Autotek 7600BTS

You'll notice R128 is completely burnt.Whats the function of that resistor and what would cause it to melt down?
Also there some jumpers installed in various places including in resistor locations.Is this normal?
Also any tips and whatnot would be appreciated as I've never worked on an old amp like this.
 

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Perry Babin

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Paid Member
2003-11-20 11:01 am
Louisiana
www.bcae1.com
Maybe Stephen Mantz (user name MOER) could tell you about the large resistor mods. I saw only one other on google and it could be the same amp.

R128 is likely part of the Zobel network across the output of the amp. Is so, they can be damaged by abusive use as well as open shield grounds (either on the RCA jacks or on the source unit input line) which cause the amp to oscillate rail to rail.
 
Hi,
R128 is indicated a 36ohm 1/4w (to be 100% sure of its wattage I should exhume&open my 7600BTS back cover) and it is on the -56V p.s. rail to Q104&Q105 (A1038) collectors at once (to 1rst A1038 through a second 620ohm resistor in series) and also R128 is connected to Q108's (D1736a) emitter looks being 1rst driver of the "triple darlington" design output.
R128 is symmetrical with R122 in the positive rail connected to its related Qs (102-103-106), and that circuit is inverting voltage to +/-16v.
R134 is the resistor 2,7ohm in the compensation network vs. speaker...

These notes are taken from schematic diagram dated 12feb1991, author S. Mantz.

Agreeing with Perry diagnostic for an open/shorted part toward ground rail in any section of the amp (abuse or careless mistake).
Added series resistors in photos seem intended to increase wattage of original ones....could it be so? are they useful indeed?...