WARNING! This article documents the repair of several old flywheel magnetizers belonging to my customers. I would like to clarify that I am a vintage electronics repairer, not a mechanic. Please, do not send me emails asking whether I can magnetize motorcycle flywheels or similar items. Thank you for your understanding!

The iconic Piaggio Vespa motorcycles have captured the imagination of generations of motor enthusiasts. One of the secrets behind the operation of these wonderful little machines lies in their magnetized flywheel. During rotation, the flywheel transfers energy to a coil, powering the spark plug and lights, which are vital elements for proper engine operation. However, when restoring an old vintage Vespa motorcycle, it is common to encounter a demagnetized flywheel. Without proper magnetization, the engine will not run. This is where a forgotten hero comes into play, the CTB flywheel magnetizer. The CTB flywheel magnetizer is an old vintage tool, once widely used in Piaggio workshops, dating back to 1965, which plays a vital role in the restoration process of vintage Vespas. This valuable instrument was designed to restore the magnetization of flywheels, giving them back the magnetic energy required for correct engine operation. The importance of this tool cannot be underestimated for enthusiasts and restorers of vintage Vespas. However, being itself a vintage piece, it is often necessary to repair and restore CTB magnetizers, which have aged along with the motorcycles themselves.

Click here to download the original vintage instructions of the CTB magnetizer in PDF format.

Initially, a customer contacted me with a request to rewind a severely damaged power transformer, which was delivered to me in a carbonized state. I clearly explained to the customer that the entire device required a complete overhaul, since it was not sufficient to simply replace the old burnt transformer with a new one without considering the serious faults evident in the electrical circuit, which also included a valve.

Despite this, I accepted the challenge of rewinding the damaged transformer. After repairing it, I shipped it back to the customer. However, the customer then encountered difficulties reinstalling it because I had changed the wire colors during the rewinding process. Since I did not have the exact original colors, I attached a clear label indicating all the correct voltages.

Faced with his confusion, I insisted that the customer send me the entire device for a complete evaluation. From the photos provided, it was evident that what was needed was not simply the replacement of the transformer, but a much broader intervention, and it was fortunate that he had not managed to reconnect it to the circuit because it would have immediately burned out.

In the various forums frequented by Vespa enthusiasts, this magnetizer is often discussed, but surprisingly few people seem to understand how it works. Moreover, some provide incorrect advice about its repair. It is important to dispel these uncertainties and provide an accurate explanation. In reality, the magnetizer circuit is quite simple, although it has some design flaws, which I will discuss later. It includes two rectifiers. One is a selenium diode bridge that provides a low voltage of 12 volts, required for the pole-finding phase, while the other is a dual half-wave rectifier valve of type 5Y3GT, not an EL34 as some have mistakenly assumed.

The operating principle of the magnetizer is to charge a battery of high-voltage electrolytic capacitors and then discharge it in a single powerful pulse to the electromagnets. These electromagnets, in turn, magnetize the flywheel. The use of high voltages, contrary to what is claimed in some forums, is probably advantageous because, according to Ohm’s law, the parasitic resistances of cables and connections become less influential at higher voltages.

It is not true that the magnetizer operates at 800 volts. Rectification occurs through a dual half-wave circuit, where a center-tapped secondary winding produces 800 volts across the ends, but only half of that voltage reaches the rectifier.

Unfortunately, I have read on these forums a series of statements that make your hair stand on end. There are stories of people claiming to have found a magnetizer that had been submerged in water for as long as 3 days, then left to dry for another 2 or 3 days, and they claimed it still worked. These people probably do not realize how dangerous such actions are, or they have suicidal intentions. It is unacceptable to handle a device connected to the mains, with a wooden base, without grounding, whose transformer from 1965 is insulated with paper and contains capacitors that can be charged up to 500 volts DC. In case of exposure to water, the drying process could take months rather than days, especially inside the transformer.

It is shocking to read such information. Be aware that these practices are extremely dangerous. Never follow such advice and, if you do not have skills and knowledge in the field of electricity, rely on an expert before working on devices of this kind. Changing a light bulb or TV remote batteries does not require the same awareness and precautions needed in these cases. Electrical safety must always be the top priority.

The condition in which I received the magnetizer was extremely critical. The selenium bridge was leaking, the 5Y3GT rectifier valve was completely destroyed, with burnt filaments intruding between the plates and causing short circuits. In addition, all eight electrolytic capacitors of the capacitor bank were damaged due to overvoltage or direct exposure to AC current. One of these capacitors was even leaking electrolyte.

It was clear that the device required a complete rebuild. Installing a new transformer on a device in such conditions would have resulted in the destruction of the new component as well. A thorough intervention was necessary to restore the magnetizer and ensure correct operation.

Since I had suspicions about possible design flaws in this device, I felt the need to obtain an electrical schematic. Unfortunately, it was impossible to find one online. Probably, the people looking for these devices are not electronics experts, not even at a hobbyist level, being mainly enthusiasts of vintage motorcycles. As a result, they are not able to reverse engineer an electrical device.

So, with patience, I traced wire by wire to extract the electrical schematic from the circuit. As I suspected, my doubts were well founded. The first problem that emerged was that the battery composed of eight 100uF capacitors, for a total of 800uF, was charged directly by the poor 5Y3GT without any current limiting. It is important to note that all diodes have a current delivery limit, and rectifier valves in particular have relatively low limits. The 5Y3GT, for example, has a limit of 440mA. However, when charging an 800uF discharged capacitor, the valve would have to withstand an initial current spike that could reach up to 1A. As a result, the poor valve was subjected to heavy stress at each activation, and it cannot be excluded that the destruction of the transformer occurred due to the failure of the 5Y3GT caused by this overload condition.

The second problem detected concerns the discharge mechanism of the capacitor bank. After charging the bank by pressing the button, the discharge should occur on the electromagnets when the finger is removed from the button. However, if the electromagnets are not properly connected, for example due to one of the banana plugs becoming disconnected, which can easily happen, the capacitor bank remains charged. As a result, when handling these banana plugs, one could accidentally come into contact with a DC voltage of 450 to 500 volts, with a current potential sufficient to be lethal. It is essential to underline the danger of this situation. The presence of such a high voltage, without a proper connection of the electromagnets, represents a serious electrical safety risk. Handling the banana plugs without awareness and precaution could have fatal consequences.

I made modifications to ensure safety and preserve the valve and the transformer of the device. These modifications are intended to make the device as safe as possible. Here are the modifications that were carried out:

• I added a limiting resistor in series with the 5Y3GT to limit the charging current of the electrolytic capacitor. In this way, the valve is no longer subjected to excessive current spikes. Although this results in a slight delay in charging the capacitor, the integrity of the valve is preserved.
• I installed a second fuse to protect the high-voltage secondary circuit. This addition helps prevent overloads that could damage the transformer.
• I inserted a resistor in parallel with the electrolytic capacitor. This resistor allows a controlled discharge of the capacitor in about 2 to 3 seconds when the electromagnets are not connected. In addition, this resistor limits the maximum capacitor voltage to 400 volts, considering that the capacitor can withstand a maximum of 450 volts.
• Finally, I connected the entire magnetizer chassis, the transformer core, and its electrostatic shield to earth ground. This grounding helps ensure electrical safety of the device, reducing the risk of electric shock or dangerous discharges.

I also carried out further improvements:

• I replaced the old selenium rectifier with a modern silicon diode bridge. This replacement improves rectification efficiency and reduces the risk of excessive failures.
• I replaced the power cord and plug with new materials. In addition, I applied a modern strain relief where the cord enters the magnetizer chassis, ensuring proper protection and reducing the risk of electrical malfunctions.
• I replaced the fuse holder with a modern model.

With these modifications, the device has been significantly improved in terms of safety and functionality. The new components and upgrades ensure reliable operation and reduce the risk of potentially dangerous electrical problems.

I am aware that someone might suggest using silicon diodes instead of the rectifier valve in your CTB magnetizer. However, it is important to consider several aspects before making a decision. First of all, I would like to point out that if I had received the entire device from the beginning, I could have recalculated the power transformer to adapt it for the use of silicon diodes. However, since I was initially provided only with the power transformer, I could not make changes without thoroughly knowing the specifications of the device. Therefore, I rebuilt the transformer to work with the rectifier valve, and this configuration must remain unchanged.

It is important to respond to those in vintage motorcycle forums who suggest replacing the valve with silicon diodes. The 5Y3GT rectifier valve is still in production and can be easily replaced. Replacing it with silicon diodes would be a mistake, because diodes have a much lower voltage drop, around 0.7 volts, while a rectifier valve can have a voltage drop of 50 to 80 volts. Replacing the valve with silicon diodes would further overfeed the already stressed capacitors of the device, increasing the risk of explosions and other significant damage, including the danger of damaging the power transformer.

If you wish to use silicon diodes in your CTB magnetizer, it will be necessary to create a custom power transformer to handle the technical specifications of the diodes. This requires deep knowledge of the device and a custom design to ensure safe and efficient operation. At SB-Lab, my specialized company, I am experienced in the overhaul and repair of vintage devices like your CTB magnetizer. I have the necessary experience to make informed safety decisions and ensure that the device works in the best possible way. I invite you to contact me for personalized advice and a complete overhaul of your CTB magnetizer.

Below is a photo of the finished magnetizer and a short video where I show it while trying to magnetize the lid of a cookie tin. Ok ok, it is funny. They did not send me a real Vespa flywheel and mine is an electronics lab, and obviously I did not have a real flywheel to test, so do not laugh!

Extreme restoration of an abandoned magnetizer

Among the most demanding restorations I have faced, this magnetizer certainly holds a special place. When it was shipped to me, it was in disastrous condition, completely brown with rust, abandoned for years in a damp environment. The original plywood base was rotten and literally falling apart. The cables were wrapped in a shapeless tangle, and the chrome plating was dull under a thick layer of oxide.

I decided to save every possible part and bring it back to life.

The process was long and meticulous:

• Complete disassembly: I separated every component, down to the last screw.
• Rust removal: I had the metal parts sandblasted to remove every trace of oxide.
• Painting: All metal parts were repainted respecting the original colors.
• Base reconstruction: I had a new aluminum base made by a workshop, faithfully copying the design of the old plywood base that was beyond recovery.
• New feet: I 3D printed new support feet, modeled on the original dimensions.
• Chrome polishing: With great patience, I brought back the original shine of all chrome-plated parts.
• Coil restoration: The spring wires of the coils, which had stretched and tangled, were rewound around a screwdriver shaft and, by gently heating them with a heat gun, I managed to restore their original spiral shape.
• New transformer construction: The power supply was too compromised, so I built a new one, sized to operate safely with the restored circuit.
• Complete rewiring: I replaced the deteriorated cables and rewired the entire circuit, taking care of every detail, including grounding every metal part to ensure safe operation.

The result? A true restoration “miracle”. The magnetizer regained not only its original appearance, but also full functionality, ready for decades of service. Below is a series of before and after photos, which tell the transformation better than any words.

Before

After (photos not in sequence)

If you are a Piaggio Vespa enthusiast and own a valuable CTB magnetizer, then you are in the right place. SB-Lab is the perfect solution to give new life to your beloved device and guarantee optimal performance.

We know that your CTB magnetizer is much more than a simple tool. It is a piece of history, a symbol of passion and tradition. That is why we take care of every detail during the overhaul and refurbishment of your device. I have deep knowledge of CTB magnetizers and extensive experience in repairing and restoring these vintage instruments. With skilled care and technical expertise, I work to bring your magnetizer back to its original performance, ensuring maximum efficiency and safety.

Do not rely on DIY solutions or inexperienced individuals. SB-Lab is the reliable choice to achieve outstanding results. Every detail is carefully checked, from electrical connections to the internal circuit, up to the fine tuning of essential components.

Contact SB-Lab for a complete overhaul of your CTB magnetizer. Do not risk damaging the device or compromising its safety. Entrust your valuable instrument to the experienced hands of our qualified technicians and enjoy the pleasure of a perfectly restored magnetizer. Do not wait any longer. Contact us today for personalized advice and discover how SB-Lab can make your CTB magnetizer like new. Restore the power and magic of your device, giving new life to your vintage Piaggio Vespa. Choose SB-Lab and make your magnetizer shine!