FAULTFINDING
©CyrusAudioLtdJun2014 13 Cyrus6DAC,Cyrus8DACservicemanualIssue1
A first test is to check the resistance between the speaker terminals. This should be
approximately14.2kΩ.Ifthevalueissignificantlydifferent,theremaybeanoutputstagefault
ofthatchannel.Acompletecheckoftheoutputstagetransistorsisrecommended.Checkalso
theDCfuses.Iftheseareblownfollowthesamecourseofaction.
If the output transistors are suspect, measure the resistance between base and emitter (the
outside pins). This should be approximately 68Ω ‐ 70Ω. If the measurement is low, the
transistormaybeblownandbothtransistorsshouldbereplacedinthatchannel.
Withtheloudspeakersdisconnected,connectthemainspower.Theamplifiershouldswitchon
andsetto standby(frontpanelLEDisred).IftheLEDdoesnotlightthemainsfusemayhave
blown or there may be a fault in the control system power supply. Disconnect the amplifier
and check the mains fuse. If the mains fuse is OK, re‐connect the mains and check the +5V
power supply forthefrontpanel control PCBattheregulator VR401 output. Ifthisisabsent,
trace the control circuit power supply back through the regulator, D448 bridge rectifier and
R421 fusible
resistor to the PCB‐mounted power transformer that supplies the power tothe
regulator.
If the amplifier has setto standby, switch it on now. The mainsrelay RL401 should click and
power is then applied to the main power transformer. The display backlight will light and
powerwillbeapplied
tothecircuitsintheamplifier.Ifthedisplaydoesnotlight,checkFS901
onthesub‐PCB.IftherelaydoesnotoperatetheremaybeafaultwithinthedrivecircuitT400
etc, or the relay itself. The relay can be tested by applying 12V to SK401 when the power is
disconnected(a6LR619Vbatterycanbeusedforthistest).
Iftheamplifierswitcheson,checktheDCoffsetvoltageatthespeakerterminals.TheDCoffset
measuredshouldreadwithin±50mV.Ifthevoltageisgreaterthanthisthenfirsttrytosetthe
DCoffsetvoltage(seepage18).Ifthe voltagewillnotsetwithinthebandspecified,the power
amplifiermaywellbefaulty.
IftheDCoffsetiswithinlimits,thenitissafetoconnectadummyloadorloudspeakers.Atthis
pointitisadvisabletomeasurethepoweroutputandTHDfromtheamplifiertoestablishifit
willmeetspecification(frontpageofthisservicemanual)anddetermineifthereisafault.
Poweramplifierfaults
DC rail fuses. If theDC offset cannot be adjusted to spec or if a power amplifier stage is not
functioningcorrectly,thendisconnectthepowerandchecktheDCrailfusesonthemainPCB.
Replace these ifnecessary. Ifthey blow again,there isprobablya semiconductorfault in the
poweramplifierorpowersupply.NOTE:‐Ifthestandbylightflashesredwhentheamplifieris
switchedonthisindicatesthataDCfusemaybeblown.
Powersupply voltage check. If the DCfusesareOK,thencheckinitiallyallthevoltagesinthe
power supply of the amplifier, including the regulated power supply ±42V. If the regulated
supply needs repair,then check againsttheschematic tofindfailed components andreplace
them.IfthevoltagescheckOKthenthereisprobablya semiconductorfaultintheleftorright
poweramplifier.
Semiconductorfaults.Thereisnoexactfault‐findingprocedureforthepoweramplifiercircuits,
howeveritisrecommendedthataDVMsetto‘diodetest’shouldbeusedtothoroughlycheck
eachsemiconductorinthepoweramplifier stage,alsosettheDVMtoohmsandcheckalllow
value resistors such as R214‐219 (left channel). Remove any suspect transistors and re‐check
themoutofcircuit.Replaceasnecessarywiththecorrectoriginalcomponentssuppliedbythe
CyrusServiceDepartment.
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