This article was written to answer the specific question of a guitar enthusiast (quoted below):
Problem with my Marshall Valvestate 2000 AVT, every time power is applied fused instantly burns out, replaced fused several times with correct ratings and still get the same results. Is it something internal that's causing the problem? What can I do to start to diagnose?"
The fact that the power supply fuse blows instantly indicates a short circuit in the power supply unit. Please ensure that the power is switched off and the amp is isolated from the mains. Additional precaution can be taken to drain the power capacitors (those big bulky colored cans) using a 3.3 K ohm resistor.
- Identify the power supply stage, characterized by a bulky transformer (coil) and a bridge rectifier. You can’t miss this stage as they have bulky capacitors to smooth the DC to the other stages.
- Ensure that this is not an SMPS (used on some variants). An SMPS can be distinguished from a normal transformer rectifier based power supply by the sheer difference in size and weight together with the fact that it is enclosed in an aluminum chassis of its own. SMPS are small and light. Classic transformer power supplies are big, heavy and exposed. (If it is an SMPS based unit simply replace the SMPS.)
- Check the wiring before the transformer stage for shorts: The first test would be to isolate the primary coil winding of the transformer. This is the coil whose two ends are interfaced to the raw mains power through the fuse and power switch. Using the multi-meter, test continuity of the mains power cord to this stage via the switch and fuse. We are checking for a short circuit, hence a 0 ohm continuity will indicate that you have a short in the pre-transformer stage. If this is the case, test the continuity on the mains cord and the internal wiring. If a short is identified, replace that component (mains cord, fuse holder, internal wiring etc).
- Check the transformer for shorts: If the pre transformer stage is clear, test the continuity of the primary coil (transformer) with the multi-meter kept on its most sensitive settings for resistance and continuity tests. If you get a resistance above 30 ohms it 'Suggests but Does Not Confirm' an OK on primary coil. If the primary coil provides a resistance of less than 30 ohms, remove the transformer and isolate it. A short in the coil can actually be smelled as the burnt enamel causes a distinct smell. Thereafter, you can connect it using a fuse in series to a rheostat, gradually raising the input voltage to primary while testing the output 'AC' voltage in the secondary if there is no marginal increase in secondary AC power you will have to replace/rewind your transformer.
CAUTION: If you do not have a rheostat please take caution and do not test it directly to the mains. It is better to have the transformer replaced or rewound.
If your transformer is OK, the next stage we' re looking for is a shorted bridge or capacitor as this may at times cause an overload on the primary stage and the blowing of the fuse.
- Check the bridge rectifier for a faulty diode: Assuming you have cleared all the above stages, the next stage to test is the bridge rectifier; remove the capacitors and isolate the bridge rectifier. Test the individual diodes. Diode tests can be performed by multi-meter continuity tests. The characteristic of the diode allows continuity in one direction and inhibits it in the reverse direction. The individual diode being tested will have to be isolated by de-soldering and isolating any one of its ends. There are four diodes that require to be tested. The diodes, if found faulty, will have to be replaced.
- Check for a leaky capacitor: The last component to be tested in this stage is the power capacitor. Usually these components can be visually assessed for leaks and blow-offs. They also emanate a 'burnt stench' when they blow off. Their blow–offs are dramatic events and are mostly heard like a firecracker. Testing them with a multi-meter is quite trivial; as you connect the leads for a continuity test you notice an impulse indication of resistance (which is actually the capacitor charging up from the multi-meters power source).
In most cases you identify the fault in the third step and it's usually to do with a burnt coil in the transformer or a short in the wiring after the fuse stage. However please take due precautions as both the primary and secondary stages deal with voltages in excess of 100V and are capable of driving high current amperages.
If you face any more challenges with respect to the same please feel free to comment or contact.