On Tue, 12 May 1998, Tony Duell wrote:
> > > I checked fuse on the back panel and it's OK. Is there a fuse on the P/S
> > > board located on the inside of the back panel?
> >
> > Yes, there is. It is in a clip on the board.
>
> I've never seen one of these PSUs, so I'm guessing here, but I've
> repaired enough SMPSUs in the past to give you some useful tips (it _is_
> an SMPSU I take it, and not a linear...)
One other thing that he might check providing the fuse is alright is the
soldering on the header connector pins. Remove the long connector from
the power supply and gently try to wiggle each individual header pin on
the board. If one does wiggle appreciably, it will be necessary to
remove the power supply - 4 screws - and resolder the pins on the back
side of the board.
I have only seen a few instances of this, but it does happen on occasion.
- don
> Remove the fuse and inspect it. You'll see one of there things :
>
> 1) The fuse is blown gently - the wire has melted, but it't not at is (2)
> below. In that case, replace the fuse and try again
>
> 2) The fuse has blown violently. Either the glass has cracked, or the
> inside of the fuse is coated in a black metal film. In that case there's
> a short circuit somewhere on the primary side. Likely _first_ failures
> are the mains bridge rectifier diodes or the chopper transistor. Alas
> there are normally other failures (resistors/capacitors) along with them,
> and unless you replace everything, your new parts will be damaged as well.
>
> 3) The fuse is fine, but the PSU won't start up. This is suprisingly
> common (I've had it twice in the last month). Most small SMPSUs run the
> control circuitry off one of the transformer outputs, which leads to a
> bootstrap problem (the PSU can't run without the control circuit, but the
> control circuit can't work without the PSU running). The solution is a
> small circuit to give the control circuit enough power to start. In a lot
> of cases this takes the form of a high-value resistor chain from the HT+
> (400V DC) line to the chopper control circuitry. And for some reason this
> resistor open-circuits. It's worth checking all high-value resistors on
> the primary side.
>
> When you've inspected the fuse, get back to us and we'll attempt to talk
> you through the PSU...
>
> -tony
>
>
donm_at_cts.com
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Don Maslin - Keeper of the Dina-SIG CP/M System Disk Archives
Chairman, Dina-SIG of the San Diego Computer Society
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Sysop - Elephant's Graveyard (CP/M) - 619-454-8412
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Received on Wed May 13 1998 - 16:21:23 BST