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.


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.


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.


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.


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.


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.


At the same time we can also 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!



Philips CD880: Buttons unresponsive

One of Philips flagships… the CD880 with an impressive selected DAC chip: TDA1541A S1 (Single Crown).

Its malfunction is that most switches on the front don’t work anymore and that open/eject is functioning erratically. Another thing is that the lightbulbs for the illuminated bars need to be replaced.

The CD open/close issues are caused by very bad conductance of the switches indicating the position of the drawer. An easy cleanup solved this issue.

The same is the case on most of the operation buttons on the front panel. These have very high resistance when pushed in all the way.
First is to disassemble the player and disconnect the front panel.

Next is to disassemble the front panel

Eject switch and left site light bulb.

Display needs cleaning, so that can be don as well.

The glue of the red display filter came loose.

Bug check: failed

Found a bug in the cd player… a real one but it had died a long time ago.


So there are a lot of switches on the front PCB, several measured high resistance.
Best solution is to replace all those switches.

I bought a bag of those. The height of the switches is very important. This cd player requires the lowest button height of 4.3 mm.

The desoldering iron, flux, cutting pliers and flat screwdriver and piece by piece the PCB lost their faulty switches.

After a lot of work the new switches were in and all was working again.


Sony CDP-227ES: Misaligned laser lens KSS-151A

An old Sony CD Player but very well reserved, came in the original box. Had been in shop storage for a very long time.

In mint condition, but unfortunately it does not read any CDs, so that’s why it’s here on the workbench.

Always happy to see good Philips components in CD Players… 🙂

It’s a double TDA1541A-R1 setup surrounded by conventional (no SMD) capacitors.

Of course Sony would use one of their own laser units, this time a KSS-151A without the geared motor for tracking, but lineair servo controlled.

Issue: Missing voltage, wobbly components…

Lets disassemble some…

Cause: Soldering/PCB

The soldering on this cd player is very poor as well as the PCB and copper traces on it.

Transistor was disconnected which caused the missing voltage.

The connecting PCB between laser unit and main PCB

Bottom side with a bunch of bad solder joints.

Issue: Lens alignment

Unfortunately the fixes of the soldering and PCBs did not (yet) bring the cd player back to life.

Further investigation on the mechanical side I saw that the lens of the laser unit was not exactly straight above the laser.
Because a new laser is not available anymore, a donor cd player was the best option.
This was found in a Denon cd player, but the laser assemblies were not really alike.

Sony assembly

Denon assembly

The laser unit as to come off, including those fragile orange flex cable solder joints.

On the left the “old” Sony laser unit, on the right the “new” Denon


No more pictures. I haven’t taken a picture of the reassembled laser and running cd player.