In this episode of The Repair Bench we are troubleshooting an Ampeg BA115 Bass Combo amplifier. This amp is part of Ampeg’s starter series and is specified to deliver 100W of power to a 15″ speaker and a small tweeter. It features a style selector with five basic tone options, three tone controls (Treble, Mid, Bass), Volume and Master gain controls, a balanced XLR line out, headphone jack (1/4″), RCA phono plug inputs for connecting a tape or CD player and a tilt-back cabinet.
We purchased this amplifier for $50 in used and non-working condition. The previous owner had purchased the Ampeg BA115 used and it had stopped working shortly thereafter, so it was used primarily as a speaker cabinet. On our repair bench we plugged it in and gave it a try and it worked initially, sounding good for about 20 seconds before cutting off. The power LED continued to illuminate but there was no sound. The chassis was then removed to begin the troubleshooting process.
As we’ve said before in this column, our first step is always to do a thorough visual inspection. With good lighting we take a close look at every component, wire and connector to look for anything out of normal. This amp looked very clean and the only thing we noticed was the dull look of one of the contacts on the headphone jack. The other two contacts on this jack looked shiny and normal, but one contact looked dull, as if it hadn’t been plated properly during manufacture.
At the time we didn’t think much about this jack, but (spoiler alert) it turned out later to be important.
Troubleshooting the Ampeg BA115
With the visual inspection completed it was time to begin signal tracing to find out where the signal stopped. For that we needed the schematic which you can download here – Ampeg BA115 schematic. The amplifier output was connected to an 8 ohm dummy load with a monitor so we could listen to the output at low level. We set our signal generator for a 400 Hz sine wave at 100 mV p-p and connected it to the amplifier input. With an oscilloscope we traced the signal through the amplifier stages. The signal flow was found to be normal through the input op amp gain stage (U5b), 5-position filter stage, buffer (unity gain) op amp stage (U4a) and intermediate op amp gain stages (U2b, U3a, U3b). In fact we traced the signal all the way to speaker terminals and found a 20 volt peak to peak signal! This made no sense – the oscilloscope was showing a large signal at the speaker output, yet when we connected the speaker we heard nothing. After consulting the schematic and giving it some thought we realized that the ground side of the speaker wire goes through the switched sleeve connection of the headphone jack. This is the connector that looked dull during our initial inspection. Using an insulated stick we pressed down on the jack sleeve connector (the metal tab of the jack indicated in the photo by the yellow arrow) and the speaker suddenly roared to life.This jack was bad, probably from the day it left the factory. The sleeve connection wasn’t closing fully (making a low-resistance contact) as it should when no plug is inserted. We replaced the jack with a new one and found that amplifier operated normally.
The printed circuit board with the replacement headphone jack was then re-installed in the chassis. The amp was then load tested by running for an hour or so on our dummy speaker load to ensure that the fix was permanent.
It took awhile for us to write this article so we’ve been using the Ampeg BA115 for more than a year now in our music schools and we really like it. It’s turned out to be very reliable and sounds good in a rehearsal room with other rock music instruments. It’s also not too heavy for times that it needs to be carted off to a gig. Though Ampeg classifies the BA115 as part of it’s starter series, we think that damns it with faint praise. These amps are worthy of consideration by all bass players.
Category: The Repair Bench