Yamaha M-70 Natural Sound Amplifier Repair

This Yamaha M-70 power amplifier came through the shop recently for an overhaul. The owner reported it was experiencing some trouble, and had a number of LEDs burnt out on the front panel power meter.

The M-70 is a truly massive power amplifier delivering a whopping 200W per channel into an 8 Ohm load, delivered by way of a staggering 8 output transistors per side bolted to mammoth heat sinks for dissipation. It’s also a very unique design, too: it has a single input, but independent A/B level controls for the speaker sets. This makes it possible to use this amp to perform perfect, level-adjusted A/B comparisons between attached speaker sets.

Yamaha was considerate to service techs in their design, too, The bottom cover unbolts, exposing the circuit traces, and the daughterboard with the driver circuitry can be removed with only a few solder connections.

The main power supply components are tight against the front panel of the amplifier, and clearance down to the millimeter is critical. As one cap was already bulging, replacing the caps in the power supply was critical, but it took two tries as the first set were less than 2mm too wide and wouldn’t fit properly.

The driver board got all new electrolytic capacitors.

The M-7o uses a pair of MV-12 varactor diodes, which are in the same family as the VD-1121, VD-1212, VD-1221 etc. varactor diodes. These are liable to fail without warning after a few decades, but are easily replaced with 2 x 1N4148 diodes in series.

There’s some active protection circuitry right on the AC lead-in which monitors overall power consumption and will kill the power if it’s excessive. The trip voltage needs to be adjusted after service.

Adjusting the power supply rails, protection trigger, bias and DC offset.

All adjustments were successful. Some of the standard lamps were burnt out but easily replaced.

After replacing those, a few LEDs in the power meter were still failing to light.

Investigation determined that the left-channel LED driver chips, the Toshiba TA7612AP, had failed and weren’t producing voltage on the blank output lines.

New chips were installed, and produced voltage, however the LEDs still failed to light. Some further testing showed the LEDs themselves were damaged and had an excessively high voltage drop, likely caused when their associated driver chips failed.

Close, but the old LEDs were red emitting with a diffusing white casing resulting in an orange color. The new Broadcom LEDs are the right light output, and right color and voltage, but their red casing produces a red color. It’s obviously mismatched. No problem, time to replace the other 34 with the same.

There we go. 

Good as new! After the LEDs were all replaced, the attenuators were adjusted for equal channel leveling.

Testing out with its companion, the C-70 Natural Sound Stereo Control Amplifier.

All set! Quite a few components replaced in this amplifier, but it’s good as new after all that service and ready to go home!