Cyrus 8 DAC - FAULT FINDING; Preamplifier faults; Control system faults; Digital audio faults

FAULTFINDING
©CyrusAudioLtdJun2014 14 Cyrus6DAC,Cyrus8DACservicemanualIssue1
Re‐testing the amplifierafter repair.After repairing theamplifier, connect aDVM to monitor
thequiescentcurrentintherepairedchannel.AsyouswitchonfromStandby,watchtheDVM
reading which should read just a few mV maximum. A high or rapidly  climbing quiescent
reading willprobably indicate that a fault still exists within the power amplifier stage. In this
case the amplifier should be switched off immediately to avoid further damage. If the
quiescentremainslow,thenproceedtosetthequiescent current accurately, referring tothe
correctsectioninthisservicemanual.
Ifthe quiescent currentsetscorrectly,continue to check and set the DC offsetvoltageatthe
speakerterminals.
If both quiescent current and DC offset settings are stable at the correct values, proceed to
connectspeakersordummyloadsandcheckthefunctionoftheamplifierwithasignal.
Preamplifierfaults
Powersupplies.Beforecommencinganyfaulttracingitisadvisabletorefertothepreamplifier
schematicsheetandcheckthepowersuppliestoeachofthepreamplifierICs.Intheeventofa
powersupplyfailure,trace thepowersuppliestothepreamplifierusingtheschematics.Note
that power to the preamplifier regulators is routed via the PSX‐R switching circuit
T401/T402/T405/T406whenaPSX‐Risnotconnected.
Nosignal.IfthepowersuppliescheckoutOK,thenpre‐amplifiersignalfaultsarebestfoundby
supplyingasine‐wavesignaltotheamplifierinputsandtracingthisthroughthepre‐amplifier
stage input selector and volume ICs to the power amplifier, and replacing any components
wherethesignalmaybelost. TheZone2outputcanhelptracesignalfaults.Ifthe testsignal
can be seen at the Zone 2 output, but not at the Pre‐amp output, then the volume control
IC100orinputselector!C101maybefaulty.IfthereisnosignalattheZone2output,thenthe
Zone2selectorIC102maybefaulty.
Controlsystemfaults
Ifthefrontpaneldoesnotpowerupandsettostandbywhenpowerisapplied,thentestthe
control system powersupplyas described inthe ‘Initial precautionsandfirst tests’ section. If
thefrontpanelinitialisesandsetstostandby,butthereareproblemswithanyelementofthe
controlsystemthefrontpanelcontrolPCBwillrequirereplacement.
If one of the control keys is not operating correctly, then check that the switch operates
mechanicallyandthatitisnotstickinginthetechnicalmoulding.
Digitalaudiofaults
If a faultis suspected withadigital input subPCB, some initialdiagnosticwork is worthwhile
before removing the PCB as this cannot  be powered for tests once it is removed from the
amplifier.
Connect a  digital input to one of the electrical or optical inputs of the amplifier. Select this
inputandcheckifthesamplerateiscorrectlyindicatedonthedisplay.Ifthesampleratereads
OK,thissuggeststhatthedigitalinputIC(IC701)isworkingcorrectly.
Checktheotherdigitalinputsinthesameway.TheUSBinputmaybecheckedbyconnectinga
computerasasourcewithanaudiosignal playing. If all digital inputsare working,but USB is
not,thentheUSBdecoderIC601oroneoftheassociatedcomponentsmaybefaulty.
IftheinputselectorICisworkingforallinputs,thencheckbothleftandrightchannelsatthe
audiooutputoftheamplifierusingoneofthedigital inputsasasource.Ifoneorbothofthe