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Hans23

Failing TI-99/4A - Starts into TI Basic, then dies

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Hi,

 

does anyone recognize this failure mode?  The machine starts into TI Basic, but it seems to take a bit longer then usual.  The cursor quickly flashes for a short period, then stops flashing.  Finally, the machine crashes, displaying just the empty blue screen:

 

 

Thanks,

Hans

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VDP and VRAM probably OK if no corruption if left on title screen. Ditto power supply.

 

Can you type and run a simple "10 PRINT" in the few seconds available?

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There are more issues. There should be a blank line between TI BASIC READY and the prompt. Can you test with an Extended Basic cartridge?

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We all know your pretty handy with a soldering iron...

I think I would try socket-ing the 9901 and putting in a know good one.

The worst that would happen, is you would have a motherboard with a socketed 9901! 😃

 

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I'm a bit further in, but no wiser:  The machine exposes a weird timing behavior when and if it goes to the start screen and to TI Basic.  The initial beep is delayed by several seconds (i.e. the start screen shows, but the beep comes much later).  Likewise with selecting TI Basic - It seems to take longer to start than usual until the beep, but I can't get to the Basic prompt anymore.

 

I have swapped the GROMs, the CPU, the VDP, the 9901, the bus mux chips and the RAM.  I have also tested all the caps and they are nominal.  Voltages are OK (I tried different PSUs), and the clocks all seem to be good as well.  I found that the TIM9904 is incorrectly labeled as being an LS362 in the schematics and the pins are not correctly labeled, which was a bit puzzling at first.

 

I'm now wondering whether there might be a source of interrupts or wait cycles that is slowing down the start process, but I'm more in fantasy land here :)  Any further clues would be appreciated (but it will be a few weeks before I can dig deeper).

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1 minute ago, GDMike said:

I bet you're wishing the 9900 was socketed now.

It is :)

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Posted (edited)

Did you swap it? Or did I miss that along the way...

Ahh. I see you did...I was thinking cpu.

Edited by GDMike

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3 hours ago, Hans23 said:

I have swapped the GROMs, the CPU, the VDP, the 9901, the bus mux chips and the RAM.

VDP RAM??? Considering the interpreter's behavior, etc.... that's where I probably would have started.

 

3 hours ago, Hans23 said:

I found that the TIM9904 is incorrectly labeled as being an LS362 in the schematics and the pins are not correctly labeled, which was a bit puzzling at first.

I believe the LS362 designation is correct! ...seen that before.

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Posted (edited)

74LS362 is a correct designation for the clock chip on the earlier consoles, as Home Automation said above.

 

What's the frequency of the crystal by the clock chip? If it is 48 MHz then the clock chip must be either a TIM9904 (no "A") or 74LS362 (they're the same). If it is 12 MHz then the clock chip must be a TIM9904A.

 

(Wondering if it is a 12 MHz crystal but someone has fitted the clock chip for the 48 MHz crystal, so it is running 4 times slower than it should be with a flaky clock?)

Edited by Stuart

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Wow, Stuart that's a theory! 😃

"You see, I never liked that basic ran to fast on the TI-99, so I took ever opportunity I could to slow it down." LOL..

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@Hans23If you have an oscilloscope, you could try to check the frequencies of the CPU (ø1..ø4 (pins 8,9,28,25)) or the VDP (/INT (pin 16) and GROMCLK (pin 37)).

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I'm away from the workshop now, but I did some measurements on the TIM9904ANL - It generates the four clock phases with 3 Mhz each, so I guess that the crystal is OK (it looks stock, same type as the one that clocks the VDP).

 

As I have the VDP RAM in sockets now, I'll try swapping it as soon as I have some replacement chips.  When I fitted the sockets, however, I swapped the chips around and that did not change the error behavior all that much, so I'm not quite hopeful regarding that.  Anyway, thank you for your help so far, I'll spend some time with the machine next weekend and may get back with further questions or diagnosis results.

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Do not forget that there is a TI-issued troubleshooting guide in the 99/4 section on WHT. It is for the 99/4, but most of it still applies with the /4A. I scanned it and put it up there a few years ago--I still need to scan the one I have that is /4A-specific (I picked it up about a year ago).

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