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- No
(or weak, distorted) Output from One or Both Channels.
This is most likely due to a failed output stage. The MOSFETs used in the output stage are not as reliable as bipolar
transistors. A shorted speaker cable can cause immediate failure. Spontaneous
failure can occur, too, usually at turn-on. Symptoms are usually spontaneous
and spectacular failure of one or more of the gate resistors attached
to the output device sockets on the heatsinks and one or both rail fuses
blown.
Solution
Upgrade the amplifier to NP100 class of operation,
or repair it.
- 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 BR1, filter caps C51/52 and power
resistor R51 may need replacing. In the second case, the RC timing network
parts R29/C30, and possibly the 555 timer chip IC1, and diodes D31,
D31 will need replacing.
- Tubes
Won't Light, Pilot Lamp Won't Light, and Unit Does Not Come Out of Mute,
but Main Fuse Behind Front Panel is Good.
This is usually due to failed parts in the filament power supply (due
to age) which is also used to power the muting and lamp circuitry. Typically
bridge rectifier BR1 has failed (100 volt 6-amp), or the connection
to the yellow and yellow/black wire connecting to the "7VAC"
points on the circuit board may have developed a intermittent connection
on boards that used the push-on connectors. We advise soldering the
wires directly to the board.
- Tubes
Won't Light, Pilot Lamp Won't Light, and Unit Does Not Come Out of Mute,
and Main Fuse Behind Front Panel is Blown.
Very likely a failed power transformer, but not always. More testing
needs to be done to be sure.
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