rkrenicki

The schematics are on Git, but there is an error somewhere in there that I have not yet found. I will compare them to the schematics from the other clone project at some point, but I am not entirely sure when that will be. I had a lot of time over COVID, which is when the majority of the work was done, but real life has gotten in the way of progress since then.

My day job has been taking up a lot of my time as of late, and I have not really had much of an opportunity to dedicate back to this. That said, Thanks for the link to the clone project! I can compare my schematics to that and see where my mistake is.

U615 does not have its "own" bypass cap on the original documents, however C608 (0.1uF), C601 (1nF), and C609 (0.1uF) are right there between the CPU and U614-616, acting as bypass capacitors for all of those chips. These are all powered by the flood-fill on an inner layer, so they are all bussed together. . I am not entirely sure of the significance of the Arrow there either. As for the Power sheet, you will note that I have removed a bunch of ferrites and coils from the power input. I did try both with and without those extra filters and I found no significant difference on power delivery, so I chose to omit them. I am not nearly as afraid of EMI compliance as TI was back in the day. The Video sheet will look a little different as well since I tried to incorporate both NTSC and PAL options. As for the rest, I did catch a few errors. It was about a year ago since I was working them, but an incorrect voltage reference comes to mind, a couple of value issues as well, but nothing too out of the ordinary.

I did stick the schematics and the base project file up on my Github here: https://github.com/rkrenicki/TI99-Motherboard It is in KiCAD 6.0 format, so you will need to be on at least that version to open it. The layout has not been cleaned up.. about half of the hierarchical sheets are going to go away in the end, but I wanted to focus on getting the errors figured out before fixing the layout. If anyone wants to peruse the schematics and sees anything amiss, let me know!

I am still needing to sort out the DIP version.. baby steps So, I finally found a bit of time in that magical center of the Venn Diagram of (Space on my workbench) and (Motivation to work on the TI) this morning. It looks like there is an issue in the system GROM area, as GREADY is constantly being pulled low. The GROMs themselves work fine in another system and the 74LS04 inverter chip tests fine, but I did swap it for good measure with no change. I ended up getting called into work at this point, but diagnostic progress has been made. I need to dig out my other power supply so I can power both the good original PCB and the non-working new PCB at the same time and do some proper side-by-side measurements. I am thinking about cleaning up and publishing the schematic portion of the KiCAD project up on my Github, so that I can get some more eyeballs on it. I am thinking I am just missing something really small here. Since the PCB layout is not working, I will not publish that part quite yet.

There has been little progress as work is ramping up now that we are approaching the end of the pandemic here. We also had a death in the family, so my workshop is now overrun with items from their estate, making it difficult to get anything done in there right now. I need to use some vacation time here soon, so hopefully I can get a day or two in the near future to dedicate to this project.

I buy car parts from Japan often, so I am quite intimately familiar with the US duty structure. In the US, there is a personal use duty exemption up to $800. If it is declared above that, then you will get hit with duty, which varies depending on what the item(s) are. The most common percentage I come across is 2.5% for automotive parts, but some things are much much higher. Sometimes, depending on how it is declared, that $800 limit will include the shipping cost, even though it is not supposed to. That said, some services on some carriers (UPS Standard comes to mind), will charge a customs brokerage charge and a "duty" charge regardless of exemption. I've been hit with a $40 Import Charges bill on a $200 item from Canada because of this. $35 "brokerage fee" and $5 "Duty", even though it should have been exempt. TL;DR: Items coming from France to the US, so long as they are declared under $800 USD, should not be subject to Import Duty.

After two weeks of night work, and beautiful weekends that meant that I ended up working outside.. I finally found some time to sit down with this again. I am satisfied that the schematics match. I did clean up some notations to make it a little easier to read, but no actual connection changes. I did build one of the 8bit Museum's Retro Chip Testers a few days ago. Turns out that the 20x4 LCD module that I had does not fully work, so I just ordered a replacement before I can go through and test most of the chips. As for the PCB itself, I did fix a few things on the silkscreen, some of the mounting holes were off by a fraction of a millimeter, and some power related fixes that were noted above. I will send this latest version off for manufacturing before I try again.

So, it looks like the ROMs are not the problem. I have not probed around since trying both sets of ROMs in both boards, so I have no theories at this exact moment. I have been working nights, so I have not had much time to work on the PCB. In the meantime, I have been continuing to clean up the schematic, as well as validating it to the Bunyard manual. So far I only found one missing connection on a bypass capacitor, and a power filter inductor that had both sides tied to +12V for some reason. I've also corrected ALL of the power delivery notation on the schematics, as they were incomplete. So.. no real progress, but after this week, I will have my days back and I can actually do some more diagnostics. In the meantime, I have ordered some new MCM6810s and 4116 RAM chips from Jameco, as well as some proper 64 pin 2.54mm pitch sockets (instead of the hacked up 40pin and 24pin socket arrangement I am using today). Also, on the next prototype PCB, I am not going to populate all of the keyboard/joystick, cassette, and IO parts, until I can get it to boot. While I could pull all of the parts off, it really isnt worth the time to do so.

I just received another Non-QI TI99 in the mail. It is pretty beat up, but it does function. I have not had a chance to open it up and socket the ROMs yet. I may actually go though and socket everything and use this as a testing reference while I am at it.

Yes, /ROMEN, /MB, and /WR all have normal looking waveforms. I went through and probed the rest of the lines, and it seems that Non-buffered A3 and A4 may have an issue as they are staying mostly low. I started pulling chips one at a time until I saw normal strobing on those lines, and it looks like the U610 ROM may be the issue here.

Alright, I think I can safely rule out a clock problem. Both with a new 0.33uH inductor and the old-stock inductor, I get a solid 2.99964MHz clock on 4 phases, both normal and inverted. I tried one of my other TIM9904's and also got the same clock exactly. Phi-0 /Phi-1 I am going to socket the RAM and ROM on my QI console sometime this weekend, to that I can test those chips. They are still big question marks to me right now.

Why, 1.21 Jigga-watts of course. I did put a copper pour keep-out area around these parts for future revisions. I also removed the values, so I can make a little grid of values nearby detailing the 9904/9904A use. So, just to be 100% clear: You are using a TIM9904/74LS362 with a 48MHz Crystal, 3.3uH Inductor and a 22pF capacitor, and that works for you? I am trying to find a datasheet for the non-A version of the TIM9904, but I have not yet found one...

Okay, that is good news. I have corrected the schematic (and therefore the silkscreen) to reflect 0.33uH. I have the correct 22pF cap (already installed) and a 0.33uH inductor on hand. Since neither of these parts are tied to ground or 12v, I cannot imagine that the board capacitance would play too much of a role here, but I can try doing a keep-out zone underneath them on the next revision. I do also have 0.33uH new inductors to try, which I have not done yet. In related news, I also tried some other TMS9900s, TMS9901s, and TIM9904s that I had on hand. The only ICs that I have not yet tested in another machine are the ROMs and RAM/SRAMs.

My original part is actually an open wound inductor. I had no color bands to go by. L602 and L207 are the only parts that I was unable to reconcile a value from the actual donor board, but yes.. there are a number of inconsistencies which I corrected along the way.