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Old 25th October 2004, 04:35 AM   #1
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Question silly bias and offset question

hi all

i recently scored a nice little aiwa 8500 integrated amp for almost nothing,it's pretty solidly built[toriodal Xformer,regulated PS] after some cleanup of the switches and relay contacts it works and sounds fantastic.

before i tried it out with real speakers,i measured for any dc offset,it's low enough to be unconcerning[below 20mV]but it's uneven between channels,with one being about twice as high as the other.the bias is presumably in similar shape..the amp is probably over 20 years old and tho not strictly necessary it could probably use a little tune up...i need some help with this,service manuals for aiwa gear are pretty easy to come by,but apparently not for this old model...

i'm not sure what type of output devices i'm dealing with,they look to be chip amps with one large package sandwiched on to the heastsink per channel,i can't read the part numbers as they are covered by steel plates securing them to the heatsink...but i do have two adjustment pots per channel on the unlabled board..i'm ASSUMING these are offset and bias trim pots but i can't turn the board over to trace the circuit without a total teardown and probably breaking something...

anyway...i have one blue and one black colored pot per channel,i've seen these colored pots used in other amps before..is there some type of standard,or rule of thumb for function based on the color?is the blue or black[sometimes white] color normally used to denote what it does?without a service manual i have no way of knowing exactly what the bias level should be set too so i guess i'll just even it out to whichever is the higher reading side if unequal and then trim the offset...tho i've not done it before,i'm familier with how to go about this and think i can handle the job,but it would be nice not to start totally in the dark,flipping a coin as to which pot to try for what..anyone care to enlighten me if possible
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Old 25th October 2004, 05:54 PM   #2
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if you are trying to adjust bias current, then the pot nearest the output transistors...the other pot will probably be the adjustment for the input stage
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Old 25th October 2004, 07:11 PM   #3
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If it's working, I would think twice about touching it unless
you have test equipment.
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Old 26th October 2004, 06:19 AM   #4
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Usually for me, if I'm sure that one pot is offset, I'll adjust it very slightly without of load of course. During this I keep a meter on an emitter resistor and another on the output. If the output current happens to change, I just put it back where it was and then continue with the offset.

If I find that the current in both channels is quite different, I'll make an attempt at evaluating which is closer to original and set both the same. If I have no idea what the original current was, but it's my own amp, I'll usually just figure out what would be a good current for that amp if I had designed it myself.

I know this is not the best practice, but it has worked for me every time with more amplifiers than I can remember.
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