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This preamp was making intermittent popping noises (brief shots of DC). It was so infrequent that 'scoping it out was not a practical option. I found the DC offset in the tone control section to be unstable, so I started there.
I pulled the 2SA913 and 2SC1913 transistors, which appeared to be cooked, and replaced them with 2SA1930 and 2SC5171, which match the originals, but with higher hfe (140 vs. 100, as tested) and higher crossover frequency (200MHz vs. 120 MHz. While doing this, I also replaced each of the 4 VD1212 on the tone control board with 2 1N4148s in series (blindly followed advice from the 'net community). All replacements done carefully, as usual.
Now, instead of getting sound with occasional popping and static (its previous state), the unit will not pass sound at all. The tone control board's DC offset won't budge from around 35mV when turning the trimpots for either channel. Before, the DC offset would change while turning the pots, and then fluctuate.
My conclusion is that either I made a mistake replacing the VD1212s and that the 1N4148 replacement won't work in this circuit, or the gain of my replacements is too high, which I doubt - I think it would still work. The third possibility is that something entirely different is going on and I'm going to spend a lot more time figuring it out.
I'd like to find a print shop that can blow the schematic up to readable print size so I can read the proper voltages.
Please post your insights.
Edits: 04/13/17Follow Ups:
Try posting on the Yamaha forum at AudioKarma. There are a bunch of knowledgeable cats there. One of them is restoring my C2A right now.
I'm a member of Audio Karma. Do you know whether the person restoring your C2A would be open to sharing which replacement parts he was successful in using in this preamp?
That makes complete sense. Thank you.
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.
...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.
First I would get direct replacements for those transistors.
Second, check all the associated circuitry of those transistors. They usually just don't blow for no reason. Recap the unit as well. Look for toasted resistors near the transistors.
My guess is your transistors choice isn't suited to that circuit, eg won't turn on and pass a signal.
Good point. I ordered new Nichicon caps for it. I'll replace them and go from there.
Finding direct replacements has been unfruitful so far. It's a heck of a lot easier to find a 12AX7 than it is to find a transistor that was manufactured a couple decades ago - the one thing that annoys me about solid state.
If I can source semiconductors that I'm confident are good teplacements, I have no problem rebuilding this preamp. It's not pretty, but it's a good unit.
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