have goes up to 3,800 program steps.Sorry to hear about your fault, I'm hoping mine just powers up with only the tape drive issues. If mine works I could grab start up waveforms if that helps.I had no idea there was a print out of the rom available. Do you remember where you found it?Either way a dump it still a good idea. The calculator seems to use non proprietary chips with the exception of the rom chips. I've been toying with the best way to dump the roms if they work. Would it be better to pop off the CPU and use an Arduino to step through the memory or pull off each rom individually and dump them?

author: Mysion
author_rank: Regular Contributor
timestamp: August 31, 2025, 08:39:57 pm
content: I've been struggling with the built in fan. It's not "stuck" but it is very stiff. I've tried flushing it with ipa and then 3in1 oil with little luck. It frees up for a bit and then gets stiff again. It's fully saturated with oil and it only helps a bit. I may need to take the fan all the way apart and hopefully it's not a one way process.Here is a first draft of the PCB. All the power good indicators are grouped near the top edge that will be visible when the machine is open. I'll use two connectors, one for power in and one for power out. By separating them it's easy to swap the power out connector with dummy loads for the first power up.I'm not happy with the rest of the layout. The 5 crow bar blocks are grouped too close together and the space is underutilized. I'm going to take another crack at it.The schematic is pretty much done. I'll change it to an 11x17 sheet so it all fits and then add a list of fuse part numbers. I'm not thrilled about using proprietary fuse blocks but there just isn't enough space for 5x20mm fuses. The good news is 2512 SMD fuses fit great on the same foot print.

author: Mysion
author_rank: Regular Contributor
timestamp: September 01, 2025, 04:22:54 am
content: Here is the cleaned up schematic. I added the OSHW logo since I intend to release this as open source. It's a fairly simple schematic.Here is the front of the board. I separated the crow bar's into separate sections and added my logo.I am much happier with the new layout. It feels cleaner than before.Here is the back of the bord. I added a link to the git hub and a section with fuse part numbers.I plan to send the board out to fabrication tomorrow. Fingers crossed for no white wires.I still need to release and polish out the github page.

author: artag
author_rank: Super Contributor
timestamp: September 01, 2025, 09:54:13 am
content: The code (or much of it) is in a patent. Comments here say it's on Eric Smith's websitehttps://www.hpmuseum.org/cgi-bin/archv008.cgi?read=19440https://www.hpmuseum.org/cgi-bin/archv019.cgi?read=156809Here is where Eric references it :http://www.brouhaha.com/~eric/hpcalc/patents/https://patents.google.com/patent/US4089059And there's quite a lot of related information here :https://www.hpmuseum.net/exhibit.php?hwdoc=146My plan is to execute some code that reads all the roms and collect the data with a logic analyser. They're soldered in on the 9815S board so I don't want to pull them. I believe the 9815S roms to be identical to the 9815A roms, but the PCB implements all the optional extensions.To get the code in there I've built a module that provides an eprom. It's just an eprom and an address decoder. You will see from the listing that it's easy to make either a full rom extension or a diag tool that will boot with minimal effort - just a couple of bytes at the start to indicate it's a diag rom. There is an official diag rom but I don't have a copy - I might eventually put an available 6800 monitor on it and add a uart, but for the moment a simple read loop will do. I started making that and my first attempt has a bug, but before I could fix it the intermittency problem started and I haven't progressed it yet. I don't think it will be very difficult once I get started, especially as I already have the logic analyser connected.Thanks for your work on the OVP module. I was thinking of something simpler - probably only crowbarring the 5V - but since you've done all the work it's probably worth copying.The fan motor is the same as those used in many fans and record players. They're usually pretty easy to take apart and don't present any problems re-assembling as long as you get the rotor central enough that it doesn't touch the sides. I guess yours has some old, dried oil gumming it up. You might dissolve it with a more aggressive solvent than IPA but you might damage the winding insulation. Probably safer to take it apart.

author: Mysion
author_rank: Regular Contributor
timestamp: September 11, 2025, 03:44:35 am
content: Thanks for all of the links! Those are great recourses. It's a bit of a shame the diagnostic tape is copy protected. I go one for the purposes of dumping but now I might be in a bit over my head.By module do you mean a module for the expansion slot? That's a good idea. Do you have a thread or any pics of your efforts? It's be cool to see what it looks like.I definitely went overboard when I chose to protect all the rails. I've had a bad experience with power supply failures at work. It may have made me a bit trigger happy.The board arrived today and it fits in the machine. I plan to assemble 2 of them this weekend. I don't need two but a spare is always nice.Getting it in isn't too bad if you angle the board slightly. It's a tight area but that was expected.

author: Mysion
author_rank: Regular Contributor
timestamp: October 12, 2025, 04:17:52 pm
content: The board assembly is complete. It came out nicely. I did some quick tests and the OVP trip points are where I want them.I'll have to remember the TL431 for future projects. It's a nice chip.It seems to fit in the calculator. Now I just need to intercept all 5 voltage rails and redirect them to the board.That will be an adventure...I'm optimistic that I can get it wired up today and power it on for the first time.

author: Mysion
author_rank: Regular Contributor
timestamp: October 16, 2025, 12:30:06 am
content: Here are the new connectors added to the board.The +15V and +5V rails have convent spots where I could intercept the rails.I had to cut there traces for the +12V, -5V and -15V rails.This is not ideal but it's easy to reverse. I only cut a short section and it would be trivial to replace it with a short wire.I reinstalled the power board into the frame and connected loads.The manual warns not to power up the supply board without a load and this should do the trick.I powered it up and the main input fuse instantly blew.None of the OVP fuses blew.The question is did I cause the problem or is it preexisting?There is a resistor on the board that looks toasty. I'll start trouble shooting there.