TI 99/4A ep 6

_source/_posts/2025-05-18-ti99-4a-restoration-ep6.md

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+---
+layout: post
+title: "Restoring the TI 99/4A, ep. 6"
+date: 2025-07-06 16:40:00 +0200
+comments: true
+categories:
+---
+
+## Where we are: funny L600
+
+I captured more traces showing voltage before L600 (i.e. at the net labeled "8" from the power supply in Sams Computer Facts) and after it (i.e. at the CPU's Vcc pin), also recording the min and max voltage.
+
+{% figure caption:"Old CPU" %}
+![Old CPU, L600 not shunted (bad)](assets/TI-resto/20250703-oldCPU-L600.png)
+![Old CPU, L600 shunted (OK)](assets/TI-resto/20250703-oldCPU-shuntL600.png)
+{% endfigure %}
+
+We are seeing:
+* very large voltage variations downstream of L600 when it's not shunted
+* interstingly, slightly different readings on the two probes even when shunted with a wire :frown:
+
+## Replacing the CPU & a provisional conclusion
+
+The hypothesis at this stage is a marginal CPU that draws a little too much current, so I'm trying to replace it with another one from eBay.
+
+{% figure caption:"New CPU" %}
+![New CPU, L600 not shunted (OK)](assets/TI-resto/20250703-newCPU.png)
+{% endfigure %}
+
+... and this time it works (and no longer shows large variations on Vcc) even with L600 not shunted :tada:
+
+So I'm considering the machine repaired, and hoping that there isn't some other issue with the power supply or the mainboard that will fry one CPU after another...