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+---
+layout: post
+title: "Restoring the TI 99/4A"
+date: 2025-03-23 12:00:00 +0200
+comments: true
+categories:
+---
+
+My father purchased that console in 1982. I was 5 at the time, and it was my very first contact with a computer.
+We used it for 5 years, till we got an Amstrad PC1512 in 1987.
+
+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
+what the "computer experience" was like back in the twentieth century.
+
+## What do we have in our hands?
+
+The machine is a European (PAL) model (6 pin DIN component video output, which we used with a PHA2037 RVB modulator).
+
+Model PHC004A (silver console), serial R030174 / 44 83 RCI.
+
+When I opened it up, I first noticed that the VDP was missing! My father mentioned a former colleague opening up the machine at
+some point... So I started with getting a replacement 9929 from eBay.
+
+When connecting the machine to a TV and powering up, I get the typical black screen and continuous tone symptom, which is
+consistent with the failure mode I remember from years back when I had last tried to use it.
+
+## First tests
+
+### Visual inspection
+
+The motherboard shows no obvious sign of damage such as corrosion or swollen electrolytics caps.
+
+### Voltages
+
+* the voltages provided by the mains transformer look good
+* the DC voltages at the output of the power supply board look good
+
+### Clock signaks
+
+The four 12V clock signals at the TMS9900 look OKish. They are not super clean, with significant ringing in particular
+in the negative region, but the frequency and the phase relationships look right.
+
+The four TTL clock signals at the TMS9904 also look about right.
+
+{% figure caption:"12V clocks" %}
+![12V clocks](assets/TI-resto/DS1Z_QuickPrint3-12V-clocks.png)
+{% endfigure %}
+{% figure caption:"TTL clocks" %}
+![TTL clocks](assets/TI-resto/DS1Z_QuickPrint4-TTL-clocks.png)
+{% endfigure %}
+
+The VDP clocks look OK:
+{% figure caption:"VDP XTAL2" %}
+![TMS9929 XTAL2 (40)](assets/TI-resto/DS1Z_QuickPrint5-VDP-XTAL2.png)
+{% endfigure %}
+{% figure caption:"VDP XTAL1" %}
+![TMS9929 XTAL1 (39)](assets/TI-resto/DS1Z_QuickPrint6-VDP-XTAL1.png)
+{% endfigure %}
+
+_No CPUCLK on pin 38 as this is an EU console with a VDP9929 VDP: pin 38 is the R-Y video component._
+
+GROMCLK taken on pin 37 the VDP loops OK:
+{% figure caption:"VDP GROMCLK output" %}
+![TMS9929 GROMCLK (37)](assets/TI-resto/DS1Z_QuickPrint7-VDP-GROMCLK.png)
+{% endfigure %}
+
+### CPU signals
+
+* :thumbsup: ~RESET~ seems to be high as expected
+* :thumbsdown: A0 and A1 appear to be rippling at odd voltage levels somewhere between logic 0 and 1
+{% figure caption:"TMS9900 address bus A0" %}
+![TMS9900 A0](assets/TI-resto/DS1Z_QuickPrint9-CPU-A0.png)
+{% endfigure %}
+{% figure caption:"TMS9900 address bus A1" %}
+![TMS9900 A1](assets/TI-resto/DS1Z_QuickPrint10-CPU-A1.png)
+{% endfigure %}
+
+* :thumbsdown: ~MEMEN~ seems to be stuck high, I'd expect it to go down regularly as memory cycles are executed
+{% figure caption:"TMS9900 ~MEMEN~" %}
+![TMS9900 MEMEN](assets/TI-resto/DS1Z_QuickPrint11-CPU-MEMEN.png)
+{% endfigure %}
+
+## Conclusion of first tests
+
+At this stage of investigation, it looks like the CPU is not working as expected.
+
+From there I see three possible next steps:
+* I'm wondering if the clock signals might be too much out of spec to allow correct operation (in which case
+  I'd need to first sort these out)
+* Could a faulty cap in the CPU's vicinity account for the symptom?
+  * I'm disinclined to do a full recap in the dark without a positive indication that that's the problem -- visually the caps look
+    all right, and those from the power supply side appear to be in a good enough state that all voltages look right
+* ~MEMEN~ stuck high and A0/A1 at weird level (or high-Z) make me suspect a nonfunctional TMS9900
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