2.3
Short-circuitprotection
Poweramp/Protections Testing/Regulating
Whenthedeviceisswitchedon,thereshouldbeanoticeabledelaybeforeitpowersup.
Youshouldbeabletoheartherelayoperating.Thelengthofthedelayshouldbeabout
twoseconds.
ThefollowingprocedureisusedtotestthecircuitthatprotectsagainstDCvoltageatthe
output:
Switchoffthedevice
Removedfusesto()
ConnectamultimetertotheemitteroftransistorT17()
ConnectastabilisedDCsource()totheoutput
Switchonthe
NowslowlyraisetheDCvoltageatthe'soutput(positiveandnegativetests)until
thedeviceisseentoswitchoff(theMuteLEDwillilluminateandtherelay willswitchoff).
Whentheswitch-offoccurs,thevoltageatT17shouldbec.0.6V.
IfthehasnotswitchedoffbythetimetheDCsourcevoltagereaches3V,thereisa
fault.
Notethatthereisanelectrolyticcapacitorintheprotectivecircuittoensureasluggish
response.Thatiswhyitisessentialtoincreasethevoltageslowly.TheexternalDCvoltageis
interruptedwhentherelayswitchesoffandafterashortinterval,duringwhichthepower
amplifierwillbeinStandbymode,itwillreturntoOperatingmode
Beforeperformingthistest,thestabilisedDCvoltagepowersupplyshouldbeset to+8V.
ThesimplestwayofconnectingtheEarthistoconnectit
tothe(Minus).Theotheroutputofthe
voltagesourceisconnectedtoNTC2.(
)
Whentestingtheshort-circuitprotection,a4ohm(250W)load
resistanceisattachedtotheoftheamplifierandan
additional1(250W)ohmloadresistancetotheearthofthe
output(seeFig.4)Asinewavesignalofc.1kHzshouldbe
appliedtotheandincreaseduntilthe
oscilloscopeattheoutputshows
DCVoltageProtection
Temperaturecontrol
⋅
⋅
⋅
⋅
⋅
Si1Si2
ProFetII
ProFetIV
ProFetII
SpeakerOut
SpeakerOut
EffectsReturn
takecaretodischargetheinputcapacitors
1Vrange
setto0V
Attention:one
pinofNTC2isconnectedtoEarth
.
Thefollowingtableshowshowtheprotectivecircuit
reactsatvariousoperatingpoints.
Heretooitshouldbeborneinmindthatthestated
voltagevaluesareapproximationsandavariationof+
or-10%ispermissible.
Page6
FanON
StandbyON
StandbyOFF
FanOFF
6V
7,5V
3,5V
4,5V
Fig.4.: Short-circuittest
(SPEAKEROUT)
TTENTION: Ifitbecomesneccesarytoreplaceabrokenpoweramptransistor,it´sstrongly
recommendettouseoriginalsparepartssuppliedbythemaufactureronly.Also,
evenincaseofONEbrokenpoweramptransistor,youshouldreplacethewhole
set,whichmeans2xor2xIRFP9240IRFP240.
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