Testing The Heatsink; Preliminary Bias Adjustment; Output Dc Offsets - Classe Audio Delta Series Service Manual

Testing the Heatsink

Before you begin, turn trimpot RV101 (or RV201 depending on the orientation of the board) on the heatsink
board all the way CCW. This will bring the biases on the output transistors to zero.
Perform a detailed visual inspection of the heatsink board. Verify that the board revision is up-to-date (the
board number can be found on the bottom side of the board just beside the break-away portion and can only
be seen with a mirror). Check the types of the MOSFETs: they should be (2S)K216 on the positive output side
of each board and (2S)J79 on the negative output side, and the types of the bipolar transistors: they should
be (2S)C3263 on the positive output side of each board and (2S)A1294 on the negative output side. Verify
that R114 is 4.7 ohms and that it doesn't touch the relay next to it. Also verify that all the relays are 12V type
(and NOT 5V type). Finally, verify that the temperature compensation transistor (Q101 or Q201) is well glued
to the heatsink.
DO NOT power-up the heatsink without a tested main amp plug-in board. Make sure that the main
amp plug-in board is seated properly on the heat-sink board connectors.
For the following tests use a "bypassed" power-supply jig powered from a variac. Make sure that the
capacitors on the supply are discharged before you connect the heatsink to the jig.
Bring up the AC gradually (in 15V steps) while measuring the main DC rails (on the big brass binding posts)
and the MOSFET supplies (+V202 and –V202) relative to ground (GND or GND201). The MOSFET supplies
should be at the same voltage as the respective DC rails. Keep a close eye on the circuits, listen for frying or
popping noises, and feel the variac for vibration caused by excess current. If you hear a capacitor pop, mark
the board and replace it. Raise the variac voltage until you measure +/-66Vdc on the DC voltage rails.
Check the biases on the output transistors. They should still be zero. If not, trim them down to zero and if that
doesn't work then power off immediately.
While the heatsink is on, verify that the Zener supplies for the DC-protect circuit are +/-12Vdc (measure the
voltages on the accessible ends or resistors R201 and R203 under the main board, relative to ground). Verify
that these resistors are 3kohms. Verify also that resistor R119 is 10k ohms (RN55D1002). If the code is not
visible then measure it.
Finally, with the heatsink off and disconnected from power, check for shorts between the chassis of the
heatsink and digital ground (on the tab of temperature sensor Q8).

Preliminary bias adjustment

If everything went well you can now proceed to a preliminary adjustment of the bias. Re-adjust the variac until
you have exactly 66.0Vdc on the DC-rails. Connect the ground probe of your voltmeter to the middle point
between resistors R108 and R113 (output) and probe the other end of either resistor. Trim RV101 CW until
the bias rises to 7mV. This should eventually bring the bias to 10mV which is the desired final value.

Output DC offsets

Measure the DC offset voltage on the output of the heatsink board (relative to ground) and verify that it does
NOT exceed +/-10mVdc.
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