punch 200s problems

bdgf2004

Member
2010-10-22 5:19 am
i have a punch 200s and when i got it it had some distortion in the left channel ( i can take pic later). i found one of the output fets (Q119/irf9540) out of tollerance. red probe to drain black to source my meter reads 792 ohms. any other time it's closer to 500 ohms. so i replaced it and checked the drivers but the distortiion is still there. any suggestions?

schem attetched
 

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bdgf2004

Member
2010-10-22 5:19 am
new fet:
black probe on leg #2 red on #3= open
red on #2 black on #3=590

old fet:
yes, with probes either way #1 is completely open from #2 and #3.

i was about to take pics but when i fired it up (60 hz wave) both channels looked very good (no load) for several minutes and then out of the blue the left (repaired channel) lost its bottom half. at this time the right channel remained perfect. i tapped and wiggled around on several components with no result. so i was getting ready to take a pic again so i left the room to get my phone and when i came back it looked okay again. i put a load on it and it distorted on the bottom half badly, so i tried the right channel and i was perfect. now i have done nothing and both of the channels have lost their bottom half ( no load). this is the first time i have had problems out of the right channel.
 

bdgf2004

Member
2010-10-22 5:19 am
the output seems to randomly change, some times it will have a full wave upon power up and other times it will only have the top half. so i don't know how accurate these results will be.

before taking out the diodes the amp was not producing a full wave on either ch.
i pulled the left ch diode out with no change.
i pulled out the right ch diode and now i have a full wave on the right ch only.

but maybe the next time i fire it up it might change again???

i've tried tapping and wiggling everything with no result.
 
Try it a few more times to see if it becomes more consistent.

If you have any freeze spray or canned air (invert can), you may be able to use that to see if any components are sensitive to temperature. You can heat individual transistors with the tip of your soldering iron to see which ones are the most sensitive to temperature.

If the right channel only distorts when the diodes are in the circuit, the problem is likely with the protection circuit components.

Did you have any open or out of tolerance 0.1 ohm source resistors?
 

bdgf2004

Member
2010-10-22 5:19 am
all the source resistors were good.

if i press on Q111 i will get a good signal from that channel but the other channel is only producing the top half. i tried to resolder Q111 with no change so i replaced it with same part # and i seem to get a good output. now Q111 gets very hot and the other channel has only got the top half of the wave.