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hello fellow enthusiast i have ac positive oscillation issue in pre drive which is emitting a loud buzz noise. it is common to both a&b and mix pre for that matter. output amp is fine and clean as i injected a 1khz in input to amp jack to by pass pre section and all was well in output as far as voltage out and clean signal, no distortion on pos & neg legs. any expertise in regards to trouble shooting pin point areas that should be red flags in pre section would be greatly appreciated. best regards to all associated with this very helpful and needed site. as in title im dealing with a peavey bass markIII series head.thanks again.
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The preamp board powr supply is on the power amp. Those 15v supplies have to get through the cable from the power amp board to the front board. Make sure the 15v supplies are getting to the ICs on teh preamp board.
If you have oscillation, look at both power supplies to see if the oscillation is riding on them.
You report that both preamp channels are affected, so isolate the problem. First scope the two post controls. Is the oscillation present there? You have a graphic equalizer section that can be switched off. Does the oscillation change in ANY way when you switch that off? Also, slide the EQ up and down, does that affect the level or tone of the oscillation?Education is what you're left with after you have forgotten what you have learned. -
hello enzo,appreciate the prompt response.yes it does go away if the compressor/eq function is turned off on that channel,i think. not in front of unit at the moment but will report back my exact findings. you've given me some morsels to chew on i just hope i don't develop indigestion!
........oh by the way i love emoticons dont you!
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HI ENZO IM REPORTING BACK ON WHAT I FOUND have unit in front of me. seems i have a 2 volt drop on neg 15 reg.my input voltage is -19v output -13v. on pos reg my voltage is input +23 out +15. i would think that -2 volts loss is a problem. can i assume i have a issue with the 1000uf 35v caps? or what tidbits do you suggest?thanks enzo....sumo skier awesoooooomeeee!
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Can you measure AC and DC volts on the input & output +15 & -15V regulators. The AC measurement will tell us the ripple & tell us if the caps are ok.
Does either regulator get overly "hot".
As a quick check, you can always temporarily add a capacitor in parallel with the 1000uF 35V caps, if the power supplies go back to normal levels, then that was your problem.
If you have the unit open & are replacing electrolytic caps anyway, why not change them all, especially the low value ones, they are not expensive.Comment
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ow ya goin' mozwell thanks for the info. how would you measure ac v ? across input and output of 15v regs respectfully?of course dc is reference to ground. what would be a bad reading in regards to ac volts?am in front of unit now can reg be bad with low input voltage?Originally posted by mozwell View PostComment
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think i found the problem thanks to mozwell!i think i need to replace 1000uf 35v cap which provides-25v to input of 7915 reg to produce-15 v output. was having major buzzing and moderate voltage issues. had a drop of -4 to -5 volts at input with the same at output and was suspecting cap. good thing was the amp output was fine and clean. good way to check is bypass pre and connect signal straight to power amp and scope to eliminate in chain. need to pick up caps tomorrow will update if that definitely solved my issues. cheersComment
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