Russell McMahon wrote:
>
> Original topic on this May 27 2001 Post by Roman was
>
>                 Re:      Re: [PIC]:PICSTART+ Hardware problem
>
> This post and Romans (see below) MAY help people with PICSTART PLUS startup
> problems caused by power supply degradation.
>
> Got sick of my PICSTART Plus not starting for about 20 minutes after being
> plugged in.
> Mine has only two ecaps on secondary side (may be older?)
>
> Replacing both failed to cure the problem.
> The first is 470 uF 16V? and is fed by the diode from the switching power
> supply. The second smooths the output proper and is fed via an inductor from
> the first. The second is 100 uF 25V as Roman says but clearly doesn't NEED
> to
> be 25V as the first is 16V and the psu is regulated 9v
>
> SO - I replaced the second with 1000 uF at ?16v. Brute force but it does fit
> and "almost" sits down on PCB.
> This ONLY JUST works. Putting mains on psu first gives 9 volts open cct. But
> when PICSTART is plugged in it powers up "most of the time" but not all the
> time. If not just unplug secondary and reinsert and it usually goes. Once
> going its fine. Clearly the system is marginal. It works for now - I may go
> looking for why some time but don't count on it. Mayhaps the cap's ESR is
> critical to the feedback loop?
>
> Mayhaps the primary DC supply (about 300 volts) is ripply but I didn't
> bother chasing it when this fix (almost) worked. (Small HV caps are less
> commonly
> to hand here than LV ones).


Hi Russell!! :o)
I've been off the PC for a while with back injury
after crashing my bike. Nothing serious, just
very irritating for the last 3 weeks if I tried
to "sit". I've got a big cushion on my PC chair
now and can do a hour here and there.

I'm not sure with your Picstart PSU problem, it
might be that the new 100uF cap you put in already
was old and had some ESR problems. I test new
caps before using them, and have taken some back
to the supplier when the "new" caps had already
exceeded their ESR spec.

With a small circuit like that I would simply
unsolder one leg of each 2-legged part, resistors
diodes etc, and measure their values. It's very
common for resistor values to drift badly when
they get hot in those little smps products.
Check for reverse leaky diodes, or diodes which
change Vf as you are actually seeing it on the
meter. Extra check anything near "heat spots"
on the board or obvious heatsinks. :o)
-Roman

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu