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debugging a small amp

Started by kin0, May 11, 2011, 11:09:07 AM

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J M Fahey

You are showing 10 red lines.
It's very tiring trying to find a unique description for each of them, such as "the short squiggly one which is to the right but not *that* much to the right of R1, I mean the end of R1 that joins C2, not the one which touches R32 ..... " and so on.
When you actually find *which* red line I am talking about, your liquefied brain is flowing through your nose.
Not many neurones alive, after that, to understand my suggestions.
Label each of them with a letter.
After all, there *must* be some reason for designers to label resistors as: R1, R2, ..... R99 ; capacitors C1, C2, .... etc.
And as Phatt said, understanding nodes; meaning node is what connects many points together, is great to understand schematics (and PCB Layouts).
In fact, schematic design software exports long lists of nodes, which describe connections within a PCB, which will later be made real through copper tracks..
And don't worry about writing
QuoteI don't understand what do you mean
, we are *all* learning here.  :tu:


You cant check a circuit like this using a multimeter in this way.  The circuit components will give you false indications. Especially if they are active.  Unless you are checking the board with no components on.


I can't unsolder all the components O.O. What I did is to check the connection between each leg, I don't think that a components will do any different.

Here is the new layout. The brown point are all the ground. F is a line which is connected to the ground in every point of it (got 3 components connected on it)


Hi Kino,
       Lets start again shall we?
We have established It makes a noise but passes no signal.

Check you have DCV in the places I've marked pins 8 and 4 are the DC power

conections, so check this first.
Next,,,Pins 3 and 5 need some kind of DC offset voltage so check it.

then we move on to signal path.

something does bother me bout this circuit:(

There is no De-coupling cap after u1 which does not seem correct.
The circled *ground I've marked* at the tone circuit maybe wrong, others here may

know more but to my mind it needs to return to the bias node otherwise it may not

work correctly.


Okay I fixed every problem I had except the false connection between E and ground. (Phatt I'll check these things if it won't work after this one).
Is there any connection there to the ground?

J M Fahey

When you find an input grounded (when it shouldn't) the *very* common cause is that the soldering heat melts the plastic insulation and lets the hot wire touch the screen.
It happens to me even today.


but there is no plastic insulation to melt O.O

J M Fahey

Don't you use screened wire for signal input?
Please post a picture so we know better what we are talking about.
We can all see the *schematic*, which is a bunch of symbols, but nobody knows *how* it was built or wired.


But the input don't touch the ground. Line E which connects the presence trimpot leg to the C3. This line somehow touches the ground and I need to disconnect it from the ground (I can't see any visual connection).


Then remove the offending parts, remelt any solder and suck any blobs away until you have a clean, clearly visible track.

Then redo those components.

Ed, without close up pics of the board and offending components this is going to take a long time for us to help.


I fixed the ground problem and now I have some problems with the electricity.

The red 9v+ (the 9v input to the circuit) get 8-9v, but the blue 9v and the VB get only 0.2v. How can I fix it?



If 9VDC at Node (D5,R13,R16) then you should have close to 9VDC at pin 8 of the IC.
If not check polarity of C11, check C12 while you are at it.


The cap polarity is okay. And the node get 8.25v (idk why not 9) but the connection of the electricity to pin 8 of the chip is very strange. Instead of going straight from the 9V to the pin (red) it goes from R13 to the pin (blue). Is this the problem? And what can be the other reason for the VB not getting the electricity?


Blue is correct. and there is always a small voltage drop across the 91 Ohm resistor.

You are obviously struggling with this,, why not take some pics of the actual board if you can?
Preferably both sides.

Ed,, check that R14 and 15 are in fact 10k (usually Brown Black Orange)
You may have got the 470k R in the wrong place and that would certainly stop it working :'(

J M Fahey

Agree, you have some PSU ripple filtering courtesy of R13 and C11.
The price you pay is a slight voltage loss.
Nothing on Earth is free.
Well, maybe true Love  <3) , but not much else.
Also agree on debugging: cut the track that joins the node R14/R15 to C12/R2 and measure voltage across R14 and R15.
Post it.