I had a spare afternoon in El Cajon, so I decided to check out some thrift stores nearby: Three independent stores and two Goodwills. At the first Goodwill, I stumbled upon a matching set of Yamaha C85 preamp (stereo receiver without a poweramp), T85 AM/FM tuner, and M85 poweramp that was just wheeled out from the back room. Given their impeccable condition, I’m guessing that this was part of someone’s estate. Here are a couple brochure photos:

The whole stack was present at Goodwill. I didn't take the receiver.

An edgy, dutched image of the amp, with the huge vu-ish meters ominously overlaid.

After a quick eBay scrub , I decided to jump on this! I picked up the tuner (\$14.99 USD) for my older brother (who’s more into radio than I am) and kept the amp (\$19.99 USD) for myself.

It’s worth noting that the amp is heavy – I almost crushed a fingernail carrying it to the register.

I also found that there was some issue with glue on the PCB that became corrosive and conductive, so I brought the amp straight to my bench when I got home. In hindsight, I should have tested the amp before taking it apart, but the impatient tinkerer in me had taken over. I didn’t even get any decent photos of the amp before taking it apart…

The following power specs are published from their service manual, with a nominal input of 1.55Vrms, 20Hz - 20kHz:

0.003% THD driving 8ohms: 260W per channel
0.007% THD driving 6ohms: 295W per channel
0.02% THD driving 4ohms: 330W per channel
50 lbs¹

At the order of 0.01%, these figures aren’t perceptually relevant for the music listener, but they are very impressively low.

Taking this thing apart was an ordeal. There are six self-tapping philips-head screws that secure the top panel to the chassis, and one of the screws felt like it was tightened with the torque of a thousand windmills. Whoever put this together must have been really angry. After coming close to stripping the screw head and trying the household rubber-band and plier-grip tactics, I decided to bypass the siezed threads altogether and drill off the screw head.

Mighty screw head is no match for mighty drill bit. I wore the PPE of goggles and fuzzy slippers. Please don't tell OSHA.

I gave a couple triumphant fist pumps before wrestling the top cover off.

Note the three sets of terminals on the rear! There are indeed selectable A/B/C outputs. I think A is wired in parallel with B+C in series.

The first thing I noticed was where the weight of this behemoth came from: two line transformers on the left of the above image. Apparently they’re wired in parallel to allow for more current before core saturation. There was a foam damping pad on top of them both that was starting to dry rot into a crumbly mess. I had already picked it off.

The AC mains voltage is then smoothed to DC with four massive electrolytic capacitors: 2x 47000uF/100V and 2x 33000uF/56V – you can see two of them at the top of the image below. All of the electrolytics in this amp are Nichicon, as expected.

There was a potential of around 60V still on one of the caps, which I discharged before diving in. I don’t want to know how many times over the discharge current might fry my body if I accidentally bridged some bits together.

Mighty conductive drill bit is mighty, but no match for possibly-dangerously-charged mighty filter caps...

And indeed, the glue had begun to take a toll on the main amp board.

Note the black burn marks and the green corrosion on the rails between the electrolytics. There's also a diode buried in the gunk below the capacitor on the left.

After trying my best to scrape off the gunk with some isopropyl, q-tips and a sharpened popsicle stick, I decided to fire up the amp. The protection light stayed off! So far so good.

Some observations and concerning bits:

The upper two filter caps are charged to 89V when the amp is on. I was too afraid to reach my probes down to the bottom two. Although I mentioned a fair bit of residual voltage that had to be discharged above, I noticed that the caps are discharged every other time I’ve turned the amp off. I guess the bleeder circuit was working every time after the first. I’ll confirm this with the schematic later.
The SM mentioned a bias voltage of 15.5mV +/- 1mV measured across emitter resistors, but some forum mentioned that my model (early serial number) had a lower spec of 10mV. For the left channel: I measured around 11mV on both resistors (not bad), but it was unresponsive to the trimpot (not good). For the right channel: one resistor was at 12mV while the other was at a near-short of 1mV (really not good), but both voltages responded to the trimpot.
With no input and no load, I measured virtually no DC offset on the left speaker outputs, but 12mV offset on the right.
Of the smaller electrolytic caps (the ones encased in the nasty glue; 1000uF/100V), the right-hand measured around 40V, while the left was at 0V.

Something seems to be shorting on the right side. I confirmed this on the scope: the negative side of the right output isn’t looking good. On both sides, the smaller trace is an input sinusoid at 1kHz 2Vpp. The good news is that the left is working fine – the larger trace on the left is an output of 60Vpp (+29.54 dB V/V gain).

My plan? I’ll replace the components on the main amp board, as it seems like no other parts are broken. More to follow…

Footnotes

I wanted to include the weight to validate my bruised fingernail. ↩︎