Fuse Inspection
Lightly blown- Very often an amp in service has a blown fuse, and is one of the first things a service tech checks. Sometimes, much can be
determined from the condition of a blown fuse. With the amp series, it is possible that the amp is OK and the fuse is the only problem. a
lightly blown fuse may indicate merely an overload condition that did not trip the overload protection circuit, or a weak
Troubleshooting Procedure (contd.)
fuse. In this case, the element is open in the center, and the ends are intact and visible. If it is suspected that the overload circuit was affected,
refer to the test in the start-up procedure. Also, make sure the unit is equipped with a spare fuse.
A failure in either the +15V or –15V supply may also cause a lightly blown fuse. If the amp had been operating normally for some
time, a preamp problem may exist. To check, remove the wires connecting to the preamp and check the power supply waveform. If OK, the
problem may be in the preamp and a prime suspect may be a TL604, if it has one. If a problem still exists, check the 15V supplies on the
power amp board.
Mildly blown- If the fuse is mildly blown, the power amp may be blown. This type of failure will often result in a fuse with a black band in
the center. Refer to Sec. 3.
Violently blown- If the fuse is violently blown, most of the fuse will look black, the glass body may be cracked an break upon removal. This
usually indicates a blown power supply. See Sec. 5.
Power Amp
Caution! - The power amp must always be operated with the outside output transistors, Q824 and Q759, firmly clamped to the heat sink with
screws through the brass heat clips with A06s inside. These latter transistors, Q823 and Q758, provide feedback for the biasing of the outputs.
Failure to clamp these parts may result in thermal runaway, as the bias current becomes several amps and the transistors operate outside their
safe operating area. Mono amps do not have Q759, but the board needs to be positioned at each end anyway. Always screw down all power
transistors to the heat sink when providing output power for any length of time.Also, the ground must be connected, which is completed by
screwing a 4-40 having a captive lock washer into the hole directly between Q824 and Q808. Make sure the lock washer is not touching the
board. Do not use a flat washer. The heat sink for the hole is not anodized.
Ohm Check of Outputs – Failed output transistors usually measure zero or low for collector to emitter. This turns out to be the two leftmost
terminals. Check all TIP33s and TIP34s. bad transistors will usually have a cracked or broken plastic body. Replace both outputs if one
measures bad. Usually, transistor replacement is all that is necessary to fix the amp, unless some other problem caused the amp to fail. These
other problems are not within the scope to this document. Note that the outputs can be disengaged by moving the clips near the transistor
terminals. The rule is, the transistor is engaged when the clip is closest to the .33 ohm emitter resistor of the transistor.
Auto Shutoff Feature – The unit may be in service to correct the problem of nuisance tripping of the output, usually caused by other
equipment on the same power circuit or by radiating equipment in close proximity. See GK Document #450-0062-01 for an explanation of the
shutdown feature and how to disable it.
Power Supply
Caution! – The two power MOSFETs, (BUK456-800A), must be firmly clamped to the heat sink when powering the amplifier. It is OK to run
them without the heat sink under no load conditions.
Visual Inspection – Look the board over for obvious problems. Severe power supply failures will sometimes result in burn marks around
R521. If so, R521 and the following may need to be changed: control chip oscillator U506, zener diode D507, and possibly Q515 and R509.
Power MOSFET Ohm Check – Power supply failure usually means failure of the power FETs. When failed, they will usually measure short
from drain to source, the two leftmost terminals. Usually the gate measures short to drain and source as well. A good transistor will measure
either open or body diode resistance from drain to source, depending on the orientation of the probes. Always replace both BUKs. Power
supply failure often results when the power amp fails. Simply replace the BUKs when this happens; no setup of the power supply is necessary,
since the characteristics of the transistors are very uniform. When replacing BUKs, cut off some length on the leads and flair the outputs
slightly. Always install the BUKs with the board firmly in place to assure proper alignment of the heat sink holes. When bringing up a power
supply that has a blown power amp, it mat be necessary to disconnect the outputs using the clips mentioned before (see Power Amp-Ohm
check of Outputs).
Oscillator Circuit
Visually inspect U506 for cracks. Whenever replacing IC s, it is always a good idea to install the new piece in a socket. Check that U515 is
OK by measuring diode drops from base to emitter and collector. R509 sometimes opens up under severe power supply failure, check that it is
low ohms. After failed parts are replaced, bring the supply up slowly with the variac as noted in the Start Up Procedure.
