3.4 KiB
| layout | title | date | comments | categories |
|---|---|---|---|---|
| post | Restoring the TI 99/4A, ep. 5 | 2025-05-18 13:00:00 +0200 | true |
Checking L600
Vcc (pin 2) of the CPU is oscillating wildly unless I shunt L600. With L600 shunted, the machine seems to start up just fine (nice beep, and title screen displays).
Desoldering L600 and testing it off circuit is inconclusive: it seems to behave just fine. Measuring its inductance with the oscilloscope (through the period of amortized oscillation with a known capacitor) as shown here shows a consistent behaviour vs a new replacement part.
{% figure caption:"Old L600 desoldered from board" %}
{% endfigure %}
{% figure caption:"New replacement L600" %}
{% endfigure %}
The old part's amortized oscillation damps down faster, which is consistent with a higher resistance (on the LCR-T7, the old part reads at 0.50 Ω, whereas the new one reads at 0.25 Ω), but both measure a period of 520 ns, which is ballpark-consistent with a 6.8 μH inductance.
I soldered the new part instead of the old one, and I'm seeing a similar behaviour as before: failure when starting up (continuous tone, wild variations at TMS9900 Vcc), apparently working fine when shunting L600.
Just to be sure, with L600 shunted I hooked up the display and confirmed that:
- the title screen displays fine;
- when pressing a key, I get the main menu;
- when pressing 1, I get the TI basic prompt, and key strokes appear on screen.
So L600 may not be the culprit, but some other connected part.
Could it be C611?
Another possible culprit nearby might be C611. Desoldering it and measuring it off-circuit, it reads just under 1000 pF on the LCR-T7, which is as expected, so I'm deciding it's good and soldering it back.
Something else near ~MEMEN~?
Next hypothesis is a marginal component sinking too much current from Vcc:
- either R607, which looks like a pullup on ~MEMEN~
- or U605 input pin 2?
A closer look at R607
In-circuit, R607 reads at 4.6 kΩ, which seems in a reasonable ballpark, so I'm disinclined to desolder it, which is fortunate because annonyingly, the new L600 is a bigger form factor than the original one, and so it's blocking access. I could desolder it and solder back the original one: since the original one measures fine, and replacing it didn't change the behaviour, I'm assuming that the problem was not there. But since the in-circuit reading looks within spec, that would really be a shot in the dark.
Vcc fighting ~MEMEN~?
Next step is to look at Vcc and ~MEMEN~ simultaneously to see if there's a correlation between changes on MEMEN and Vcc variations.
Finging: The Vcc variations do not appear to be correlated with ~MEMEN~: there are big oscillations on Vcc even when ~MEMEN~ is stable (whether low or high).
{% figure caption:"Vcc vs ~MEMEN~ / no shunt on L600: KO" %}
{% endfigure %}
{% figure caption:"Vcc vs ~MEMEN~ / shunt on L600: OK" %}
{% endfigure %}