WhitakerAudio J C Whitaker 20 W Stereo Audio Amplifier, Руководство пользователя

Страница: 148 / 194

148
WhitakerAudio
The time-delay circuit of Q201 through Q203 can be divided generally into two elements—
the voltage ramp/trigger circuit (Q201 and Q202), and the output driver circuit (Q203). Several
tests can be accomplished without removing the PWB, as detailed below:
• Check Q203, RYL201, and the front panel Auto-Off switch and associated wiring.
Connect a test lead to the +25 V dc power supply source on the power management PWB.
Use the schematic diagram to identify a convenient source. Attach a 1 k
Ω
0.5 W resistor
to the end of the test lead. Confirm that the front panel Auto-Off switch is enabled. Touch
the end of the resistor to the gate connection of Q203 from the foil side of the board. If the
output circuit is working correctly, RYL201 should trip and shut down the amplifier.
• If RYL201 trips but the amplifier does not shut down, then RYL203 has failed to close.
More about that later.
• Assuming the Q203 output circuit performs correctly, efforts can be focused on the ramp/
trigger circuits of Q201 and Q202. Refer to the schematic diagram and check for typical
operating voltages. This can be accomplished with the power management PWB in place.
• One of the challenges in checking the operation of the auto-off circuit is the long time
delay needed to determine if the problem has been fixed (about one hour). For testing
purposes, the negative lead of C201 can be unsoldered from the PWB while still in place,
and a small value capacitor (e.g., 5
μ
F at 25 V) can be tack-soldered in place from the foil
side of the board. This will greatly shorten the time needed to check operation of the auto-
off system. Be sure to observe proper polarity. When troubleshooting the auto-off circuit,
remove CONN402 on the power management expansion PWB. This will allow the auto-
off circuit to run without being reset by audio present at the speaker terminals.
The function of RYL203 is to enable the protection chain (auto-off, high-voltage status, bias
voltage status, and high-temperature sensor). If the relay fails to close, the amplifier will operate
but the protection systems will be bypassed. RYL203 is triggered by SCR Q206. At power-up, the
voltage on the anode of D208 slowly increases owning to the RC time constant set up by R211
and C207. Once the voltage has built up to the break-over point of the zener diode, current is
supplied to the gate of Q206 to trip the relay. Note that this time delay needs to be at least a few