LED/LDR - How to read in schematics???

[doug0147]

I am building a DOD EF440 from GGG. I assembled the whole thing and unfortunately it doesn't work. I double and triple checked all components, polarities, IC, circuit board traces, and any thing I could think of. I decided I want to try out all the components on a breadboard to get a working circuit. My problem is I'm not sure how to read the LED/LDR on the schematics. It has 5 legs and I'm not sure what goes where according to the schematics. I made sense of the side that has 2 legs, but I'm a little confused about the side with 3 legs. Can any of you guys explain this to me. Thanks 

[biggy boy]

Hi
How about a link to the schematic?


Glen

[doug0147]

Sorry about that. Here it is.

http://www.generalguitargadgets.com/pdf/ggg_ef440_sc.pdf

[Mike Burgundy]

for the two legs, the one with the dot is the LED's cathode - connects to ground.
The three legs are the resistor bridge, and wired as you see it on the schem - the outer legs are the outer connections, the center leg is the center tap, as per http://www.datasheetcatalog.org/datasheet/perkinelmer/VT500.pdf page 49. Does that help? You can find a LOT by googling for datasheets.

[doug0147]

Ahhhh Haaaaa! I get it now. Thanks.

Is there any simple way to test if the thing is working correctly?

[Mike Burgundy]

Sure. Make sure you have the polarity of the LED correct, and use a current limiting resistor as you would with a regular LED. Make sure you do NOT exceed 30mA.
Clip a DMM across the outer two leads or outer/inner lead, set to resistance, and connect and disconnect power to the LED. If the resistance changes (lots), you're good. If you want to get picky, you can compare the two halves (use the center lead and either of the outer ones) to see if they match up.
It should turn "ON" (LED on, resitance drops to a few 100 Ohms) quite quickly, OFF (LED off) somewhat slower, to several 10k's to 100k after a second or two, between the outer legs.
hih

Edit:
That said, what are the symptoms of your problems? The pedal, I mean.
if it's completely dead, there's at least something other than the vactrol going on.If it doesn't, er, envelope filter (as in wah), but it does pass audio, the vactrol and it's supporting circuitry are suspect.

[doug0147]

Thank you Thank you Thank you Mike 

[Mike Burgundy]

sorry - crossed my edit - see above

[doug0147]

Well this is whats seems to be going on.

If I power it up sound goes thru but no wah effect. The knobs don't do anything either. If i power it off (disconect battery), but leave the bypass switch in the "on" postition the sound sound still comes thru, but much less. I don't know if that makes any sense. I checked the whole circut board and I can't find any leads touching.

One other thing, I solderd on most of the components about 4 or 5 days befor I did the LED/LDR, the diodes, one resistor, and all external wiring. The board was a little oxidized and I had a bit of a hard time getting the solder to stick to some places. But despite that, I did a continuity check on all the suspected solder joints and did not find any thing wrong.

[Mike Burgundy]

'Kay, might be the vactrol, but it might just as well be something else. After you've checked the vactrol, do the basics - you've checked suspicious soldering connections, so that's done for now (although it might just be one that's not under suspicion. You never know...) Check the opamp's pin voltages and post them here. If that doesn't yield anything, trace the signal (do a forum search for signal tracer, or alternatively there should be something in schematics I think - it's basically a cap with some sort of passive limiting device - I've often done it with a tube amp (watch it - might get loud and/or pop) and just a 100n cap and some wire- and see how far the signal goes where. Especially if it reaches the envelope detector (pin 3 in, pin 1 out). The other half of the opamp seems to be fine.
It's normal for *some* signal to get through - there's a path looking at the schematic -if you consider the opamps to be dead ends - between input and output. Won't be much, but it's something.

[doug0147]

Well, I'm at work right now, but I did check the op amp pins earlier and I found some strange results. Pins 2 and 3 ( I'm pretty sure those are the ones) had low voltage ( about 2.5) while the others were around 4.something. I should of posted this info earlier.

[Mike Burgundy]

Please recheck. Those voltages look wrong, but we'd better make sure exactly which pins they occur at.
if the first opamp is off, that points to nothing (probably) being wrong with the vactrol, but with something in the envelope detector ckt - either the opamp or wiring, maybe a shorted cap (C2?).

[doug0147]

I forgot to check the voltages last night, but I'm positive they are around what I posted. What you said C2 makes sense

[doug0147]

OK here are the readings on the opamp

BAT 8.82

1 - 4.42
2 - 2.57
3 - 3.76
4 - 0
5 - 3.57
6 - 4.41
7 - 4.42
8 - 8.83

I swaped out c2 and still got the same thing

Now I'm new to electronics, but this seemed strange to me.

The side of C2 connected to the pot had -8.82v (i put the + side of the probe on batery + and the - probe between the cap and the pot) the side of the cap connecte to R4(10K) is -2.33 or +2.57. The current doesn't seems to be affected by R4 and pin 2 on the opamp ends up at 2.57

Does that make any sense?

[Mike Burgundy]

Okay, that makes sense.
You will benifit hugely from some reading on basic opamp circuit design - search in the beginner forum/project, there's loads of links.
To recap quickly, most opamps want a bipolar supply - so you have -V, ground, and +V. If there is no input (inputs at ground level) the opamp WILL force its output to ground (0V) no matter what, sucking in or putting out current if it needs to to level the voltage to 0.
Since pedals mostly work with batteries (so you have ground and +V), there's a simple trick to "fool" the opamp into believing it has a dual supply. This is done with a devider network of two identical resistors between V+ and ground - effectively deviding V+ in half between the two resistors. Call this Vr (for reference). This is R7/8 in your schem.
If you present this DC voltage level (Direct Current - it stays at the same level, verses Alternating Current (AC) which swings around "0", such as a signal) to the opamp + input, it will accept this as a virtual ground and "think" it has a bipolar -4.5V/+4.5V supply.
But you then have a different "opamp ground" compared to real ground - to prevent nasty problems you *isolate* those bits with capacitors - they block DC but let AC (such as a signal) through. They have resist less with increasing frequency - depending on their value.
So, Vr keeps (have a look at the schem) IC1b's inputs at 1/2 supply voltage.
They also keep the + input of IC1a (pin3) at 1/2V+ through R3/R2. The IC will now want to force it's output to a steady 4.5V as well, and when that's done, there's no current flowing.
C1,2,3 and 8 isolate DC levels. Since there's a steady DC level, R4 doesn't carry current and doesn't drop voltage. That suggests the cap to be ok, by the way.
Looking at the voltages, something's definitely off at pin 2. lift C2 out, measure again. If this changes things, C2 is off or there's a short across it. If nothing changed, remove power and measure for resistance to ground at pin 2. If you don't get something really high, there's a short or the IC's bad. Take out the ic and measure again at pin 2's pad on the pcb. If it's still low, there's a short somewhere. If it's high now, the IC's bad.

Keep at it, keep chipping away at possibilities eliminating them one at a time, keep learning, read up and see if you can get a little insight in the hows and why's - it will really pay off in the long run as well.
Good luck

[doug0147]

Thanks Andrew,

That was a load of good information. I'll let you know how it turns out.

[Mike Burgundy]

Andrew? 

[doug0147]

Whhoops . Sorry about that Mike. I guess I had that name in my head.