Need help for Boston AK60GB bass amp

[Roly]

Doc he mentioned a "comp" control which I assume is compression, and the LM13700 being an OTA would be the obvious compression stage.

[DrGonz78]

Yeah I thought that chip might be used for the compression but I was not sure. Anyhow he said that he already changed that chip. Still it could be something in the circuit there that is the culprit. Since the boards out and ready, I thought maybe look at the first stage of the input. Really would be nice if he could scope or trace the signal, at least, around the first part of the preamp. What you thinking Roly?

Edit: Plus he did say "when you max the volume, comp and the bass knob volume the sound is small and distorted and loud sound occur randomly." This makes me think that it is later in the preamp around the area of the bass pot and compression circuitry. Still could just be a bad solder joint somewhere.

Double Edit: Perhaps take a better close up pic of the bottom the pcb board around that area so we can see the condition of the solder joints around that part of circuit. Bass pot and comp. pot. Also, the solder joints on the actual pots need to be in question even.

[Roly]

What you thinking Roly?

I'm thinking that if I had just a couple more billion dollars that sexy young women would be throwing themselves at me.

I'm thinking that logically the grounded resistor in the op-amp NFB loops going open due to a dry joint would turn it from a gain stage into a voltage follower.

I'm thinking that my intuition that this has something to do with the OTA compressor stage has no rational basis.

I'm thinking that having nerves in teeth seems like a really bad and pointless design idea (but that I'm glad somebody chickened out so I got to see the dentist yesterday instead of next week).

I'm thinking that a powered computer speaker would make a reasonable signal tracer when a CRO isn't available.

[renzy101]

thanks for the help I have no signal tracer I will make one as you suggested.
I will make an update when I already have signal tracer.
can you teach me how to use the signal tracer beco'z I have no idea how to use it.
does I have to hook up the first wire to the speaker amp board and the other wire for tracing the board correct me if I'm wrong?

[Roly]

When you don't have an oscilloscope so you can see what is going on with signals, then a signal tracer is the next best thing because it allows you to hear what is going on.

A powered computer speaker (battery or wall wart) is essentially a signal tracer; all it needs is a way of clipping it's ground/common to the amp ground/common, and a bit of a flying lead off the hot plug connection so you can probe around a bit.  A simple way to do this is rat a suitable socket out of something and connect a clip lead to ground and a length of wire to the hot side, and plug the computer speaker in.

The general idea is that you feed a signal in the amp input, say from an MP3 player or similar (and it's handy to have an adapter lead to make a good connection), then starting from the inside of that socket probe along the signal path.  If the input socket is good (and they often aren't) you should hear sweet music.  Move along to the input pins of the first op-amp (or base of the first transistor) and you should hear the same, if not you have a break or a short somewhere between.

When you move across the first stage, from the op-amp input to its output, or transistor base to collector, you should need to turn the tracer volume down because there should be an increase in signal level.

This is a fairly simple amplifier without complex path switching and so on, so it should be fairly simple to move along the board from the input end, hopping from op-amp input to output, getting an increase in signal at each stage (depending on control settings of course).

You will notice that the signal drops a lot across the tone controls, but should pick up again right after.  Do not try and probe past the input to the main power amp as the voltages present are likely to make your computer speaker unhappy, but by the time you get there you should have either confirmed that the preamp is working as intended, or located the stage where the signal vanishes or gets horribly distorted.

If you want to make up a more formal tracer for frequent bench use then it would be a good idea to have a DC blocking cap, a fixed resistor to protect the tracer from excessive signals, and a pot right on the front end to allow probing of high signal levels around the output, but if just for this amp then don't use a computer speaker you value and be careful only to probe op-amp inputs and outputs you have identified from the datasheet or amp circuit/schematic.

While you are tracing it would be a good idea to disconnect the amp speaker - it will only be a distraction and you may accidentally make some very loud noises.

Something you need to do is identify the chips used in the preamp.  These may be single op-amps, but they might be duals, so you need to get the type number from the top and identify them.

The vast majority of op-amps use the same pin arrangement (which is something to be thankful for), and these are almost always shown in the top, or component side, view.  The notch identifies the pin 1 end and they very often have a dimple to identify pin 1 itself;


(you can ignore the offset null connections)

...and for a dual...


Keep us posted.   :tu:

[renzy101]

thank you very much for the guide it is very helpful on my part.

[renzy101]

I already find the distorted sound in one of the op-amp when I  feed the signal in the amp input - and + IN and also the OUTPUT of the op-amp the other side of the op-amp is working I attach a photo so you can see where it is does.
does it has to do with the resistor or the diode?

[DrGonz78]

First de-solder one leg of each of any component you want to check with the multimeter. On ohms settings do any of the 3 resistors measure out of tolerance? If you have a diode checking feature, do any test bad? That compression pot far RIGHT leg(in reference to the picture) solder looks a bit cold>>?? Take a better close up pic to see the tracing of the circuit on the bottom of the board around that area, plz. Cold solder joints can cause all sorts of problems.