NOOB question about IC debugging

[half_smith]

Hello, I have 4 successful builds under my belt. A Mutron, Ibanez Compressor, Klon clone, Phase90...

Now I'm struggling with this MXR Distortion+ from GGG

When I plug it in:
LED=YES
Clean pass through=YES
Effect on=very low signal, no "effect"

There is 9v flowing through the circuit, but my guess is the IC (LM741cn originally, currently trying a NTE 941, new 741 on order)
The voltages at the IC were similar with the 741 (before I blew it attempting some tests involving shorting pins 2/6 or something I didn't quite understand)

9v in = 8.94

IC "741"
1- 7.38
2- 3.56
3- 5.58
4- 7.39
5- 7.39
6- 7.07
7- 8.94
8- 0

Seems to me the pin 1, 4 should be 0, and pin 3 seems high...

I just don't know what this means, is my IC shorting or not grounded?

I've gone over every solder, component value and polarity, looked for bridges, scraped the pcb, removed all "optional components"
Please point me in the right direction!

Thank you

ps, I am not sure about posting the schematic or wiring diagram from GGG website? but if you need to refer to them they are on the MXR Distortion+ project page over there.

[blackieNYC]

Pin 4 should be ground. Do you have ground from the sleeve of a jack to the board?

[half_smith]

blackieNYC thanks for the reply!
I get continuity between the grounds on the jacks and board, and from Pin4 to all the ground points.

My suspicion is that all 3 ICs I've put in there are bad. But could something in the circuit be causing them to pop?

[idy]

Bad ICs are very unlikely.

There is no way for pin 4 to be connected to ground and also showing 7.39 volts.

If Pin 4 is connected to ground, then ground is at 7.39v.

Take the chip out of the socket. You should find 0v on pin4 and "mid" (nearly 4.5) on pin 3. Your meter may "load" this and make it read a bit low.

When you have satisfied this condition, then upon reinserting the IC, you should also see "mid" voltage on pins 6 and 2.

[idy]

And welcome to the forum!

[half_smith]

yep, there is 7.41v on Pin 4 and the Ground.

I just can't figure out how that is happening. Where am I likely to find the source?

[half_smith]

And thanks for the great forum, I've been lurking for a long time

[idy]

GGG doc shows jumper for R16. I can't find that part on either the D+ or 250 you can also build on this pcb. Maybe try that. Another jumper missing on the far right upper corner, can't read the part #, next to the big red box cap. You've left out the 1.5M "pulldown" resistor R1, but that's not your problem.
Also missing is C5. Not your problem, but it is there to cut super highs and prevent oscillation.

Go over the solder, jacks look doubtful.

You say all the ground points are connected ton pin 4...is the battery or power supply negative attached to this point also? (Hint: The answer is almost certainly "no.")

[half_smith]

There is a jumper on the top of the PCB across R16
R1 and C5 are labelled "optional" on the GGG Docs so I removed them while debugging. Along with D8 and any extra clipping diodes I was planning to try.

There is not continuity between Battery Neg and Pin 4

The signal passes cleanly through the jacks when not engaged.

I will continue to go over the solders and recheck values, but can you tell me specifically what would make the Pins 1 and 4 show near input voltage, and Pin 2 be very high?

Thanks again

[antonis]

yep, there is 7.41v on Pin 4 and the Ground.

That can't be so..
(or else, you should measure about 8.2V on pin 3..)

[antonis]

then ground is at 7.39v.

Compared with another more grounding ground..?? 

[duck_arse]

edit : pllllleeeeeaaassseeeeee - link to YOUR build docs, circuit, etc, every time. I've just looked at half a dozen docs that didn't mach your board.

please tell us the markings on your two caps at C3 and C7.

[BJM]

edit : pllllleeeeeaaassseeeeee - link to YOUR build docs, circuit, etc, every time. I've just looked at half a dozen docs that didn't mach your board.

I think it's this one:

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

The voltages on the pins are also mentioned. It says pin 1, 5 and 8 are not connected btw, I suppose Half_smith's measurements on pin 1 and 5 are not good?

[idy]

You have confirmed that battery negative is not connected to pin 4.

Why not? You probably have this set up so you need to have a plug in the input jack to establish this connection (battery - to ring, grounds to sleeve). Trace this. Battery - to jack ring, jack sleeve to ground wire to pcb, pcb G to pin 4. Where is the trouble?

[half_smith]

Thank you all for the replies

duck_arse - thanks for looking into it, C3 and C7 are both "WIMA 1000/100-" which I believe is .001uf

idy - you were correct, Pin 4 does connect to G - only when a plug is in the input Jack.

BIG NEWS!
when I measure the IC with an input plug inserted things look more reasonable:

741
1- 0v
2- 4.15
3- 2.97
4- 0
5- 0
6- 4.5
7- 8.95
8- 0

but effect still very weak and not distorted.

Volume Pot works, goes from nothing to barely a signal at full power
Gain Pot does not do anything audibly (but I can hear it sweeping as though it is connecting)


I was not sure I could post the GGG links here.

I followed the MXR Distortion+ schematic/wiring diagram from GGG, the board is a little more complicated with many "optional" traces needing jumpers, and the kit came with a handful of different value resistors and capacitors... For debugging I have reduced the components down to the bare minimum.

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

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

http://www.generalguitargadgets.com/pdf/ggg_dist2_plus_lo.pdf

[ElectricDruid]

The voltages around the chip look a lot better, but it still sounds like you're getting no gain.

I'd make sure that the bottom end of the gain pot is actually connected to ground. If it weren't, you'd have a unity-gain buffer, basically - somewhat underwhelming! Turn the power off, and do a continuity check from ground to the two pins on the gain knob that are connected together. I note that there's a long path from the supply ground to those on the GGG wiring diagram -

http://www.generalguitargadgets.com/pdf/ggg_dist2_plus_lo.pdf

It goes from the supply ground to the board, to the level pot, to the outputted jack, to the input jack and *finally* to the gain pot. If any of those joints are bad, you'll have problems with gain.

[antonis]

And make R2 & R3 100k or better 47k..

[ElectricDruid]

Antonis, why?

I agree that it might be a better choice of component value, but I don't think either of us think that's why this pedal isn't working.

[antonis]

Just for good design practice, Tom..

R3 is set in parallel with R4 and that's not a good thing for bi-polar inputs op-amp exhibiting a "significant" input bias current..
It also dictates marginally low C1 cap value (due to high RC time constant) hence less effective Vref AC ground..

[duck_arse]

C3 and C7 are both "WIMA 1000/100-" which I believe is .001uf

please, indulge me. I see your red caps with black markings, which Wima tells are FKP-2, PCM 5, from the datasheet. the 1000pF has the same smaller case measures than the 10nF, I assume your C2 is also Wima. so your C3 and C7 look "too big" to be 1nF caps, at 100V, made by wima.

also, if you had caps at much higher value than 1nF at C3 and C7, they will filter great gobs of your signal straight to ground, no matter how good your IC voltages looked.

a quick way to shut me up is to lift both those caps out of the board and see if you get full, screeching, unfiltered output.