I thought it was about time to write here about my DVD player and my CD player.
My DVD player is a fairly nice Cambridge Audio device, and when it started playing up I didn’t really want to throw it away and replace it with a cheap and tacky model from Tesco. But it was fairly obvious what was going wrong. It would occasionally crash on power-on, and this was getting progressively worse. After some time it even refused to recognise some previously working DVDs. This immediately suggested to me that it was having problems with the power supply. So I opened it up and immediately confirmed my suspicions when I discovered that two of the smoothing capacitors on the output side were popped. A quick replacement later, and all is great.
After all the recent success of capacitor replacement, I thought it might be wise to have a go at fixing my old Marantz CD53 CD player. It’s a rather nice CD player which I’ve had for about a decade now, but I’ve been using its digital output as the analogue output had severe distortion on one of the channels that would kick in after a while.
So I pulled it apart, hoping to find a popped capacitor on the audio output. No such luck. What was worse was that I plugged it all in and used it for some time without being able to reproduce the problem. But as soon as I put the lid on and put it in a more permanent position, it started playing up again.
Well after pulling the lid off again everything seemed fine once more, so I heated it up with a hair dryer. Lo and behold, distortion. Cooling it down with a fan made the distortion go away again. Great, a reproducible problem.
Better still, I found a scanned copy of the service manual on the internet.
Some poking and prodding with a scope showed that the problem was in the mute circuit. This circuit is used to silence the output when the player isn’t actively playing music. This is obviously in Stop/Pause mode, but also it is briefly used between tracks. The line which was enabling the mute transistors was floating at a temperature-dependent voltage, way above what it should be. When the temperature (and hence voltage) got high enough it was partially enabling the mute transistors, leading to heavy distortion.
I tracked the temperature dependence down to a PNP transistor which was pulling this line high against a resistor. The voltages on all of the pins were pretty much the same on the left and the right channel, save for the output, and holding a hot soldering iron next to this transistor changed the voltage. So I concluded that this transistor must be the faulty component and ordered a replacement.
When the replacement arrived I duly soldered it in and tested the player again. The fault remained. I was mistaken in my diagnosis.
Quite a lot of head-scratching and measuring later, I realised that the supply voltage to this circuit was actually too high, and as the base of the PNP transistor was being fed via a resistor, the voltage drop across the resistor and hence the base voltage of the transistor was going to depend on the supply voltage. After some careful reading of the circuit diagram and a fair bit more poking about with a scope and a multimeter I concluded that the only possible thing that could have gone wrong here was the zener diode in the power supply for the mute circuit. This supply was a simple low power voltage regulator which used a single zener diode as a voltage reference.
So I replaced the zener diode. Twice. The first time round an unfortunate short circuit caused by an adjacent pin poking through a piece of insulated wire resulted in the diode letting out the magic smoke. The second time, I double checked everything before applying power and this time it worked. No distortion, no floating voltage, fixed.
The one final issue was the CD tray. It was no longer able to properly open/close. My initial idea was that dusty storage had hampered lubrication. There was indeed a fair bit of resistance in parts of the travel. After some experiments with silicone grease, I discounted that as the cause of the problem. It turned out to be simply that the drive belt had worn and was no longer providing sufficient tension. I replaced it temporarily with an elastic band from some christmas cracker toy, to allow some time to order a replacement.
The replacement has now arrived, but for time being the elastic band is holding.