Ampeg BA115 Bass Combo Amplifier Repair

| May 22, 2013 | 11 Comments

Ampeg BA115 100 watt bass amplifierIn 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.

Our first look inside the BA115 chassis

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.

A close up of the jack that shows poor plating on one of the three connections.

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.

Category: The Repair Bench

Comments (11)

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  1. avatar Joe Kolla says:

    Well done, great troubleshooting! You saved me a lot of time, the BA115 that was brought to me had exactly the same problem. Thanks!

  2. 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

  3. I just got my 1156HP back from the repair shop and it had the same problem as yours and the other fellows. Like you, I bought mine used and non working. Many articles I have read talk about the tweeter problem with the BA115 and some owners have disconnected the tweeter and say that the amps work and sound fine. The tweeter was ok in my new used amp and not being a tech I took it to a tech. I didn’t think it’d be that expensive of a repair and I was right. It was that darned headphone jack.

    It sounds fine now

    Thanks for your article.

  4. avatar Lars-Åke Nilsson says:

    Hi friends . I am trying to get hold off a shematic for Torque TB50 bass amp. I whould be wery gratefull if someone out there could help me with that. Thanks in adwance. // Lars-Åke

  5. avatar Alex says:

    I have the exact same problem with my BA115, upon opening the chassis I noticed the exact same connector looks ‘dull’. Would you be able to point me in the right direction for ordering a replacement headphone jack connector?

    Thanks

  6. avatar Tim says:

    I have the BA115HP. The sound is distorted. I opened the chassis and cleaned the headphone output and it worked fine. When I put the chassis back into the box it started distorting again. I bought this amp new and it has been on on going problem. No one has been able to figure this out. Happy for any help I can get.
    Thank you

  7. avatar Ken says:

    Tim, your amp is still under warranty. Contact Ampeg and they’ll point you in the direction of a service center.

  8. avatar Larry Edwards says:

    Waiting for someone to diagnose the “STYLE” 5 position switch/circuit that also has been a PIA
    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?

  9. avatar Paul PETCAVAGE says:

    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

  10. avatar Russ says:

    Believe it or not I found a chassis to one these amps in the swap shed at our local dump today.
    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!

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