It's gonna be something uber-simple I reckon. Re-soldering all chassis grounds tomorrow too. Is the frequency any clue? I can't think of one but someone might have a revelation. No change although it wouldn't have explained the PI as seemingly the beginning of the 6.3Khz signal. I mused the spkr ground was ineffective and possibly floating a little - removed jacks, cleaned/abraded sockets/washers/chassis - no change.Īlso wondered if weird conductivity of crap at 6L6 socket bridging heaters AC to grids or plate - cleaned. Pretty sure I tried grounding signal after the mixers but will check in the morning after nightshift.ĭid try this arvo with FB lead removed from speaker jack - no change. Grounding the yellow lead pre 220k mixer yields same oscillation. Tim, no resistive/dummy as such, just workshop 8ohm spkr. Put another way, how would YOU approach tracking this damn noise?ĭuane, I did consider swapping the plates but held off reasoning it likely went well for 50-odd yrs and I've not introduced any change there. I've prolly left out other clues, but can U guys start throwing ideas ? Look under board around PI and vib roach eyelets - no solder blobs, rubbish.Ĭheck earth to speed/intensity pots - good. Separate white bias lead away from wire loom exiting board(with yellow/blue/brown leads - all vib leads to speed/intensity pots) - no change. lead picking up something like an antenna? Lift Yellow lead from intensity pot leaving the eyelet end alone - squeal goes, ground yellow lead to chassis - squeal returns. Lift the blue and yellow V1, V2 signal supplies to the 220k summing resistors - squeal persists at variac 125v and upwards. with V1, V2, V4 OUT, even B+ to V3 removed. Now cannot get variac above 125v before awful oscillation commences (the old CRO tells me it's 6.3kHz, sine wave.). Solder sucked the 100k eyelets - no underside solder pooling.
New 100k metal film and crackling gone (Waiting 82k to arrive). Isolate to PI, suspect orig carbon comp plate resistors 100k + 82k. Looking promising.ĦL6 + PI tubes in, no signal, much crackling commencing at about 110v supply. Power up (no tubes) and voltages throughout reaching schematic. I replace filter caps, cathode, bias caps, new 470R/3w screen resistors, new roach + 3x oscillator caps( 0.01, 0.01, 0.02uf), new vibrato cloth wires (white,blue,brown,yellow), disconnected ground switch, new RCA jack. Vib RCA jack changed to a 6.5m mono jack. Vib roach + vib parts missing, filter caps leaking., V6 socket changed, pilot light globe + red bezel missing.Vib intensity pot gone, rest pots look orig. It's a 1964 AB763 export blackface Bandmaster.
1967 Marshall JMP 50, earliest configuration for this amp, E元4s. I bought amp chassis in dishevilled cond. Fender 1964 Bandmaster Head & 2x12 Cabinet.
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