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working on a 300T. amp came to me w/ 2 of the 100ma power tube fuses blown. it now has new power tubes and those tube fuses have been replaced. at first, after biasing/balancing, it works fine. but it started cutting out until it eventually stopped passing signal entirely.
i reseated all of the ribbon wire harnesses and checked that all of the knobs and buttons were in the appropriate positions. i turned it back on and still no signal. i used a signal tracer to determine the input signal is definitely getting through XV1 and XV2, but not reaching XV3. signal also gets to tuner out jack consistently. i poked around on the board w/ a chopstick to see if i could find any loose connections or bad solders. no luck. removed XV3 to see if the signal would pass that far w/out that tube in place. still nothing.
when i re-inserted XV3, the amp suddenly worked again.....for a few minutes. it seems that once i give it any volume it cuts out again. not all at once...it will cut out for a second then come back, then cut out for a second and come back, etc. if i nurse it along w/ the volume low i'm pretty sure i could keep this pattern up for a long time. but i've found that after the first cut out, if i turn the master up a hair, the signal will come back... this will work for the first 2-3 cuts before the amp goes silent.
i'm inclined to think this is a bad transistor or solder joint. but, i REALLY don't want to take the board out to resolder everything. so i'm checking here first. thoughts?
i reseated all of the ribbon wire harnesses and checked that all of the knobs and buttons were in the appropriate positions. i turned it back on and still no signal. i used a signal tracer to determine the input signal is definitely getting through XV1 and XV2, but not reaching XV3. signal also gets to tuner out jack consistently. i poked around on the board w/ a chopstick to see if i could find any loose connections or bad solders. no luck. removed XV3 to see if the signal would pass that far w/out that tube in place. still nothing.
when i re-inserted XV3, the amp suddenly worked again.....for a few minutes. it seems that once i give it any volume it cuts out again. not all at once...it will cut out for a second then come back, then cut out for a second and come back, etc. if i nurse it along w/ the volume low i'm pretty sure i could keep this pattern up for a long time. but i've found that after the first cut out, if i turn the master up a hair, the signal will come back... this will work for the first 2-3 cuts before the amp goes silent.
i'm inclined to think this is a bad transistor or solder joint. but, i REALLY don't want to take the board out to resolder everything. so i'm checking here first. thoughts?
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