Condor build, have I fried my TL074?

[Killthepopular]

I'm building this:
https://2.bp.blogspot.com/-Hh9jwaRaHuE/V5XmosfDYsI/AAAAAAAAA0M/Avf1G1ewR2suT_zSvZIO4hTBdWHM3wZqgCLcB/s1600/Condor%2B2.png

I Missed out a couple of the cuts because I was mainly working from the layout that showed the components and two of the cuts were sort of "invisible" on that layout. I'm talking about the one underneath the diode and the one underneath the 22k next to the opamp, third row from the top. I also missed the jumper that sits under the trim pot.

When I tried the pedal out with a battery the IC chip got really hot. I smelled burning. I touched the IC chip and it burned my finger so bad I had to run it under the tap for a while and it hurt for the rest of the day. I measured the battery voltage and It had gone from 9v to about 1.5v in a matter of minutes, i.e. instant depletion of a fully charged battery.

I saw my mistake, completed the 2 missing cuts and one missing jumper. I've double checked to make sure that I've got all the cuts and jumpers in the right place. I've also tested the cuts and jumpers with a multimeter to make sure that the jumpers are conducting and the cuts are not.

I can now test it out without anything overheating. I get a signal but it's quiet and distorted and occasionally gets super loud if I strum hard. So sound is coming out but something is obviously very wrong with the circuit.

Here's the voltage readings for the TL074:
1. 4.62v
2. 4.62v
3. 4.56v
4. 9.16v
5. 4.52v
6. 4.62v
7. 4.61v
8. 4.62v
9. 4.62v
10. 4.60v
11. 0.001mv
12. 4.60v
13. 4.62v
14. 4.62v

From the fact the TL074 got so hot my suspicion is that I've toasted it. I don't really know what the voltage readings are supposed to look like...
Maybe I damaged the diode? seeing as i missed the cut under the diode...

I Dunno if there's any other components that might have got roasted....

It's possible that I've still got something wrong with the circuit but I have double checked my layout at this point so I'm inclined to believe that damaged components are a more likely suspect.

[EBK]

I'm taking a peek at that layout now.

First thing I can tell you is that missing the cut under the diode didn't hurt anything. 

I'm looking at the other errors now....

Here is what missing that other cut created:

[MaxPower]

As far as I can tell the op amp voltages look fine, assuming the third and fourth should also be around vref, but I'm not 100% sure. 

[EBK]

I saw one more sneaky bit on that layout.  Did you get the double jumper in the lower right?

TL074 has output short circuit protection, which should keep it alive at these voltages, I think.

[Killthepopular]

Did you get the double jumper in the lower right?

Yeah I got that and tested it to make sure it was conducting too.

I should also mention that I'm using this circuit to test it:




So it sounds like you guys reckon the chip is probably ok? I could order a new one for £2 to find out but thought it might be a pointless waste. I had just assumed that if the chip was hot enough to burn my finger it was also hot enough to have damaged the internal circuitry.

Any other components I might have damaged? Guess I should just triple check everything at this point...

[Killthepopular]

Here's a photo. The underside has been flipped in photoshop so that it looks like the schematic.




Things got a little crowded on the right side. I did put the wires in the correct holes though, just had to bend things a bit.

The copper trace came away on that one jumper under the IC socket so I soldered the wire directly to the leg of the socket instead of to the adjacent hole.

The JFET is 2N5457 (bitsbox didn't have j201 in stock).

[patrick398]

The jumper in the bottom right of the layout is a double jumper, hence the little dot. Seems like you only have a single jumper unless i'm missing something on the main side of the board

EDIT: Having said that it looks like your using solder bridges deliberately instead of jumper wires. In future i'd recommend having jumper wires on the component side of the board. Put them in before any other component. This also allows you to run a blade between the tracks and remove any potential bridges

[patrick398]

Is that ceramic cap under the IC actually 3n9?

[Killthepopular]

It looks like you're using solder bridges deliberately instead of jumper wires.

For the links on adjacent tracks I used clipped off bits of wire, the excess from the legs of components. It looks like solder but it's still a bit of wire. For links that span several strips I used insulated wire to avoid unintentional connections.

[Killthepopular]

Is that ceramic cap under the IC actually 3n9?

It has 392 written on it.

The jumper in the bottom right of the layout is a double jumper, hence the little dot. Seems like you only have a single jumper unless i'm missing something on the main side of the board

The blue wire is stranded wire. I put half the strands in the one hole and half in the other. My multimeter tells me that this link is conducting so it should be fine.

[MaxPower]

Is there a cut underneath the blue wire (double jumper) near the lower right corner?

[Killthepopular]

Is there a cut underneath the blue wire (double jumper) near the lower right corner?

Yep.

I've read the debugging sticky and decided to re-do my voltage readings.

Battery: 9.19v

TL074:
1. 4.49v
2. 4.49v
3. 4.43v
4. 8.90v
5. 4.39v
6. 4.48v
7. 4.48v
8. 4.48v
9. 4.48v
10. 4.46v
11. 0.2mv
12. 4.46v
13. 4.48v
14. 4.48v

Diode:
A: 9.13v
K: 8.88v

JFET:
G: 0.2mv
S: 89.6mv
D: 118.8mv

I also checked out the electrolytic caps and found that the positive poles were higher voltage than the negative poles.

To re-cap, the problem is that i get very little sound and it's distorted unless i strum hard where I suddenly get a REALLY LOUD SOUND, which is also really badly distorted. The effect is similar to what i hear when i try and force too much gain through a germanium diode.

[Slowpoke101]

Your voltages seem to be good apart from one really important part.
The JFET's Drain should be much higher, somewhere around 4V. This voltage is set by the 100K trim-pot.
9V is applied directly to one leg of the trim-pot (leg is on the same copper track as pin 4 of the TL074 - so it should be 9V ). Check for a break in that track and for short circuits.

Those WIMA 1nF capacitors...Are they actually 1nF ? They're rather large.

Another thing to keep in mind is the JFET may have a different pinout to what you may expect. Quite often the Gate can be transposed with the Drain.

[Killthepopular]

The JFET's Drain should be much higher, somewhere around 4V. This voltage is set by the 100K trim-pot.

This comment made me realise I hadn't really spent much time fiddling with the trim pot. It turns out there's a narrow spot where you get a really clear sound coming out even though you get almost no output at all if you rotate a mere 20 degrees in either direction.

It Works! Sounds pretty good too. I'll play with it for a bit to see if i want to do any mods, then I'll box it up. Thanks for the help guys.

[anotherjim]

Glad you got it there. I was going to wade in with the JFET thing because it causes a lot of trouble. At least you didn't have dodgy ebay fakes to contend with.
The trim pot in the drain is probably the simplest way to get any random JFET from a bag to play nice. It isn't ideal, because it can be so finicky, but is practical.

I think those Wima caps are 1000pF (aka 1nF) polypropylene (aka PP). They are chunky little beasts. Good caps though.

[Killthepopular]

I'm doing some mods before I box it up.

http://www.runoffgroove.com/condor.html

There's a notch at 400hz. My main electric guitar seems to have a resonant peak at 600hz. I was thinking it would be nice to shift the notch up to 600hz so it can carve out that resonant peak and make my clean tone (for my main guitar) that much nicer.

Currently tinkering with the Big muff section of Duncan's Tone Stack Calculator. I have some spare 47k resistors hanging around. Looks like replacing the 100k next to the 15n with a 47k will give me a subtle notch at exactly 600hz. I'll try it.

Here's a visualisation of the original vs the mod.

[PRR]

> notch at 400hz. ...be nice to shift the notch up to 600hz

As a *general* guide: to change EQ frequency, change the caps.

The frequency is both caps and resistors. But the resistors are chosen to suit circuit impedance, and sometimes DC bias needs (not here). And pots only come in a few values. Leave pots and resistors alone. Change caps by 400/600 or 0.67X. 0.15 becomes 0.1, 1nF becomes 0.68nF, etc. For stuff with just a few caps, nearest 10% value is close enough.

[Killthepopular]

As a *general* guide: to change EQ frequency, change the caps.

The frequency is both caps and resistors. But the resistors are chosen to suit circuit impedance, and sometimes DC bias needs (not here).

I've made numerous filter mods by swapping resistors and never really noticed a negative difference. Also ROG themselves suggest changing the four resistors in the LPF for getting more or less high end.
I'll bear it in mind in future anyway. I always wondered what the difference was between changing caps vs resistors.

[Killthepopular]

I think I want a higher HPF (say at about 150hz) but with a more gradual roll off, giving me less of the 120hz without totally killing the 80hz. Would I be right in thinking I could achieve this by replacing one of the caps with a jumper and lowering the value of the remaining cap? I have already lowered the 47n caps to 27n, so I would be lowering it from 27n. OTOH, An RC filter calculator actually tells me that I'd need about 59n of capacitance in conjunction with the 18k resistor to make a 150hz filter...

I'm guessing I'd jumper the second capacitor, so that the signal hits a cap, then the resistor then carries on to the opamp?



I think the 1st cap is the one on the left of the 18k, so I would remove and jumper the cap on the right of the 18k. This is all guess work btw.

EDIT:
ok, i just went ahead and tried it. It seems to have done what I thought it would do. There's more bass but not WAY more bass, just a bit more of the very low bass. I'll now start lowering the remaining cap value to remove the excess bass. I think I probably have a 22n somewhere...

[Killthepopular]

Just wanted to report that I boxed this up a week or so ago and everything works perfectly. My main hiccup was in missing those 2 cuts and one jumper and also in not setting the trim pot correctly. The boxing up stage was actually quicker and easier than I expected it to be.

Anyway, it's been a positive experience building my first pedal. If i were to do it again I would make more effort to source metal film resistors just to get the hiss as low as possible (I'm gonna buy one of those packs with like 10X30 different MF resistors) and I would also socket the Jfet and try out a J201 as well as the 2n5457 which I ended up using (because bitsbox was out of J201s). Also I've heard that you can get less noise by using the correct resistor instead of using a trim pot so I might have done that too. It's too much effort to go changing it now and I'm 90% happy with how it turned out so I'll just leave it as is.

Here's a summary of the mods I did to the original circuit:
Notch: replaced a 100k with a 47k for a shallower cut at 600hz instead of the deep cut at 400hz.
HPF: Jumpered 2nd cap (47nf) and replaced 1st cap (47nf) with 18nf for a shallower bass rolloff.
LPF: replaced one of the 4 22k with a 13k for more top end.

The overall result sounds more 1 by 12 inchy, and less 4 by 10 inchy.

Next build will probably be an EP preamp/boost which I'm considering putting in a smaller enclosure.