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In Reply to: RE: Yamaha C2A -- anyone intimately familiar with this preamp? Repair advice needed... posted by LowRedMoon on April 13, 2017 at 16:35:21
Sorry, have never worked on C2A, these are general comments regarding popping, intermittent crackling static type sounds. Most times it is a connection problem, including the wipers in controls, cracked resistors, cold solder joints etc. Much less frequent, semiconductor (transistors, diodes) or capacitors. Changes in temperature can exacerbate it, due to expansion/contraction.
Any insulated tool is good for prodding, flexing etc., to see if you can cause the sound. Potentiometers, including bias, as long as you know how to adjust them, see if they make similar noise turning them, clean or replace if they do. Take your time, sometimes you have to touch/flex something in a certain way to replicate the problem. Then when you have, it can take time to isolate its source.
Side comment about appearance of power devices. Many, many years ago, just starting doing service, suspected some devices due to discolouration, for me at least, complete waste of time.
Follow Ups:
...you may also want to narrow it down to a section and shotgun the section (that is, replace all the parts). Parts are cheap and time is expensive.
Also, examine and reflow the existing solder joints.
Finally, if you're working on a differential amp pair, the transistors should be matched. We had that issue in a Yamaha C-80. Solution required a $30 (ouch) matched pair of FETs if I recall correctly.
Right - I'm going to put the transistors back in and start over. It's much easier to find the cause of noise that is constant, now that I'm reasonably competent with an oscilloscope. I'll do some poking. Thanks for your reply.
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