Now my brother plays guitar in a band. Some time ago he bought a used Laney VH 100 R Valve guitar amp. He really liked it but started to have an annoying problem with it. It would blow the internal fuse on a very irregular basis, just after hitting the power button. So, as you can imagine, for a band which plays live, unreliable equipment is not tolerable. So after changing the fuse a few times, he brought it in to an amp technician. When he picked it up, he was told everything would be fine now, there was some loose part inside.
So it went well for a few months and then again, fuse blown. Back to the puzzled technician. After weeks he was told to pick it up, the defect was not found and the technician did not know what was going on.
So I told him, I wanted to take a look, although I did not work with guitar amps before, but I have a colleague, who used to build his own amps, so I could probably ask him for help. To prepare, I downloaded the service manual and took a look. Since the fuse only blew, when it was switched on, the problem seemed to lie within the power transformer. Or perhaps related to the valves heating up? I live in an old building, and I have had problems with the fuse of my flat, when I switched on a (more or less) powerfull electric device, like a vacuum cleaner. I thought it might be related: The guitar amp is quite powerful, perhaps the Transformer pulled too much current when switched on? The irregularities would be explained by the phase, at the exact moment you switched it on. When the sine of the voltage is at a maximum, or minimum, the initial current would be larger. Could that be so simple? I took a look at the schematic:
Now, it seems, that after the plug an NTC is put as surge guard before the fuse. Now, that means, that the initial idea was off the table. The NTC does prevent exactly this. It is a resistor, which looses its resistance, when it gets warm. So, when you switch on the device, the NTC prevents the transformer from pulling an immense amount of current, which woudl blow the fuse. during the process it warms up and after a short amunt of time its resistance is down, so that the amp can work without restrictions. After switching it off, it cools down, regains its resistance for the next time. This is one reason, why you should not switch off an on a device without some time in between. If there are such elements in the circuit, they would be still hot and unable to protect the circuit from too much current.
Further down it is in a bit more detail. In the red box we have the parts for european power. There is, right before the T2A fuse an NTC. So upon opening, there should be two NTCs, one for 120V and one for 230V.
Now it was hard. What could it be? The transformer? Some problems further down? Or was the NTC shortened out somehow? Was it perhaps simpler and there was the wrong fuse used? There are different types of fuse. The T2A which should be used is not as fast as other fuses, it needs, not unlike the NTC, warm up before it blows. This is to prevent it from Blowing if there is a very short peak. It is indicated by the T in front of the number, which is the current, above which it blows.
When the amp arrived, I first looked at the fuse. It was a T2.5A. So it was already sturdier, than it should? Why? Was the technician hoping, that the wrong fuse would not blow so fast? It did not help, I needed to take a look. Together with my colleague, we opened it up.
Now, inside the red box, there is the power socket and the fuse. Aaaannnd.. no NTC. huh? Well we looked, and it was not there. Either the NTC made it in the circuit in a revision, or it was taken out by someone, or it was not installed in the first place. Nevertheless, no surge guard there. So, that made the repair extremely easy. We looked and found the correct part:
Then I took the amp home to install it:
And fixed everything up with shrinking tube, placing the NTC where it would not touch anything and has enough space to get the heat away:
I also ordered 10 fuses, just in case, from an electronics supplier. There, they are incredibly cheap compared to what they cost in a music store. But that was unnecessary, the amp works now like a charme, no fuse was blown after that. If you want to learn some more about guitar amp electronics, I strongly recommend the youtube channel of Uncle Doug, which is incredibly entertaining.
