Cresta AS-0216 repair

The second “moving message” LED display project I started with was a Cresta AS-0216 (probably) with a defect on the left side of the display.

Cresta AS-0216 display

The characters on the left are filled up in the square block of 5 vertical lines. I got two “keyboards” with the display. So that maybe can become usefull in the future.
Since these kind of displays work with shift registers along the with of the displays, that was the first thing to take a look at.

I opened up the display from the side. The cover and pcb just slides out of the frame.
The system had a green controller board on the left and a brown board with the displays, driver transistors and shift registers.

The shift register used is a CHILONG 3100110005, a 24 bit shiftregister which is unfortunatly made of “unobtainium”. 😉

CHILONG 24 bit shift register

The bad news was that one of these chips had failed. I swapped the chip most left on the PCB with one one place to the right (installed IC sockets for both). The issue moved up 24 bits to the left, so that’s a win!

This way I was certain that the chip was faulty and that I have to look for a replacement.

A replacement was nowhere to be found, so I thought why not make one myself…

The 24 bit shift register design

I’ve started with using a breadboard and two 74HC595 shift registers in daisy chain setup. But that didn’t work so wel because the 595 uses a latching output that will push the data out on an extra clock pulse, so the data was lagging on clock pulse behind.

Since the 595 wasn’t the right IC to used, I switched over to the 74HC164 shift register, a 8 bit shift register without a latching output:

In the following video you can see it in action as soon as I turn on the power on the breadboard (the flickering of the LEDs is because of the video frame rate)
Two 164’s in series gave 16 bits extra of data! So we need three 164’s in series to replicate the CHILONG.

2x 8 bit shift register in action

The schematics and board design

Time for some designing in Eagle CAD…

Schematics

Because the board uses a 6V power supply, I had to figure out if the system wil work with 5V TTL type shift registers. Luckily the output of the CHILONG chip drives small transistors, so lowering the power rail a bit was not a problem. I simply added a diode in series of the 6v power supply to get to a safe 5.4v for the shift registers.

Also I added a startup reset circuit using a 10k resistor and 100nF capacitor to ground.

Next challenge was to design a board with the size of a 28 dip IC. I’ve came up with this:

After ordering the PCB and all parts, it looked like this:

And the end result…

There was only one challenge to put it all back into the enclosure…
The new shift register PCB in it’s IC socket was to thick, it bumped against the aluminum frame enclosure. So I removed the IC socket, soldered the board directly on the PCB, trimmed a bit from the edge of the PCB and it fitted perfectly in the aluminium case.

Text Lite mm500 keyboard design

The first time I started playing around with a display, is a mm500 from Text Lite. I bought this display without cables, software or even a proprietary keyboard. It’s a display previously used at PTT Post, the former Dutch Postal Service.

I’ve tried to connect it via RS232 and a USB to serial converter with no luck.

The next option was to see if there are keyboards available, found this image on the internet:

It used a ribbon cable that connects to the micro processor bord. It should be a matrix type keyboard, so I started measuring signals with my oscilloscope and logic analyser. This way I could figure out which lines were on the X axis and which on the Y axis.

button test

After long testing and trying I figured out pretty much all keys and functions. After that the shift+key special functions started to take shape. They appear to be somewhat different than on the keyboard display I found online.

I then translated that into a schematic and eventually in a PCB (needed one, but order minimum was 5 @ JLCPCB 🙂 )

Trying it out…

“IT WORKS!!!”

Since I wanted to keep the costs to a minimum, a keyboard case was not really an option. So I ended up having a sheet of plastic cut up on a large laser cutter.

Good enough for when we need a new text loaded on the display.
Also mixed up two keys (pauze and backspace) so maybe redo the laser cuting once more and also make a back side and screw it all together. Or even a printed sheet on top…

This display is now used in the Makerspace facilities in our town.

Rotel RCD-951: Distorted sound on one channel

On the workbench I got a nice Rotel CD player with an error in the analog output. Digital output was working fine. The right channel sounded distorted.

First let’s put in an ordinary CD and listen to the sound and view both signals on the two channels off the scope.

The blue wave form looks like an audio signal, but the yellow signal definitely does not.

Let’s see what a 1kHz signal looks like:

That is not what it should look like. Is it the DAC that is missing some bits or is it the digital signal that is feed into the DAC.

I grabbed my USB datalogger and connected a few clips to the digital input of the DAC.

How does the left and right channel look like when an 1kHz signal is played.

Digital signal of 1kHz mono

Left and right are digitally not identical because it still is a two channel recording. But the data in both the left and right channel look the same. It’s not like there are bits missing.

New DAC

The DAC in this player, is a BurrBrown PCM69AP. I ordered a new one and soldered it in.

Fixed 1kHz signal

And it’s working again!

Output stage

Another view on the DAC with the output stage. High quality Rubycon Black Gate capacitors.

Final test and ready to go back to the owner.

Music Hall Trio CD Receiver: repair & review

This time on my workbench is a CD Receiver of the brand Music Hall.

Music Hall produces HiFi / High-End-ish audio gear. For those who do not want a huge stack of separate devices, Music Hall introduced this entry level All-In-One CD-player, Tuner and Amplifier back in 2007.

I came across this device on a second hand trading website. I have had heard of Music Hall before and I new they make quite well build devices. So that would be a nice way to take a closer look and have it restored. I haven’t decided yet if this all-in-one will stay on my workbench shelf for testing purposes and replace the Philips 22AH209/309 combi and Grundig CD-909 or that I will sell it again.

Lets start with a first impression:

Black case with a thick aluminium front.
A three-in-one… hence the name “trio cd receiver”, I would say 🙂
Inside the Music Hall

B&O Beosystem 2500 (type 2606): CD not recognized

On my workbench is a B&O Beosystem. The symptoms are: CD is not recognised, CD won’t start spinning.
In most cases these machines have worn belts for the glass doors, CD cover and tape deck. Most also have doors that need adjustments. This one was no exception, but I discovered that the CD was also not working. Let’s see what is going on:

To understand where to look for the defect, we need to know the steps it is going through upon initialisation. The steps are for most CD players the same, except for the start of the spindle motor that turns the CD. In some cases the motor is started in the early phase, but mostly when a cd is “seen” by the laser. That is: laser light is reflected onto the CD and picked up by the laser pickup sensors.

The steps are as follows:

  • When CD tray/door is closed
  • Move Laser to starting position*
  • Turn on laser light
  • Start focussing sequence (Depending on system, repeat this several times, sometimes move the laser tracking position.
  • When CD is detected, start spindle motor.
  • Look for lead in track and read TOC (Table of Contents)

*) Linear tracking and radial tracking servos move the laser to the start position when this process is started. That is not the case when motor/gear tracking systems are used. With that system the laser is mostly moved to the beginning of the CD upon power on, upon CD tray close/open and upon stop command.

Focus sequence not working

With the knowledge of these startup steps I started to test the system with and without a CD in the player. The spindle motor was not started and the CD was not recognised.
Without a CD in the player, I could see whether the laser tries to find a CD and tries to focus on it. That wasn’t happening at all, also the laser was not moved to the center of the CD. So that makes sense that the CD is not detected. I know that this Philips based system uses no feedback on laser light “on”, so it would try to focus anyway regardless of whether the laser is on or not.

After I mechanically loosened the CD cover, I could safely try to confirm that the laser in deed was working. Gently because I didn’t want to damage my eyes. I had a quick look at the laser from a distance and from an angle while keeping the laser in the shadow of my hand. A little faint red dot was enough for me to see and know that the laser was fine.

Measurements

I started to see whether there is any voltage on the tracking and focus servo, there was nothing on there.
Next I checked the schematics to see how the tracking and focus servo are driven:

Focus and Tracking servo schematics

There is one half of a dual opamp used for driving the focus servo, the other half for the radial tracking servo. I first checked the positive and negative power lines, both were not present. In the schematics the negative power line is pictured on the focus servo opamp, the positive is on the tracking opamp. Both power lines have 1 Ohm safety resistors. Those were both defective. That was not directly visible on the resistors as shown in this picture:

As a precaution I removed and replaced four of those resistors. Then it showed that at least one had been a bit to warm.

New resistors installed

And the CD is working again!

To see whether the laser current is still within specifications, I looked up the procedure in the service manual. The voltage should be measured over R16, which is a SMD type resistor.

The voltage was perfectly within specs, so no need for adjustment.

MHZS CD33F Modified Tube CD-player (recap/remod)

Recently I’ve bought myself a Chinese MHZS CD33F tube CD-player which was in working condition, except for the remote. The remote was already taken apart and the CD-player itself had already been modified by the previous owner. This modification was based on several articles found on the internet.

What he intended to do was the following (if I can recall it all):

  • Replaced the output electrolytic capacitors with 10uF ClarityCap film capacitors
  • Remove the opamp and bypassed it
  • Removed the electrolytic capacitors in the audio path between DAC and opamp/tubes.
  • Removed blue LEDs underneath the tubes
  • Changed a few resistors to compensate for the absense of the opamp

Despite his pretty low level of knowledge and skills he actually did manage to modify the CD-player and it was working! Unfortunately it was not the best attempt of modifying a CD-player, that I will show you further on this page.

Read more MHZS CD33F Modified Tube CD-player (recap/remod)

ReVox B225 CD Player: Distorted sound

Today on the workbench: A beautiful designed ReVox CD player. Based on one of the first CD players of Philips and uses a CDM1 transport and two mono DACs TDA1540p.

This service friendly machine didn’t come in for a little maintenance, no it was brought in with a common symptom for a TDA1540 like CD player: Distorted sound.
To me this sounded like an easy fix, because I’ve seen this error a lot and it’s commonly cause by a power supply voltage that is to low. In most cases that is the -18 V or -17 V volt line, created by a voltage doubler circuit.

In order to know what we are dealing with, I opened up the device, did a visual inspection, found nothing disturbing and connected it to my test amplifier and have a test run.

Let have a look inside…

It appears there was a bit more going on. The right channel was in complete silence and the left channel experience a heavily distorted audio sound.
So that could be two defects in one. I decided to have a look at the distortion first, the dead channel next.

I’ve measured all the voltage rails and checked it with the service manual / schematics. All are OK and within specifications. So there goes the theory of a bad negative voltage line.
Since there is no digital out, I couldn’t directly rule out half the CD player. Luckily the device was so well build, I could easily take out the DAC board and put the scope on the digital signals.
At first they seem fine, there was a signal on all the required data and clock lines. But a scope isn’t really suitable to view multiple digital signals, even with a 4 channel scope. That’s where this device comes in:

It’s a cheap 8 channel USB logic analyser that can show 8 channels of digital data with a theoretical maximum of 24MHz.

The data signals that are the input of the DAC board is looking fine, all the signals are there and all looks very much like the signals in the service manual. This signal is then fed through the digital filter and 4x oversampling IC SAA7030.
So, lets see how the signals look like after they are processed by that chip.

SAA7030

What is it that this chip does? Well, it does the oversampling, so it will multiply the clock frequency by 4, does the digital filtering and creates new clock and strobe signals.
So basically what is expected to see at the output is that it will have quadrupled the data bitstream for left and right (not exactly, because of quantisation, etc) and created quadrupled clock and strobe signals.

I’ve recorded these signals (except the main clock, because I already ran out of the 8 channels on the logic analyser) with two different sources:

  • 1 kHz calibration signal, which is a function of this CD player
  • 1 kHz test signal from a test CD (Burosch)

Here you see the 1 kHz calibration signal with a Data clock in, Strobe (which indicates the start/end of a word) and two data channels.
What we would expect at the output is a repetitive data stream, like the output of channel L, but not a constant bitstream like channel R.
But that’s not all, the Data Clock out is not constant and is very slow and not constant.

With the test CD as a source the left and right channel may vary, but that is not uncommon. The outputs are similar as with the calibration signal.

Conclusion

That the right channel does not give any signal makes sense based on what we see here. So the cause of that could be this SAA7030.
The heavy distortion is likely caused by the inconsistent signal on Data Clock out. So it looks like both errors are caused by the same circuit: the SAA7030.

Since the SAA7030 is a very independent chip with not that many surrounding components like capacitors or oscillators, it most likely this chip is defect. Let’s have closer look:

 

After I soldered the SAA7030 out of the PCB there was definitely signs that it got very warm.

I’ve installed a IC socket and put in another SAA7030 which I salvaged from a CD104 PCB. Installed the DAC PCB and there was sound from both channels again and the distortion was also gone.

 

Philips FA-91: One channel no sound

This time on the workbench, a Philips FA-91 midi set amplifier, which is part of the FW-91 set.

Philips FA-91

The problems with this amplifier: Only sound from one channel, other channel is completely silent.

At first I expected blown output transistors in the power amplifier section, so I started to disassemble the device to get to the power amplifier, which is not an STK-type amplifier, what some would expect with small midi sets, but this is a conventionally build class A/B amplifier. Definitely a higher quality than other midi sets.

I measured the transistors quickly for short circuits using the continuity tester on the multimeter. All seemed fine. So powered up the amplifier using a series lamp. I provided a test signal to the amplifier and discovered that there was only audio going into one channel, the other was complete silence.

Pre-amplifier

So that looks like a pre-amplifier problem.
Time to take a look at the service manual, which was a bit hard to find. It is registered as “70FA091”.
The error becomes a bit more challenging since all audio is controlled by several audio ICs.
First an audio selector LC7821, then an equaliser (LC7520) and balance control (LC7536), analog volume control and finally to the power amplifier.

When going through the menus of the equaliser, I discovered that the equaliser was set to -10db for all frequencies. After that I had a look at the balance setting, which is also controlled by the equaliser. This one was also set to one channel only.

This can only be caused by memory settings, not deliberately set by a user, so a factory reset is a good start.
To do that, we need to enter the Service Mode:

Entering Service Mode

In the service mode we can reset (CLEAR) the EEPROM.
Default parameters will then be stored in memory.

CLEAR the EEPROM

At the same time we can also test the EEPROM…

TEST the EEPROM

That gave a PROM – DEF error message. So it seems like the EEPROM need to be replaced…

Replace the EEPROM

The EEPROM is a small 8-pins IC that is located on the front PCB.

Disassembly of the amplifier
EEPROM removed
New EEPROM installed

After a reset and successful test of the EEPROM, the amplifier is fully working again. 🙂

Myryad Z140 Tuner: no display

Today on the workbench a special brand, Myryad. It’s a FM Tuner with RDS.
The defect is that the display does not illuminate anymore. The tuner does work and produces sound, so it’s only the display that is not working somehow.

No light what so ever, lets look what’s inside

Nice layout of the PCB with sections of the several block from a block diagram, like the VFD (Vacuum Fluorescent Display) power supply

I measured the voltage of the VFD power supply that is usually an AC or high negative voltage and is connected to the outer most pins of any VFD display. The meter reads 2,7V.
For my tast to low for a display like this. Because there was no voltage indication anywhere and no service manual or schematics at hand, I didn’t really had anything to go with.

As I took a closer look at the power supply I discovered something that I already was expecting.

One of the two 220uF capacitors didn’t look so healthy anymore. I took it out and it was very obvious that this capacitor is gone.

The capacitor completely lost its capacity. For what I can tell without the schematics this is a voltage doubler circuit. The first capacitor in this circuit will fail sooner or later because of heavy use.
Just to be sure, I replaced both 220uF capacitors.

Reassembled the PCB and fired the tuner up. Done!