Broken 75w solid state bass amp

[aidan]

Hi all,

Wondering if anyone could tell me what might be causing my bass amp to sound how it does in the link below:

https://www.dropbox.com/s/8cpwjzhmc1ztbhk/Broken%20amp%20ashton%2075watt%20solid%20state.mp3?dl=0

The recording is a balanced line out straight from the amp, so the issue is not the speaker

Cheers,
Aidan.

 

[Bo Deadly]

Bad solder joint maybe. What is the amp? Something that has a schem? Open it up and trace back through the circuit with a scope if you have one.

 

[aidan]

It’s an Ashton ba-75. I got it back when I was at school so it’s probably like 10-15 years old.

It’s also working sort of like a gate; no matter what level you have it at, it won’t start making sound until you play it fairly hard.

I have found a schematic for the power supply and poweramp section but not the preamp board; as it appears to have the xlr out coming straight from the preamp circuit board, bypassing the poweramp section and it still comes out distorted and gate-like.

Haven’t been able to find a schematic for the preamp, any ideas on where to find that?

Unfortunately don’t have an oscilloscope or anything apart from a basic budget digital multimeter.

 

[scott2000]

I have found a schematic for the power supply and poweramp section

https://s20.postimg.cc/4pdb1axkd/Ashton_BA_AMPsection.jpg


in case it helps

 

[PRR]

Get this (will take over a minute to be ready):
https://elektrotanya.com/ashton_ba150_preamp_sch.pdf/download.html
Also attached.

That's a stand-alone preamp with a bigger number, but may be the same designer and general thinking.

Check the bi-polar power supplies. If one has failed it may sound as you describe.

 

[aidan]

Thanks PRR

A lot of the ba150 preamp seems to be exactly the same as the preamp section on my ba-75. Even down to resistor values and everything.

I used the audio probe method to dig round the preamp circuit, and, for the same values as attached in the ba150, the sound seems to get distorted at the op amp - fine at the 100picofarad cap at C2, then super loud and distorted at the op amp output and the negative feedback loop- resistors R3 and R4 and the 120p cap C3.

The transformer is putting out a pretty neat 15.3 volts going into the preamp section. Perhaps I should try the +15 and -15v regulators? Seems like the op amp is getting too much input??? The same distortion and gated effect until you blast the input volume.

Also, how might I  check what voltage the opamps are getting into them and putting out with just my digital multimeter??

Cheers,
Aidan.

 

[PRR]

> Perhaps I should try the +15 and -15v regulators?

Why not?

 

[aidan]

When u said bipolar power supplies did u mean some nonpolarised capacitors, or the the 4700uf 50v filter capacitors coming off the transformer after the rectifiers, or capacitors for the power supply coming off 15v into the preamp section? Or something else??

 

[scott2000]

When u said bipolar power supplies did u mean some nonpolarised capacitors, or the the 4700uf 50v filter capacitors coming off the transformer after the rectifiers, or capacitors for the power supply coming off 15v into the preamp section? Or something else??

your op amp's pin 4  and then pin 8  should have + and -  vdc.........referenced to ground

you could check there.....