Casio watch will not receive time signal

 Casio 3495 G-Shock Men's watch

This wrist watch is very popular as it receives the current time via radio waves. In this case it was set to receive DCF77 but can also do JJY, BPC and WWVB for Japan, China and USA respectively.

The watch worked find but did not receive the time signal anymore despite it still being under warrenty. Casio wanted 50 Euros for repair! Not worth it.

The case was removed from the back of the watch in a watch vice.

After removing the four screws, the back came off and looked like this:

A view under the back of the watched showed two electrodes:

On a closer look at the back of the case where the electrodes touch, I could make out corrosion of some kind.

The case was cleaned using Napthalene, being careful not to touch the rubber seals.

After that, the time signal was received and the watch is back in working order.

Adjustable fuse that can be reset

 Those of you who have read my previous posts on safety on the bench, will be familiar with the article I wrote on safety when working on tube radio repair.

My home wiring system has a 10A resettable fuse on the sockets in my shack. That is 2,300 Watts of power! 

However, with all my test equipment running, I barely use 2.5A. So I wanted a way to adjust how much power is delivered to my bench using an adjustable fuse that can be reset.

Most people will think, why do I need this as I have FI protection on my circuit that will detect a ground fault quickly? Well, if you read the article in more detail you will see that a part of my bench is not referenced to ground and for a very good reason. In a case like that, the FI will not see a fault. So the dim bulb gives protection but a resettable fuse is the ultimate current limiter.

I also committ the ultimate no no, which is connecting extension cords in series. This can easily lead to an overload situation. However, I do this because I need the length and the large number of low power devices spread all over my bench. I also want the added security that I do not overload the first extension cord in the link.

So, I came up with the following concept to limit power and make it adjustable:

All components are from F&F.

• Controller EPP 620
• Current Transformer TI-30/5
• Relay PP-1P

The controller is set to mode C:

I use Relay R1. When Imax is reached, R1 latches and connects terminal 7 to terminal 8 (NO), effectively cutting off current to the rest of the circuit (default mode is terminal 7 to terminal 9 = NC). It can not be reset without turning the power off and on again, which in my case means, unplugging it and plugging it back in. So it is fail-safe.

The current sensing transformer is connected to terminals 1 and 2.

The EPP 620 can switch a maximum of 2 x 8A. So to avoid overload, a relay PP-1P was added, which can switch 16A. More than enough for this application as the main board breaker trips at 10A.

The setup was tested with different loads and works exceptionally fast, tripping very, very quickly. The adjustment is very sensitive and can be set as low as 800mA.

Dell Laptop with broken hinge

 A broken hinge is a normal occurance in a laptop. Everything parts ways and puts enormous strain on the plastic housing which causes it to break apart.

So the laptop was opened and immediately it was clear that the plastic housing holding the screw dowells had broken apart. A common occurance. The manufacturers make the springs so tight, that it is only a matter of time before the hinges break off.

I tried to use epoxy to put everything back in place. This was the expoxy I used.

I used it to reset everything in place.

Sadly, the repair did not hold. The expoxy was left 24 hours to cure, but after 3-4 laptop openings, it was broken again. Stronger measures were required.

The dowels were removed and holes were drilled through the casing and M3x30 screws were inserted and nuts placed on the other side.

It is a nasty solution to a terrible engineering problem from Dell.

However, it holds the hinge and is indestructable.

Happy customer.

Tecsun S-2000 World Receiver Won't Receive On External Antenna

 The Tecsun S-2000 World Receiver is a radio that has AM, FM and SSB listening capabilities on the LW, MW, SW, VHF and AIR bands.

This radio was bought second hand online and at first glance was in working order using the external aerial whilst scanning the FM broadcast band. However, the owner stated that the radio was silent on the short wave bands using an external aerial. This can be plugged into the side of the radio.

Usually a long wire is hung up outside and attached to the short wave input, then the mechanical switch is moved to select the external aerial instead of the internal one. The short wave listener can then enjoy distant stations, especially at night.

In this case the owner stated that not even hissing could be heard after switching to the external aerial. The radio simply went dead. 

The user also stated that the display also did not show the swich over between the internal and external antennas, despite the switch having been selected.

So I first tried to simulate the fault on my test bed, and connected the external jack to my radio set tester and injected a normal 7.000 MHz signal at -113 dBm. This is normally more than enough to hear a good steady test tone and sure enough, there was the test tone loud and clear. Bang on frequency. Other bands were tested, all worked perfectly!  SSB, AM, FM. Everything worked perfectly. So where was the problem the owner was having? How come the radio worked just fine for me but would not work for him? Curious....

We chatted and talked about what was different about my setup compared to his. We tried my outdoor longwire aerial instead of the test set. However, that worked just fine too.

Then we talked about my dim bulb, current limiting power supply (read more about it in this article) which also has a variac to limit the voltage. I use this often when testing older tube radios. It just happend to be set to 110V as I noted that his power supply for the radio can do 110 and 220V. So we decided to up the voltage from 110V to 220V and immediately his problem was reproduceable!

The problem was due to the power supply and did not involve the radio at all. By increasing the mains voltage, this must have added pressure to the electrolytic capacitors in the power supply making the power supply a little bit too noisey for the radio's internal aerial switching electronics to work correctly. So the power supply was opened.

 The caps were removed and tested for leakage.

Replacement caps were installed and the radio retured back to life again, as good as new.

Broken 24V Switched Mode Power Supply

My LED light stopped working this week. I am quite fond of it as it is quite a "quiet" one. By that I mean, it does not produce so much electrical noise as most LED lights do these days. As a keen amateur radio enthusiast, that is important to me. So my son openend the plastic brick and we went in search of the fault.

Switched mode power supplies can store high voltages even after they are unplugged from the mains supply. At 230V there can be DC voltages in excess of 315V stored inside. Be sure you know what you are doing and know how to safely discharge these voltages first, before working on any kind of mains powered devices.

The first thing I do, when fault fixing switched mode power supplies, is to test the transistors, diodes and bridge rectifier components with a test meter set to diode test mode. Fluke published a good article explaining how to do this. If everything tests good, I then go and test all the electrolytic capacitors using an ESR meter. I am looking for shorted components.

A quick look over the PCB showed, that the rectifier that converts 230v AC to DC, had one leg physically broken off; So a new one was ordered and soldered in place.

Power up on a dim bulb test bench supply, showed no power output. So debugging continued. I drew up this version of the PCB in Adobe as a .pdf to help with the debugging process. It shows the bottom of the PCB.

The orange line on the right shows the division between high voltage on the left and low voltage on the right. A quick test, showed no low voltage at all on the right hand side. So the problem must be on the left. The end of the red and black lines should show high voltage DC. This was also not present in the circuit. So the tracing went back to the newly soldered in bridge rectifier at the bottom left of the picture. The middle two pins should have 230V mains AC voltage but there was nothing. So the line voltage was traced from where it came into the PCB (see the blue lines). However, it stopped at the four pin component on the bottom left corner. Turning the PCB over, we can see that this is a choke (top left).

Zooming in, we can see that a leg has broken off.

So, the leg was re-attached to the thin wire of the choke and the component re-attaced to the PCB using hot glue. Finally, the case was glued back together and the LED light is working again.

Canton Subwoofer Dead on Arrival

 The Canton family of speakers are very popular and good value for money for things like a home cinema. 

This one came into the bench dead on arrival. It simply made no noise at all. So my amplifier was hooked up to it to reproduce the issue. This is always a good idea, as it rules out a possible issue with the customers amplifier and allows me to ensure the fix has actually worked.

Once opened, the circuit board was immediately visible. A search on the internet showed there were various mods of the Canton PCB out there and finding the exact one would be a challenge. I finally settled on this one, as it matched my version of what was infront of me most.

I used my vintage Heathkit signal tracer to follow the input signal and see where it was being lost.

Why? Well, this is a set I had purchased in USA years ago and had refurbished and overhauled and brought back to life. So it was fun to get a chance to use it again. It is an old tube operated amplifier, that allows and engineer to trace a signal using the probe and to work out on which component, the  signal stops and goes no further. In this case, the signal was being lost at the cross-over potentiometer.

A good strong signal was going in but absolutely nothing as coming out. A test with my meter showed open circuit no matter where the potentionmeter was positioned. So any easy fix is to join the output of Op-Amp IC1A to the input of IC1B.

This is what the potentiometer looks like.

There is only one seller on ebay currently selling this item used, removed from an older device. However, the cross-over setting is rarely used anyway and the potentiometer can be bypassed. Should the user wish a full repair later, we can look into that option. So for now the R-3 to R2 legs of the potentiometer were bridged using solder.

Once the subwoofer was re-assembled, it was tested using a 40 Hz signal from my signal generator.

The subwoofer was back to life and operating as normal. Time to screw everything back together and return it to it's owner.

Icom IC-251E Dead On Arrival

 The IC-251E is a fairly good 2m CW / SSB and FM transciever made around 1980-1983. The British Mutek company sold a high-performance front-end board for this radio (Design by Ian White, G3SEK). This was because the front end tended to be slightly deaf. It is a double conversion, super hetrodyne design with intermediate frequencies of 10.7 MHz and 455 KHz. Interestingly, the 10.7 MHz is available on the back on the device and can be viewed on a scope.

This little set was given to my local club by an OM who had retired from the hobby. He said it was in perfect working order. It lay for a few years and then it was decided to dust it off and use it as a loaner set to a new OM who had just passed his entry level certification. So that they could immediately get enjoyment out of the hobby on the local repeater or in USB. So it was plugged in and tried out. Instantly, the set went POP and magic smoke came out. The set was dead. So the club passed the set to me for repair.

I first verified the problem, but by this stage, the radio just did nothing. However, plugging it into 12v showed the set worked perfectly. So obviously the problem was confined to the power supply and most probably the electrolytic capacitors. So the set was opened. Getting to the power supply was a big job. It involves removing everything. Rear frame, both circuit boards, pulling out the power supply and disengaging multiple plugin connectors.

Before removing a screw, pictures of the connectors were taken. All of these must be removed, so it is important they all go back in the same places. Many of the connectors have the same size and pin numbers, so it is very easy to plug the wrong connector into the wrong slot on the re-build.

Once that was documented, the tear down could begin. Everything must come apart and I mean everything!

What we are left with, is the power supply unit at the rear of the case.

From the circuit diagram, we can see that the power supply is quite simple. However, the two huge electrolytic capacitors were the first suspects for trouble in an older radio that has sit some time before being turned on again. The start up should always be done using a current limited variac to avoid potentially devistating damage to the circuitry.

As can be seen from the picture above, the electrolytics are 200 uF at 220v. They were desoldered and tested on a home made ESR meter and proved to both be bad. 

New ones were procured and the size difference is remarkable.

As the Nichicons are much smaller, longer legs must be left to fit the differing hole size in the PCB. Owing to the fact, that the tear down was so labor intensive, the new capacitors were tested on my ESR tester to ensure they were indeed good, before soldering them in and putting everything back together.

Before re-assembly, I always sort the screws that I removed. This helps me work out which screws go where. This is especially helpful if you have had to wait a long time for parts and the memory of taking everything apart is a distant one.

Finally, things are coming back together and the radio can be tested before finally installing all the covers and panels.

The radio was tested on the tester and found to be within spec. This radio is now ready to go to it's new home. I hope the new OM will have fun with it.