Troubleshooting - Mesa/Boogie Subway WD-800 Owner's Manual

ANSWER:
In Class D amplifiers, the analog audio signal is converted into a PWM (pulse width modulated) pulse train representa-
tion of the analog audio signal (similar to the A/D converter in a PWM digital audio recorder) at +5 volts conventional logic level. This
PWM signal is level shifted upwards by large switching power MOSFET transistors (that operate non-linearly to avoid waste heat) to
high voltage and current. The resulting high level PWM signal then passes through a high power "low pass reconstruction filter" that
extracts the original but level shifted (amplified) audio signal from the PWM signal which feeds your speakers. Class D is not "digital",
it is a non-linear analog technology, sharing some fleeting similarities with its digital counterpart. (Disclaimer: This is a simplified
description, there are many details omitted, but the basic operational function is accurate. There is a lot of engineering that goes into
making a high performance, robust, safe and reliable design.)

TROUBLESHOOTING:

In the event that your amplifier appears not to work correctly, often enough the problem is not with the amp, but a related piece of
equipment, so it's necessary to take a deliberate, systematic approach to troubleshooting in order to effectively identify and correct
the problem. Yes, believe it or not, we have seen all of these things many times.
SYMPTOM:
No audio output:
Is the power LED lit and do the tally LEDs over the function switches work?
NO: Possible causes to check in this order are
1. Verify the power source, verify that the IEC power cable is not damaged and is fully inserted into its socket,
2. Verify that nobody unplugged your amp or turned off your power strip when you weren't looking (if so, blame the guitar player,
turn on and play.).
3. If these do not solve your problem, it's possible that your amplifier has failed so call our customer service department and we
will help you get this resolved.
YES: Possible causes to check in this order are:
1. Defective speaker cable or cabinet (test known good speaker cable and cabinet)
2. Defective bass or cable (test with known good bass and instrument cable)
3. Defective pedals/effects/pedalboard (plug bass directly into amplifier input, bypassing all devices in front of the amp)
4. Mute switch engaged or mute footswitch turned on (turn mute function off)
5. Problem with effects loop (for models with effects loop, a cable plugged into the effects return will interrupt signal, there is a
problem with an effects device or patch cable, there is an intermittent connection due to dust or debris inside the effects return
jack. Try bypassing effects loop with short shielded patch cable.
6. Gain or Master controls turned down (turn the controls up)
7. Will the amp play with a signal fed into the AUX INPUT or EFFECTS RETURN jacks? If so, the problem is with the preamp.
It's possible that the tube/valve has failed. (substitute a known good 12AT7 or 12AU7 into the V1 position and see if this
restores audio)
8. There is indeed a problem with the amp, call our customer service department and we will help you get this resolved.
SYMPTOM:
Distorted audio output:
1. Defective battery in active bass causing instrument's onboard preamp to distort (replace battery)
2. With very hot 18V basses, it's possible to overdrive the amp's preamp when in passive mode (switch to active mode)
3. Gain structure problem if using effects (if the amp has an effects loop, which operates at line level, it's possible to overdrive
some effects that are built exclusively for instrument level. Try unplugging effects from effects loop.)
4. Slightly intermittent cable in system (this can be an instrument cable, patch cable or loose screw in SpeakOn plug)
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