LCD Screen for Tektronix TDS-544A

Here a story how I replaced a CRT screen with LCD one.

Thanks a lot to the document named "TDS 520B Mod CM Digitizing Oscilloscope Component Service Manual", it reveals some details on scope's guts.

After my lovely CRT screen became a piece of glass, I started to look for alternatives our Chinese brothers can offer.
Not too many.
According to the Service Manual, the scope resolution is 640x480. But the problem is that nowadays it is not very easy to find an affordable LCD with such native resolution. 6.5" 640x480 screens are mostly used in cars and industrial equipment so they are not as cheap as widescreen 800x480 ones. For example, 4:3 screens start from $150 when 16:9 start from $60 with controller.

So I bought AT070TN90 7" 800x480 LCD panel with a controller for less than $70 with shipping from a US seller on eBay.
Luckily controller has 4:3 mode so LCD can display 640x480 picture without any pixel transformation issues. But the bad thing about 4:3 mode is that controller fills unused areas with blue. It sucks. I need to cover it with something. But I have to make a front panel for this screen anyway.

Finally I got new screen working. It took some time to complete. So here what else I needed:

• anodized aluminium panel (eBay)
• Radioshack perfboard
• 12V DC/DC converter (eBay)
• 1-ft VGA cable (eBay)
• aluminium bar to make nuts
• #4-40 tap
• some wires, screws, and washers
• epoxy glue
• nylon PCB stands

AT070TN90

Realtek LCD controller board

Frame was cut from anodized aluminium:

Last test

Radioshack perfboard, 12V DC/DC converter and LCD controller with 1-ft VGA cable

Where to get some power for the LCD

OSD menu buttons. LCD power wires connected to J574 are on the top.

 Finally I put it all together:

The resutl

But for some strange reason some pixels at the bottom of the screen are missing. No way to correct it using OSD menu, but it doesn't affect the signal graph area.

I'm satisfied (but CRT screen was much better). It is a nice feeling when you bring a good device back to life.

Digital Amplifier: First Run

It is time to turn my first amplifier board on.
Here is my first test setup:

Don't blame me for the power supply, I just scavenged transformer and capacitors from an old Sony tuner. My main concern at the moment is to test if my amplifier can produce any sound but not a sound quality which will be affected by such a power supply for sure. I will use a switching power supply soon, but I have to find a suitable one first.

It took some time to setup my BeagleBone to play music on MiniDSP USB Streamer. I will post details on that later, when I settle it out. Someone would ask me why not to use Raspberry? At the moment it doesn't support USB 2.0 audio devices well.
MiniDSP USB Streamer might be not the best in a family of USB->I2S converters since it doesn't have dedicated low-jitter clocks but it has 8 I2S channels. I'd like to test what is better - to have crossover on 5548 chip or to have it on BBB. I'll by better converter later if 5548 wins.

So what about the sound?
First pictures from the scope scared me...
Look at this (sorry, my digital scope just died):

Text is not readable on this picture but it shows PWM frequency on the output: ~1.25V at 384kHz.
Tried to switch from AD mode to BD ant got things look even worse:

What a disappointment!

But guys from TI forum told me it is expected and not a big deal. Also I found this video saying it is OK. I need to build a simple RC filter to check it out.

So I connected a junk speaker.... And spirits of electronics let me to hear the sound!

Then I found a couple of things I'm not very happy about:

• Inductors heat up to 50 degrees Celsius even in idle mode (but PWM is on)
• Electrolites heat up to 43 degrees Celsius. This temperature is far from critical but I'm afraid it would shorten capacitors life.

And a good thing: amplifier can easily output 25W RMS for a while without a real heatsink.

What about sound?
It sounds. My junk speaker did its best and I didn't hear any suspicious clicks, pops or any saturation that would be different from the same speaker powered with analog amp.
To test it in a real life I need to take the following steps:

• assemble more boards (at least one since my speakers are still two-way)
• put them on a real heatsink
• put ceramic capacitors right under electrolytes to reduce ripple current a bit
• calculate biquad coefficients for crossover
• write some python code to initialize TAS5548 with crossover coefficients
• buy a suitable switching power supply with switching frequency far beyond the audio band

still a lot of things to do... Why do people have to sleep?

TEK Scope screen is dead

Bad thing just has happened!
I was watching strange signal shape coming out from my just assembled amplifier board when picture on the scope began to jump and scary pops and clicks started to come from my nice scope.
Thunderbolts inside a scope is not a very good sign, right? So another challenge is going to begin.
To be honest I'm not a fan of scope repairing... But high voltage transformer has died tragically.

Couldn't find a transformer like that on the Internet, found just a couple of LCD kits for almost thousand bucks. Well, I believe I can do it for less because the scope has a VGA output and it should be possible to connect a regular LCD to it.
I'm afraid a new LCD would not have better picture than CRT had, but I don't have a lot of options.

I know some people who have such color Tektronix TDS scopes have dead CRTs, so maybe I can find such a folk and give my CRT to him...

Soldered: TAS5631B amp

Here is my first amplifier board completed:

I put SMD inductors vertically while they supposed to lay on the PCB:

I have two good reasons for that - first of, it saves a lot of very limited space and the second - if the warm up it would help them to cool down (questionable, but makes me feel better).

Looks fine. Especially having in mind it is my first SMD project. Let's connect all my stuff together and check if it is able to produce any sound...

TAS5548 PWM Modulator board

I started with PWM modulator. What is the reason of soldering the whole amplifier board if PWM doesn't work?
And even more, my setup requires only one PWM modulator board and I have ten available. Plus 5548 has its pins on two sides only which probably can make the soldering a little bit easier.

Well, it was not that scary,
I'm not a soldering guru and I was scared about SMD components, especially chips with 0.5 and 0.6 mm pitch. But it turned out to be not such a big deal. Thanks to Steve from EEVBlog for his video tips on how to solder smd chips.

Not bad for the first try

The only thing I really struggled with is crystal oscillator. It has its pads underneath and I couldn't place it well. But it works and I don't want to re-solder it.

Here is my test setup:
BeagleBone Black + MiniDSP USB Streamer + my PWM modulator:

It is a whole different story how to setup BBB...
Adafruit has python libraries to drive I2C bus so here my first python program ever:

import Adafruit_BBIO.GPIO as GPIO
import Adafruit_BBIO.PWM as PWM
from Adafruit_I2C import Adafruit_I2C
 
def i2cPrint32(x): 
 bytes = i2c.readList(x, 4)
 print "%X %X %X %X" % (bytes[0],bytes[1],bytes[2],bytes[3])

i2c = Adafruit_I2C(0x1A) # see notes about I2C address below

print "5548: 0x%x = %X" % (0x01, i2c.readU8(0x01))

### UnMute
print "============="
print "CTRL:"
ctrl = i2c.readU8(0x03)
ctrl = ctrl & ~0x10
print hex(ctrl)
i2c.write8(0x03, ctrl)


### VOLUME ###
print "Master Volume:"
volume = [0,0,0,0xAF]
x = 0xD9
i2c.writeList(x, volume)
bytes = i2c.readList(x, 4)
print "%X %X" % (bytes[2], bytes[3])

it umutes the modulator and sets some non-zero volume.
And here is the output:
5548: 0x1 = 4
=============
CTRL:
0xa0
Master Volume:
0 AF

After unmute I can see 50/50 PWM duty cycle:

And when input signal is applied:

So looks like it works. On first try!

But there was a caveat with 5548 I2C addresses. My BeagleBone didn't want to work with 5548 chip at the address from its datasheet - 0x34/0x36 (depending on jumper). Luckily Linux has a command to scan I2C buses so my device was found in a couple of minutes (googling):
root@arm:~/py# i2cdetect -r 1
WARNING! This program can confuse your I2C bus, cause data loss and worse!
I will probe file /dev/i2c-1 using read byte commands.
I will probe address range 0x03-0x77.
Continue? [Y/n] y
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- -- 
10: -- -- -- -- -- -- -- -- -- -- 1a -- -- -- -- -- 
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
50: -- -- -- -- UU UU UU UU -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
70: -- -- -- -- -- -- -- -- 
I put this address 0x1A into my python script and voila!

So it's time to build an amplifier board.

PCBs arrived

Oh my! They are gorgeous!
I must think this way because they are my first SMD-PCB-babies. And I have ten of them, so I have enough spare ones if I mess one up.

Right away I found a wrong label: "5431 Amp v.1.0". OK, let's put it this way - "it was made intentionally to confuse enemies".

Scope Cleaning

All capacitors were removed with "twisting" method. I don't have hot tweezers to desolder large SMD capacitors and I'm afraid I would damage more pads if I used a single soldering iron. Almost all the traces left in place, I lifted just two pads which is not bad having almost hundred caps and all these leakages which could damage everything around.

After all capacitors were removed I had to clean all the acid residue. I found a lot of PCB cleaning stories. Seems that most popular way is to use Simple Green general purpose cleaner.
Simple Green + toothbrush + baking oven at about 150F made it virgin clean.

All capacitors are replaced with new low ESR leaded ones.

Before caps replacement

After all the caps are replaced. Still have Acquisition board fail...

Error log shows a mysterious error

I still have failed acquisition test, but scope seems to be working. 

Unfortunately I couldn't find where the problem can be. The only suspicious area is around U1602, where were a lot of corrosion. I replaced this opamp with a new one but could not find where one of the legs connected. Seems to be a broken trace. I would be happy to fix it but could not find where to connect it. Need to try to find out...

It  looked not very nice...

But I think I can live with non-calibrated scope.