
Ampeg BA115 Bass Combo Amplifier Repair
- May 22, 2013
- in Guitar Kit Builder - Repair Bench
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.
Symptoms
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.
Visual Inspection
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.
Conclusion
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.
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excellent job on troubleshooting. I am trying to restore a holmes bass combo amp model “rebel 112 b” . I’m having a hard time locating the schematics and I tried every site I could think of. Any help would be greatly appreciated. Thanks
NO repair shop will touch it because parts cannot be found.Cool huh? Without even taking it in they refuse to work on it.
Story line- Bought the BA115 fron Guitar Center after trading in a
fender tubed bass head.like 1969-no workie.
That was 2007.Amp worked fine until the 5 position switch for “style”
spun around and did nothing.The amp was stuck in one of those styles,and that was it.So I ordered a rotary switch,it’s controlling a DSP chip on the main PCB.$18 shipping from Loud Technologies.
After replacing the switch,the amp sounded great,but while locating the exact placement of the knob,there was a pop,and hey whattayaknow,
turning the style knob did nothing again.
That’s where it is today because that chip is NLA
Anybody have a workaround? Jumper wire to where?
Well done, great troubleshooting! You saved me a lot of time, the BA115 that was brought to me had exactly the same problem. Thanks!
I took it home and looked it over. Fired it up and no sound. Found this site on the web and BOOM! it was the same issue!!! Thank you! Now I’ll be building a small box with a speaker and I’ll have a BA-155 for free!
BA`115
Tim, your amp is still under warranty. Contact Ampeg and they’ll point you in the direction of a service center.
It sounds fine now
Thanks for your article.
Yo compre un combo BA115 HP usado, cuando lo prendo por algunos minutos da un sonido, tocando mi bajo como si tuviera una distorcion y luego se va ese ruido que solo lo hace cuando toco las cuerdas del bajo queria saber si este problema es igual al que se refieren mas arriba para ver como lo puedo solucionar me comentaron que podia ser el ajuste de voltaje del amplificador este descalibrado y tenga que poner el voltaje correcto puede ser asi muchas gracias Daniel