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36 lines
1.5 KiB
Markdown
36 lines
1.5 KiB
Markdown
---
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layout: post
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title: "Restoring the TI 99/4A, ep. 6"
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date: 2025-07-06 16:40:00 +0200
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comments: true
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categories:
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---
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## Where we are: funny L600
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I captured more traces showing voltage before L600 (i.e. at the net labeled "8" from the power supply in Sams Computer Facts) (<span style="background-color: #f8fc00;">yellow</span>) and after it (i.e. at the CPU's Vcc pin) (<span style="background-color: #b800b8;">purple</span>), also recording the min and max voltage.
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{% figure caption:"Old CPU, L600 not shunted (bad)" %}
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{% endfigure %}
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{% figure caption:"Old CPU, L600 shunted (OK)" %}
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{% endfigure %}
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We are seeing:
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* very large voltage variations downstream of L600 when it's not shunted
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* interstingly, slightly different readings on the two probes even when shunted with a wire 🤔
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## Replacing the CPU & a provisional conclusion
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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.
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{% figure caption:"New CPU -- it works!" %}
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{% endfigure %}
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... and this time it works (and no longer shows large variations on Vcc) even with L600 not shunted :tada:
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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...
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