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


Hans23

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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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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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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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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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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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