Discharging capacitors, PSU-testing with dummy loads

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exxos
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Re: Discharging capacitors, PSU-testing with dummy loads

Post by exxos » Fri Sep 21, 2018 8:49 am

stween wrote:
Fri Sep 21, 2018 8:40 am
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!
Probably best start a new thread, explain the story, post a image of your board etc
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Re: Discharging capacitors, PSU-testing with dummy loads

Post by stween » Wed Oct 03, 2018 9:52 pm

One other question I think relevant to this thread: I've re-capped my SR98. I haven't swapped out R201 or R202 yet.

A quick sanity check of my work so far against the load resistors shows a very nice 5.0V on the +5 line. The 12V line is kicking out 11.25V.

You said earlier in this thread (emphasis mine):
exxos wrote:
Wed Aug 30, 2017 10:00 pm
Loading the 5V alone should be enough to start the PSU as the feedback opto is coupled to the 5V rail. The 12V rail is likely based on turns ratio on the core (which is also why the 12V is mostly "screwy" sometimes with output voltage). But one should really load the 5V and 12V for initial testing. Once thats verified, load the other voltages (once the wire colour codes have been identified).
What did you mean by that?

I'm going to go back in to swap out R201 and R202, but I'm curious if 11.25V is in some way typical or if I should go review my solder joints while I'm in there.

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Re: Discharging capacitors, PSU-testing with dummy loads

Post by exxos » Wed Oct 03, 2018 10:29 pm

stween wrote:
Wed Oct 03, 2018 9:52 pm
What did you mean by that?

I'm going to go back in to swap out R201 and R202, but I'm curious if 11.25V is in some way typical or if I should go review my solder joints while I'm in there.
I mean exactly what I wrote...

There is no regulation on the 12V at all. Some PSU's had a diode in series with 12V because voltage was to high.. then later revisions I assume used a different transformer and voltage ended up lower (to low) and diode was removed to compensate.

Regulation is on the 5V rail only.. transformer acts like a "x2" on the 5V rail (well x2.3) . So if 5V was exactly 5V, you would get 5x2.3= 11.25V. If the 5V rail was 5.5V then 5.5x2.3 = 12.65V.

They are just cheap PSU's... about 11V for the 12V output is typical. Unless you are using some really old floppy drives which use 12V.. then its hardly going to matter what voltage the 12V is anyway.
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Re: Discharging capacitors, PSU-testing with dummy loads

Post by stween » Wed Oct 03, 2018 10:43 pm

Interesting. Thanks for the description!

I had a moment's doubt because I don't recall such a low voltage on the 12V line before the work. But, perhaps it was closer to something like 5.2V and 12.0V; that'd made some sense.

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