[rescue] SUN IPC PSU - still not working after recap

[John Hudak]

HI:
I am not familiar with this particular PSU but can offer some general
advice.
The SMPS ( I assume these are SMPS) need a load to operate. There are lots
of rules of thumb for this but I generally use that the (resistive) load
should be at least 10% rated current output for all voltage outputs.
THe larger filter caps are mainly for filtering. If you are seeing no
voltage either the switching ckt is not operating or for some reason OVP or
OCP crowbar ckts are being triggered.  The circuitry in general has lower
power transistors that constitute the oscillator part of the SMPS, as well
as steering diodes.  I'd check the transistors first.  They usually fail
open.  Also, as part of the feedback circuit, there are smaller (size wise)
caps that should be replaced as will. (they help to phase shift the
feedback to allow oscillation.
Checking the OVP and OCP ckts can be a challenge because hard to determine
what (component)  is shutting down.  One approach is to isolate the crowbar
ckts by temporarily removing the SCR that clamps the circuit. This PSU may
use a different approach so my advice may not be applicable.
HTH


On Fri, Jun 6, 2025 at 8:43 AM Jan Liška via rescue <rescue at sunhelp.org>
wrote:

> Hello,
>
>
>
> I’ve started recapping my IPCs using locally sourced low-ESR capacitors.
> However, both PSUs I’ve recapped so far exhibit the same issue:
>
>
>
> They power on for a very brief moment (about 0.1 seconds), the 5V rail
> voltage rises to 2–3V, and then the unit shuts down. I checked the units
> for cold solder joints and significant corrosion damage on trails but found
> none.
>
>
>
> I’ve been advised that either one of the TLP634 optocouplers may be
> faulty, or the capacitors I used might not be truly low-ESR as claimed by
> the seller.
>
>
>
> I doublechecked the capacitors and they seem to be OK - I used mostly
> Panasonic low-ESR capacitors like CE 3300uF 16V EEUFR1C332L. I replaced all
> capacitors except for the largest 400V one.
>
>
>
> Before I proceed with replacing more components, I would greatly
> appreciate any advice.
>
>
>
> Thank you!
>
>
>
> Jan
> _______________________________________________
> rescue list - http://sunhelp.org/mailman/listinfo/rescue_sunhelp.org
>
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