repairing any Counterpoint amplifier, see the Safety Guidelines for High
Voltage and/or Line Powered Equipment at http://www.repairfaq.org/sam/safety.htm
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.
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.
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.
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.
discharging the main filter capacitors, remove both of the Rail Fuses
for the channel to be repaired from their clips on the circuit board.
original MOSFETs can now be removed after first prying off the plastic
covers. They are held in place with a silicone rubber cement.
installing the new MOSFETs, use a small amount of thermally-conductive
silicone heatsink compound between the device and the heatsink.
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).
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.
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.
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
the DC Offset (DCO) trimpot for that channel to the center position.
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.
the DC voltage from the frontmost (P-channel) gate line to ground. This
voltage should be about -3 to -5 volts.
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.
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.
a 1 ampere Fast-Blow fuse into the repaired channel's positive Rail
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.
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.
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.
the amplifier to an autotransformer
and set the AC voltage to zero.
on the amplifier.
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.
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
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.
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.
the DC Offset pot to reduce the Offset Voltage (indicated on the first
voltmeter) to less than 10mV.
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
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.