stage center reverb grounding question

[chaliapin]

Hi all,

I am very warily posting this as I have almost zero understanding of electronics but am trying to learn. I have been trying to build the SCR effectively by a 'painting by numbers' approach by simply hole-boarding the pcb layout as best I can. Did that, and was unsurprised when it didn't work. Chip gets hot, I suspect a short, anyway, I am learning how to use my multimeter and following debugging tutorial. What I am really not understanding is the grounding of the circuit, I built it in this format

http://www.generalguitargadgets.com/diagrams/stage_center_reverb_lo2.gif?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

but in this pcb layout all the grounds seem to be connected, fine, but on the scematic there is a differentiation between circuit and chassis ground. I have tried to google this but I am still not understanding this difference between layout and schematic, specifically in reference to the ground. Any advice on the whole circuit is appreciated but I understand that I need to slog through the debugging with the meter.
thanks in advance,
Mike

[bluebunny]

"Chip gets hot" is not good!      Do you have the IC the right way round?  Pin 1, marked by a dot or notch, is towards the topmost edge of the board shown in the GGG diagram.  The TL074 is almost rotationally symmetrical.  Put it in wrong and all the opamps are still in the right place.  But the power connections are swapped.

I'm sure R.G. will step in with a good explanation of this ground vs. that ground.  From your beginning debugging point of view, you should have continuity amongst all "grounds".

BTW, welcome, and good luck with the learning and the building.  You'll do lots of both around here!   

[chaliapin]

Hi, thanks for the reply, IC is now the correct way round but I made a few mistakes and hooked up the batteries before I found them, I have 3 spare chips but am carrying on debugging with the original in as I don't want to fry a new one. the Schematic is here

http://www.generalguitargadgets.com/diagrams/stage_center_reverb_sc.gif?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

I basically don't understand the difference between chassis and circuit grounds, is one at 0v and one at -9v?

[bluebunny]

I basically don't understand the difference between chassis and circuit grounds, is one at 0v and one at -9v?

No, both are essentially the same thing.  There should be continuity amongst all the "ground" connections (including the -ve terminal on the battery).

BTW, if the IC was the wrong way round, it may now be damaged.  Perhaps someone else better qualified might be able to predict whether it was likely to have died.  If it became too hot to touch, my guess is RIP IC.

[chaliapin]

ok, great, thanks. Seems that isn't the problem, I shall press on, I am aware that the IC is probably fried but I am trying to find the mistake(s) without putting in another and frying that one too,
Mike

[chaliapin]

ok, am struggling to get my head around the most basic stuff but am searching for obvious mistakes I may have made.

Q. Where there are several wires in the diagram going to ground, can I connect two components and run one wire, ie. the two pots, instead of each running to ground I have connected the ground pins with a short wire and run a longer one to ground? I'm pretty sure this is ok but am double checking.

Q. The LED indicator appears to have stopped working and there is no continuity accross it, this isn't affecting the (non) functioning of the circuit right?

Q. With the batteries set up as pictured I am only getting about 3v reading across the two batteries, I assume this is bad right?

I know these are deeply simple questions, I hope I'm not being annoying, I have visually checked the circuit a lot and checked for solder bridges with the meter but still NOTHING on the audio output so am sure something is badly wrong. I've checked all the resistors are doing their job and that is all the progress I have made so far, advice appreciated,
Mike

[bluebunny]

1. Yep.

2. Nope.

3. Yep.

Btw, "continuity" across an LED (or any diode) will be in one direction only.

You might want to build an audio probe to find out how far your signal does get.  Search the forum for details: you'll find it turns out to be dead handy to have in your toolkit.

I hope I'm not being annoying

Nope. 

[PRR]

Hot chip and dead batts mean you have the polarity reversed. Re-check chip Power pins + and -, trace back through diode (stripes), to battery lead color codes. Measure voltage from ground to each chip power pin, note polarity.

> I have visually checked the circuit a lot

I once had a large mixing console on my repair bench for 7 months until I found the (my) screw-up. Get it up against your nose. Walk away from it and go fishing. Ask your brother or dog to look it over with a fresh eye. (I once explained a problem to my dog and the answer came to me.)

[chaliapin]

thankyou both, I will progress as reccomended. Just checking that the way the batteries are wired in the above ggg link is correct? Yeah, clutching at straws I know. The batteries show normal voltage when not wired into the circuit.

[chaliapin]

from looking at this

http://www.generalguitargadgets.com/pics/stage_center_inside1.jpg?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

I think I've built the circuit upside down as it were, I've made it as if the pcb image was taken looking at the soldering side and 'through' the pcb it's self to see the components, for example, when looking down onto the component side R1,2 and 3 are on the left and it looks as if they should be on the right. I'm guessing I can't just rotate the chip and, in fact, I need to start again. Is bending the legs over on the IC and then flipping over going to work. Hmm.
Thanks for the help so far.

[chaliapin]

ok, I tried flipping the IC and hey, some sound! Very pleased! However, the signal is over-amplified and distorted and the un-reverbed signal is not making it through the circuit. I shall do some more testing with the meter as I'm now feeling alot more positive. If I still need help I expect I should start a new thread with all the voltages etc?

[bluebunny]

I think I've built the circuit upside down ...

Layouts tend to show the view from the component side - as if the PCB substrate were transparent.  If you've flipped your chip onto its back and bent its legs backwards, I think you might end up confusing yourself more.  It's making my head spin just to think about it.      I'd start again.

[chaliapin]

I didn't bend and flip, just rotated as it looked to me the only non-symmetry was the polarity which was what needed reversing, I assume that apart from that the four parts of the chip are interchangeable? Ihahven't had time to do any more checking but the voltage across the batteries was up to 18 or so as I understand it should be, I think I can proceed as per the debugging post.