8
Reliability Verification 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. See the
previous page for the 170 and 186 boards with some test points noted.
1. Adjust the bias control (R163 on 170 Board) in both channels for 12mV +/- 1mV measured
at the bias test points (J23 along 170 board back edge) after unit has idled for a few
minutes. This is with no signal and no load. The Pot and test points are clearly marked on
both the top and bottom sides of the 170 board.
With Full AC line voltage applied to unit, it will pull around 130W from the line (1.6A at
120V). Measure for DC offset on both output connectors, it should be less than +/- 50mV.
2. Verify and adjust the fan speed if needed. On the 186 board, short pin 1 to pin 2 on J3.
Adjust R1 for 28V (+0V -1V) across J1 and J2. Remove the shorting jumper on J3. All the
test points and pots are clearly labeled on the back side of the PCB and are easily
accessed with the main heatsink sub-assembly installed.
3. 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
carefully verify symmetrical “flat-topping” on the waveform.
4. Reduce the output levels, install a 0.1µF 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).
5. Remove capacitive loading and minimize sine output. Verify and re-adjust the bias if
required. Note that the bias will not drift appreciably in a unit that is functioning properly.
6. Connect the amplifier directly to the AC line and connect an 8Ω dummy load to both
channels. Each 8Ω dummy load should have a minimum power rating of 500W. Bring the
sine wave level up on both channels and verify symmetrical clipping. The output will clip
somewhere between 150V - 180V pk/pk depending on how stiff the line is. Clipping
should be as described above. Add the 0.1µF capacitive loading and verify clipping is still
well behaved.
7. Individually load Channel 1 and Channel 2 with 2Ω . Each 2Ω dummy load should have
minimum power rating of 1500W. Clipping should be symmetrical, well behaved , and
occur somewhere around 120V - 140V pk/pk. Verify that clipping is well behaved after
adding the 0.1µF capacitive loading. Reduce the output level to zero and remove the
loading.
8. Next verify the two different short detectors. Perform these tests first on channel-1 and
then repeat for channel-2. On the 170 board, short J18 pins 1 and 2 (this defeats the
average power SOA detector and allows the transient SOA detector to be tested).
Adjust the output level to 60V pk/pk and short the channel-1 output line. Channel-1’s
front panel “short” and “protect” LEDs should turn on and the respective channel will
mute for about 5 seconds. On the 170 board, remove the short on J18 and install a short
on J19 pins 1 and 2 (this defeats the transient SOA detector and allows the average
power SOA detector to be tested). Adjust the output level to 60V pk/pk and short the
channel output line. Channel-1’s front panel “short” and “protect” LEDs should turn on
and the respective channel will mute for about 5 seconds. Remove the shorting jumpers
on channel-1 and repeat above testing on channel-2.
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