Coxx GG-30 with low volume issue

[Superpunky]

Hi everybody,
I have a Coxx GG-30, a 30W combo solid state with a low volume issue which is driving me crazy . The guitar can be heard only with all volumes (including equalization controls) at max level.
I tried to search for the schematics online, but they are impossible to find and there's not an official site. Maybe it's a rebrand?
The circuit board is based on an LM1875T and uses two 072D IC for pre and effects i guess (attached images).

Here is what I did/found:
- The low volume issue affects also the heaphones output
- The volume is low both on clean channel and with overdrive. I tried to use both inputs (high and low Z).
- I made a reflow of almost every soldering
- Potentiometers are OK
- The power supply is OK, +- 25V are everywhere needed
- I tested the resistors and they seem to be ok
- I tested all the capacitors onboard and none of them seems to be shorted
- Update: i tested both 072D. All 4 OpAmps are working correctly

Considering that I don't have an oscilloscope here at home, are there any tests that you can suggest to do to find out what's happening? Also, is there a way to find the schematics online?

Thanks!

[Jazz P Bass]

Ok.
So you measured the + & - 25Vdc supply but what is on the opamp supplies?
Pin 4 & Pin 8.
What is the actual output of the LM1875 (Volts AC) with the speaker disconnected?
What is the signal amplitude at the input pin ? (1 Vac would be nice)
With a + - 25 Vdc supply the IC should be capable of sourcing 20 watts into a load.
(that would equate to a 12.5 Vac RMS output signal into an 8 ohm load)

[Superpunky]

The opamps are supplied with +-15V (+15V on pin 8 and -15V).
I also tested the AC voltage of the various pins of the LM1875. The tester measured almost 0Vac on output pin 4 (?) and on both + and - inputs (pin 1 and 2). Am I doing something wrong? I made this test with the speaker disconnected and with the speaker connected, same results.
What source could I use other than the guitar to test the signal? I don't own a function generator

[Jazz P Bass]

You will have to trace out what is supplying the input signal to the LM1875.
Most probably one of the opamps.
Use the schematic that I attached to verify what input pin they are using. (Pin 1 or 2)
It should be a capacitor coupled input (C12?)
If so, you can lift that cap to isolate the LM from whatever is driving it.
(this may be called for if the LM is toast & dragging down your supply signal)

[Superpunky]

I tried to trace the inputs of LM to find where the signal comes from. I poorly drew a schematic (in attachment, sorry for low quality). It turns out that the signal comes from pin 1 of IC3.
As suggested, I lifted C23 to isolate the output of the opamp in IC3. I measured 0Vac and 0Vdc on pin 1 both before and after taking out C23.

The really weird effect is that if I play the guitar without C23 I still can hear the speaker working with the same volume as before.  I can't understand where does that signal come from.
The output on the speaker is 0,2Vac

[Jazz P Bass]

C23 looks as if it is a part of the Reverb circuit.
There has to be a separate input for the dry signal.
You show the tone control going to an opamp.
That may be it.
There is no label of the output pin. Pin #7?

[phatt]

If the amp worked before and then started to loose volume it could be a corroded contact sw.
As the amp has an External speaker output socket then check that tab on the socket. These elcheapo sockets are a common issue for fading volume loss.
The external speaker likely disconnects the internal speaker when used so wise to check.

Insert a guitar plug in and out a few times or just clean the contact points,, if you have an external speaker maybe try that as well.
It's a long shot but the amp might be working just speaker fading out due to a bad contact in that socket.

That relay has got me puzzled Any clue as to what it switches?
Phil.

[Superpunky]

I controlled and cleaned all the contact switches, they are ok on guitar inputs and outputs. I tested the output with an external speaker as suggested, the volume is still low. The external speaker plug is not designed to disconnect the main speaker. Only the headphones disconnect the main speaker. Odd...
Anyways, all the possible ouputs have a low volume.

The relay is a double contact. It switches when the overdrive switch is turned on, so I guess that it sends the signal to different paths on different opamps. The low volume issue affects both the clean and the overdrive channels, so I think that the following opapms are working. I'm still working to find out the schematics to track the signal path, but it's quite difficult.

[phatt]

OK if the speaker contacts are intact then I can see you have 2 likely fail points the *power chip and the relay*.
You need to find the path of the main signal, if it goes through the relay then that may not be passing signal. I'm assuming the OD switch activates a control voltage to the relay so that may have failed.

(I'm guessing, The Reverb path is likely a separate path to dry signal and not passing through the relay ,, hence you can faintly hear it)

As Mr Jazz has mentioned you really need to isolate the input on that LM1875 power chip then hot wire a signal into it to see if it works. I'd test that first because it's a common fail point and not too hard to test out.
If that works then you need to find the signal path in the preamps/relay/reverb.

Maybe wise to spend some time to draw out the circuit,, or at least part of it.
When I'm stuck this is how I work out a schematic.
Most single side PCB's are semi translucent so with a strong light under the PCB you can follow the tracks from the component side. and draw them out. often requires several attempts and lots of waste paper.
It might seem hard and does take time but it's a good way to get the brain to connect how it all works.

Older PCB's are laid out in (human) logical format while some later pcb's use computer lodgik and they can be a real challenge to back engineer.
Phil.

[dmeek]

The way I draw a schematic from a pcb is to scan or photograph the foil side, flip it mirror image in the computer, print it on paper and draw the components on.
Works great for single sided boards. Double sided is a little more trouble. SMD? Forget it!

If you can post photos of both sides of the circuit board I can do it for you.

[Superpunky]

OK if the speaker contacts are intact then I can see you have 2 likely fail points the *power chip and the relay*.

Yes, the relay is working. I already tested all resistors, capacitors and opapms of the circuit. Maybe there can be some undetected short between the tracks, but my suspects are on the LM1875, so I ordered two from China. For that price (about 1€) it's worth a try.
Furthermore, R13=68k, R15=68 ohm, C12=224n, so the gain in this configuration goes up to Av=1000. It's difficult to measure the input signal without an oscilloscope. At this point it's easier (and really cheap) to make a guess and change the LM.

The way I draw a schematic from a pcb is to scan or photograph the foil side, flip it mirror image in the computer, print it on paper and draw the components on.
Works great for single sided boards.

I'm working on that, I will share with you the results as soon as I have done. Meanwhile, I attach the images of the back side and of the component side.

[Jazz P Bass]

Well I see that they highlighted the Reverb section.
So you can ignore the one op amp. (IC3)
I also see that it looks like the Gain control is snapped off.
If so, set the amp for Clean and look at the output pins of IC1 Pin 1 & 7.
I would think one output is for Clean & the other is for OD.
From there it should go to the relay.
The relay output should go the the LM1875/ Pin #1.
You do not have to have a scope.
A decent volt meter set to read Volts ac should suffice.

[dmeek]

Both the External Speaker and Phones jacks have switches that could cut out the speaker, also check the .1 ohm resistor R16.

[Superpunky]

Both the External Speaker and Phones jacks have switches that could cut out the speaker, also check the .1 ohm resistor R16.

Wow, man! Thanks for your work, good job! I owe you some beers 
I was working on the reverse engineer as weel, I made the model with Target 3001!, but I exceeded with the number of pin/pads of the free version so it was not possible to generate the schematic. At least now I have a nice 3D model  of the board

Actually, the switch of the headphones jack cuts the "ground" for the rest of the chain, this doesn't happen with the switch of the external speaker. In fact, if you notice, the + signal of the line out jack goes in series with the ampli speaker. I don't know if the LM appeciates this
The resistor R16 is ok. It measures exactly 0.1ohm.


I finally managed to make some decent measuring on the signal:
- My test guitar generates 40-50 mVac at full volume
- The exit of the first opamp (pin 1) is between 300 and 400 mVac
- The exit of the second opapm (pin 7) is between 700 mVac and 1Vac
- The + input of the LM (pin1) measures always 25 mVac, with or without the guitar signal, so there's something wrong
- before R11 (exit of treble pot) the signal is ok and is up to 200 mVac
These measures suggest that the LM1875 is broken and is dragging down the signal (consider that I already tried to disconnect C23, so it can't depend on the reverb section. C11 and R12 are ok as well).
So, now I'm waiting for the replacement LM from China, I will update you asap

[Superpunky]

SOLVED!

Sorry for long time without update. I received the LM1875 today, I changed it and the problem was still there   
I decided to remove the surrounding capacitors and measure them one by one. It turns out that C12 was damaged, but not in short so I didn't detect it before. It measured some kohm in resistance and 0,5 uF instead of 4,7 uF. That damned capacitor drove me crazy.
Anyways, I changed it and the amplifier is now loud again.

I would like to thank a lot everyone that helped me, in particular dmeek that made a lot of work with reverse engineering.  Without the shematics I would have been lost. I hope that these posts will help people with the same amplifer.