However, here's one thing that seems off... If I load the +5V rail with just 500mA, the rail has dropped as low as 3.9V 🤔​

I'm not sure how hard you're supposed to be able to drive that rail, but flipping through the schematics it seems like the +5V is the main supply, and the transformer winding driving the +12V and +5V combined is rated for 1.35A. So I'd expect to be able to pull half an amp from that without anything starting to sweat...

(Also, it seems unlikely that TI'd put that whole costly switching section in there, if it only needs that little current.)

I should check the ripple as well. The big smoothing capacitors are probably past their prime.

But that will be for another session; this is as far as I'll get tonight. 🤓​

#TI99 #retrocomputing #electronics

Remember that sagging Texas TI-99/4A I had under test recently?

Well, I measured the ESR of all the caps (in circuit, so not perfect, but a decent sanity check), and they all looked perfectly fine. Then I went on to hook up the scope to the +5V output, intending to measure the ripple. But before getting so far, I noticed something: the voltage line I saw on the scope did *not* drop as I turned up the load, like what I'd seen on the DC electronic load last time.

The load supports separate sense wires for measuring the voltage but I rarely bother, so it was showing the voltage it measured at its input terminals. So... that voltage drop happened between the PSU and the load?

But seriously, it would have to be some pretty bad cables to drop in the ballpark of 1.5V at a meagre 500mA...

#TI99 #retrocomputing #electronics

So I measured them, and guess what. They *were* indeed some pretty bad cables.

One of them measured at 1.5Ω, the other at 1.2Ω.
So 2.7Ω at 500mA - which works out to a drop 1.35V.
And with the unloaded voltage of about 5.2V, minus the calculated drop in the cables, we arrive at 3.85V - which is almost exactly what I read off the load (3.91V) previously. Eureka.

I found another set of cables that measured at just 0.1Ω and re-ran the test...

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And yes: this PSU is performing just fine. 😅​

At half an amp's load, 5.19V is maintained. At 1A, 5.15V. At 1.5A, 5.11V.

Lesson learned: fat lot of good your nice piece of kit will do you if you skimp on the cables. 😬​

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@Retrograde who needs cables. You are a sack of conductive fluid. Just lick your fingers and bridge the connection!