Repairing a BBC Master #2: Basic steps

First, do no harm. Second, check the voltages.

That’s how the Hippocratic Oath starts when adapted for electronics.

If the device under test is not getting the proper power supply, then you can’t reasonably expect it to work. But there’s something I wanted to check, even before I apply power, and this turned out to be a learning opportunity.

The BBC Master has many powerful connections.

With the power off, I put the digital multimeter (DMM) into continuity mode and stabbed the probes into the +5V and GND connections. I wanted to check that there was no dead short on the main power lines (which might indicate something like a failed capactor or chip).

There was a short. The DMM was beeping like a bastard.

Huh. I checked the power connectors on my spare motherboard and, sure enough, no short. Had I already found the problem?

I disconnected the power leads that run from the power supply unit (PSU) to the motherboard. The short disappeared. So then I checked the power leads themselves and there was the short again. Had the PSU blown in some way.

With the leads still disconnected, I turned on the power and checked the voltages. The PSU was working fine, providing both +5V and -5V.

Now, I don’t know much about power supplies and am nervous about working on them. So I popped over to the Stardot forum to get a bit of expert advice. It turns out that what I was seeing is perfectly normal. The ever-helpful ‘1024MAK’ told me: “The Master uses a switch mode power supply unit (SMPSU) and these have load resistors across the output lines. Also some multimeters on some ranges will see the electrolytic smoothing capacitors as short circuits until they charge up.”

So, nothing to see here, please move along.

Next time I’ll check the chips…

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