10
Bias and test procedure
After the unit has been repaired, the following should be done to assure long term reliable
operation. If a distortion analyzer is present, distortion specifications should be verified.
1.
Adjusts bias in both channels (R51 & R60) for 30mV +/- 2mV at bias test points (J5 & J6)
after unit has idled for a few minutes. With Full AC line voltage applied to unit, it will pull
around 55W from the line (700mA at 120V). Measure for DC offset on both output
connectors. DC offset should be less than +/- 50mV.
2.
Apply a 1KHz sine wave to the inputs and verify that the unloaded outputs have a
waveform that is symmetrical and undistorted. Drive the outputs into clipping and verify
symmetrical “flat-topping” on the waveform.
3.
Reduce output levels, install the 0.1uF capacitor jumper from the output to ground
connections, and verify that clipping behavior is proper. Verify that no high frequency
oscillation occurs near and at clipping (parasitic oscillation).
4.
Remove capacitive loading and minimize sine output. Verify and re-adjust bias if
required. Note that the bias will not drift appreciably in a unit that is functioning properly.
5.
Connect the amplifier directly to the AC line and connect an 8
Ω
dummy load to both
channels. Bring sine wave level up on both channels and verify symmetrical clipping. The
output will clip somewhere between 90V - 110V pk/pk depending on how stiff the line is.
Clipping should be as described above. Add the 0.1uF capacitive loading and verify
clipping is still well behaved.
6.
Individually load Channel 1 and Channel 2 with 2
Ω
. Clipping should be
symmetrical, well behaved , and occur somewhere around 60V - 80V
pk/pk. Verify that clipping is well behaved after adding the 0.1uF
capacitive loading. My boss told me that I should include a picture of
“well behaved,” so here is a picture of my boy Joe.
I hope this clears up any misunderstanding.
7.
Remove loading and reduce output level to 20V pk/pk and short first
channel one and then channel two. When one channel is shorted the
product should pull around 250W from the AC line. If the power pulled
from the line is substantially higher (290W or more) look for failed parts in the VI limiters.
Immediately
remove the short when the power consumption is excessive to prevent re-
damaging the output stage!
8.
Place amplifier in bridge mode and connect 4
Ω
loading to bridge outputs (across both
“+” output binding posts). Slip some card stock between the heatsink-outlets and chassis
sides. Monitor one of the outputs, and adjust for a 45V pk/pk sine output (250W of output
power bridged). After a few minutes the fan will begin running fast (heatsink at 60°C)
and a short time later the amplifier will mute (heatsink at 86°C). The “Hot” LED will come
on. Remove card stock and after a few minutes the amplifier will come out of mute
mode and the “cold” LED will return.
9.
Disconnect loading and remove drive. Reconnect amplifier to Variac and confirm that
the idle consumption is roughly 55W or 700mA, as before. Connect the amplifier to
speakers and verify that it sounds OK with music.