Hello to everyone can somebody help me my bass amp was not working properly when I plug my input jack and start to play the sound is low and distorted and randomly pop a loud sound to the speaker and I can't find a schematic diagram on the internet can you please help me thanks in advance.
here are the pics..
Welcome renzy101.
Despite the "Boston" tag it looks like it is made by Soundking in China;
http://www.soundking.com/html/Products/Category_display/Guitar_Amplifier/2011/0323/2037.html (http://www.soundking.com/html/Products/Category_display/Guitar_Amplifier/2011/0323/2037.html)
I'd try writing to them <Sales@soundking.com> and asking for a circuit.
Meanwhile,
- disconnect the speaker and measure the voltage across the output connections, should be less than half a volt;
The two big caps C29 and C32 should have very similar voltages across them, say around 35-40 volts.
The smaller caps C33 and C34 should also have very similar voltages across each, but somewhere around 15 volts.
If it has a Pre Out and Main In you can try;
- plugging a known good lead into each to bridge them,
- take the Pre Out to another amp and see if the Pre is clean,
- take another amp Pre Out to this Main In and see if the power amp is clean.
Take the metal plate off the face of the main chip amp and find out its part number (e.g. LM3886)
Report back with your findings.
Roly thank you for the reply
You are right maybe it is a Soundking in China they are very similar.
By the way as you said I measure the two big cap C29 and C32 both are 40 volts, and also C33 and C34 are also 15 volts.
I test to plug my bass into the loop IN input and it is ok but if I try to plug it to the Master Input the problem arises, it is a LM3886TF.
It sure seems that it is pointing here to something on the preamp section of the amp. However, you just plugged the bass directly into the effects IN jack when testing. We really need to know the outcome when you plug a regular cable into the in and out(bridged) and then plug into the main input. That will help rule out the grounding jack on the effects out. Also, as Roly had said you can plug the pre out into another amp and we don't know if you had tried that either. This needs to be officially ruled out. After you perform the full test then we can conclude that the it is before the effects loop on the preamp.
Edit: I am assuming that you did not try all the methods listed by Roly and that all of these tests need to be explained better.
I plug it to the Main In and Pre Out and test it to another amp both are clean.
Well if the preamp is okay into a different main amp, and the main amp is okay driven by a different preamp, and a simple good lead between Pre Out and Main In is okay, then that would appear to only leave the bridging switch contacts on the Pre Out/Main In sockets themselves.
Apply contact cleaner, work plug in and out of each a bunch of times to scrub the contacts, see if that makes any difference.
I cleaned it but no success what should I do next?
Try connecting a different speaker and try the original speaker on a different known working amp. Got to make sure the speaker is good.
I connect different speaker but It's still the same. :(
Okay yeah I was just working in my shop and it just occurred to me that it is probably not the speaker. Sorry I had a brain fart there... Okay so it is actually looking like that the preamp is the problem. What type of op amps are in that amp? We need to check if there is the correct voltage getting to the preamp section. If you know what type of op amp it is then we can look at which legs are the v+ + and V- -. Many times it is 15vdc on the preamp rails, but not always. Lets find out.
Edit: BTW are all the knobs on the preamp doing their jobs?
Actually reading this whole thread again I can see why I am getting so confused here... Anyone else? I mean we need to still really isolate whether this is a preamp or power-amp issue. This is strange.
Edit: Ok this amp has an effects loop, line in & out and an input jack. We need to know that when you say "Main In" that it means Line In. Or if you mean input jack as you normally hook up your bass. So we feed the line out of good amp into the Line In or the Input jack of the bad amp, what happens? Is there a difference? Just gotta understand this better thats all.
Work done:
1.) I connected to INPUT to LINE OUT result - working without speaker
2.) I connected to LOOP IN to LINE OUT result - working without speaker
3.) I connected to LINE IN to LINE OUT result - working without speaker
4.) I connected to LOOP IN with speaker result - working
5.) I connected to INPUT with speaker result - not working (when you max the volume, comp and the bass knob volume the sound is small and distorted and loud sound occur randomly.
OP AMP
LM3886TF - 1 PCS (connected to the big heat sink)
LM13700M - 1 PCS (16 PIN located at the back of the motherboard)
TL072CP - 3 PCS
Okay perfect and good job getting these results. :tu: Sorry to get confused but we really need to make sure what we saying here about the tests. This clearly shows that the problem is on the preamp and now we can focus on that part of the circuit. Now without a schematic we will need to be tracing the signal through those op amps and using data sheets of the components to see where to find the signal. If you look up tl072cp you will see that it is an eight legged critter and Vcc- is on leg 4, while Vcc+ is on leg 8. Check this data sheet here... page 4
http://html.alldatasheet.com/html-pdf/28807/TI/TL072CP/99/4/TL072CP.html
So now you need to measure for DC voltage at each of those points of the op amps. DC voltage readings will be taken from chassis ground. Sounds like you know how to test voltages already, right? Just ask if you have a question here...
Also, the other chips the LM3886TF is the audio amplifier and is part of the output side, so we are not really concerned about it right now.
LM13700M chip looks like it might be used as input and output buffers for effects loop etc. Look at the data sheet on this one and note the voltages we have at pins 6 & 11.
http://www.ti.com/lit/ds/symlink/lm13700.pdf
Right now we just want to verify the voltages to the IC's and make sure the preamp power supply is good. This really will not tell us if the chips are good or bad. However, it will help to know that the power is good or not. Later signal tracing will probably need to be done, have you done anything like that before?
I already test the voltage 15v and I also replace all the 3 tl072CP and also the LM13700 chip.
So those parts can be assumed to be good since we replaced them already. The noise that you are hearing with the occasional pops and distortion makes me think of a bad signal cap. Not saying that is what is going on here but it also could be a bad solder joint. So can you trace some of the signal through the board? I mean it helps having a schematic, but if you take some time the signal path can be understood just by looking closely.
Anyhow lets just first re-flow solder on the film caps. Start by re-flowing the caps C1-C27 around IC1 and see if that changes anything. I mean at this point signal tracing is the only way to really trouble shoot to find where we are getting this bad noise in the circuit. Sometimes I have found those film caps to have issue's that make pops and distorts the sound. It would be best to use signal tracer to help locate where the failure is occurring, do you have that capability?
Doc he mentioned a "comp" control which I assume is compression, and the LM13700 being an OTA would be the obvious compression stage.
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.
Quote from: DrGonz78What 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.
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?
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;
(http://www.engineersgarage.com/sites/default/files/imagecache/Original/wysiwyg_imageupload/4214/Image-19.jpg)
(you can ignore the offset null connections)
...and for a dual...
(http://cds.linear.com/image/10906.png)
Keep us posted. :tu:
thank you very much for the guide it is very helpful on my part. :)
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?
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.