Wharfedale Pro PM 600 Burnt Circuit Board

When an amplifier board is this far gone, the options for repair are rather limited. Simply changing the power transistors and burnt resistors may not be enough, as there are many other transistors that may be partially damaged. You could end up spending huge amounts of time and money and still not have a working board. Therefore it will be more cost-effective to simply get a replacement board and have a qualified engineer install it.

Another possible option would be to buy an amplifier kit from the auction site and install that; however, you may need to build a split-rail power supply for it. In addition, you will need an amplifier board that can operate on the voltage that the transformer provides otherwise you may end up having to buy a new transformer as well. As you can see, this option also has costs in terms of time and money.

The final and cheapest option would be to build a 15 V split-rail power supply on a separate board and use that to power just the mixer panel. This will allow you to use the mixer functions but you will not have a power amplifier. This option might be a good choice for those who already have a power amplifier but need the mixer functions. This way, one can feed the output from the mixer panel to an external power amplifier. I decided on this option and it worked well for me, and I was also able to recycle the 15 V positive and negative voltage regulators from the burnt board.

Studying the burnt amplifier PCB, I discovered that the design was fine, however, I suspected that the previous owner overloaded it. The strange thing about this short was that the safety fuse did not blow, and the current continued to flow through the shorted components. When I plugged this amplifier in, there was smoke coming out from the cooling fan grille and a strong smell of burnt PCB.

The Amplifier... Is on fire...

This reminds me of that Amy Holland song "She's on fire" except it is the amplifier that is on fire. :-)

The person who sold this said that it was repairable and the fault was minor, but of course, it was not. He also forgot to mention that the amplifier was producing smoke and was a potential death trap! It was lucky that I bought it and not someone else. Since I am a qualified and experienced engineer; I knew how to work with it safely, however someone less qualified would have been in trouble. He was also kind enough to send me a burnt out bass speaker. This of course adds more weight, which I end up paying for in hefty courier charges, whilst he gets to clear out his mess and make money out of it.

Interestingly, I found all the transistors to be short emitter-collector, however, they were also short collector-base, and any resistor on the base was either burnt or open. Since there was no decoupling between stages, almost every transistor had high voltage DC going through it. This information is of course very useful, because if you were building high-reliability circuits for avionics and spacecraft, inter-stage decoupling using capacitors can block DC paths, thereby isolating the current.

This Article Continues...