Help with RG's Neutron Filter

Started by ringworm, May 27, 2009, 12:48:05 PM

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ringworm

My first complicated build and it hasn't turned out well. RGs Neutron Filter www.geofex.com/PCB_layouts/Layouts/neutronpub.pdf
I'm getting A LOT of distortion and noise, not all of which can be down to not being in an enclosure yet, it's VERY noisy. I can hear some sweeping in some of the background noise but it's not coming through in the guitar. The only sign that it appears to be functioning in any way is a seriously noisy 'blatty' sound on some settings. I've taken all my readings in the bandpass mode.
Changes I made: I mounted the rotary switch to the board adding traces from the appropriate lugs to wires and existing on-board connections.
Here's a pic of the pcb i edited http://i26.photobucket.com/albums/c143/ringworm_1974/neutron_filter-pcb.jpg
I also used a 10uF non-polarized cap off the bypass switch.
I used two vactrol vtl5c3 and a 7660 for the charge pump.
I used resistors on the lugs of two pots to make up the odd pot readings using the Analog Alchemy EMH.
I can't get good enough sound to make adjustments with RX but there's a 3.3k resistor in for the readings.

When I read the voltages a lot of them are rising and falling too which makes posting readings difficult but here I go...
this means a falling signal <
this means rising >

Battery: 9.74v

U1
1 0.00
2 0.00
3 0.00
4 0.08 <
5 0.17 <
6 0.47
7 0.16 <
8 0.00

U2
1 0.00
2 0.04 <
3 0.17 <
4 0.30 <
5 0.61 <
6 0.20
7 0.12
8 0.15

U3
1 0.15 <
2 0.16
3 0.50 <
4 0.40
5 0.60 <
6 0.14
7 0.26
8 0.28

U4
1 0.00
2 0.00
3 0.02
4 0.15
5 0.20 >
6 0.00
7 0.00
8 0.15

Q1
E 9.74
B 9.02
C 0.20

D1
A 0.09
K 0.14

D2
A 0.14
K 0.17

Help!  :(

R.G.

You have (at least) power supply problems. The power pins (Pin8 = + and pin4=-) of the opamps do not show 9V and -9V.

Did you put jumpers in the places on the PCB where either a jumper or inductors go for providing power to the signal part of the PCB?
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

ringworm

Yes, I had jumpers in place. I've just removed them 30 mins ago and put chokes in (470uH). I'm starting to get pads lifting there now, I've done some repair but I  may need to etch the board again  :-[ It didn't make any change to the sound.

R.G.

Well, one thing's for sure - it's not going to work until you have approximately +9V on the + power pins and -9V on the - power pins. That's an absolutely necessary precondition.
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

ringworm

Ok, thanks for that. I think I've been reading  the voltages wrong with my multimeter. I was holding the black pin against the -9v pad, when I hold it to the ground connector on the ring tab of the input jack I start to get voltages (doh!) BUT they are reading negative on both sides of the op-amps, any ideas?

R.G.

Quote from: ringworm on May 27, 2009, 04:20:02 PM
Ok, thanks for that. I think I've been reading  the voltages wrong with my multimeter. I was holding the black pin against the -9v pad, when I hold it to the ground connector on the ring tab of the input jack I start to get voltages (doh!) BUT they are reading negative on both sides of the op-amps, any ideas?
Yes. That's what happens when there is no voltage supplied to the + voltage pin, or if the + voltage pin is shorted to ground by soldering or wiring issues.

Have you read "Debugging: What to do when it doesn't work" ? I wrote that because there is a logical order for debugging that is most efficient. The absolute first thing to do is to verify that the proper points on the PCB are getting the proper power supply voltage. There is utterly no point in doing anything else until you know the power supply is good.

In this case, you are feeding in power from a battery (?) at 8V, the charge pump seems to be making -V out of it, but the +V is not getting to the circuit, as close as I can tell from your description.

If it were me, I would do this.
- fire it up, place the black lead on the input jack ring as a ground point, as you already have.
- using the red probe, measure the voltages right where the battery comes onto the board. Are they correct? That is, does the black lead have 0.000V on it and the red lead batttery voltage (and you gotta get a fresh battery; 8V is nearly exhausted)?
- if so, measure the voltages on the two jumpers (or inductors if you installed them now) to the left of and below U3; the one to the left of U3 should be -8 to -9V, and the one under U3 should be +8V to +9V.
- check: are Q1 and R20 in place? Is the pad which R20 connects to grounded? If not, Q1 is *by design* holding the + voltage off the board.
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

ringworm

No, I haven't read your article, will do that. I just read the "what to do..." on this site. This is my first proper de-bug.
I fired it up again and can see a problem now, on the three supply pads the ground pad reads 0.00 the -9v pin reads -8.81 (battery is 9.47) and the +9v reads 0.00. I've tested the battery snap and it's fine, it reads 9.47 when disconnected from the board but with the black lead connected back to the board the red lead goes to 0.00 again. So is there some kind of short between the -9v supply and the ground of the circuit? I can't see any shorts on the copper side (I always check this by holding a torch through the other side to pick up any details) or on the top.