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Jun 11, 2014, 10:25 PM
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Yamaha surgery

The old Yamaha HTR-5230 had problems ever since year 3. Its volume control finally broke, after 14 years. It has too many complex mechanical solutions for simple problems.

A modern replacement would be $200 with no speakers, all to replace a single pot. A search for a single pot connected to an ADC revealed this instead:

The pinout on the board:

+10 - +12 MOTOR
-12 - -12
+12A - +12 OP-AMP

Another 12 pins are where the 6 audio channels are routed through.

So basically, the volume control is actually 6 independent pots directly in the signal path of the 6 audio channels. They connect to 6 non inverting el cheapo NEC C4570C op-amps. A lot of effort was spent shielding the pot connections with ground lines, powering the motor from a different rail than the op-amps.

The motorized volume control was real slick, until it broke. It gave smooth, dampened tactile feedback.

The force used to dampen the motion was transferred through a vastly underrated shaft. After 14 years, it broke.

The module is disassembled, revealing

A simple circuit allowing each pot to control 2 channels.

All 13 lines required by the volume control are broken out

to a $4 2 channel pot. There is no easy way to get a single pot to control 6 channels. It would require 6 digital pots capable of 24V range.

Only 2 channels have been traced out. It would take many hours to trace out all 13 lines.

The board was held in by the volume control. Now it can only be stuffed in.

For now, having 2 functioning channels is good enough. It's much noisier, because the control isn't shielded. The 6 channel mode was originally useful, but as rent increased & space decreased, there was no longer enough room for 6 speakers.
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