punch 200s problems

[bdgf2004]

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

 

[Perry Babin]

It may be leaky. Compare the resistance between its gate and other two legs to the other IRF9540.

 

[bdgf2004]

everything tested ok on the fet except what i previously stated

 

[Perry Babin]

You'll have to remove it from the circuit to check it.

 

[bdgf2004]

the fet was out of the circuit before testing. ive already replaced it and still have the distortion

 

[Perry Babin]

I don't know how you tested the FET. Did the new one read open in one direction (leg 2-3)?

Did the failed FET read open from leg 1 to each of the other legs?

 

[bdgf2004]

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.

 

[Perry Babin]

That's 0.590? 590 would seem to indicate resistance which is entirely different.

Do you have negative rail voltage on the IRF540s when the bottom half isn't being produced?

 

[bdgf2004]

i hate this new internet service we have!!! i have already replied 4 times and none of them have posted!!!

yes i have negitive rail voltage at the fet. -27v on lef #3

 

[Perry Babin]

Remove Dx04 from one or both channels. Does that allow the amp to produce the negative half of the waveform?

 

[bdgf2004]

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.

 

[Perry Babin]

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]

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.

 

[Perry Babin]

What if you push on the board near Q111 but not on Q111. Do you get the same results?

 

[bdgf2004]

started out with my finger and narrowed it down to Q111 with my probe the surrounding areas didn't seem to make a difference. since i have replaced it the signal has been perfect fot that channel, it just gets hot.

 

[Perry Babin]

What gets hot, the outputs in that channel or Q111?

 

[bdgf2004]

Q111 and i just noticed that Q212 (mpsa56) in the other channel is very hot

 

[Perry Babin]

Is there any DC voltage across any of the gate resistors for the outputs (no audio)?

What is the DC voltage across R148 and R149?

 

[bdgf2004]

no voltage across any of the resistors you mentioned

 

[Perry Babin]

Are R148 and R149 within tolerance?