| |
- No
(or weak, distorted) Output from One or Both Channels
The MOSFETs used in the output stage are quite fragile compared
to bipolar transistors. A shorted speaker cable can cause immediate
failure. Spontaneous failure can occur too, usually at turn-on. The
signs of a failed output stage are: no output from the failed channed,
blown fuses in the failed channel (look inside the amp), and generally
there will be at least four fried gate resistors - on the SA-20 these
are located on the side heatsink, soldered to the MOSFET sockets, and
on the SA-220 they are located on the side circuit boards, under flameproof
sleeving. When a channel fails, it usually does so spectacularly,
with smoke and sparks. Click here to
read about the repair options for a blown output stage.
- No
Front Panel Pilot Lamp
Unplug
amplifier from the wall and check the main fuse, located inside the
unit behind the front panel.
- If
all the tubes are cold and dark, and the fuse is blown, then the
main power transformer may have failed. More testing will
need to be done, but there is nothing that can be done at home.
- If
fuse is NOT blown, check the center-front tube. If it is lit and
the other five tubes are not, then the filament supply has failed.
The bridge rectifier, filter caps C50/51 and power resistor R50
may need replacing. These parts are all on the main circuit board.
- Goes
Into Mute (red lamp), or doesn't ever come out of mute
This can be due to either failing or failed parts in the filament power
supply (due to age) used to power the muting circuitry; or due to failing
parts in the muting circuitry itself. Impossible to diagnose further
without examining the amplifier. But in
the first case, the bridge rectifier, filter caps C50/51 and power resistor
R50 may need replacing. In the second case, the RC timing network parts
R70/C70 and possibly the 555 timer chip, IC1, will need replacing. These
parts are all on the main circuit board.
|
|
