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Yamaha C2A -- anyone intimately familiar with this preamp? Repair advice needed...

Posted by LowRedMoon (A) on April 13, 2017 at 16:35:21

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.

Thanks!

Adam

• schematic at HiFi Engine   (Open in New Window)

Edits: 04/13/17

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