This absolutely mint-condition solid state bass amp head came through the shop for a quick once-over. More significantly, the owner wanted one of those new-fangled IEC AC power jacks installed in place of the existing power cord.
These units are highly regarded by many bass players. Some consider them to be an equal to the tube-based models in the realm of versatility and sound. And they are sure a lot lighter to carry around!
The rear panel is straight forward, with everything you need for a decent bass amp setup. More goodness from St. Louis Music Electronics!
Inside, we have the interface to the front panel and some power transistors. The unit with two flag terminals is a thermal switch that opens when the heat sink gets too hot.
Here we have more power transistors and the driver transistor pairs.
Power supply capacitors and the choke are found on this end of the board.
The power transformer is bolted here, and the input protection circuit board is mounted on the side of the chassis.
Input jacks and controls are found in this view, facing the back of the front panel.
This is the rest of the front panel. Note the big solid state rectifier for the power supply in the center foreground.
American-made Texas Instruments semiconductors are widely used throughout this unit. Here is a preamp chip.
The original line cord is removed with a snip. The wire remaining inside the chassis will be soldered directly to the IEC connector. The big chunk of insulation will be removed. The AC wiring will then be dressed in the same manner as the rest of the under-chassis wiring.
This blue IEC connector will be installed in the rear panel where the strain relief bushing goes.
A rectangular hole will be cut to mount the IEC connector. The pencil lines show up pretty well in this view.
A magnet is positioned to keep the metal chaff from the sheet metal nibbler away from the electronics.
This is the rough-cut hole. More filing will gun-smith this into a rectangular shape. Note the handle on the magnet visible through the rectangular hole.
Here, we are trial-fitting the IEC connector.
The location of the hold-down screws is marked with a center punch.
Now the holes for the screws are drilled, as you might have guessed.
The machine screws are torqued, mounting the IEC connector in its new home.
The magnets have really done their job. None of these filings will be left loose inside the chassis!
The cut ends of the power cord inside the chassis are stripped and tinned.
As a nice touch, some heat shrink tubing of the correct color is slipped onto each wire. Pure cosmetics!
These are tacked into place for now. Before the final joint is made, I’ll verify that the wires go to the right place!
Wiring orientation is confirmed as correct! The soldering was completed and the heat shrink tubing is shrunk into place.
Here, I’m checking the functionality of the low-level signals to feed a bi-amp setup. Note the Marshall Stack!
This unit is ready for the 21st century. The modular cord makes setup and transportation more convenient.
Thanks for reading all the way to the end!
CONTACT – David Latchaw EE
Please follow and like us: