Need help for Boston AK60GB bass amp

[renzy101]

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..

[Roly]

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

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.

[renzy101]

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.

[DrGonz78]

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.

[renzy101]

I plug it to the Main In and Pre Out and test it to another amp both are clean.

[Roly]

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.

[renzy101]

I cleaned it but no success what should I do next?

[DrGonz78]

Try connecting a different speaker and try the original speaker on a different known working amp. Got to make sure the speaker is good.

[renzy101]

I connect different speaker but It's still the same.

[DrGonz78]

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?

[DrGonz78]

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.

[renzy101]

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

[DrGonz78]

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?

[renzy101]

I already test the voltage 15v and I also replace all the 3 tl072CP and also the LM13700 chip.

[DrGonz78]

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?