Equalizer problem

[martinez_90]

Hi everyone, I've finished today an stompbox EQ from generalguitargadgets.com (here's the link: General Guitar Gadgets - 6-Band Graphic EQ) and of course it doesn't work :/

Why? I really don't know, I've checked twice every route and connections between the elements so it ain't that. But there are 2 weird things which make suspicious:

1. Look at the schematic:


there are two electrolytic capacitors there which are connected to each other PLUS to MINUS on the same route (as in the layout)! It's an absolute contradition for me.

2. Look at the layout pic:


Ok, there's GND on the bottom where e.g. jacks' GNDs are connected and so on but why the hell the "B" route on the pic where for exaple minus of the diode is connected is NOT connected with the main GND? I'll add that minus of the diode on the schematic is of course connected to the GND.

There's a voltage on the components but there's another weird thing- when i'm measuring voltage on the +9V input ("A") and touch second end of the voltmeter it displays 9V but when i touch it to the suspicious "B" route it displays only 8V.

What's wrong with that? Please help, I have no other ideas about that. :/

[defaced]

What does "it doesn't work mean"? Does the LED come on? Are you getting output? Are you getting proper voltages on your ICs? Have you tried to power the board directly with a single 9v battery to help rule out off board wiring mistakes?

The caps are connected + to - because they're stacked to create a bipolar supply. You'll also see this in tube amp power supplies but for a different reason, series caps with balancing resistors (the 10k resistors in your schem) increase the voltage the pair of caps can handle. Now notice that the node where the caps connect is grounded and there's a + and - supply line derived from either cap. This is a pretty standard configuration. If you grounded the B route you'd have a short which would cause some issues. A is your + supply line, C the ground, and B the - supply line. I've not gone through the effort to see if the ICs are powered correctly, but the supply on the schem is right on the layout.

Considering the source of the schem/layout, it seems to me that it'd be a verified design/layout. Have other people successfully built this? Right now it looks like you're seeking a design related problem when you should be looking for a wiring/solder error. Are you sure you installed everything properly and have everything that's off the board wired correctly? More than a few of us have read a 1K resistor as 10K and vise verse because the orange band looks alot like a red band when it's 3am and you've had too much coffee and solder fumes.

[Enzo]

They are connected plus to minus, but that point is also ground. Of you use two batteries, the top one goes from ground to +9, while the bottom one goes from ground to -9. Now if I were connecting a capacitor from ground to -9v, I would want the - end of the cap on -9v and the + end of the cap on the ground. And that is exactly what you see here.

It is not what they are doing here, but we quite often see caps in series in tube amps. It is a way of using two lower voltage caps to create a higher voltage capacitance.

All your op amp circuits are referenced to ground. In the two battery situation, that is zero volts. In the one battery situation, you will note the battery does not conect to ground, and the ground potential is established by the two 10k resistors with a cap across each. That ground is then sitting at half the battery voltage.

SO your -9v is conected to B.


I doubt voltage is your issue here. It is a lot more likely you miswired the switch or put a wrong component value in the board. If you have more or less 9v between A and B, then you should also have 9v more or less between pins 4 and 11 in each IC. Check it. If you do, then ther is no power issue.

Put your meter black probe to ground. Now measure DC voltage to A and to B. DO you get more or less +4.5V and -4.5V?

SInce the project is not working, we need to isolate the problem. I would first verify the board works. I would either have good power already having just tested, or otherwise wire a battery clip directly to points A and B. Directly, as in no power jack, no wire run to the input jack, just directly. Now connect the input jack directly to the input on the board - do not wire in the stomp switch. ANd finally wire the output jack directly to the output on the board - again not through the switch.

Now try it. If it works that way, we know the board is OK. If it doesn;t then we need to work on the board.

If the board works, then carefully rewire the jacks, switch, etc. One potential source of trouble if you have not used a 3PDT switch before is with 9 pins you could turn it 90 degrees and it looks about the same. But wired that way wouldn;t work.

"Doesn;t work" is not very descriptive. In what way doesn;t it work? Does it pass signal?

[martinez_90]

I've done some measurements as you said: between GND and A I've got 9V but between GND and B I have only about 6V. In addition between A and B or 4 and 11 pin of the IC I have about 3V.

I've connected directly input and output signals to the board, I'm using a DC jack and 9V power supply so I've connected + from jack directly to the board and - directly to the board GND.

I'd like to add that I unfortunately connected the B to the ground before so maybe it broke the ICs due to the short.

I wrote "doesn't work" and it means that I have no signal at the amp.

[defaced]

Remove the ICs and re-measure your voltages. Verify that the diode across A and B is in the right location and the right orientation.

[martinez_90]

After removal of ICs between GND and A is still 9V, between GND and B is 0V, between A and B is about 8,9V.

Diode is correctly oriented, however it's a 1N4007 diode instead of 1N4005 but according to the datasheet it shouldn't disturb the working of circuit (the only difference between these diodes is max volatage- 300V for 4005 and 1000V for 4007).

I think sth got messed up coz at earlier time I remember there was 4.5V on the one of this routes (probably C, on A as I remember there was always 9V).

[Enzo]

Is B shorted to C?

[martinez_90]

WAS for some time.

[defaced]

Now that it's not shorted, what are you voltages with the ICs pulled?

[martinez_90]

The measurements on the ICs' pins display various values around 7-9V on each pin:

IC1: pins: 1,2- 8.4V, 3- 6.9V, 4- 9V, 5 - GND, 6- 6.9V, 7- 8.5V, 8,9- 8.5V, 10- 6.9V 11- 7.4V 12- 6.9V 13,14- 8.5V
IC2: pretty same as IC1

[martinez_90]

After the exchange of the ICs, diode and connecting minus from battery to B I've got demanded +4.5V on A and -4.5V on B.

Still no signal, even the smallest. May incorrectly potentiometer or freq capacitor cause absolutely no signal?

I have at last no ideas.

[Enzo]

OK, YOU have the power supply straight. Now with the two ICs installed, and facing the right direction, check the four corner pins of each. They should all be at about zero volts DC with respect to ground - your C conection is ground.

[martinez_90]

I've got 0V on each corner pins EXCEPT of 7 pin of the IC1 and 8 pin of the IC2- they have about -3V.