Yamaha R840 is part of Micro Component System MCR-840 and comes with either CD-Player (MCR640), DVD-Player (MCR840) or BluRay-Player (MCR940). The players are connected via RCA but need a proprietary “System Connector” cable. The remote control is different for MCR640.
The system comes with Yamaha NS-BP300 speakers in black or white and sound good.
You can add Bluetooth capability via iPod-30pin dock or RCA

Self-Diagnostic Mode: unplug device, turn balance completely to the left. Then press Pure-Direct button while plugging in the socket. Check error code in the service/schematics manual.
Try resetting the error first in self-diagnostic mode, the device may function again (at least for some time).

Schematics / service manual:
https://elektrotanya.com/yamaha_ns-bp300_r-840_mcr-640_mcr-840_mcr-940-.pdf/download.html

The stereo receiver has a standby circuit, which is permanently powered and uses a film capacitor (C13 with 22nF, 630V). Due to aging and impulse spikes the capacitor dies, the yellow LED just blinks and the receiver does not turn on.

http://www.hifi-forum.de/viewthread-220-3031.html#15

Replace with Polyester dielectric capacitor (MKT type), or better use one with polypropylene (MKP type). The voltage rating may also be higher than 630V.

'Protection Mode' problem:

There are four 68k 1/8W SMD resistors setting the constant current of the circuit. Replace them with >=1/4W metal film resistors (you can use THT too). Unfortunately, they are on the underside of the main PCB, so you have to remove a lot of components to get to the four 68k resistors R552, R553, R622, R623.

See
http://www.hifi-forum.de/viewthread-220-8212.html
and
http://www.hifi-forum.de/viewthread-220-3031.html#32

Note: A-S1000 has similar issues.
R-N602 also has the Protection Mode problems.
Probably more Yamaha systems can be repaired in that scheme.

The system connector is used for a connected disc player. It enables the RCA-input for the player and also transfers signals like power-toggle or remote control.

It uses proprietary 8‑pin DIN connector (find the cable under name “MINI DIN 8-PIN MD8”).
The pinout is as follows:

#TODO <insert pinout table here>
#TODO <insert signal protocol here>

Make sure the input on the iPod dock (like Bluetooth adapter) is pre-amplified - (the cheap adapters are commonly not)!