Dynamo Radio Repair

When I was browsing a local antique store, I found this pretty old, but interseting radio. Apparently, it is very useful for emergency situations, since you don’t need to plug it in or even bring your own batteries. That is because you can charge it up in three different ways. First, you can leave it outside to charge using the solar panel when it is sunny outside. You can also crank this shaft, which is a dyanmo generator. And finally, you can plug it into a 3VDC power source to get it to work. The rest isn’t that special. There is an antenna that extends a few feet. You can also select between FM and AM channels. And finally, there is an audio jack at the back as well. The only problem is that it doesn’t work as it should. So the question is: should we fix it or scrap it? Let’s find out.

Let’s begin by figuring out what exactly is wrong with this thing. The first time I powered it on, I thought that it worked, but quickly realized that it has an incredibly short runtime only a few seconds. The front panel reads that the radio should be left in the first charging option when not in use, which is the solar option. That made me realize that my recording light was the thing that was briefly charging it up. The radio wouldn’t stay powered though even with the light source. I tried sunlight as well, but got similar results.

What about the dynamo charging, that should work right? Well, no. I cranked this thing for a while and didn’t even get it to turn on. So something is definetly wrong. To make matters worse, the antenna acutally broke off the radio. Not really a big deal, but kind of annoying to say the least.

I tried one more thing that I thought would surely make the radio work. I put in the optional batteries in the slot located at the back of the case. They were both AA rechargable batteries. And they did sort of work. They were able to power on the radio, but with poor quality. The audio was very quiet and attempts at turning up the volume did not work. Strange considering that this is a very consistent power source. This means that there is an internal power delivery issue.

So I grabbed my screw driver and started to open the case up. There were six screws on the back which were easy enough to get out. Upon opening it, I immediately recognized a few things. First, the yellow and red wires were obviously the positive and negative connections to the battery pack. The white wire led directly to where the antenna used to be. We can also see how the dynamo charging was implemented here. There is a motor attached to a gearbox. Close inspection shows that the gearbox is meant to speed up the rotation when it finally reaches the motor. The motor then spins and generates a current on its terminals. In case you are confused, apply a current to the motor will make it spin, inversely applying a torque to the motor will generate a current. Continuing, we can also see the two sockets for the 3V charging and the headphone out. At the top, we find the solar panel, nothing too surprising there. So far, at least visually, nothing seems to be wrong. Although, the age of this design meant that a ton of flakes were coming off, making a huge mess. Everytime I moved it, it seemed like more stuff would come out.

Anyways, we can only deduce so much by looking at the back of the board. So I starting taking it off, but had problems doing that. I looked for screws and found a couple of them. But the board simply wouldn’t move. I looked around and realized that there was a screw that somehow blended in and caused me to miss it. So I removed that one too. But the board still wouldn’t budge. I checked again for more screws, but didn’t find any. That was when I realized that the dials on the front were stopping me from moving the thing. I thought that I could simply just pull the dial off of the board using pliers. Unfortunately, that went wrong and I ended up destroying the component underneath. This is really annoying design because it is almost impossible to remove the front case without removing the dials, but to remove the dials, you need to remove the front case. Because of the physical damage I caused, this was looking more like a scrap, but I kept going anyways.

Thanks to the destruction of the dial, I was able to access the other side of the PCB. As expected, there are a ton of inductors and capacitors, it is a radio after all. There was even a sneaky component that the designers put underneath the IC. There is also another, larger inductor that isn’t directly attached to the board and was instead glued directly to the case. The speaker that is used is a 1/4 watt 8Ohm speaker. The big blue cylinder that we can see is actually the battery that should be charged by the charging methods we disucssed earlier. It is a two-cell, 2.4V 200mAH Nickel-Cadmium battery.

We can finally get to arguably the most informative component, the IC. There is only one IC on this board and it is a Sony CXA1191. Looking at the datasheet, it is a one-chip FM/AM radio IC, which is to be expected. Its voltage range is 2 to 8.5 volts. It provides detailed pin descriptions with little schematics to describe the equivalent circuit. It also provides an example schematic, which will be very useful for any debugging we may have to do. Using these observations, it is time to try to get this thing to work.

The first thing that I must do is replace the component that I broke earlier. Based on the front panel, the example schematic in the datasheet, and visual inspection, I deduced that this component is a combination of a volume potentiometer and a power switch. My meter shows that the destruction has messed up what is left of this component and the terminals are shorted together. At this point, it’s better to simply replace it. However, this is the first time I’ve ever seen a component like this, so I simply decided to split it in two. I will use a switch and a potentiometer separately. To replace it I first removed all of the old stuff. I then cut and stripped two lengths of wire to attach to the new switch. Then I cut more wire for the potentiometer. After soldering the potentiometer, I put electrical tape over some of the exposed contacts and then hot glued it to the board. With that out of the way and the board back to its inital state, we can search for the main problem.

The radio seemed to have an issue holding any sort of charge, so let’s begin with the battery. The age of the battery should be another indicator. I took the battery out of its spot for inspection. Since this is a 2-cell NiCd, we should be getting around 2.4 volts. NiCds have a very stable voltage throughout their capacity, so a dead battery should have a very low voltage. Measuring the voltage across it reveals that it is 1.5 volts. So, 0.75 volts per cell. That is a considerable amount lower than the usual 1.2 volts. Not to mention that the IC requires at least 2 volts to properly function. So as a test, let’s apply 2.4 volts directly using my power supply. And it works exactly as expected!

Great! Now we just need to find a replacement for this battery. I don’t have anything shaped like this on hand. In fact, I don’t even have any NiCds on hand either. So, I had to look for another solution. A LiPo most likely would not be suitable for this application due to its sensitivity. Coin cells wouldn’t be great either. What I did have, however, was a few rechargable NiMH batteries, which should work in a fashion similar enough to the NiCd.

Before we put the batteries into the circuit, however, let’s see how exactly they will be recharged. Starting with the solar panel, I put it underneath my recording light. Measuring the voltage on the output gets us 1 volt out. I’m sure that it would be higher were we to place it in direct sunlight. The motor is similar, spinning it outputs a few volts on the terminals. It helps too that the shaft is connected to the gearbox for a higher RPM. Now that we have power inputs, how are they connected to the battery? Well, the three-state switch, as we know, switches between charging methods. The connection isn’t direct because if it were, the battery would force the motor to spin. To find out what is inbetween the battery and the chargers, we should look at the board. As we can see, there are two diodes, one for the solar panel and one for the motor. This is to make sure that current only flows into the battery and not the other way around.

So, I got a AA battery case to hold the new batteries. Directly connecting the new batteries in place of the old battery was successful. But using this battery container would be a bit inconvenient, so I came up with another solution. Remember the optional battery slot at the back of the radio that doesn’t work? Well, we can rewire it to directly connect it in place of the old battery instead and use our new batteries to accomplish the same task. And that concludes all of the electrical issues, and now it is time to reassemble everything. But first, I have to make a spot for the new switch that was added to the radio. To do that, I just drilled a hole using my drill press. Now it’s actually time to put everything back together. And after doing that, it looks good as new and works just as well. Glad to see this radio in proper working order again!

Well, I hope you enjoyed this video. Maybe it has inspired you to repair something you have? Anyways, please consider subscribing so you can see the other videos that I make. Have a good one.