Mysteriously, this modern Ampeg bass combo amp quit working. Could the Unbrokenstring Crew make it right?
The unit appears to be completely dead, with power present at the fuse holder. Let’s go inside to take a look around.
More goodness from St. Louis Music.
This picture documents the wiring polarity on the main driver coil. This needs to be right when reassembling the unit!
And the one on the right is the ” + ” terminal.
The screws that hold the chassis in the chassis come in from the sides of the enclosure. The captive nuts in the chassis have sharp corners on them, which snag the Tolex covering that wraps inside the enclosure. If you ever venture here, beware! TIP – slip a thin scraper or putty knife between the chassis and the cabinet to keep this from happening.
The circuit board is separated from the front panel of the chassis. Note the white nylon spacers on the jacks to the right and the inside-toothed lockwasher on the rotary encoder to the left. These come off now and are stored with the knobs lest they fall off and get lost on their own.
Do you see what I see? This white wire carries the AC power neutral to the main circuit board. The flag terminal came off the main circuit board.
The solder used in this unit is ‘lead-free’ and compliant with RoHS, the directive to remove harmful substances from the supply chain. This kind of solder is brittle, so solder joints made with lead-free solder often fail from cracking stress.
This is the other side of the circuit board. The solder fillets are OK but the mechanical joint failed entirely.
Rosin activated flux was added and the old solder removed.
Here is another failed joint undergoing rework. This one is at the DC common point of the amplifier.
While we’re at it, let’s look for other failed joints. Can you see these? These are still electrically OK but will fail soon.
Let’s put this guy back together. The shiny metal plate next to the circuit board is the heat sink for the power semiconductors in this unit.
These pics were made earlier to document the location of the flying wires attached to the circuit board.
These wires carry DC power to an off-board circuit.
That shiny metal heat sink gets a new coat of silicone heat sink compound, to minimize thermal resistance to the chassis.
Power is applied. Look, we have an indicator light now!
Before final assembly, let’s take one last look around. See those components stapled to the rear of the cabinet? That is the crossover network for the tweeter. Yes, I said stapled.
This is the pair of wires that go between the chassis and the crossover network. When the chassis is installed, this hole will be resealed with RTV to control the moving air behind the main loudspeaker.
All back together except for the grille, which you saw. This unit works very well for a solid state unit. It is loud, and light-weight. The future holds many more years of service.for this unit.
Thanks for reading all the way to the end!
CONTACT – David Latchaw EE
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