Newbie needs debugging tips for DOD Envelope Filter 440

[SolariEGO]

Hi guys,

I'd be really thankful for tips to debugging a DOD EF 440 (kit from GGG), schematic at http://www.generalguitargadgets.com/pdf/ggg_ef440_sc.pdf. The box has absolutely no sound in effect mode, normal sound when bypassed.

The only modification I made was to change the red indicator LED to a yellow led, thus also changing R17 from 1K to 348 Ohms. LED works OK, 3PDT switch measured and found OK, but the IC readings are strange.

Meter readings:

Out of circuit battery voltage = 8.88 V

Voltage at the circuit board end of the red battery lead = 8.78 V
Voltage at the circuit board end of the black battery lead = 0 V

IC1 (JDs readings from the instructions file at http://www.generalguitargadgets.com/pdf/ggg_ef440_instruct.pdf in parenthesis)
Pin 1 = 8.03 V (4.34)
Pin 2 = 6.63 V (4.30)
Pin 3 = 0,66 V (4.37)
Pin 4 = 0 V (0)
Pin 5 = 8,66 V (4.30)
Pin 6 = 0.61 V (4.37)
Pin 7 = 5.43 V (4.36)
Pin 8 = 4,59 V (8.7)

D1
A = 0 V
K = (different readings every time, all under 2 V and dropping fast)

D2
A = 0,9 V (different readings every time, all under 2 V and dropping fast)
K = 1.31 V

OC1
A = 1.31 V
K = 0 V
Legs (from anode to cathode side)
1 = 4.45 V
2 = 4.55 V
3 = 4.64 V

Any hope, doctor?

[Krinor]

Pin 8 = 4,59 V (8.7)

I find this to be a bit bewildering...
Have you checked the whole circuitboard for possible shortings ?
I just measured my own DOD440 (also from GGG) and with a battery at 8,48V I get 8,46V on pin 8.

Edit:

Oh, and have you checked that the IC is alligned the correct way ?

[SolariEGO]

Hei på deg, Kristian!

Yes, several of the pins have values that differ a lot from the reference file at the GGG site, but I don't have the knowledge to interpret them. The board has no solder bridges, and the IC is correctly aligned.

Steinar

[Krinor]

Hei igjen Steinar 

Usually the problem is obvious once you spot it. Check again. And again. 
Check your electrolytic caps too; C3, C4 and C5. Are you 100% sure that absolutely everything is correct according to the layout ? If so, then there might be a faulty component. JD Sleep will send you a replacement free of charge if that is the case.

Here's a link to the datasheet for the IC. Maybe this could be usefull in understanding the workings of the op amp:

http://www.ortodoxism.ro/datasheets/texasinstruments/tl022.pdf

Post a gut shot, and maybe someone else spots the problem ?

Lykke til.

-Kristian
(Bergen)

[SolariEGO]

Hei igjen, igjen!

Yes, I've checked, double checked and triple checked, but none the wiser – guess there's no way around actually learning this thing...

Posting a gut shot as you suggested, although it reveals my sloppy wiring and probably not much more.

[Krinor]

No, you're right. That looks nice and clean.
Guess we'll just have to bump this one untill someone else sees it. 
Maybe you should try to desolder the ic and change it for another one. And maybe put in a socket also.
You'll find what you need here:
http://www.smallbearelec.com/StoreFront.bok

[PerroGrande]

There isn't anything immediately obviously from looking at the picture you posted...  However, the voltages you posted are clearly not what is expected...

So -- let's start at the most obvious problem spot -- pin #8 showing only ~4.5 volts.  This is the V+ pin for the IC and, based on the circuit board, should be wired straight to the 9v supply.   So -- for whatever reason, you don't have V+ where you should (pin #8), but you've got it two places you don't want it -- pins 1 and 5.

The most likely cause of this, apart from a fried IC, is some sort of physical problem -- solder bridge, etc, etc,.

Remove the board, turn it over, and post a picture of it, if you've got a free moment.  I'd like to get a look at the solder connections on the underside.  While you're doing that, carefully re-heat the connection on pin 8 to make sure that the solder has flowed and is making good contact with everything (pin and board).

Also -- check for other "physical" stuff...  Such as -- wires pinched, lugs from a jack touching something they shouldn't (like the outside of a pot, or the case), etc, etc. 

[jrem]

you sure you have the right value for R13?  Something is dragging down the voltage on pin 8, a fried IC wouldn't drain current, IMO.  They just sit there looking stupid with full voltage on the supply pin.  Maybe you have a 100 ohm in there for R13, pulling the voltage down?

Or how about this . . .  put a 1 ohm resister between the wire and the center lug on the 'level' pot, put a volt meter across the resistor, and the reading is the current draw through the LDR (or whatever it is) (you can do it across the 100k resistor but you divide by 100k instead of 1, so the reading is different by 100,000).  Is that diode draining current?  If so, it's on.  If not, you're not losing it there.

Also, this is where a signal generator and a scope come in handy.  Just plod along the signal path until you find where it goes terribly wrong.

Let's see the underside of the board . . .

[SolariEGO]

Thanks a lot for your advice and patience, guys,

PerroGrande and jrem's interest in the underside of the board spurred me to compare it with the PCB layout on the GGG site – and bingo! A big ol' bridge in the copper tracing of the board itself, between one of the R2 points and the trace between the positive side of C3 and pin 1).

I hadn't spotted it before, because without comparing it to the original layout, it really looked as if it was supposed to be there (the board is one of GGG's old school etched ones – and a bit rustic). Fixed it, all readings OK, and now the Harlemizer (http://www.cubanconnection.no/images/hammond125Borama.jpg) is quacking it's way on to the pedal board – thanks again for your time and patience!

[PerroGrande]

Glad to help!   Enjoy your pedal

[Krinor]

Excellent. Nice name for it too.