Distortion as note fades out.

[Axtman]

I am working on an Ampeg BA115 bass combo amp. I notice that as the note fades out, the sound becomes distorted. What would cause that?

[dmeek]

Probably crossover distortion. Need more bias current.
Not sure if this is the right schematic.

[TomCarlos]

I have this schematic in my collection -

BA115_Schematic.pdf

[Axtman]

I set the bias to 5mV, as it says in the schematic. The pot is VERY sensitive. The bias voltage keeps creeping up slowly. Ideas?

[Axtman]

The rails test only 36v instead of the 40v on the schematic.The low voltage 16v tests fine.

The main filter caps tested low. They are supposed to be 4700uf. One tested 4300uf, and the other tested 4100uf. I changed them out, and the voltage did not change. It is still 36v.

[pdf64]

as the note fades out, the sound becomes distorted. What would cause that?

Those symptoms line up with some types of speaker faults.

The rails test only 36v instead of the 40v on the schematic

Secondary and non regulated rectified voltages are determined by the mains voltage and transformer ratio / copper loss.
Always make contemporaneous mains voltage measurements, and the secondary V AC.

[Tom Phillips]

Those symptoms line up with some types of speaker faults.

I have experienced that too. Always worth checking by plugging in to a known good speaker. My thought is that this issue can be caused by a slight voice coil rub (or a loose voice coil winding) that causes only a slight noise that is masked when the signal level is high.

[g1]

The rails test only 36v instead of the 40v on the schematic.

Please post the schematic that you are using and is correct for your version of this model.
Are you on a bulb limiter?

[Axtman]

g1,

TomCarlos posted the correct schematic diagrams in his post.

I was using a light bulb limiter until I was sure that nothing was going to blow up. Then I switched over to plugging directly into outlet strip.

BTW, with the light bulb limiter the voltage was plus/minus 34 volts.

[g1]

Is the +/- 36V right at start up or after the bias creeps up from 5mV. How high does it climb to?

[TomCarlos]

For ease of viewing, I created JPEG versions of page 1 and 2.


Update - and just a side note .... Periodically, I like to have large paper versions of a file. So I take the JPEG to Office Depot and use the self serve machine to create 11x17 copies. If you have a JPEG, it will auto size to fill out the sheet. If you have PDF files, they print as a smaller size. So this is another reason to convert to JPEG. If you do not have Adobe Standard, you can try using the Microsoft Snippet tool or similar.

[Axtman]

The voltage is +/-36 volts when the amp is initially turned on. I adjust the bias voltage down as it creeps up. It starts out at about 3.5v. It takes about 2 minutes to stabilize. Could there be a bad cap?

[x-pro]

Check C31, C20

[x-pro]

I notice that as the note fades out, the sound becomes distorted.

Feed the signal from the oscillator to the "CD input" and watch an oscilloscope of the response to an equivalent load at the output of the amplifier.

[The Dude]

As stated above, the first thing I'd check would be the speaker. This sounds very much like coil rub to me.