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125 lines
4.6 KiB
Markdown
125 lines
4.6 KiB
Markdown
---
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layout: post
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title: "Restoring the TI 99/4A"
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date: 2025-03-23 12:00:00 +0200
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comments: true
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categories:
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---
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My father purchased that console in 1982. I was 5 at the time, and it was my very first contact with a computer.
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We used it for 5 years, till we got an Amstrad PC1512 in 1987.
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It has been non-working for a number of years, and today I'd like to bring it back to a working condition, if only to show my kids
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what the "computer experience" was like back in the twentieth century.
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## What do we have in our hands?
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The machine is a European (PAL) model (6 pin DIN component video output, which we used with a PHA2037 RVB modulator).
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Model PHC004A (silver console), serial R030174 / 44 83 RCI.
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{% figure caption:"Type plate" %}
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{% endfigure %}
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When I opened it up, I first noticed that the VDP was missing! My father mentioned a former colleague opening up the machine at
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some point... So I started with getting a replacement 9929 from eBay.
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When connecting the machine to a TV and powering up, I get the typical black screen and continuous tone symptom, which is
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consistent with the failure mode I remember from years back when I had last tried to use it.
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## First tests
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### Visual inspection
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The mainboard shows no obvious sign of damage such as corrosion or swollen electrolytics caps.
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{% figure caption:"Motherboard (front)" %}
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{% endfigure %}
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{% figure caption:"Motherboard (back)" %}
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{% endfigure %}
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{% figure caption:"Motherboard (PCB version marking)" %}
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{% endfigure %}
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{% figure caption:"Closeup of video output section (EU model)" %}
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{% endfigure %}
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### Voltages
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The PSU board shows no obvious sign of damage, either.
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{% figure caption:"PSU board" %}
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{% endfigure %}
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* the voltages provided by the mains transformer look good
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* the DC voltages at the output of the power supply board look good
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* voltage readings at CPU pins:
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* pin 1 (Vbb) -4.88 to -4.64V
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* pin 2 (Vcc) +4.8 to +5.2V
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* pin 27 (Vdd) +11.4 to +11.9V
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### Clock signaks
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The four 12V clock signals at the TMS9900 look OKish. They are not super clean, with significant ringing in particular
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in the negative region, but the frequency and the phase relationships look right.
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The four TTL clock signals at the TMS9904 also look about right.
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{% figure caption:"12V clocks" %}
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{% endfigure %}
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{% figure caption:"TTL clocks" %}
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{% endfigure %}
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The VDP clocks look OK:
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{% figure caption:"VDP XTAL2" %}
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{% endfigure %}
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{% figure caption:"VDP XTAL1" %}
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{% endfigure %}
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_No CPUCLK on pin 38 as this is an EU console with a VDP9929 VDP: pin 38 is the R-Y video component._
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GROMCLK taken on pin 37 the VDP loops OK:
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{% figure caption:"VDP GROMCLK output" %}
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{% endfigure %}
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### CPU signals
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* :thumbsup: ~RESET~ seems to be high as expected
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* :thumbsdown: A0 and A1 appear to be rippling at odd voltage levels somewhere between logic 0 and 1
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{% figure caption:"TMS9900 address bus A0" %}
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{% endfigure %}
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{% figure caption:"TMS9900 address bus A1" %}
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{% endfigure %}
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* :thumbsdown: ~MEMEN~ seems to be stuck high, I'd expect it to go down regularly as memory cycles are executed
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{% figure caption:"TMS9900 ~MEMEN~" %}
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{% endfigure %}
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## Conclusion of first tests
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At this stage of investigation, it looks like the CPU is not working as expected.
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From there I see three possible next steps:
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* I'm wondering if the clock signals might be too much out of spec to allow correct operation (in which case
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I'd need to first sort these out)
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* Could a faulty cap in the CPU's vicinity account for the symptom?
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* I'm disinclined to do a full recap in the dark without a positive indication that that's the problem -- visually the caps look
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all right, and those from the power supply side appear to be in a good enough state that all voltages look right
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* ~MEMEN~ stuck high and A0/A1 at weird level (or high-Z) make me suspect a nonfunctional TMS9900
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