Thursday, August 3, 2017

Mesa Boogie Mark II power supply wiring.

Here's an excerpt from the schematic diagram for a Mesa MK II that I recently brought back from the dead. It had a damaged power cord that needed replacing and as I got to digging into the schematic and mapping out the wiring scheme I uncovered something that bothered me.

As I understand it, the correct way of wiring a three conductor cord to an amplifier is as follows. The black lead from the power cord (hot) goes to the tip of the fuse, then to the switch, and then into  the power transformer. The other primary lead is connected to the white, or neutral, wire.

No wire nuts here chillun. This ain't residential. Do a good wraparound splice with an inch of bare wire on either lead, solder well. and use two courses of heat shrink tubing.

The white, or neutral is connected to the power cord and out we go to the plug.

It has always been my understanding that the first thing the hot lead should see is the tip of the fuse holder. That way, if the fuse blows for any reason whatsoever, the hot lead is disconnected.

If, on the other hand, the fuse is on the neutral side, then potential will always be present in the primary side of the power transformer and that means danger of electrical shock lurks therein.

Needless to say, I revised it and removed all the excess trash-a line reverse switch, a thermistor, and the so called death cap. I disabled the courtesy outlet and used the existing hole in the panel for a SPST switch so that the owner can switch off the fan if he is recording.

It now looks like this. The fan will not function until power is applied to the power transformer primary through the appropriately placed fuse and switch.

And of course the "No adjustable bias! Use only Mesa tubes! Let us handle this! Begone!" business was something they did not think was that important back in 1980 when this thing was built.

Note the nifty bias adjust pot. You can always incorporate this feature into any amp you like if it does not have an adjustable bias.


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