Odd TI-99/4A fault

[jjne]

I was given one of these machines, a PAL model, without any leads or history, and have been able to determine that it is kind-of working. 

 

It tends to boot to a black screen most of the time but occasionally will boot to the full welcome screen, at which point I can go into TI BASIC. It seems to boot more reliably if I quickly switch off-on from a black screen, so I'm thinking either dirty contacts or the reset circuit not working right? 

 

Once in I can enter as many lines of BASIC as I like, and run code. However as soon as you type RUN there is some screen flicker, as if there's a voltage sagging somewhere. I've tried both the built-in power supply fed by two DC sources (think this should be fine, I've done this on other machines without incident), and also a ZX Spectrum +3 supply with the -12V brought down to -5 with an LM337 - same deal, so I don't think it's the power supply rather something odd happening on the main board.

 

Due I think to the apparent voltage sagging which I have not been able to trace (the supplies *seem* OK to the board), whenever several lines are executed, or a loop is run from BASIC the machine freezes and/or resets to the welcome screen. 

 

The caps, both in the power supply and the main board check OK for capacity/ESR, although I will change these as a matter of course. 

 

Is this a common problem? I'm sure this one is viable just a few issues. 

 

Any help gratefully received 🙂

[jjne]

Just to add to the above... 

 

I guessed that this might be a scratch RAM issue, so replaced both chips (admittedly with cheap AliExpress ones) - exactly the same behaviour - intermittent startup, when it does boot it (usually) seems to do what it's supposed to do. 

 

Caps seemed fine but replaced as a matter of course - no change. 

 

The video RAM was socketed (partially with some old 1979 ceramic parts) - these test fine in my RAM tester - next step here may be to swap them out for ones on a ZX Spectrum board. 

 

I was trying to ascertain what is and isn't working in BASIC, and found something interesting that might point to the issue. 

 

If I enter:

 

10 FOR T=440 TO 550

25 PRINT T

30 NEXT T

 

This goes through a few iterations, beeps and resets to the welcome screen. 

 

However if I insert a call sound into line 30:

 

30 CALL SOUND (200,T,2)

 

This works reliably, every time. Sound and video just fine. 

 

If I reduce the duration from 200 to 150, the machine crashes. 

 

So it's a timing issue somewhere. 

 

Does any of this make sense? 

[jjne]

Should also point out that I've poked around with a scope, and there's nothing immediately obviously jumping out on that either - I wouldn't expect there to be though as this falls under the category of unreliable rather than broken for the most part... 

[Gary from OPA]

Can you post some pictures of the motherboard.

 

It could be timing issue, the 9904 and it's crystal, since it does work for a while.

[jjne]

Thanks for the reply. I will post some pictures later today. 

 

I have ordered some replacement TIM9904s and crystals, along with some spare 4116s. However I don't think they'll be necessary (they weren't expensive). 

 

I acquired an original cartridge (Blasto - I just wanted something to test), and once you get beyond the intermittent start-up, this game runs perfectly. I played it a couple of times and left the demo running for a good half hour and it was fine. 

 

So I think this proves that the system is fundamentally sound... but BASIC crashes randomly (well almost randomly). 

 

So my current thought is the BASIC ROM is marginal and needs replacing. I have a programmer and some EPROMs, so I will look into the possibility of adapting a 27128 or similar to these (slightly odd-looking) ROMs over the weekend. 

 

To be honest the unenhanced BASIC is all but useless so I could just use the computer as-is once I've built a ROM card for it, but I always like stuff to work fully. 

Edited September 6 by jjne

[Gary from OPA]

Try taking the 3 GROM chips out and cleaning the pins on them, I find after many years, the pins on these chips will turn black. And 99% of your ti99 operating system runs out of them including your ti basic.

 

See this picture for the location, they are already socketed and just above the big 9900.

 

[jjne]

Oh it's the GROMs is it? Right well that has saved me a chore... 

 

I'll look into those later - although if they are screwed up I don't think they're easily replaced are they? 

 

This TI is a right weird architecture... I have no problems getting my head around Commodores and Sinclairs but this is just baffling. Thankyou! 

[HOME AUTOMATION]

2 hours ago, jjne said:

...along with some spare 4116s.

These sound more likely to be the culprit.

[jjne]

2 hours ago, Gary from OPA said:

Try taking the 3 GROM chips out and cleaning the pins on them, I find after many years, the pins on these chips will turn black. And 99% of your ti99 operating system runs out of them including your ti basic.

 

See this picture for the location, they are already socketed and just above the big 9900.

 

... is the right answer! 

 

Just did this and the machine is now running perfectly. It looks like this might have been done before as the top of the GROMs was badly tarnished, the bits further down less so. Used some contact cleaner and scraped with a small screwdriver. 

 

I thought I'd ruined it at first as the first time I did this it did not boot at all, but giving it a second pass proved to be the fix. Now it boots correctly every time and I've put a few BASIC programs through their paces, all fine. 

 

Will need to remember this one as doubtless it'll reoccur... 

 

Thanks very much indeed for the heads up, I'm not sure I would have found that without scattergunning everything and/or buying a second TI,and these machines aren't cheap in the UK (unlike the US) because they were quite rare even back in the day here. 

 

Now need to get this FinalROM99 board built so I can have fun with the old beast!

Edited September 6 by jjne

[jrhodes]

5 hours ago, jjne said:

... is the right answer! 

 

Just did this and the machine is now running perfectly. It looks like this might have been done before as the top of the GROMs was badly tarnished, the bits further down less so. Used some contact cleaner and scraped with a small screwdriver. 

 

I thought I'd ruined it at first as the first time I did this it did not boot at all, but giving it a second pass proved to be the fix. Now it boots correctly every time and I've put a few BASIC programs through their paces, all fine. 

 

Will need to remember this one as doubtless it'll reoccur... 

 

Thanks very much indeed for the heads up, I'm not sure I would have found that without scattergunning everything and/or buying a second TI,and these machines aren't cheap in the UK (unlike the US) because they were quite rare even back in the day here. 

 

Now need to get this FinalROM99 board built so I can have fun with the old beast!

You'll want to go with the FinalGrom, not the FinalROM99. FinalROM99 only does ROM-Only carts (games mostly). FinalGrom allows ROM, GROM, and ROM+GROM cart images to be used

You already have a memory expansion sidecar? Some of the cart images for the FinalGrom expect it to be present for loading the cart image into memory.

[+Lee Stewart]

13 minutes ago, jrhodes said:

You'll want to go with the FinalGrom, not the FinalROM99. FinalROM99 only does ROM-Only carts (games mostly). FinalGrom allows ROM, GROM, and ROM+GROM cart images to be used

You already have a memory expansion sidecar? Some of the cart images for the FinalGrom expect it to be present for loading the cart image into memory.

 

That would be the FinalGROM99 vs. the earlier, more limited FlashROM99.

 

...lee

[RickyDean]

17 minutes ago, jrhodes said:

You'll want to go with the FinalGrom, not the FinalROM99. FinalROM99 only does ROM-Only carts (games mostly). FinalGrom allows ROM, GROM, and ROM+GROM cart images to be used

You already have a memory expansion sidecar? Some of the cart images for the FinalGrom expect it to be present for loading the cart image into memory.

Sounds as if he already has a FR99 board, so he will get some use out of it. I plan on using the one I've got for the grandkids to play games from

[HOME AUTOMATION]

FinalROM 99

 

FlashROM 99

 

FinalGROM 99

 

 

 

 

[+OLD CS1]

15 minutes ago, HOME AUTOMATION said:

FinalROM 99

 

FlashROM 99

[+OLD CS1]

dun dun dun dun dun dun dun dun dun dun dun dun dun dun dun dun

Flash!  ROM!