Duo / TurboDuo Low Volume Sound Fix - Ultimate Guide

[Fidde_se]

To pinpoint and guide where and why there is low sound on PC-Engine Duo (after re-cap)


Mostly suited for Duo and TurboDuo (Not Duo-R/RX)

For this guide i talked to Artemio and he kindly updated
his 240p Test Suite with new functions which can be used when also troubleshooting the audio on
Duo-R/RX, Super CD-Rom², CD-Rom²/IFU.

When you read this the updated 240p Test Suite will probably be released and/or you can try out the Beta.
http://junkerhq.net/xrgb/index.php?title=240p_test_suite
https://sourceforge.net/projects/testsuite240p/files/PCE-TG16-SCD/

Use the CD-ROM2 or the Super CDROM2 version (for now use "240pSuite_1.04_SuperCDROM-test.zip")

In there you could play a beep before, left, middle, right but now you can also play PSG (as before)
left, middle, right, but also CDDA (Stereo) going to each channel and ADPCM (Mono) going to both channels.




We all know about the bad capacitors plaguing the Duo and TurboDuo,
and after changing them it's very common to hear that there are still
problems, low audio, missing a channel, no voice sampling or cd-audio.

Common practice is while changing the caps to also clean parts of the
board around some of the more leaking caps, checking vias and also lifting
and cleaning underneath the op amps, the 4558, and that's what this is
mostly about the 4558 as there is many Duo's out there which these
problems after cap change, and it's time to put an end to that.


So first change the caps, clean, then read this when something is not
sounding correct.


First we have to explain what an OP-Amp is, or to some extent, it stands
for "OPerational AMPlifier", another word for it would simply be
Comparator as that's what it does, and this is actually already at the
heart of our problem, we see it as signal and ground in our heads but a
comparator see's two inputs and comparing them, usually these kinds of
circuits uses what's called a negative feedback, meaning a portion of
the amplified signal is sent back to keep the circuit in check, if one
of these two inputs has a rupture in it's signal path it's not amplifying
right, and to make it worse on the Duo they are cascaded with signals
from each audio source, one for ADPCM, CD-Audio, HuCard (DIN) and finally headphone amp.


Additional Info:
Headphone Potentiometer: (203B) Min->Max
Pin 1-2, 0-27K
Pin 1-3, 0-13.5K
Pin 1-4, 0-13.5K
Pin 1-5, 0-13.5K
Pin 2-3, 0-13.5K
Pin 2-4, 0-13.5K
Pin 2-5, 0-13.5K
Pin 3-4, 0-0 (GND)
Pin 3-5, 0-0 (GND)
Pin 4-5, 0-0 (GND)

Main OP-Amps in this discussion:
IC503 ADPCM
IC521 CD-Audio
IC506 DIN (if lifted then Headphone will still get PSG but no CDDA+ADPCM)
IC507 Headphone Amp


All 4558:
Check that pin 4 has ground connection.
Check that pin 8 has power (≈7.3V)
Power for all 4558 IC506, IC507, IC503, IC521 (also IC203 CD part and IC202 CD part but 4560) gets its regulated power from
Q304 (SMD Mark BR, located in the middle of the cap jungle beside the two main regulators) Japanese SOT-23 (SC-70) incoming
≈8.2V on pin 3, outgoing ≈7.3V on pin 2 and pin 1 should ≈8.0V, IC107 4558 (CD part) however does not get regulated voltage
from Q304 but is powered directly from the same source as Q304 is, meaning IC107 pin 8 will be directly connected to Q304
pin 3, and that pin gets it's voltage from the same pin as IC303 7805 (IN).


So PSG from HuC6280 goes up threw visas and under the HuCard slot comes back to the sec side beside the slot
each with a test point, then going towards IC506 going threw C879 (right) and C878 (left), after those capacitors
there is a new testpoint, then threw R611 (1K left) and R612 (1K right), then going threw R631 (15K Left) and R613 (15K Right) going threw the vias into IC506 but also getting signal from the CDDA/ADPCM.

IC502 OKI M5205 (ADPCM)
Should get ≈5V on pin 18.
On pin 10 the signal goes to pin 6 on IC503 but first threw C652, R657 (1K), R688 (270K).

IC503: ADPCM
Pin 1 ≈3.6V
Pin 2 ≈3.6V
Pin 3 ≈3.6V (connected with pin 5)
Pin 4 GND
Pin 5 ≈3.6V (connected with pin 3)
Pin 6 ≈3.6V
Pin 7 ≈3.6V
Pin 8 ≈7.3V

The signal goes from IC503 pin 3,5 to IC521 pin 3,5

IC521: CD-Audio
Pin 1 ≈3.6V
Pin 2 ≈3.6V
Pin 3 ≈3.6V (connected with pin 5)
Pin 4 GND
Pin 5 ≈3.6V (connected with pin 3)
Pin 6 ≈3.6V
Pin 7 ≈3.6V
Pin 8 ≈7.3V

IC506: DIN
Pin 1 ≈3.6V
Pin 2 ≈3.6V
Pin 3 ≈3.6V (connected with pin 5)
Pin 4 GND
Pin 5 ≈3.6V (connected with pin 3)
Pin 6 ≈3.6V
Pin 7 ≈3.6V
Pin 8 ≈7.3V

IC507: Headphone Amp
Pin 1 ≈3.98V
Pin 2 ≈3.98V
Pin 3 ≈3.6V
Pin 4 GND
Pin 5 ≈3.6V
Pin 6 ≈3.98V
Pin 7 ≈3.98V
Pin 8 ≈7.3V

Headphone Output:
Left channel comes from pin 1 on IC507 (threw C678, 100uF and R693, 120ohm, Right channel comes from pin 7 on IC507
(threw C679, 100uF and R694, 120ohm. If you get all the correct sound in both channels on the DIN then there is
something with or around IC507 focus on that.

You can have headphones on (max volume) and listan after a buzz when you touch pin 2 or 3 on IC507 with something
metallic (Left channel) and pin 5 or 6 (Right channel), if voltages are correct and you hear a buzz on each channel
that op-amp should be OK.

With the same headphone test you can try out IC506, pin 6 buzz to the right, pin 2 buzz to the left, if voltages and
buzzing on each channel then that op-amp should be OK.

Continuing headphone buzz test on IC521, touching pin 6 buzz right, touching pin 2 buzz left, and touching IC503 should
give buzz in both channels (adpcm mono) on pin 6 (loud), pin 2 (weaker), you can nearly with just the buzz test locate
where the problem is.

DIN: Audio
Left on pin 3, Right on pin 4.
Going threw L602 and L601 respectivly.
After that going threw R608, R610 (1K) respectivly.
Then passing threw C604 (10uF right), C610 (10uF left).
From C604 to IC506 pin 7, C610 to IC506 pin 1.

As these op-amps are cascading then if any voltage are wrong on one and making audio low it can be the previous one thats
not working correctly, you can try lifting one and see if some the sound become "loud" again usually when lifting IC506,
(they should be lifted at some point anyhow to remove excessive electrolyte fluid).



Signal into IC506 pin 6 (right) comes from R630 (15K) then to IC504 pin 1, and IC506 pin 2 (left) comes from R628 (15K) then to IC504 pin13.

There are several replacements for 4558 (SO-8), the original is made by JRC but RC4558 (Texas), NJM4558 (New JRC) or LM4558 (Diodes) should work fine.

The HuCard sound (PSG and Simple Voices) comes from HuC6280 pin 17 (Left) pin 18 right



With the 240p Test Suite you can test each channel individually or each soundtype separately,
different volume between left and right or different sound coloration, sometimes it's easier
with headphones and there you can also listen closer for different kinds of static and popping
sounds (that usually comes from leaked electrolyte charging and discharging on the board),
and if your laser is ok you can test the cd audio track to see that it sounds correct (stereo separation on some of the sounds, bongas to the left), and finally a final solution to test the
voice sampling (ADPCM) without the need to find a correct game a wait for the sound to heard,
just hit play and you'll know if it works and it should sound the same on both channels with
the same volume.


Duo 4558 HuC6280:

Large: https://www.retrosampling.se/Temp/PCE%20DUO%204558/Duo%204558%20HuC6280.jpg

Duo 4558 PRI:

Large: https://www.retrosampling.se/Temp/PCE%20DUO%204558/Duo%204558%20PRI.jpg

Duo 4558 SEC:

Large: https://www.retrosampling.se/Temp/PCE%20DUO%204558/Duo%204558%20SEC.jpg

Duo 4558 Volume Knob:

Large: https://www.retrosampling.se/Temp/PCE%20DUO%204558/Duo%204558%20Volume%20Knob.jpg

240p Test Suite:

Large: https://www.retrosampling.se/Temp/PCE%20DUO%204558/240p_Test%20Suite.jpg



This values are taken from the Duo with serial: 21031541B
Pictures before recap.

Made by Fredrik Nyqvist 1-3/2018 SWEDEN
And many thanks to Artemio Urbina for updating
his 240p Test Suite with CDDA and ADPCM tests.

[choijimmy]

Awesome. Really appreciate for the detail pictures

[ClodBusted]

Thank you so much! I'll use your information tonight to finally fix that weak Redbook audio of my Duo.

When you read this the updated 240p Test Suite will probably be released and/or you can try out the Beta.
http://junkerhq.net/xrgb/index.php?title=240p_test_suite

Link doesn't work.

[Fidde_se]

Thank you so much! I'll use your info tonight to finally fix that weak Redbook audio of my Duo.

When you read this the updated 240p Test Suite will probably be released and/or you can try out the Beta.
http://junkerhq.net/xrgb/index.php?title=240p_test_suite

Link doesn't work.

The link works now, some DB updates to his site.

[Fidde_se]

How's that CD Audio working out for you, ClodBusted?

[ClodBusted]

Haven't found time to fix it, but I might get back to the bench on Sunday.