General Electronics | SB-LAB di Bianchini Stefano

Moto-Tester Guzzo Oscar: history, operation, and repair of a rare 1960s auto-electrician diagnostic instrument

5 December 2025

How the Guzzo Oscar Moto-Tester works

The clearest information about its operation comes from the contributions of some experienced users in an old thread dedicated to this device. The Moto-Tester performed two fundamental types of tests:

Test of internal and external HT coils: The device generates a pulse signal that simulates the action of the breaker points and the capacitor. By connecting the HT coil to the Moto-Tester output and the spark plug cable to the spark gap (the pair of adjustable points at the top), you can check the consistency of the sparks and the maximum arc distance. Tables existed that indicated the ideal spark length depending on the type and brand of coil.
Test of lighting and charging coils: The flywheel coils are placed on the red side supports. The Moto-Tester generates an alternating magnetic flux that simulates the magnetic field of the flywheel magnet. This makes it possible to connect an equivalent load in watts and measure the voltage produced by the coil under realistic conditions. In the more complete models there was also an rpm regulator to simulate different engine speeds.

Forum experts emphasize that a simple resistance measurement is not enough to evaluate an HT coil. A coil may appear perfectly “normal” to the tester (for example 1.2 ohm on the primary and 9500 ohm on the secondary) but still have shorted turns, compromised insulation or problems that appear only at high rpm or when hot. The Moto-Tester was created precisely to go beyond simple static tests.

An incredible coincidence: two broken Moto-Testers in the same month

It is common to see rare devices in the workshop, but what happened with these Moto-Testers was truly unusual. In a very short time, two different customers, with no connection between them, brought me their Guzzo Oscar Moto-Tester for repair. Both devices were faulty, but with completely different defects. One of them had also been heavily tampered with in the past.

The real stroke of luck was having them on the bench at the same time: many parts were damaged, burnt or missing in one unit but present in the other, and vice versa. By cross checking real data and comparing original wiring with the tampered one, I was able to reconstruct the internal operation with a level of precision that would otherwise have been impossible. Without this coincidence, neither unit would have returned to working condition.

The burnt transformer problem: reconstruction and upgrade

One of the two Moto-Testers had an additional major fault: the power transformer was completely burnt. With no official documentation available, I was able to rebuild it by calculating it from scratch thanks to measurements taken from the working unit.

The original transformer of the older model was an autotransformer. In the more recent model, however, Guzzo Oscar adopted a traditional isolated transformer. By replicating the structure of the newer model I achieved a real upgrade in terms of safety and reliability.

Transformer vs autotransformer: differences and risks

Traditional transformer: primary and secondary are galvanically isolated. The device is separated from the mains and is much safer to use and repair.
Autotransformer: a single winding with taps. No isolation from the mains. The internal circuit may be at mains potential, with much greater risks in case of faults or accidental contact.

Switching from an autotransformer to an isolated transformer significantly improves operational safety. For this reason, in my reconstruction I reproduced the transformer of the more recent model: a concrete improvement over the original. The new replacement transformer is identified with code 25S127 and can also be ordered separately for those who need to restore other Guzzo Oscar Moto-Testers.

In the photo below you can see the transformer I rewound using the original bobbin, laminations and straps of the device. Those who order one will instead receive a completely new transformer.

Warnings for those who want to repair it themselves

The Moto-Tester is an electrical device powered by mains voltage. It is not a tool for beginners. Before powering it with a new transformer:

The circuit must be fully repaired and checked.
All faulty semiconductors must be replaced (in both units I received, diodes and transistors were almost all open or shorted).
Never power a device that is still faulty: the new transformer would burn immediately.
Experience with dangerous voltages is required. If you are not fully aware of what you are doing, it is better to contact a professional.

Conclusion: The Guzzo Oscar Moto-Tester is a valuable testimony of the professional equipment used by Italian auto electricians in the golden years of mopeds. A simple yet ingenious tool, capable of diagnosing real faults that escape traditional testers. Its rarity makes it a fascinating object to restore, but also extremely demanding without documentation. The coincidence of having received two faulty units at the same time made it possible to bring them back to life and to reconstruct much of the information that would otherwise have been lost. A small piece of Italian technical history that deserves to be preserved. Anyone who needs the replacement transformer 25S127, or has a Moto-Tester to restore, can contact me through the website. I will be happy to help.

Building an ignition coil tester for cars and motorcycles with a spark gap

23 September 2025

Stefano Bianchini

Recovering the Spark Gap

The visual heart of the project is the spark gap, which lets you observe the spark produced by the coil under test. To find it, I started… at the junkyard: I salvaged an old 1950s coil tester, bought for literally a couple of coins because it was nothing more than a rusty wreck. Perfect for my purpose, since I only needed the spark gap.

The only flaw was the black bakelite plate on which everything was mounted: it had become slightly conductive. Measuring with the megger, I found a leakage resistance between the two tips of about 100–110 M?. It may seem high, but at spark-plug voltages that’s enough to drain all the energy from the coil. To fix this, I 3D-printed some insulators and inserted them into the original plate, simply widening the existing holes and reassembling everything. Problem solved.

Power Supply and Circuit

To drive the coils I had a custom transformer wound, model 25S121, and built the circuit on a standard perfboard. The schematic is free: anyone who wants to assemble it can order the 25S121 transformer by contacting me. Click the image to view it in high definition.

Everything is housed in a Gewiss enclosure, easy to find, inexpensive, practical, and sturdy. For cooling, I added a fan salvaged from a PC: even though the transistor dissipates only about 3 W, without ventilation the temperature would rise above 100 °C despite the heatsink. With the fan, the operating temperature stays around 35–40 °C. Remember to make an extra hole for the air to exit.

Key Components and Protections

The main capacitor is a 1.5 µF Audyn Cap MKP 600 V: I used it because I had it on hand, but any 1.5 µF 630 V polyester capacitor will work. To protect both the capacitor and the BUX85G transistor, I added silicon carbide TO-220 diodes, model C3D02060E. They are essential in case of faulty coils that may send high-voltage discharges back to the driver circuit: without these protections you risk damaging the circuit and causing failure.

Essentially, the circuit is a relaxation oscillator that charges and then discharges the 1.5 µF capacitor into the coil with pulses of about 300 V at very high speed, a principle similar to that of Porsche 911/930 ignition control units. This driving method ensures effective testing of any 6 V, 12 V, or 24 V coil.

If You Can’t Find a Spark Gap

You don’t necessarily need to salvage an old one. Anyone with a mechanical workshop and a lathe can easily build one using simple brass rods. I removed mine from an old wreck with an autotransformer, full of terminals that leaked to the chassis: live voltage was everywhere, making it impossible to reuse, and the analog meters were already out of order.

Safety Warnings

Please note that this circuit operates at potentially lethal voltages. Anyone without adequate electrical knowledge should not attempt to build it. This article is for educational purposes only, and I refer to my Disclaimer page for further warnings.

Conclusion

This small project combines creative recycling, vintage technique, and modern practice. If you’re passionate about classic cars or motorcycles and enjoy experimenting, building an ignition coil tester with spark gap can be a fascinating challenge. I can only supply the 25S121 transformer, but I remain available for advice and technical clarifications.

Happy building—and watch out for the sparks!