User`s manual

(2) If the mains fuse blows immediately upon power up, you likely have blown rectifier
diodes. These can easily be replaced by any competent technician. They are located in
the upper right corner of the chassis, below the voltage selector and mains fuse, mounted
on blue standoffs (when replacing the diodes, be careful not to to break these standoffs.
It is best to carefully cut the diodes out, then unsolder the leads to avoid stressing the
standoff). Here is a picture showing the diodes on the standoffs:
The HT rectifier diodes are on the left, connected to the filter capacitor, and the bias
supply diodes are on the right of the standoffs. Early Sabres used two bias supply
diodes, later models used only one (with some different value components on the main
board as well – see Service bulletin 1 for details). The Sabre uses 1000V 3A FR307 fast-
recovery rectifiers (available online from Digikey at http://www.digikey.com), but you can
substitute standard 1N4007 diodes if necessary. For added protection against future
rectifier failure due to voltage surges, use two diodes wired in series in place of each of
the diodes (this is addressed in service bulletin 2). Be sure to replace all the diodes, not
just the bad one, in case the amplifier was subjected to a voltage surge, which may have
marginally damaged one of the other diodes.
There is a small chance fuse blowing can be caused by a bad filter capacitor or power
transformer, but this is highly unlikely, unless the amplifier is very old or has been
subjected to misuse, line voltage surges, or incorrect setting of the mains voltage switch.
(3) Next, install the power tubes. If the fuse blows, one of the output tubes is bad, and the
entire set should be replaced. If the fuse doesn't blow, and the amp works fine, it was
likely just a nuisance fuse blowing, and the amp should be monitored carefully for the first
hour or so of playing just to make sure everything is okay. Never keep feeding fuses to
an amp that is blowing them, you'll likely cause permanent damage. Stop after the
second one blows, unless you've isolated the cause as I've described it above.