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TECHNICAL NOTE
Date 15th Jan 09
ref 203
Raised by: TC
Distributed to: as required
Soundtracs – Digico(UK) Ltd. Unit 10 Silverglade Business Park Chessington Surrey KT9 2QL England
Tel: +44 1372 845600 Fax: +44 1372 845656 email: support@digiconsoles.com
SD7 REPLACEMENT OF POWER COMBINER DIODES
It has been found certain older SD7 mixers shipped prior to April 2007 may have a problem associated with
the PSU power combiner diodes, resulting in unreliable engine operation in the event of a PSU failure.
If a mixer is considered at risk of this problem, replacement diodes will be provided from the factory.
Procedure
First the performance of the diodes presently fitted can be checked. If measured OK, then the diodes need
not be changed.
Lift the upper master panel.
Remove 8 x 2mm hex stainless panel fixing screws.
Lift the front edge (nearest the faders) and prop up, or have an assistant hold the panel in the raised position.
Switch the mixer on with both PSU 1 and 2 powered.
Under the master section see the power distribution / controller PCB.
At the left end of the PCB the main power cables can be seen. Adjacent to these are the connection screws
to the combiner diodes, arranged as 4 pairs.
Inputs are to the left, outputs to the right. The measurement is made input to output.
Starting at the pair to the rear of the console measure the volt across the pairs of screws. (see picture)
The 4 pairs to the rear are for the 5V supplies, the remaining diodes, to the front, under the fader panel, are
for the +12V and -12V. The diodes on the +5V supply should read up to 250mV. Bad diodes are typically
400mV.
The voltage drop across the 4 pairs for +/-12V (nearer the front) are slightly less, and should read less than
200mV. The reading on the pair at the front is reversed to the remaining diodes.
If it is determined the
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diodes require changing proceed as follows.
Panels
Prop up the upper master panel to allow access to the power distribution PCB from above.
Release the master fader panel (8 x 2mm hex screws), also to allow access from above.
Multiway plugs
Disconnect all multiway/plugs. Note the location or mark all connectors with their correct positions,
as normal for such work. These comprise 15way and 6 way power plugs, ribbon connectors and a
small molex header, and 2 x PS/2 connectors. See the keyboard and mouse connections are
identified on their respective plugs.
Several connections are accessible only from under the fader panel.
Note certain headers on the PCB are NOT normally connected, and should be recorded as
unused.
Power cables
Next disconnect the power input cables. Note these are colour coded. Tape these together by
stud as they are removed.
Each stud has a self locking not, shakeproof washer and plain washer top and bottom together
with 1 or 2 cable eyes fitted. Note the cable eyes are arranaged as “upper” and “lower”
orientations.
st
Note also that the cables from the 1 PSU are NEVER to be connected to to the same terminal
nd
stud and the 2 PSU. Taping the cables together by stud will ensure this remains correct.
The nuts are 8mm A/F. The use of a miniature socket rachet wench or “stubby” rachet nut driver
will make removal of these much easier. A nut driver with long flexible or articulated shaft can also
be used. Use of a conventional fixed spanner will prove to be a slow method.
Heatsink/PCB
The complete PCB and heatsink assembly is screwed into the floor of the mixer with 4 screws
through the flat aluminium plate (use 5mm flat blade screwdriver) . Note these are captive screws
and will remain in the heatsink panel when undone.
When this is released, remove the complete assembly from under the upper master, taking care
not to damage the controller circuitry on the PCB.
PCB removed from the mixer
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Removing the diodes
With the assembly on the bench remove the PCB fixing screws (M3) using a PZ1 screwdriver.
There are 12 fixing screws with shakeproof washers for the PCB.
Next remove the 16 screws (M4) and split washers from the combiner diodes. Use a PZ2
screwdriver. This will allow the PCB to be removed leaving the diodes stuck the heat sink.
PCB removed from the heatsink
leaving diodes
Note the diodes are arranged as 4 combined pairs in 4 separate packs. These are not screwed to
the heatsink but located on sticky pads made of a special thermal conductive foam.
Take careful note that the 4 diode packs are arranged as 3 oriented in 1 direction and 1 in the
reverse, as seen by a slot at one side and hole on the other side.
Diode packs as left on the PCB
Now carefully remove the diodes and the special foam, taking care not to damage the foam. Any
excess silicone oil can be wiped off the heatsink. Discard the diodes.
Diode pack and pad removed
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Fitting new diodes
The new diodes are fitted directly the PCB. Take the new diodes and loosely fit to the PCB, using
th
the existing screws and split washers. Again note 3 packs are oriented 1 way and the 4 the other
way.
Replacement diode packs fitted to PCB.
Note orientation!
Turn the PCB over and GENTLY tighten the diodes.
IMPORTANT NOTE: It is very easy to over–tighten these connector screws, at the risk of
breaking the diode pack plastic moulded bodies. These should be tightened finger tight only, to
ensure a good electrical connection, but not for mechnical strength.
With all 16 screws tightened turn the PCB back over and apply the 4 thermal pads to the back of
the diodes.
Replacement diodes with thermal
pads in place
Replace the PCB onto the heatsink. Line up the fixing holes before allowing the diodes and their
pads to contact the heatsink, to avoid moving the thermal pads. When lined up fix the PCB with 2
screws and recheck the thermal pad alignment.
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Refit and tighten all fixing screws with shakeproof washers. Check all thermal pads are a snug fit
and square on the diodes.
View of correctly fitted thermal pads
under replacement diode
Recheck all screws are in place.
Refit the heatsink/PCB to the mixer
Replacing the assembly is largely the reverse of the removal procedure.
Fit and screw down the heatsink.
Begin by replacing the power cables. Again note the order in which the power cables are fitted.
IMPORTANT NOTE: power output cables from the 2 different PSU’s must NEVER be fitted to the
same connection stud.
The order to replace the connections from the bottom up are plain washer, first terminal eye,
second terminal eye (if fitted), plain washer, shakeproof washer, nut.
Tighten the nut. This should be tightened hand tight only, to ensure a good electrical connection,
but not for mechnical strength.
Having reassembled the power cables, replace all 15way and 6 way power plugs, ribbon
connectors and small molex header, and 2 x PS/2 connectors. See the keyboard and mouse
connections are identified on their respective plugs.
Recheck all connections are correctly installed.
Power up and retest the mixer operation. Fully check engine switch over operation and that both
engines continue to operate with eather one (only) PSU in operation.