Discharging capacitors, PSU-testing with dummy loads

[Pacman]

Thanks

[IngoQ]

Thanks for confirming. The pinout should be correct.
I've just posted the "how to" here. I hope someone finds it useful.

Wow, that is great and very detailed... Thank you for this effort!

[CoraDias]

Hi..i am a new user here. As per my knowledge those resistors were to test the STF/E PSU so other voltages wouldn't be tested. You can still use them to test the 5V and 12V rails. The -12V needs something like a 330R resistor to try.I am not sure what he colour codes are for the MSTE PSU, I only had the PSU on loan for a short time and I never thought to document it.Though I would assume red is 5V and yellow is +12V.

[exxos]

Though I would assume red is 5V and yellow is +12V.

Yes

[stween]

Hey; I recently picked up an old STFM to repair.

Far as I can tell, the PSU is handing out 3.5V on the 5V lines, and 9V on the 12V lines. But I want to measure properly, and I'm going to load test with the resistors for sale in the store.

So that I'm sure I'm measuring the correct thing, and following Pacman's (excellent) image: I ought to hook the resistors up across ANY 5V+GND pair and 12V+GND pair, and then I'll be able to get a reliable measure from any of the pins?

A couple more questions:

First: would old and dried out capacitors (no leaking, no blown tops) lead to the low voltage output I seem to have measured so far? The actual soldering work is not a problem and the caps are due for replacement anyway, but there's no point doing that work if there's a bigger problem.

Second: I don't usually recap hardware that is so close to the mains voltage. Let's say I screw up and I don't make a good contact across one of the capacitors. What's the biggest likely risk to the hardware here?

[exxos]

Far as I can tell, the PSU is handing out 3.5V on the 5V lines, and 9V on the 12V lines. But I want to measure properly, and I'm going to load test with the resistors for sale in the store.

The PSU will malfunction without a lot.

So that I'm sure I'm measuring the correct thing, and following Pacman's (excellent) image: I ought to hook the resistors up across ANY 5V+GND pair and 12V+GND pair, and then I'll be able to get a reliable measure from any of the pins?

Yes.

First: would old and dried out capacitors (no leaking, no blown tops) lead to the low voltage output I seem to have measured so far? The actual soldering work is not a problem and the caps are due for replacement anyway, but there's no point doing that work if there's a bigger problem.

If you are not measuring with the load, voltages you are reading are a little irrelevant...

Second: I don't usually recap hardware that is so close to the mains voltage. Let's say I screw up and I don't make a good contact across one of the capacitors. What's the biggest likely risk to the hardware here?

The PSU would likely return on and off due to overvoltage protection.

[stween]

Far as I can tell, the PSU is handing out 3.5V on the 5V lines, and 9V on the 12V lines. But I want to measure properly, and I'm going to load test with the resistors for sale in the store.

The PSU will malfunction without a lot.

I forgot to say: this was while connected to the main board. Power makes it through to the monitor port but nothing kicks into life. So I have a load for the PSU, but I'm going to load test with the resistors just to eliminate the mainboard and get a clean reading.

So that I'm sure I'm measuring the correct thing, and following Pacman's (excellent) image: I ought to hook the resistors up across ANY 5V+GND pair and 12V+GND pair, and then I'll be able to get a reliable measure from any of the pins?

Yes.

Okay!

Second: I don't usually recap hardware that is so close to the mains voltage. Let's say I screw up and I don't make a good contact across one of the capacitors. What's the biggest likely risk to the hardware here?

The PSU would likely return on and off due to overvoltage protection.

Okay, so it'll mostly protect itself. I shouldn't expect blown caps if I do a bodge job and miss something. Also, the PSU will discharge the caps over a short period by itself, right?

[exxos]

Okay, so it'll mostly protect itself. I shouldn't expect blown caps if I do a bodge job and miss something. Also, the PSU will discharge the caps over a short period by itself, right?

I would check the 400V caps with voltage on meter to make sure its safe.

Bad caps can cause spikes in the voltage, where over voltage protection kicks in and turns the PSU off. Once the voltage drops to a safe level, the PSU turns on again.. if the caps are bad, this happens very fast and the voltage output will be wrong. Mostly likely showing voltage lower than it should.

[stween]

Bad caps can cause spikes in the voltage, where over voltage protection kicks in and turns the PSU off. Once the voltage drops to a safe level, the PSU turns on again.. if the caps are bad, this happens very fast and the voltage output will be wrong. Mostly likely showing voltage lower than it should.

Okay, that sounds valid; thanks.

I'm pretty willing to just do the soldering work and see how the PSU behaves once I'm done, I'm really just making sure there aren't any obvious "gotchas" when doing the work (other than the obvious: don't electrocute myself). It'll take me a couple weeks to get to this project, but I hope to update with good news when I get to it

[stween]

Well okay, maybe this won't be so obvious. I had a few minutes so I tested with the resistor load, and I get nice +5V/+12V readings. But first I realised I had a bad cable on the multimeter, and so I assume I had bad readings last week.

Replacing the caps will obviously be a good plan in any case, but I'm expecting I'll need a deeper dive to figure out what's up, and why the system refuses to boot!