Resistor/Fuse replacement (DEC H7104-D)

Mattis Lind mattislind at
Tue Mar 22 02:29:58 CDT 2016

>>> 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.

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.
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
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.


> - Josh

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