Trying some board debugging

[Duewester]

@InfiniteTapeGood to hear you are still plugging away. I don't have 6810s or other 2 to try piggy backing. I swapped all 3 GROMS, no joy.

I've removed and socketed the processor, and I have two replacements on order just in case it's the processor.

I've been going through the check list and checking all the gates and so on. Hard to say one way or another when I suspect the processor or ROMs.

[matthew180]

7 hours ago, Duewester said:

The reason I got led down the 10.xxx road was that it had a very low p-p (even for 5v).

 

You have to be careful when trying to measure a passive crystal since they are very sensitive to capacitance, and just probing with and O-Scope can change, reduce, or even stop the oscillation.  You usually need a very high impedance probe with a spring ground-lead attached near the probe tip.

 

Also, the voltages on the XTAL1 and XTAL2 pins are not necessarily logic signals or 5V.  The datasheet for the 9918A indicates that the XTAL waveform is 0.8V low to 2.75V high (minimum).  I would be very surprised if the problem is the VDP crystal.

 

[speccery]

On 10/10/2022 at 9:00 PM, Duewester said:

@speccery I'm interested in your grom replacement setup. How can I get one for testing or otherwise contribute to your research and development.

Thanks for the interest! The easiest way for now is me to send you a tested one. I need to build a few these for others too. Will send you a PM when I have some ready for shipment. I just got a whole bunch of PCBs from manufacturing and a load of components for this and other projects, the only missing piece is time....

[Duewester]

1 hour ago, speccery said:

Thanks for the interest! The easiest way for now is me to send you a tested one. I need to build a few these for others too. Will send you a PM when I have some ready for shipment. I just got a whole bunch of PCBs from manufacturing and a load of components for this and other projects, the only missing piece is time....

Thank you @speccery. My Frankennine is just the machine I have in mind for this.

[Duewester]

The saga continues.

I put in a new clock crystal for the VDP. Proper timing now (time to redo the checklist).

Did some DeoxIT work on all the chips with sockets.

While I am waiting, I found the keyboard that came with this 99 had a bad trace on the INT5 line. Found a broken trace at a jumper point. It was lifted UNDER the solder. I put in a jumper line and now I need to disassemble a working 99 so I can test the keyboard. It will NOT be the Frankennine. That has my homemade keyboard in it and the connector is just 15 female wires that I'm to lazy to mess with. Colossus is my F18 machine so I guess it's Guardian this time around. Might even do a video upgrade (replace 9918a with 9928a and TMS with all that goes with that) on it. I was gonna do the video on this broken 99 but, I suspect the broken 99 will have enough done to it by the time I finish. 

Time to take trash to the dump and pick up medicines, Y'll have a great day and stay safe.

[Duewester]

Well, against my better judgement, I tried all the socket chips in another unit and they all work. So,... even though several TS tips pointed to bad VDP, not the VDP, 9904, GROM0, GROM1,  GROM2 or sound chip (SN94624N). I've removed and socketed the processor. The installed processor was the Korean version (S9900P AMI) which is a new one to me. I had purchased (I thought) two TMS9900NL processors only to realize the reason they cost $16 was I had purchased 10. So anyway. One was a TMS9900NL-40 from 8505 with -40 marked over. The printing was Identical to another NL-40 from 8801. Both made in the Philippines. The others are an assortment from the Philippines and Singapore. None has made a difference in the black screen and annoying screech.

Sorta good news is that somewhere with all the DeOxit and chip swapping the static in the black screen (usually appears after a minute or so) has stopped.

I fear that the 6810 chip in U608 may be bad. I have a very cheap ( by cheap I mean low resolution) home made (from parts sourced at Adafruit) thermal camera and the this chip shows a big hotspot in the middle whereas the other ROMs are cool. The VDP looks warm as does the processor and 9904 but overly so. All the video rams look the same - no hot spots.

[HOME AUTOMATION]

10 hours ago, Duewester said:

So,... even though several TS tips pointed to bad VDP, not the VDP, 9904, GROM0, GROM1,  GROM2 or sound chip (SN94624N).

I don't think the last three are needed to operate.

[Duewester]

They aren't. I was afraid one might be pulling down a voltage or something because I initially saw a degraded signal on GROM0. It cleared up with some DeOxit on all the GROMs. I've been running without the sound chip since I started working on this one because of the noise.

I'm just starting to get frustrated and worried. If it's a ROM,....

[Duewester]

Well if I didn't live near a conservative Christian college town, I'd run naked through the streets, "Eureka!".

Bad SRAM - not the one someone previously replaced - AND a flaky VDP - lasts about 30 seconds. The VDP didn't show up until I installed two replacement 6810's that finally showed up.

Doing a burnin on the new VDP (with a heat sink laying on top of it). So far the screen looks pretty good (given that I'm engineered into a HDMI monitor with the worst aspect ratio). Of course it goes blank after a few minutes on the intro screen because I don't have a key board hooked up yet. I'm using my thermal camera to check the temps and so far, all look great.

This has been an expensive adventure but a great experience (so far). Now at least I have; a nearly 100% socketed in Ti-99 4a for future endeavors, a logic probe (didn't trust my ability to read my Hantek O-scope), an XGecu Pro T48 Programmer/chip tester, nine extra SRAM chips, ten TMS9900NL (actually eight with two NL-40's) processors, four extra OR gate chips and four multiplexor chips and an assortment of other misc chips.

If this had been a single issue or a VDP that didn't flake out so fast, it might have been easier to determine it was the SRAM in U609. If I had left the F18 installed, it too might have revealed the SRAM issue. If a frog had a rubber butt, it'd bounce when it lands.

Now I just have to beat my wife to the mail and pay off the credit card before she finds out how expensive this has been.

Thanks to all who have sat patiently through my meanderings and offered their advice.

[Duewester]

Well, maybe not so done after all. This poor thing had/has more issues than it deserves.

Finally got to go through the boot up and No "Select 1 for Basic".

Goes right to "Insert Cartridge" which works great with a Extended Basic Cartridge and a FinalGROM99 cartridge.

I've seen this somewhere here before, so time to start digging.

[HOME AUTOMATION]

I believe this is expected when GROM1, is out.

[Duewester]

28 minutes ago, HOME AUTOMATION said:

I believe this is expected when GROM1, is out.

@HOME AUTOMATION thanks for the tip. I been looking through the forums and haven't found anything else.

Well, time to take it back apart, again. I put the GROMS in another machine and they worked - at least I think they did. The machine booted up but I never tried to select anything. Very likely I missed a pin or something when I reinstalled them into the bad machine. Happened before.

Even if this isn't the problem, the machinr boots up FinalGROM and my TiPi is working. So.... not a total loss so far.

[Duewester]

You called it @HOME AUTOMATION, pin 8 Furthest away in my field of view, was bent out.

Thanks.

[HOME AUTOMATION]

Ha... Ha... Happened to me once, long ago, while fiddling with a Missile Command machine!📡🛰️🚀

[Duewester]

I gotta go back through all the forums and pull all the tips for my repairs into one document.

You folks here have been an inspiration and a great source.

I never imagined when I started down this rabbit hole that I would feel so satisfied with the process.

[+dhe]

23 hours ago, Duewester said:

Now I just have to beat my wife to the mail and pay off the credit card before she finds out how expensive this has been.

Yea... I know that feel. But, I had to spend a $1000 dollars in tools and parts, to fix the $25 console I bought at a garage sell!

[InfiniteTape]

I finally got my workbench set up again and got my failing unit out. I socketed both the scratchpad rams and tried the spares I bought. No dice. I've got some TMS9900's on the way, but that seems unlikely to be the fix. That leaves the roms or something far less likely. I'll probably go back and walk the troubleshooting tree again since it's been so long.

 

I will say: The Hakko desoldering gun is an amazing upgrade!

Edited November 7, 2022 by InfiniteTape

[Duewester]

I bought the T48 programmer/Chip tester in anticipation of learning to program the ROMs. I even have some adapter boards coming from Arcadeshopper. The tester did help me verify my OR gate chips, mux's, and other logic chips were all good. 

[+dhe]

48 minutes ago, InfiniteTape said:

I will say: The Hakko desoldering gun is an amazing upgrade!

+1

[Duewester]

Yep, a desoldering gun AND a hot air gun just barely preceded this project and turned out to be essential. I purchased them for a video ram problem on another machine.

An assortment of dip sockets is beneficial.

[InfiniteTape]

I've gotten my hands on a couple of 9900's in case I need to swap them. Does anyone have a recommendation on sockets for those? I seem to remember the pin spacing is a little unusual.

 

Also, I know Thierry's page is out there for replacing the console ROMs, but it's complicated by the inclusion of in-circuit programming support. I don't really have a need for that extra capability, so does anyone know of a simpler design for swapping the console ROMs with EEPROMs?

[+dhe]

You'll have to chase it down, but the console ROM's are compatible with TMS2532.

   https://html.alldatasheet.com/html-pdf/103331/TI/TMS2532/24/1/TMS2532.html

@atrax27407 can correct me on the part number.

 

  He might even be able to make you up a handful to have er.. on hand.. as backups.

 

   You can also program them yourself, but the challenge is finding a programmer that will give you the 25V programming voltage needed.

[atrax27407]

I can do the EVEN and ODD console ROMs. As a sidelight, Tony Knerr found out years ago that. although they are listed as 25V, the TMS2532s program very nicely at 21V. An adapter is needed (check my MiniPro thread for details). Or contact me for ordering details.

Edited November 11, 2022 by atrax27407

[Duewester]

I purchased the T48 programmer because it will do 25v (if I read the spec's right). I also got some 2532/2732 adapter boards from Arcade Shopper. All with the thought I might end up replacing my ROMs. Been lucky so far that I don't have to head down that rabbit hole.

[atrax27407]

The T48 doesn't support TMS2532s natively. However, with an adapter, it should do them if you use a TMS2732A as a template.

Edited November 12, 2022 by atrax27407