Mercury Epiphone Valve Junior - Discharging Capacitors in Tube Amps (Continued); Testing and Discharging Procedure

46 Epiphone Valve Jr.•Mercury Upgrade Kit
ThisprojectanditsdocumentationistheresultoftechnicalinvestigationsmadebytheengineeringstaffofMercury Magnetics.Thedisclosureoftheinformationhereinmaypertaintoproprietaryrightsand
thefurnishingofthesedocumentsdoesnotconstituteanexpresedorimpliedlicensetousesuchmaterials.
2. Digital multi-meter.Capableofreading500
voltsDC.
3. Capacitor discharge tool.Ajumperwire
consistingofalligatorclipsontheends,witha
resistorinseriesinthemiddleofthejumper.
4. Needle nose pliers.Withverygoodinsulation
onthehandlesforholdingthealligatorclipsof
thecapacitordischargetool.
Alwaysmakesureyourtoolsareingoodcondition.
Ifthere’sanyquestion,replacethem.Cracked
insulationisoneoftherstthingstocheck,and
checkoften.
TurnthepowerswitchOFF,andplacethestandby
switchintheONposition.Thisisn’tthesameas
havingyouramp“onstandby.”
Alwaysunplugtheampfromthewall.Unplugging
fromthewallwillnotdraintheltercapacitors
completely,butagainthisisforourownsafety.
Unpluggingistheequivalentto“lockingout”before
workingonadownedmachine.
Becauseltercapsarealmostalwaysofthe
electrolytictype,they’re“polarized”andtherefore
haveapositiveandnegativeend.Wecanidentify
thepositiveornegativeendsbylookingforthe
“polarityindicator”printedonthecap’sside.
Ourrstobjectiveistocheckforresidualvoltage
withamulti-meter.Insomecasesmostofthe
voltagemayhavealreadybeendrained.Many
amplierswillincorporate“bleeder”resistors,which
willdrainthecapacitorchargeautomaticallyin
afewminutesaftertheampisturnedoff.Don’t
dependonthisresistortodothejob.Neverassume
thecapshavebeendrainedtoasafelevelwithout
checkingwithareliablemulti-meter.
Here’sacoupleofwaystomeasuretheresidual
voltagecontainedintheltercapacitors.Notethat
thesearealsoequallyvalidfordrainingthecaps:
1. Across the capacitor.
2. From the positive side of the cap to the
chassis.Thisisessentiallythesameas
draining“across”thecapssincethenegative
sideisatthesameelectricalpotentialasthe
chassis.
3. From the chassis to a tube pin.Thiscanbe
doneatpin3ofanypowertube,orpins1or
6ofanypreamptube.Byshortingthechassis
tothecorrectpinthecapswillbeforcedto
drainthroughtheplateloadofthattube.This
preventssparkingfromhighcurrent.
IMPORTANT NOTE, HOW NOT TO DO IT: Inold
electronicsbookstheytellyoutouseascrewdriver
withaninsulatedhandletoshortoutthecaps.
“Shorting”withascrewdriverwillalmostinstantly
drainthecapstozerovolts,causingsparksfrom
highcurrent.Thisisnotacceptable.Ifthereare
severalhundredvoltsstoredinthecaps,be
preparedforanicesizedspark.Itwillmostlikely
leaveablackmarkonthechassis,andinextreme
cases,partiallyweldthescrewdrivertothechassis.
Now we’re ready to start testing and
discharging
Setyourmulti-metertoreadDCvoltage.Adjust
themeter’srangesothatit’sabove500volts.Clip
theblackleadofyourmulti-metertothechassis.
Usingonlyonehand,withtheotherhandfarfrom
thechassis(preferablyinyourpocketorbehind
yourback),measuretheDCvoltageacrossone
oftheltercapacitors.Youcanadjustthemeter’s
rangedownifyoucan’tgetaclearreading.AnyDC
voltagereadingslessthan5voltsareharmless,so
theampcanbesafelyworkedon,butdoadouble
checkofallthecapsjusttomakesure.Ifnotwe
mustdrainofftheresidualvoltagewitharesistor
untilitissafe.
Usearesistorvaluesomewherebetween10Kand
100K,withapowerratingof5Worso(ahighpower
ratingisnotreallynecessaryforheatdissipation
reasons,butmainlybecausethehigherpower
resistorsarephysicallymorerobustandwon’ttend
tobreak).Thelargertheresistorvalue,thelongerit
willtaketodischarge.Ifyourcapacitorsdonothave
adangerouslyhighvoltage,sayaround30volts,
feelfreetouseasmaller1K1Wattresistor.
Nowtakeyourcapacitordischargetoolandclip
onesidetothechassis.Alwayscliptothechassis
groundrstbeforeconnectingtothecircuit.If
youcliptothecircuitrstyouwillhaveahotwire
hangingoutofyouramp,whichisverydangerous.
Onceyouhaveonesideclippedtothechassis,
takeyourneedlenoseplierswithgoodinsulationon
thehandlesandholdthealligatorclipthatwillgoto
thecircuitwiththeneedlenosepliers.
Clipthistopin1or6oftherstpreamptube.This
seemstobethesafestwayofdischargingthe
capacitors.Ifyou’reworkingontherstpreamp
tubesocket,thenclipyourdischargetooltopin1or
6ofthesecondpreamptube.Waitafewminutes
(about5minutes,iftheamphasbleederresistors
thistimewillbeshorter)then,whilethedischarge
toolisstillinplacerecheckallthecapacitorswith
yourmulti-meterandkeepthatotherhandbehind
you.Keepcheckinguntilthevoltagehasreached
asafelevel.Note:Thisproceduremustbedone
everytimeyouturntheamponthenoff.
Remembertoremovethedischargetoolbefore
poweringontheamp.Makeitabrightcolorlike