Biasing Counterpoint SA-12, SA-100, SA-20 and SA-220 Power Amplifiers  
 

Before repairing any Counterpoint amplifier, see the Safety Guidelines for High Voltage and/or Line Powered Equipment at http://www.repairfaq.org/sam/safety.htm

On Counterpoint SA-type amplifiers, an output stage that has failed is indicated by a large DC offset at the speaker terminals and fused gate resistors on the MOSFET sockets. Shorted speaker wires is the main cause of such a failure: the high internal losses of the MOSFETs pretty much guarantees that they will fail when their current limits have been exceeded.

Final failure of the entire output stage typically occurs at the next turning-on of the amplifier, when the weak device exhibits a drain-to-gate short. This causes full drain voltage to appear on the gate drive lines, which fuses the gate protection Zener diodes and gate resistors. On some occasions, the muting relay may also be fused, causing the gate drive lines to be shorted to ground. Replacement of all eight MOSFETs per channel (SA-20/220) or all four MOSFETs per channel (SA-12/100), all gate resistors, all four protection diodes and (rarely) the muting relay is indicated.

New replacement MOSFETs for these amplifiers are not available - the originally specified parts are no longer available from any supplier and equivalent parts with the desired zero temperature coefficient point of less than two amperes are not being made. Finding a small quantity is of no use: it is not possible that they would be matched: Counterpoint purchased thousands at a time, heated them to 80°C, sorted them into similar Vgs ratings then placed them in paralleled groups of four and tracked them for equal current-sharing for 24 hours. Such device matching is critical, since the SA series of amplifiers do not use any ballast resistors in the source circuits to force the MOSFETs to share current (such resistors degrade the current-output and, in this design, the sound dramatically). The best I can do for repair purposes is provide tested, used MOSFETs from amplifiers sent to me for upgrading to NP100 or 220 class.

MOSFET Replacement Procedure

ALWAYS DISCHARGE THE AMPLIFIER'S MAIN FILTER CAPACITORS THROUGH A 10 TO 50 OHM HIGH POWER RESISTOR BEFORE ATI'EMPTING ANY REPAIR OR PARTS REPLACEMENT ON THE OUTPUT STAGE.

  • After discharging the main filter capacitors, remove both of the Rail Fuses for the channel to be repaired from their clips on the circuit board.

  • The original MOSFETs can now be removed after first prying off the plastic covers. They are held in place with a silicone rubber cement.

  • When installing the new MOSFETs, use a small amount of thermally-conductive silicone heatsink compound between the device and the heatsink.

  • Check to be certain that the replacement MOSFET mounting screws are firmly tightened. They should be tightened almost to the point of damaging the mounting sockets. A poorly tightened MOSFET may overheat and fail (each device dissipates about 22 watts at idle).

  • Replace the gate resistors and protection diodes. Do not overheat the diodes during soldering. Check them with a ohmmeter after installation to be certain that they still show normal cathode/anode resistance.

  • Check the grounding of the MOSFET main gate lines by using an ohmmeter from the gate lines (the common connections of the front gate resistors, and the common connections of the rear gate resistors) to ground. The reading should indicate less than 1 ohm, which shows that the muting relay is correctly connecting the gates to ground, preventing any gate bias from being applied during tube warm-up.

  • Using an ohmmeter, check the DC resistance from the cases of the MOSFETs to ground. The reading should be well over 1,000 ohms. If the reading is less than this, one or more of the MOSFET cases is shorted to the heatsink. Remove the MOSFETs, one at a time, until the problem is eliminated. Examine the mounting surface of the MOSFET, the mounting screws and the insulating pad for damage. Correct the problem and re-check the case-to-ground resistance.

  • Set the DC Offset (DCO) trimpot for that channel to the center position.

  • Turn on the amplifier and apply nominal AC Mains voltage. The front panel LED should light RED. After about 70 second, the front panel lamp will change to GREEN. If the lamp does not change color, the 555 Time IC (U1) should be replaced.

  • Measure the DC voltage from the frontmost (P-channel) gate line to ground. This voltage should be about -3 to -5 volts.

  • Measure the DC voltage from the rearmost (N-channel) gate line to ground. This voltage should be about +3 to +5 volts.

  • If either gate line reads zero (0) volts, then measure the DC resistance from that gate line to ground. If you read a short, either the relay needs to be replaced, or the bias circuit's thermal sense transistor, Q1, may be shorted to the heatsink.The relay is located on the circuit board near the output stage, and is a 5-6dcV DPDT relay. Q1 is a small 2N2222 TO-92 transistor. On SA-20's, each channel's Q1 (or Q101) is mounted on the output stage heatsink and connected to the main circuit board with a three-wire cable On SA-220's, it is part of the side board assembly mounted to the heatsink. If no short is indicated, it is possible that Q1, is shorted. If the voltages are greater than indicated, it is possible that the bias transistor, Q1, is open. Again, be certain to discharge the main filter capacitors before replacing any of the above components.

  • After ascertaining that the gate voltages are correct, turn off the amplifier and discharge the output stage main filter capacitors as described above and bias the amplifier as described below.

Biasing Procedure

  • Install a 1 ampere Fast-Blow fuse into the repaired channel's positive Rail Fuse location.

  • Into the negative Rail Fuse location install a 1-ohm 2 watt resistor which has been soldered across an open (blown) fuse for easy installation. This resistor will be used to sense the output stage's bias current.

  • If there is no Output fuse (or this fuse is bad), install a good one into the Output Fuse clips located at the rear of the circuit board.

  • Connect a digital voltmeter (20 dc volt range) across the speaker output terminals, from red to black. This meter will be used to monitor the DC offset.

  • Connect a second DVM across the l-ohm current-sense resistor you installed into the Rail Fuse location. Set this meter to the 2 volt DC range. This meter will be used to monitor the bias current.

  • Connect the amplifier to an autotransformer and set the AC voltage to zero.

  • Turn on the amplifier.

  • Slowly increase the Mains voltage to 1/3 nominal and observe the bias current meter. The reading should remain less than 10mV. If there is a reading higher than this, either the MOSFETs have been installed incorrectly, one or more gate resistor has not been connected or is open, or there is a faulty MOSFET.

  • Slowly increase the AC voltage to 100% nominal Mains value. If the DC offset reading begins to exceed 5 volts, it is possible that one of the MOSFETs is faulty.

  • Wait for the amplifier to unmute.

  • If the bias current indication exceeds 0.5 volts (equivalent to 500mA), attempt to reduce this voltage with that channel's BIAS trimpot. If the trimpot has no effect, it is likely that Q1 is faulty. If the trimpot does control the bias current reading, but you cannot reduce the indicated voltage to 0.5 volts or below, the value of R60 needs to be reduced. Try placing a 100K ohm resistor in parallel with R60. This should bring the bias reading into a controllable range.

  • Adjust the AC Mains voltage to within 2% of nominal and adjust the BIAS pot to obtain a bias current reading of 0.45 and 0.5 volts. This voltage will initially climb to a somewhat higher reading and then begin to drop as Q1 controls the temperature of the heatsink by modifying the gate bias voltage.

  • Adjust the DC Offset pot to reduce the Offset Voltage (indicated on the first voltmeter) to less than 10mV.

  • Allow the amplifier to heat up. Occasionally readjust both readings as needed until the amplifier has settled. We recommend a minimum of 4 hours of observation and adjustment before trying the amplifier with music (and cheap speakers).

  • After everything has settled, turn off the amplifier, discharge the output stage filter capacitors and install new Rail Fuses. For the SA-20/220, use 6-ampere fast blow types. For the SA-12/100, 4-ampere parts are a good size. In the case of difficult to drive loads, you can increase the fuses up the 20A (SA-20/220) and 10A (SA-12/100) with reduced reliability.