DIAGNOSE, OPEN, AND REPAIR YOUR OLD AMP

OPEN . IT UP

Many amplifiers are simply wooden boxes with metal boxes screwed inside them. The Acoustic 150 here only needed four screws taken off the top. and the metal box of guts slid right out. Make a diagram if you need help remembering where the parts go.

Look inside the amplifier, checking for burn marks, loose wires, and structural damage. Burned up components usually indicate trouble. If they are resistors, use the multimeter to check that the resistor values are still correct. Broken hardware can usually be fixed or replaced. Rotted plastic can be replaced with hard metal. Pop rivets. PEMs. cable ties, or a squirt of Silicon RTV can replace many broken fasteners and mounts.

Now we’ll want to look at the power supply and measure the bulk filter capacitor for replacement. Start at the power cord and trace it through the power switch to the large, heavy power transformer. At the output of the transformer is the bridge rectifier, a diode-based sub circuit that converts the ebb and flow of AC current into rough DC.This is fed to the BFC. which smooths the voltage out. Locate the BFC and note the maximum capacitance, working voltage, and surge voltage specs printed on the side. For capacitance, you can match your replacement BFC to the same value, or try one with a higher value to reduce hum. Don’t go with too high a value, though, or you’ll put too much stress on the full wave bridge rectifier. Next, we’ll measure the voltage values needed for this cap in practice.

CAP CODE

This capacitor is rated at 3900 microfarads, with a maximum working voltage of 75 volts, and a maximum surge voltage of 95 volts.The capacitor’s manufacturing «born on» date is decipherable from the last four digits of the cryptic number at the bottom, which probably denotes the year and week — in this case, the 17th week of 1973.

CHOOSE AND REPLACE YOUR . BULK FILTER CAPACITOR

Capacitors fail when too much voltage is placed across them or when too much AC current is passed through them. So it’s important to determine the maximum voltage and current requirements for the bulk filter capacitor, which handles the incoming power. This can only be done while the amplifier is operating at a full load turned up to eleven. Since this would ordinarily make a tremendous amount of noise, we fed the amp’s output into an 8-ohm power resistor, to simulate a speaker with 8 Ohms of impedance.

Speaker cable 8-ohm power resistor

WARNING: The load resistor gets very hot and smokes a bit! That’s why we let it rest inside the metal chassis, so it wouldn’t start a fire. We kept the fire extinguisher handy during this project!

a. Disconnect the BFC’s positive terminal, set the multimeter to measure AC current, and connect it as shown in the first diagram below, across the capacitor’s two terminal connections. Use a clip and take care that the wire does not fall off. as this could blow the amplifier! At full power, our amplifier puts 3.31 amperes (A) of AC current through its BFC. There¬fore our replacement BFC should probably have a ripple current spec of at least 3.5 amperes.

b. Reconnect the capacitor, switch the meter to DC voltage, and measure across the dummy load resistor. Ours read 74 volts (V). The voltage rating on the capacitor is 75 volts, so there is not much margin in this old amp.

c. Armed with the values you need, buy a replacement BFC.

The one we chose for our Acoustic 150 amp is the 4k microfarad. 100V AA Series Panasonic Computer Grade Capacitor, which is rated at 100 VDC and 4.12 amperes of ripple current.

We bought it online from Digi-Key. This capacitor has a 2″ diameter, so it fits into the old.

REPLACE THE OTHER . CAPACITORS

The second most heavy-duty capacitor an amp needs is the output cap. which converts the all-positive signal into the plus-minus wave that speakers use. As with the BFC. the amp’s other electrolytic capacitors can be replaced after measuring or calculating the voltages they need to be able to handle. Or you can just replace them to match what’s already there and what you already know about the amp. You don’t need to replace ceramic capacitors, because they don’t wear out.

Like batteries, electrolytic capacitors have polarity. Look for little + or — symbols near one of the terminals. Don’t get the terminals reversed, or the capacitor won’t work. Worse, it can get very hot or even explode!

a. A with a 4-ohm load. So we chose another 100-volt AA Series Panasonic capacitor, this one 10kF. because it could handle about 10A of ripple current. The replacement was physically much larger than the original. so we bought a mounting bracket and found a spot inside the amp where the cap would fit. connecting it via wires, with heat shrink tubing over the solder splice.

b. Unsolder and remove the capacitors on the printed circuit board. You can measure or calculate the voltage on each before choosing replacements. or make an informed generalization. Since this amp uses a 75-volt supply, we figured that 100V capacitors would work just fine.

Other than the BFC. the other capacitors in the amp have low ripple current. We used ordinary electrolytic capacitors with the same values, but with a 100V voltage rating.

CHOOSING ELECTROLYTIC CAPACITORS

Important electrolytic capacitor specifications include:

A value or capacitance, in microfarads (pF)

A working voltage, i n volts (V)

A maximum or peak voltage, in volts A maximum ripple current, in amperes (A)

Sometimes low frequency response can be extended and hum reduced by using a capacitor with a greater value, but it’s safer to replace capacitors with the same value.

For maximum reliability. a good rule of thumb is to use capacitors at 50% of their rated voltage. Don’t exceed 75% of the maximum rated voltage.

Ri pple current specs are harder to find. They are not generally marked on the capacitor itself, but are buried in a table on the capacitor’s datasheet.

CLEAN UP THE . POWER ENTRY

The power supply starts at the power cord, then goes through the fuses, power switch, transformer, full wave bridge rectifier, and bulk filter capacitor. The original power supply used a polarity switch and several special capacitors across the AC line. A more modern way to do the same job is to use a filtered line module. A good module contains the equivalent of all these capacitors, and has another filter element called a common mode choke. Line modules are rated to carry a maximum current. Line modules also use three-prong I EC connectors, which means the power cord can be removed and easily replaced if damaged.

a. Unplug the amp and expose the power supply by unscrewing the side panel. On this amp. the panel hinges back with all the wires still attached.

b. Make space for the line module to fit into. Our amp had an AC socket on the back to provide power to another piece of equipment. To create enough room for the module, we removed this extension outlet and filed the hole to widen it. Use duct tape and a piece of paper to keep the filings out of the amp — otherwise they might cause short circuits.

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