Sunday, April 17, 2016

Yamaha DGX-620 LCD Repair/Retrofit

UPDATE: Ulrich Borghoff did his repair slightly different and sent in photos. Thanks Ulrich!

Here's a Yamaha DGX-620 keyboard with a broken LCD. Judging from the fact that there are quite a few videos and tutorials on how to exchange a broken LCD with the proper spare part, I'd reckon these LCDs break quite easily and often.

 (Broken LCD)

(Original LCD module: The slightly kink in the flexfoil is enough to ruin the module, this slight damage is, unfortunately, not repairable.) 

I was a little surprised to get a quote of 130€ ($150) for the replacement (Yamaha Part # WG299100), and I was looking for a replacement that's cheaper. As this is a fill-in instrument, and long out of warranty, I wouldn't mind not getting the original part. Also I don't mind the tinkering.

Enter Pollin, a company selling electronic remnants, such as bags with 1kg of capacitors, sold as “perfect for tinkering”. By chance, they sell a “LCD-Modul NAN YA LTC79H202T50K, 240x320” (order #121307, €4/$5) which, on closer examination, is electrically 100% compatible to the Yamaha display. Only with a slightly different, smaller shape, and with a different pinout. Also, they sell the exact same flex-cable that Yamaha uses to connect the LCD to the CPU board: Flexprint-Kabel AXON FFC1.00A14/0200L5-5-10-10 (#562251) (€1).

(Pollin: Product Page)

(cheap replacement LCD)

Looking into the datasheet of the LCD provided by Pollin and the Yamaha DGX-620 service manual easily found online, it's clear that electrically the display is compatible.


(Pollin LCD Module)

FR (first row) is FLM (first line marker), LP (line pulse?) is CL1 (data latch), XCK (x? clock) is CL2 (shift clock). Everything else has identical designators. Sanding off the back of the replacement LCDs PCB a little to have good bonding to epoxy glue...

(replacement LCD, before gluing down flex cable)

(sanding off the back of the PCB and the white flex-cable, so that the epoxy sticks better, you can see the residue from a first attempt: The blue plastic on the flex cable didn't stick)

(jumper wires)

Ugly patching of wires from the 14-in cable to the pads on the LCD according to pinouts shown above. After soldering the wires (which is hard on the flexfoil, as the plastic melts and the copper traces of the flexfoil will move around, creating shorts) and emitting a few prayers to the deity of choice, a picture appears! :-)

(first signal on replacement LCD screen)

Then I added the display to the original plastic part from which I removed both the LCD logic and all the light spreading works. The complete LCD module almost fits in the cutout for the old glass, only a small plastic ridge had to be removed with a xacto-knive.  I had to add a small slot for the flexcable, to feed through and added a hole for the big blob of goo by which I glued down the cables so that they don't become loose.

(cutout for big lump of wires and goo)

The end result: A replaced LCD screen for €4. But I had to invest a few hours of work, which in this case I didn't mind.

(replaced screen on the Yamaha keyboard)

Update: As Ulrich pointed out, one can directly solder the cables for the LCD backlight to the cables going to the two-bin connector on the power supply on more recent models of the keyboard. Some onlder ones have high-voltage for an electroluminiscent foil, though, so beware.

Here are Ulrich's Photos:

Saturday, April 02, 2016

Superlux HMC660 Headset and PS 418-S Phantom Power Adapter

The HMC660 Headset made by Superlux is dirt-cheap, and not that bad, for the price. Certainly beats some other headphones sold for €39. But even though it has a XLR connector, it doesn't really work that well with phantom powered microphone inputs. That's because its Electret Capsule is directly connected to the XLR pins, which sometimes works, but generally doesn't.

From Chris’ Miscellanea

The microphone part is glued, but after heating it gently to about ~40..50 °C with a hair-dryer or a heating gun used for heat-shrink-tubing, it can just be pulled apart by hand.

From Chris’ Miscellanea

The proper way to adress the connection issue is to use a proper phantom-power adapter for electret microphones, luckily this taiwanese producer of cheap (and a little crappy) audio electronics also has something in store. The Superlux PS 418-S adapter!

Inside, this little marvel looks like this: Mostly through hole, one-sided PCB, waves-soldered.

From Chris’ Miscellanea

The schematics mostly adheres to the often copied "Schoeps" design, here's my attempt at the schematics.

From Chris’ Miscellanea

I think I got the type of Q1 wrong. It's probably wired as an emitter follower (the voltage gain of the drawn common emitter amp of 22x would be unreasonably high) with Q1 being a KSA1015 PNP Epitaxial Silicon Transistor. Q2 and Q3 are likely KTA1268

The PCB is small enough to fit it inside one of the earcups of the HMC660. Just cut open the direct connection that go from the 6-conductor XLR & TRS cable (braided copper, yellow, green) to the 3 wires that make their way into the gooseneck (white, black, red).

From Chris’ Miscellanea

From Chris’ Miscellanea

Here's the PCB inside a piece of heat-shrink tubing, but in the end, I opted to just hot-glue the bare PCB into the cup. Also some foam (from inside the PS-418's small shipping box) was added to prevent the whole thing from rattling around.

From Chris’ Miscellanea

Here you go. 39€ + 19,90€ for a only halfway crappy headset ;-).

Wednesday, March 23, 2016

W&T 36201 Isolated USB to RS485 Interface: Fix "automatic" RS485 Transmit mode

Some time ago, I bought three used W&T 36201 isolated USB to RS485 interfaces. They have pretty good build quality and are intended to be mounted in control cabinets (on “DIN Rails”). You can see them in this photograph, it's the blue thing.

From Chris’ Miscellanea

And while I'm pretty impressed by their build quality, I found out that even for (normally pretty expensive) industrial kit, they get the control of RS485 transmission wrong.

RS485, in short, is a pair of cables named A and B which carry a differential signal. This means that the signal in one wire should always be the inverse of the signal in the other wire: If voltage on A increases, the voltage on B should decrease. The state of the bus is “1” when the voltage on B is higher than the voltage on A, and the state of the bus is “0” when the voltage on B is lower than the voltage on A. A RS485 bus, when idle, is kept at the level corresponding to a “1” bit by the termination voltage, which pull B to a higher voltage than A, keeping an impedance of 100 Ohms on the differential pair A and B.

When data is exchanged over the bus, the first thing being transmitted is the stop bit, at “0” level, then the first byte, ... until transmission is completed. Naively, one would think that a driver only has to supply the “0” level and let go of the wires, making the termination resistors pull them to “1” again.

Unfortunately, these resistors aren't typically able to do this fast enough, especially for higher baud rates, and active drive of the bus therefore must be maintained throughout the complete transmission.

Thus, we need some control signal that tells the driver IC when to turn on its output drivers, and there are several methods for this.

  1. some RS485 adapters cheat by only transmitting the “0” in full, and only output a very short “1” runt pulse to charge up the bus, leaving maintaining the idle voltage to the termination resistors,
  2. some RS485 adapters use a serial control line (RTS, DTR) as a control signal and
  3. many USB to RS485 adapters use a signal provided by the USB-UART for exactly this purpose.
2 has the downside that one typically has problems being fast enough in switching the control lines over USB, and 3 is obviously the preferred version.

Unfortunately the W&T 36201 interfaces only do 1 and 2, so I tried to modify them to provide for 3.

Here's a (very slow) 9600 baud transmission via the unmodified interface in "automatic" mode. The driver is only activated (blue) during 0-bits.

From Chris’ Miscellanea

The used USB to UART bridge fortunately has a TXEN pin (#16), which we jumper over to the line normally carrying the #DTR (inverted) signal that can be used to implement method (2). The chip is mounted on a separate small PCB which makes working on it pretty easy:

From Chris’ Miscellanea

From Chris’ Miscellanea

Unfortunately the polarity of TXEN is high-active, but as built the converter needs #DTR to go LOW to transmit. So we use a spare gate of the HC04 hex inverter conveniently located on the baseboard. One has to scratch open the very short trace between the pullup for the optocoupler and the via left to it.

From Chris’ Miscellanea

After this modification, set the device to computer-controlled transmit control (normally DTR, now TXEN). As can be seen on the scope, now the transmitter IC drives the bus for the whole duration of a character.

From Chris’ Miscellanea

Sunday, February 28, 2016

ASUS RT N12E Bootup

Debugging a problem with a ASUS RT-N12E 300Mbps Wireless N Router, I soldered a pin header to  J15 (next to the ethernet magnetics handling the yellow "LAN" ports).

Pinout is Vcc, GND, Tx, Rx. Vcc is the square pad, Rx and Tx as seen from the N12E soc. Port runs at 38400 bps.

Here's the bootup messages, just in case anyone is interested.

$ cu -l ttyUSB0 -s 38400

========== SPI =============
 ------------------------- Force into Single IO Mode ------------------------
|No chipID  Sft chipSize blkSize secSize pageSize sdCk opCk      chipName    |
| 0 c22016h  0h  400000h  10000h   1000h     100h   86   39      MX25L3205D/E|
Set 8196C PHY Patch OK

---RealTek(RTL8196C)at 2013.12.09-17:34+0800 version v1.1f [16bit](390MHz)

####return_addr: 0x05010000, root_bin_offset: 0x050dd012
Jump to image start=0x80500000...
decompressing kernel:
Uncompressing Linux... done, booting the kernel.
done decompressing kernel.
start address: 0x80003600
RTL8192C/RTL8188C driver version 1.6 (2011-07-18)

Probing RTL8186 10/100 NIC-kenel stack size order[2]...
chip name: 8196C, chip revid: 4
eth0 added. vid=9 Member port 0x10...
eth1 added. vid=8 Member port 0x1...
eth2 added. vid=9 Member port 0x2...
eth3 added. vid=9 Member port 0x4...
eth4 added. vid=9 Member port 0x8...
[peth0] added, mapping to [eth1]...
init started: BusyBox v1.13.4 (2014-09-18 18:08:32 CST)

##flash.c free apmib ##
Init Start...
wan_disconnect: option all

##system/sysconf.c free apmib ##

##flash.c free apmib ##
===== Set parameter for BSMI test=====
Init bridge interface...
wait for bridge initialization...
syslog will use 64KB for log(7 rotate, 1 original, 8KB for each)
route: SIOCDELRT: No such process
route: SIOCADDRT: Invalid argument
Init Start...
Init Wlan application...

##flash.c free apmib ##

##flash.c free apmib ##
update wps state to -1

WiFi Simple Config v2.9-wps2.0 (2014.09.18-10:09+0000).

Init Wlan application...

##flash.c free apmib ##

##flash.c free apmib ##
update wps state to -1

WiFi Simple Config v2.9-wps2.0 (2014.09.18-10:09+0000).

Register to wlan0
iwcontrol RegisterPID to (wlan0)
Register to wlan0

##system/sysconf.c free apmib ##
iwcontrol RegisterPID to (wlan0)
Init Firewall Rules....
firewall clean prerouting
firewall add iptables rule
No wan ip currently!
System TZ ENV = GMT-8

##system/sysconf.c free apmib ##
start infosvr
WLAN0_WLAN_DISABLED=0 ##flash.c free apmib ##
sh: ##flash.c: unknown operand
# Start httpd!
Start wanduck!

# <-- :-="" a="" b="" here="" rootshell="" s="">

MemTotal:          11144 kB
MemFree:            5164 kB
Buffers:             224 kB
Cached:             1128 kB
SwapCached:            0 kB
Active:             1672 kB
Inactive:            716 kB
Active(anon):       1036 kB
Inactive(anon):        0 kB
Active(file):        636 kB
Inactive(file):      716 kB
SwapTotal:             0 kB
SwapFree:              0 kB
Dirty:                 0 kB
Writeback:             0 kB
AnonPages:          1044 kB
Mapped:              716 kB
Slab:               3064 kB
SReclaimable:        176 kB
SUnreclaim:         2888 kB
PageTables:          164 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:        5572 kB
Committed_AS:       2220 kB
VmallocTotal:    1048404 kB
VmallocUsed:         264 kB
VmallocChunk:    1048136 kB

# cat /proc/cpuinfo
system type : RTL8196C
processor : 0
cpu model : 52481
BogoMIPS : 389.12
tlb_entries : 32
mips16 implemented : yes

# busybox
BusyBox v1.13.4 (2014-09-18 18:08:32 CST) multi-call binary
Copyright (C) 1998-2008 Erik Andersen, Rob Landley, Denys Vlasenko
and others. Licensed under GPLv2.
See source distribution for full notice.

Usage: busybox [function] [arguments]...
   or: function [arguments]...

BusyBox is a multi-call binary that combines many common Unix
utilities into a single executable.  Most people will create a
link to busybox for each function they wish to use and BusyBox
will act like whatever it was invoked as!

Currently defined functions:
arp, ash, bunzip2, bzcat, cat, cp, cut, date, echo, expr, false,
free, grep, gzip, halt, head, hostname, ifconfig, init, ip, kill,
killall, klogd, ln, ls, mkdir, mount, ping, poweroff, ps, reboot,
renice, rm, route, sh, sleep, sync, syslogd, tail, telnetd, top,
true, umount, vconfig, wc, zcip

Sunday, February 21, 2016

Linux/udev: Unbinding from one kernel driver, and rebinding to a different one. Automatically, using udev.

Quite a lot of USB devices claim to be a human interface device, the rationale being that you can access them on Windows without the need to supply your own "proper" driver.

Under Linux, one can easily unbind the usbhid driver from a particular usb device, but this gets tedious after a few dozen times. But there's a workaround using udev and a short script to rebind to a different kernel module.

First, create a udev rule, e.g. in /etc/udev/rules.d/

ACTION=="add", DRIVER=="usbhid", SUBSYSTEMS=="usb",
ATTRS{idVendor}=="04d8", ATTRS{idProduct}=="00de",
RUN+="/usr/local/sbin/ %p %k /bus/usb/drivers/mcp2210"

(all in one line)

Then use the following small script, stored as /usr/local/sbin/ to do the actual work:


if [ "$#" != 3 ] ; then
echo "Usage: $0 sysfs_path kernel_name new_driver_path" >&2
echo "" >&2
echo "To be used in udev rules:" >&2
echo "   RUN+=\"$0 %p %k /bus/usb/drivers/mcp2210\"" >&2
echo "which rebinds a particular device to a new driver." >&2
exit 1

set -e
logger -t "$0" "Rebind device $1($2) to driver $3."

cd "/sys$1"
echo "$2" >driver/unbind
sleep 1
echo "$2" >"/sys$3/bind"

Sunday, November 29, 2015

Focusrite Scarlett 6i6 and Pulseaudio

Oh, pulseaudio, how do I love thee...

The Focusrite Scarlett 6i6 is a 2 Mic, 2 Line, 2 Digital input, 4 Line, 2 Digital output audio interface and is presented to the Linux machine as a 12 output, 6 input USB2.0 class compliant soundcard. Strange, but that's what it is. Now, pulseaudio, not very smart in the first place, gets confused and only ever wants to use this card as a "multichannel" output, with outputs 1/2 deprived of bass, and output 6 (typically mapped to the s/pdif output) being a subwoofer. Not useful when listening over headphones.

After looking through the non-existing pulseaudio-documentation regarding udev, and searching on the web, I've came up with a workaround. This is at least useful to use the card as a normal stereo output.

To get output on the 2nd headphone output, use alsamixer to map Master 2L and Master 2R source to PCM1 and PCM2. The same applies to the S/PDIF output (Master 3L/R).

Bug filed.

➜  ~  cat /usr/share/pulseaudio/alsa-mixer/profile-sets/focusrite-scarlett-6i6.conf 
; Based on native-instruments-traktor-audio10.conf

; Focusrite Scarlett 6i6 has 6 physical inputs/outputs
; Inputs
;  2x  Microphone/Line on front
;  2x  Line on back
;  2x  S/PDIF coaxial on back
; Outputs
;  2x  Headphone (Out1/2, Out 3/4) on front
;  2x  Line on back (Out 1..4)
;  2x  S/PDIF coaxial on back
; It's presented as a 6 input, 12 output interface to the PC, so
; we have to create some mappings. Actual routing is configurable
; in ALSA mixer. I prefer a 1:1 mapping to physical outputs.

auto-profiles = no

[Mapping analog-out]
description = Analog Outputs
device-strings = hw:%f
channel-map = left,right,aux0,aux1,aux2,aux3,aux4,aux5,aux6,aux7,aux8,aux9

[Mapping analog-in]
description = Analog Inputs
device-strings = hw:%f
channel-map = left,right,aux0,aux1,aux2,aux3
direction = input

[Profile output:analog-out+input:analog-in]
description = Analog Duplex
output-mappings = analog-out
input-mappings = analog-in
priority = 100
skip-probe = yes

➜  ~  cat /etc/udev/rules.d/99-pulseaudio-local.rules 

SUBSYSTEM!="sound", GOTO="pulseaudio_local_end"
ACTION!="change", GOTO="pulseaudio_local_end"
KERNEL!="card*", GOTO="pulseaudio_local_end"
SUBSYSTEMS=="usb", GOTO="pulseaudio_local_check_usb"


# The Focusrite Scarlett 6i6 only runs as 12 playback and 6 capture channels!
# (mappable using the internal mixer)
# Bus 002 Device 004: ID 1235:8012 Focusrite-Novation Scarlett 6i6
ATTRS{idVendor}=="1235", ATTRS{idProduct}=="8012", ENV{PULSE_PROFILE_SET}="focusrite-scarlett-6i6.conf"


Monday, November 02, 2015

Behringer "X-Air" XR18 Teardown

Here's a teardown of a Behringer XR18 (sometimes spelled X18R) Compact Digital Mixer. There's a full album with all pictures I took.

To open the device, you have to remove the rubber bumpers on the sides (which can be replaced with rack-ears). Two of the tiny screws on each side don't have to be removed, this lets the bottom shell of the case keep more mechanical strength. Then two halves slide open, an L-shaped top-plus-front-side part, and the rest enclosing sides, bottom and back side.

From Behringer XR18 Teardown

In the inside, you can find the power supply, a digital board (+ analog outputs) and a stack of three boards mainly responsible for the analog mic/line inputs. The topmost board (which I didn't remove because it's attached to the case by all the XLR connectors) mainly has connectors and the headphone amp. The middle board carries circuitry for the actual mic preamp. The bottom board has a few opamps and the ADCs for the analog inputs.

From Behringer XR18 Teardown

Power Supply

Generally the manufacturing is pretty nice and everything looks produced to tight tolerances. I could see no gaps, nothing loose. The only downside I see is the powersupply and the mains connector: The supply gets very hot during normal operation (and no devices running on phantom power were connected during the few hours it was in operation before opening the case). The mains connector is isolated with an additional layer of sticky tape, and fastened with hot glue. This looks rather sloppy.

From Behringer XR18 Teardown

Interestingly Behringer (the Music Group) designed their own supply!

From Behringer XR18 Teardown

Digital/DSP (+analog output) PCB

People are generally annoyed by the lack of support for WPA/WPA2 with the builtin WiFi access point. It's essentially a Microchip MRF24WG0MB Module

From Behringer XR18 Teardown

The USB audio interface seems to be powered by a XMOS 16L7C10 1000 MIPS, 128 kB SRAM, 32-bit multicore microcontroller and its associated Microchip USB3340 Enhanced Single Supply Hi-Speed USB ULPI Transceiver.

From Behringer XR18 Teardown

The actual user-facing functionality obviously is orchestrated by a Freescale MCIMX253DJM4A i.MX25 400 MHz, single core, ARM 926EJ-S, the network is connected with a standard Microchip LAN8720A Small Footprint RMII 10/100 Ethernet Transceiver.

From Behringer XR18 Teardown

From Behringer XR18 Teardown

The eight XLR analog outputs are fed by a CS4385 Cirrus Logic octal 24 bit DAC, the CS4272 Cirrus Logic 24 bit stereo audio codec feeds the headphone output and digitizes the line input (ch 17/18). [it's the only ADC/DAC left after accounting for the 8 analog outputs and 16 analog inputs and corresponding octal converters].

From Behringer XR18 Teardown

Routing of signals probably is taken care of by the logic inside this Xilinx XC6SLX4 (Spartan 6) FPGA. It's the smallest of the Spartan 6 family -- and while it has 216 kBit of blockram and 8 "DSP Slices" which feature multipliers, adders and accumulators, I doubt that it does any computational work. Maybe it scales the output level or so, though...

From Behringer XR18 Teardown

There's a truckload of New Japan Radio Corporation NJM4580 Dual Opamps used inside the XR18, and
a bunch of them seem to take care of driving the differential XLR analog outputs.

From Behringer XR18 Teardown

Mic/Line ADC

The most interesting chip on the ADC board (bottom-most in the three-board stack) is the Cirrus Logic CS5368 octal 24bit ADC. There are two of them, for 16 inputs in total. There are also a bunch of NJM4580 (1+1/2 for each channel), likely for a final level adjust and to drive the differential inputs of the ADCs.

From Behringer XR18 Teardown

Mic Preamp

The board with the mic preamps is rather interesting. The layout seems to be a conventional frontend using discrete transistors as first stage amplification and a NJM4580 as a differential amplifier. There are two Coolaudio V411 analog switches per channel to adjust the gain, and quite a few shift registers to provide the control inputs to the analog switches.

From Behringer XR18 Teardown

From Behringer XR18 Teardown

From Behringer XR18 Teardown

From Behringer XR18 Teardown

Connector PCB

The connector PCB is very uninteresting. One very thoughtful detail, though, is that they drilled holes beneath the XLR/TRS combo jacks. If you ever had to desolder one of those because the TIP of a TRS connector broke off you appreciate this. I did not remove this board, because it's held in place by dozens of screws on the XLR jacks.

From Behringer XR18 Teardown

A OPAMP labeled JRC8074A seems to be the headphone amplifier.

From Behringer XR18 Teardown