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Sony WM-D6C Walkman Pro DC-DC converter repair

This story starts with the long drive from Cambridge, UK to Warsaw, Poland. I like to be able to listen to music to while away the hours in the car, and I decided to use cassettes. Why? Our car radio is faulty, so much of the time there’s hardly anything to listen to. It has a CD player, but almost all of my CDs are stored away, having long since been converted to MP3s. There’s a handy AUX IN jack, so I can plug in my smartphone. But there’s simply no way to operate a smartphone without looking at it, and I’m not taking my eyes off the road at Autobahn speed.

My solution? Cassettes! I’ve got lots of them, generally high quality recordings, which I’ve never digitised, so they’re not stored away. They’re easy to operate with one hand without looking at them, too. But the car has no cassette player. Sorry, had no cassette player. A little judicious eBay shopping got me a Sony WM-D6C Walkman Professional in immaculate condition for a somewhat lower-than-average price because it didn’t work.

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The WM-D6C is widely acknowledged to be one of the finest portable cassette machines ever made. It’s pocket-sized, if you have large pockets, and has sound quality and features that rival full-sized hi-fi cassette decks. It can also record, which is extremely unusual for a Walkman-format machine.

This particular example is a very late one. It doesn’t have the posh amorphous head of the original models, but the electronics are mostly easy-to-access surface-mount components rather than the gruesome bird’s nest of wire-ended parts that the early models had. I remember servicing an early one for a student radio station and it wasn’t a lot of fun. I think this one must have expired quite early in its life and been left on a shelf, because there’s no perceptible head wear and the casing is unmarked.

Putting batteries in and pressing play resulted in the ‘BATT’ LED coming on but absolutely nothing else. No clicks in the headphones, no motor whirring, nothing. Fortunately the service manual is readily available on line. ‘Supplement 4’, dated 2001, accurately describes my example.

Browsing the circuit diagram revealed one of the secrets of the WM-D6C’s excellent performance. Most Walkman-type cassette machines used a pair of ‘AA’ cells, so all the electronics had to run from just 3 volts. That’s common enough in 2018, but back in the day it was a real challenge, so the capabilities of the motor and electronics were compromised. The WM-D6C not only runs from four ‘AA’ cells, for a 6 volt supply, but does even better. Almost the first thing it does is step up that supply to about 11 volts. That rail then runs nearly everything, including the motor and audio circuits. A nice generous supply voltage is a good start for getting top performance, especially with 1980s-era technology.

A quick prod with the multimeter revealed the problem. This boosted supply was entirely absent. Seeing as how it powers most of the machine, that would explain the lack of results. The supply rail comes from a much-feared component, the DC-DC converter (CP304). Inscrutable in its little screening can, labelled ‘SONY’ on the right hand side of the picture of the Walkman’s entrails below, it’s often considered unrepairable.

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The service manual includes a somewhat misleading diagram of its innards. I think the diagram is actually back-to-front, showing the output and input swapped, because that’s the only way it makes sense. The NPN transistor makes a boost converter in a variation on the classic ‘joule thief‘ circuit, and the PNP one with works with the zener diode to regulate the output by depriving the switching transistor of bias if the output voltage rises too high.

WM-D6C DC-DC converter

The converter wasn’t too hard to remove and dismantle, given reasonable desoldering tools and a powerful iron to unsolder the can. Here’s what’s inside. There are components on both sides of the board, and a certain amount of grey silicone which is easy enough to peel off. Back in the 1980s this would have seemed intimidating in its compactness, but it’s easy to work on given modern tools.

Finding the fault was a case of looking for the ‘usual suspects’: there were two tantalum bead capacitors sitting there looking guilty. The one on the input was short-circuit, which had killed off the 22uH inductor connected to pin 3, the large green component on the right.

I replaced the faulty components, using a higher-voltage-rated tantalum and a ceramic chip in parallel to replace the capacitor, and a surface-mount inductor with bits of wire soldered on to it. The values aren’t very critical and I just used what happened to be lying around. A quick test, giving it 6V from a bench power supply, revealed a healthy 11V or so at the output.

After reinstalling the converter and reassembling the machine (watch out for the little ‘speed tune on/off’ knob at the back) it worked! It shows signs of having had attention from the phantom twiddler. The head azimuth adjustment screw was tightened up, but good quality sound returned when it was properly adjusted. The peak level meter seems rather unenthusiastic so may need adjustment, and I haven’t checked the recording bias yet. There’s also a forest of little surface-mount electrolytics waiting to dribble corrosive ooze all over the PCB, but that’s a job for the long winter evenings. For now, it’s working. Being able to play cassettes has turned out to be unexpectedly useful. We rediscovered a tape of nursery rhymes from Domowe przedszkole, a classic Polish children’s TV programme, which granted us peace on a long trip recently!

Solidlights new LED upgrade

Repairing a floppy disc drive

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Why on earth would anyone want to repair a floppy disc drive? It’s quite a while since most of us bade them good riddance and started using USB sticks and Flash memory cards. However, I still use floppies from time to time, mostly with my trusty BBC Micro which still sits in the corner of the workshop.

Recently I was asked to recover some documents from some old 5.25″ BBC Micro floppy discs. The documents themselves were in an unusual format, about which more another time, but the first step was to simply get the data off the discs. The discs were 80-track ones, and I have a pair of Chinon FZ-506 80-track drives for the Beeb.

Out of all the various Beeb drives I’ve had over the years, I’ve kept these two because they’re housed in a compact casing with a mains power supply, and they have handy 40/80 track switches on the front, not hidden round the back. One drive has always been a bit reluctant to start spinning, but for occasional workshop use that wasn’t a problem. I’d got in to the habit of just opening and shutting the door a little which would kick the motor into action. However, when it came to intensive use backing up these old discs, which needed both drives, the failure to start became a real pain. I didn’t have a spare drive, and finding another one (especially in Poland) isn’t easy these days.

My curiosity got the better of me and I decided to open up the drive and find out what was wrong with it. It wasn’t hard to take apart and I soon had the motor revealed. Here’s a photo of it sliced into its component parts.

It’s a ‘pancake’ motor, so called because it’s (nearly) flat. The shiny silver bit on the left is the turntable which drives the disc, and it sits in a bearing. The next layer is the circuit board containing the windings and controller circuitry, and below that is the rotor which is a multi-pole magnet on a steel disc.

The motor is controlled by a Mitsubishi M51785P motor controller chip. The chip’s data sheet revealed that the motor has three phases, each of which has a coil to drive the rotor round and a hall effect sensor for feedback. This particular one is arranged with two coils per phase, but occupying 6/7 of a revolution, so the motor goes more slowly than the chip is driving it. At least, I think that’s what’s going on. Here’s a closeup of the circuit board. You can see the six coils, and the coloured wires I soldered on to measure things while the motor was running. Because of the way it’s built, it’s impossible to access most of the circuit board while the motor is assembled.

The controller chip seemed to be doing all the right things: its oscillator was running, and the outputs to the coils were doing sensible things. The coils themselves were all undamaged and measured the same resistance as each other. But I noticed something odd about the hall effect sensors. There are three of them, HG1, HG2 and HG3. I noticed accidentally that if I shorted together the two wires taking the output of HG1 to the controller, the motor still ran but sounded very rough. Not surprising. The same happened if I shorted the output from HG3. But shorting the output from HG2 had no effect at all. Aha! Only HG1 and HG3 seemed to be having any effect on the motor. I swapped HG1 and HG2 just to see what would happen, and the fault moved to HG1. That proved to me that I had a faulty sensor, not a faulty chip.

Where to get a replacement sensor, though? This drive was made some time in the late 1980s, and I couldn’t find hall effect sensors in today’s electronics catalogues which would fit mechanically and electrically. I had a rummage around the workshop and found a scrap 3.5″ floppy drive. A squint at the circuit board revealed a suspiciously hall-effect-looking device of the right shape and size nestled next to the spindle rotor, used for index sensing. Well, it had to be worth a try. I extracted it and fitted it to the 5.25″ drive in place of the faulty one.

Success! The motor now ran more smoothly, and shorting each of the hall sensors in turn had roughly equal effects, so they were now all working. Best of all, the motor started reliably every time. Interestingly it wasn’t as quiet as the other drive, but I suspect the scavenged hall sensor is optimised for magnetic fields from the side rather than the front, given how it was mounted, so it’s probably not perfect.

The last job was to realign the head slightly, because this drive was a bit fussy about reading some discs. I found a disc that it struggled with but that the other drive would read every time, and tweaked the position of the head stepper motor each way a little until this drive read that disc reliably. You can see in this photo that the stepper motor has elongated mounting holes, so it’s possible to loosen its screws (there’s another one just out of shot to the right) and turn the motor a few degrees to adjust the position of the head.

After all this work, the drive read all the discs I asked of it without any problems. I hope it’ll be OK for the next decade or two.

Denon DVD-1720 DVD player power supply schematic and repair

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A couple of weeks ago, my wife and I wanted to watch a movie. I went to put the DVD in the player, and was disappointed to find that the machine was dark and didn’t respond to any of its buttons. It was broken.

It’s hard to say that a DVD player is really worth fixing. This one, a Denon DVD-1720, is about 7 years old, and wasn’t expensive when it was new. However, I was curious to know why it had stopped working, and that alone was reason enough to delve inside.

Considering its low cost (I think it cost about £100 when new) it was very neatly constructed. Removing a few screws allows the top to come off (careful of sharp edges) and the plastic front panel just unclips with a bit of gentle persuasion, revealing the guts.

Before we go any further, I have to insert a health warning. Dealing with DVD player power supplies has health risks. There are dangerous voltages present inside. If you don’t have the right equipment or don’t know what you’re doing, you can get a nasty surprise, a terminally broken DVD player, a serious injury or be electrocuted. None of those are fun, especially the one which results in death.

Almost all the electronics are on one big motherboard, apart from a few buttons and lights at the front and the fiddly digital stuff to do with the actual DVD reading, which sits on the green board on pillars in the middle. Undoing the screws from the green board, a few on the motherboard, all the ones on the back of the machine, and unplugging three connectors, makes it possible to wiggle the board free. It’s a bit of a squeeze and the board is fragile, so don’t blame me if you break it.

The power supply is at the rear right corner. Denon have kindly marked in white the area with dangerous voltages in it.

There’s not much to it. It’s clearly a switch-mode flyback converter. The transformer is much too small to be an ordinary mains transformer, and there are no inductors on the secondary side. I did the obvious checks – the fuse wasn’t blown, and the three big pale blue safety resistors measured OK. All the diodes dioded too. Time to put my reverse engineering hat on and dig deeper.

The PCB is nicely marked with the component identifiers on both sides:

so it wasn’t too hard to draw out the circuit diagram.

It’s a pretty elegant design, using only three transistors. The circuit appears to be a blocking oscillator centred around Q1001 and the transformer. At startup, Q1001 is allowed to conduct, and current starts to flow in the transformer primary. At a certain current, the transformer’s core will saturate so the current can’t increase any more. The voltage induced by the changing core flux in the feedback winding at the bottom of the transformer will suddenly drop, creating a pulse which, fed via C1029 to the gate of Q1001, switches it off. At that point the energy stored in the transformer core has to go somewhere, and it finds its way out through the secondary rectifiers into the outputs. The basic scheme is very similar to the Joule Thief, a simple blocking oscillator for driving LEDs from batteries.

The two transistors Q1003 and Q1008 seem to be involved in regulating the supply. Q1003 can cut off the gate drive to Q1001 in response to three things: the current in R1001 getting too high, the optocoupler IC1001 conducting too much (which indicates that the output voltage is too high) or Q1008 stopping conducting. The latter seems to be a way of shutting down the supply if its output voltages get low, or it might be a cunning standby mechanism involving some more stuff on the secondary side which I haven’t investigated. C1032 seems to be there to make sure that the power supply starts up.

I powered the bare motherboard from an isolating transformer (don’t try this at home, folks, unless you know what one of those is and how to use it) and measured some things. There was precisely no activity going on at all, apart from 300V on the reservoir capacitor C1004 and Q1001’s drain, as expected. I first laid the blame on the little electrolytic C1032, since they’re the most unreliable components in any modern electronics, but tacking another 10uF across it didn’t help. Holding my breath and briefly shorting C1032 didn’t bring anything to life, either.

Then I measured some of the DC conditions. Everything around Q1008 was OK, with R1096 and R1034 merrily feeding electrons into Q1003’s base and keeping Q1001 switched off. High-value resistors are the next most suspicious components in a circuit like this, especially when they’re teeny-tiny ones. I checked around the 1.8 megohm R1005 and R1006 which pull up Q1001’s gate. Clearly it couldn’t work without them. Lo and behold, my 10 megohm input meter showed 250V at the junction of R1005 and R1006. That can’t be right – the voltage should be more like 150V, since the resistors are the same value and basically connected straight across the 300V supply. I pulled R1006 out of its hiding place and measured it – it was open-circuit. Gotcha!

Soldering in a replacement 1.8 megohm resistor for R1006 brought the whole thing back to life. The secondaries seemed to have sensible voltages on them (3.3V and 12V at a glance) so I reassembled it far enough to test. It lit up, accepted a disk and played it. Success!

It’s a credit to Denon’s neat design that there were no screws left over when I put it back together. The whole machine is now back in action for the sake of a 2p resistor.

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