The PC Engine Duo has a sound system that mixed audio from various sources.

I have yet to reverse engineer the entire circuit, but have done a good amount of it.

A lot of the circuit users op amps, and a good replacement is the Texas Instruments RC4558.

Audio can be provided from:

In short, many 4558 Op Amps feed audio through a multiplexer (BU4053BCF) to mix the various sources together, and they get sent out to the headphone port through an op amp and to the AV port through an op amp.

A quick overview of the typical issues I have encountered are:

There are several places the audio is generated from. I will cover them below.

The HU6280 is the PC Engine HuCard system, and it generates audio for that part of the system.

The audio comes out of pin 17 (Left) and pin 18 (Right). They travel over the IC506 and connects to the IC506 amp that drives the AV Port Out.

The HuCard through pin 2 sends audio directly through C883 > R616 (3k) which then goes through several places like R613 (5.1k) to the IC506 Right In- pin.

I have not yet tracked back where Super CD audio comes from.

The AV port op amp is a 4558. It is the IC506.

This amp is right before the AV port.

IC506 Left In- (pin 2) goes through several paths:

IC506 Right In- (pin 6) goes through several paths:

The IC506 Left In- and Right In- join together through two resistors R627 and R629 and the center of those two resistors taps off and goes to the mono audio output Z0 of the BU4053BCF (pin 5).

IC506 Left In+ (pin 3) and Right In+ (pin 5) goes through three paths:

The IC506 Left Audio Out comes out (pin 1), joins back to LEFT- (pin 2) through R691 (1.5k), carries on through +C610- > R607 (47k) which pulls to ground. The signal out of C610- then carries on through R610 (1k) > L601 (inductor on top side) > LEFT pin AV port.

Also, a NPN transistor Q604 is connected to the left audio output Collector, and Emitter to ground, so basically pulling the left audio signal to ground, when the Q604 (base) is high.

The IC506 Right Audio Out comes out (pin 7), joins back to RIGHT- (pin 6) through R692 (15k), carries on through +C604- > R604 (47k) which pulls to ground. The signal out of C604- then carries on through R608 (1k) > L602 (inductor on top side) > RIGHT pin AV port.

Also, a NPN transistor Q603 is connected to the right audio output Collector, and Emitter to ground, so basically pulling the left audio signal to ground, when the Q604 (base) is high.

In short, sending Q604 base high will disable left audio by physically sinking the Left audio out on the AV port.

Sending Q603 base high will disable right audio by physically sinking the Right audio out on the AV port.

Both disable signals meet at D601+.

The left audio Q604 (base) goes through R605 (2.2k) through -D601+

The right audio Q603 (base) goes through R606 (2.2k) through -D601+.

The D601+ goes to several sources. The primary one being the PNP transistor Q308.

The PNP transistor Q308 ultimately pulls Q604 (base) and Q603 (base) high disabling audio:

The Q308 (Collector) connects to the D601+.

The Q308 (Emitter) > -D303+ > 5V Output of IC302 > +C310- > Ground keeping the emitter high ready to send it to the collector.

Also, Q308 (Emitter) > +C325- > Ground

So, when the Q308 (base) is low it will turn off audio.

So from what I have currently traced, the audio should never turn off unless the 5V and 7V rail are down or traces are damaged.

The headphone port has left and right audio and ground. There is no separate spring switch that disables anything.

The amp by the headphone port sends the audio out to the headphones.

TODO

TODO

As well as turning off the AV Port audio, the headphones are also shorted to ground via the same trigger (Q604 base going high):

Another amp that receives audio, but I haven't traced yet to where it goes is IC503.

It is a 4558 Op Amp.



Another amp that receives audio, but I haven't traced yet to where it goes is IC521.

It is a 4558 Op Amp.

Another amp that receives audio, but I haven't traced yet to where it goes is IC203.

It is a 4558 Op Amp.

The audio sources come into the BU4053BCF multiplexer.

All three enable pins are sent high by the NEC D91317GD pin 81.

The volume balancer is responsible for balancing the audio coming in from IC203 (yet unknown where its input comes from).

It then sends volume controlled audio out to the AV Ports Amp IC506 Right In- and Left In-.

I have yet to track all these pins to where they go.