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K200A-1 Power Amp Rescue Needed [message #27251] Fri, 19 April 2019 13:28 Go to previous message
thetragichero is currently offline  thetragichero
Messages: 33
Registered: April 2019
Member
hello i picked up this early 1968 k200a-1 bass head (by ross inc on the front, no 200) for a song because the owner said all it was doing was crackling
measuring the voltage on the speaker at -12V, i can see why!
i checked the power transistors using the resistance setting on the multimeter and the only one that tested odd was the one closest to the power transformer (resistance between emitter and collector around 1M, shorting base to collector and measuring again gave around 77k. all the other power transistors measured >4M and 700k respectively). replaced that transistor with a 2n3055 i pulled from an organ and that gave me -20V across the speaker terminals
i'm having a hard time finding a schematic for this unit as the ones on the site do not match up. boards along the back of the chassis are PC702 and PC602. there's no internal reverb or anything. there does not appear to be a transformer on the output as the K200-Ax schematic would lead me to believe
another thing of concern is that it is fitted with 5 power transistors, even though there is a chassis hole for a sixth. obviously an accurate schematic would work. any help would be much appreciated

i took voltage readings on the power transistors (from left to right)
C -18V -18V 38V 38V 38V
B -33V -33V -12V -13V 26V
E -33V -33V -12V -14V 26V

filter caps look relatively new, and besides being attached to the chassis by a zip tie and hot glue (some tagboard would make me much more confident in its durability), i measure 38V and -32V respectively

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