Resistor/Fuse replacement (DEC H7104-D)

Josh Dersch derschjo at
Tue Mar 22 12:53:48 CDT 2016

On Tue, Mar 22, 2016 at 12:29 AM, Mattis Lind <mattislind at> wrote:

> >>> Good news - mine worked so fingers crossed for yours too. I now have a
> >> functioning PSU again though I've not tried it back in the chassis
> yet...
> >>
> >> Nice!  My replacements arrived today and unfortunately I did not have
> > such good luck.  No smoke or fire, but now I get nothing at all out of
> the
> > supply.  The whine is gone, but there's nothing output at all.  The
> > fuse/resistor didn't blow (it's still got continuity across it) and the
> > transistor I replaced is still fine, but there must be something else in
> > the supply that's causing issues...
> >
> > Blargh.  I hate working on power supplies.
> In this case you have an excellent technical manual and a good schematic
> which should help a lot. Working with big SMPSU without schematics or
> technical manuals is a not fun.

Yeah, I have actual original copies of the printsets, too.  I have no
excuse, I just get nervous working on these things.  I suppose eventually
I'll get used to it.

> So here is my piece of advice directly from my head.
> 1. The PSU has really two AC inputs wired together in the input harness.
> You can separate the startup supply part from the SMPSU part. Connect the
> startup PSU to a separate AC input and the SMPSU part to a insulation
> transformer, a variac and lightbulb in series.

2. Check that the startup PSU, that uses a normal 50/60 Hz transformer
> gives the correct voltage. +11 if I remember correctly.
> 3. There is a circuit that monitors the input rectified 300VDC voltage and
> enables the relay when it has reached a proper voltage. It is a soft start.
> Disable it for temporarily. Good idea to check that is working though. If
> not the soft start resistors will become overheated when trying to run it
> at full load.

The relay (and the circuit driving it) appears to be working -- I hear it
click on at power-up (faintly, over the roar of the fans) and click again
after power-down.  I'll hopefully have some time this week to play around
some more, thanks for the suggestions!

I'll note that my earlier statement that I was getting "nothing" out of the
supply turns out to be slightly inaccurate -- with a load, I get nothing
(well, 0.07v) out of the 5V supply; without a load the voltage slowly
increases (maybe 0.5V per second).  Didn't let it run long enough to see
where it ends up ;).

Thanks again,

> 4. Now you need to have the SMPSU section connected to AC inlet. On one of
> the daughter boards there is a switching bias supply. Check that it
> generates the +/-12 V and +5V.
> 5. Is the main SMPSU switching logic making a nice square wave signal
> output. The control board is yet another daughter board.
> 6. I disconnected the output terminals from the H-bridge to the transformer
> and connected a dummy load to be able to have a look at the output
> waveform.
> 7. Reconnect the transformer. The main switch produces +5V and +38V. The
> 38V is then used on the daughterboards to create all the other voltages,
> +/- 12V, +/- 15V, +12V SB, +5VSB (depending on which supply). Is there 38V
> here?
> 8. There is a crowbar circuit on the +5V output terminals. Check that it is
> not tripping.
> Please note that when working with the +2.5V supply it gets the bias
> voltages from the +5V supply. So either get that one working first or
> supply bias voltage from a regular PC supply.
> /Mattis
> >
> >
> > - Josh
> >
> >

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