Webb Steel Guitar Operating Manual - page 14
4
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Reconnect Preamp/Power Amp PCB connector plug.
•
Visually inspect all wiring again for any problems.
Factory Electronic Inspection Procedure
Ground
•
Check for ~0_ resistance between chassis ground, output jack ground, input jack ground, and AC
plug Pin-3 ground. (Note: <0.5_ – OK)
•
Check that there is infinite resistance between chassis and both AC plug prongs 1 & 2.
Power Amp
•
Connect AC cable and turn on unit power. (Note: Reverb cable is not attached for this testing.)
•
Check DC voltage of Speaker Outputs. DC Offset should be <30mV with no dummy load.
•
Connect oscilloscope to output with no load.
•
Apply a 500Hz audio signal into rear panel Power Amp Input and increase level to view signal.
•
Check for any intermittence by tapping the Power Amp PCB resistors and bumping chassis.
•
Connect 4_ resistive dummy load to Speaker Outputs.
•
Test Output Power. Output should be approx. 31.5VRMS= 235WRMS @ 4_.
•
Check sine wave for clipping. There should be slight noticeable filtering.
•
Check crossover distortion with 5KHz low input signal (for 2V output).
•
Power Output Test: input sensitivity 680mv. @ 1KZ. Output Level into 4 ohms 31.5 vrms. @ 1KZ, 235
watts RMS.
• Note: If the output test reading is different from the above, trouble-shooting is required. Use
appropriate test equipment.
Control Settings for Pre-amp & Power Amp
•
Use High Input for testing.
•
Sensitivity: 10
•
Volume: 0
•
All EQs: 0
•
Tone Selector: 1
•
EQ Switch: Off
•
Input a 500Hz 360m V RMS audio signal into the front panel High Input and increase the Volume
control to view signal.
•
Check for intermittence by tapping on PCBs resistors, the board itself and the chassis.
•
Check that Low Input is about 50% level of High Input.
•
Check Selector Switch. Settings 1 and 2 should have very little change; gain will increase on setting
3. Settings 1 & 2 = 0.5Vp, Setting 3 = 2.5Vp
•
Check for slight distortion on setting 3, noticeable at peak of waveform.
•
Check that all Preamp controls work. Treble and Bass controls have little effect on signal @ 500 Hz;
however Mid-Range should increase from 0.5Vp to 1.5Vp.
•
On reverb connector plug, check reverb input red wire for signal.
•
With reverb control set to maximum, touch metal object to reverb output green wire to view noise/AC
hum. (Note: Reverb output goes directly to the Power Amp.)
•
Turn on EQ and verify that all controls work.
•
Verify that Preamp Output is functional.
•
Re-attach all wires with ties.
•
Tighten all PCB and front bezel screws.
•
Indicate any scratches/blemishes with stickers.
•
Note the problems corrected (if any) and tag amp.
Audio Inspection
•
Connect Speaker.
•
Turn on amp.
•
Listen for turn-on/off transient noise, white noise, AC hum.
•
Test all controls for full range and effect.
•
Test all inputs, pre-amp output, and speaker outputs.