Rat problem

[bleubleu]

Hi!

I just built a tonepad RAT. I used all the specified parts, including the LM308 opamp. Sorry in advance for the total lack of details, I will post some more details tomorrow.

Problems :

• No sound when activated, bypass is fine.
• Sucks an amazing 120mA of current when turned on.
• The PCB gets hot after a few minutes

I cant post voltages right now because I just killed my last battery while doing experiments. I double checked components values, inspected for solder bridges, triple checked the wiring. I seem to be dropping a lot of voltage across the first 47ohm resistor (which is similar to http://www.diystompboxes.com/smfforum/index.php?topic=57699.0), but in this thread, the guy had the diode backward. I verified the diode and it is properly oriented.

Any idea before I start measuring voltages ?

Mat

[Papa_lazerous]

voltages would be great......BUT there is obviously something seriously messed up so rather than draining your battery and damaging something on the circuit you should have another serious look at it.

did you use a pcb from tonepad or your own vero layout?  If you used there PCB post pictures of both sides nice and zoomed in.

Could be a bad trace somewhere, bad soldering shorting something or a component misplaced or reversed in polarity

if you did your own layout it could be all the above plus a bad layout not following the circuit.

without pics and or voltages you are screwed 

[SonicVI]

I would guess you have a short somewhere, maybe a solder bridge?

[bleubleu]

Ok, here you go with the pics!

It is the tonepad PCB with a little modification to select between 2x4148, 2x4148+Ge and LED clipping (aka Multi-Rat). I have 3 SPDT switches so I can choose which type (or combinations of) I want.

Please bare with me, I AM NOT GOOD AT SOLDERING.

I am investigating a new debug method I call "DEBUG BY HEAT". I leat it plugged for 5 minutes, then I started to feel heat coming for the first 47ohm resistor. Only this component seems to get hot.




Mat

[Papa_lazerous]

Well I cant see exactly whats going on with your little mod on the bottom of the pcb you etched there. mostly because I cant see the off board wiring...

Just a couple of suggestions to help you out.... Try removing all your mods from the cicuit for now and just build to spec. then see if it works.  If it does great you can figure where you went wrong.  If not at least you know the problem is withing the main circuit.

Just noticed you havent socketed the IC, to be honest I dont always. But at the same time I can solder very well and know I am not going to fry something.  You definately need to practise on the soldering.......

The de-bug by heat method... I like it haha. well you are getting heat through the 47ohm Resitor for a very good reason.  Its the first component after the positive terminal on the battery.  So all current flowing through that circuit flows through that one resistor!!  Its like you have a short to GND somewhere or a reversed diode.

[bleubleu]

Cool, thanks for the tips!

I'll come back tomorrow with some voltage reading and more info.

Mat

[Rick]

Yeah, you were not kidding about the solder joints. They are too low heat chunky looking. Try a better iron or higher heat and good clean tip.
The joints should be round and shiny to properly adhere the component to the board. This is a must do !

[R O Tiree]

You've connected the battery the wrong way around. Either that, or the 1N4001 diode is the wrong way round. Check it out. Seriously.

See my reply to this thread to see why: http://www.diystompboxes.com/smfforum/index.php?topic=59975.0

The clue is the fact that you're pulling 120mA and that the 47R is getting hot. These 2 together say that the current is going battery -> diode -> 47R -> battery or the other way. Depends on whether it's the battery the wrong way round or the diode. Before you re-connect the battery again, check that diode and the battery polarity.

You might like to think about getting wires of different colours for your off-board connections. In addition to making it more attractive, it is far easier to debug. I use black wires for 0V, red for +9V, purple for input signal, yellow for output, etc.

[Mark Hammer]

1) Good idea to take some alcohol and a discarded toothbrush and scrub the flux off that sucker so you can see where there might be some solder bridges.

2) These days, I take a Q-tip with some diluted liquid flux and wipe it over the traces as soon as the board is etched and drilled, and before it gets tarnished.  Then I take teeny dabs of solder and tin the entire board.  I also check the leads of all components being installed and verify that they are clean and shiny.  If not, I give them a gentle scrape with an X-acto knife to make them shiny.  As someone who has been acquiring and hoarding parts (a lot of them surplus) for damn near 30 years, that may well be more a hazard unique to me than to folks whose parts bins contain only the shiny stuff.  Between the tarnish prevention on the board, and the tarnish check on the parts, that tends to result in nice solder flow and assured conductivity, as well as joints  that are less bulbous than what one might have seen on a 1973 E-H pedal.

For sure something is providing a low resistance path where there ought not to be one, though, given the current draw you note.

[bleubleu]

PROBLEM SOLVED! 

First, thanks to everyone who replied so fast!

The problem was a defective LM308. For a quick test, I installed a TL071 and remove the 30pF cap and it worked perfectly. I'm going to the store tomorrow first thing in the morning to get a new LM308 for a more authentic sound.

I bought it on ebay for a few cents and never really trusted it or could it be my amazing soldering skills that burned it, who know. Is it possible that a broken LM308 creates a short between pin 7 and 4 ?

Lessons learned (note to self) :

• Always use sockets for ICs. It protects them while soldering and easier to test other ICs.
• Get better at soldering.

Mat

[R O Tiree]

Well, that'd do it. Maybe the previous owner wired it up backwards and fried it? Who knows? The main thing is, you figured it out.

Sorry I accused you of a noob mistake.

[bleubleu]

Actually I did learn a lot by reading your posts. The one on the diode thread was very detailled. My electronics skills are very very humble and everything I read realy really helps.

Mat

[oldrocker]

You might want to socket the chip.  That way you know you didn't fry it when soldering and it's easier to change out.

[bleubleu]

This thread is pretty much closed. But just for the record...

I bought a new LM308 and it sounds great, but the testing I did with a TL071 was not all that bad. It does sound pretty good, but it might need some tweaking. The TL071 sound much more "trebly" and seems to be louder than the LM308. The tends to create feedback and some noise on higher settings, but it creates a pretty good distortion. Replacing a few caps and/or resistors might fix this, but this is way out my league.

Mat

[petemoore]

I bought a new LM308 and it sounds great
  open IC placement I grab a socket, fine touch/high heat/quick solder can put them in with no socket/no damage, but I like to bake the joints a little more than what I think an IC can take.
  I agree with the more heat/less solder, let the 'to be joined' pieces heat by using a small amount of solder to get the heat 'moving' [transferring from the iron tip, through solder, to pad and leads], then after the joint pieces have had a chance to heat, add just enough solder to encompass everything, I like to use little dabs at a time..how fast it melts on the distant point of the joint pieces tells me enough info about all joint component temperature, probably 1/3 the amount your'e using is sufficient to provide a good physical structure.
  Adding solder cools the joint, continuous solder adding doesn't allow proper heating, the solder, leads, pads all need to reach..the temperature which they need to reach [so solder sticks to all surfaces].
  The solder should be smooth looking, it is apparent that some solder melted when the other side was solid.
  the ones at the left-bottom area look heated and have formed uniformly smooth surfaces.
  The ones that look more like 'the Alps' look like the joint pieces wer never uniformly raised to the correct' temperature.
  If it's working it's working, but I would reflow most of the joints, just heat until everything looks 'uniform' as in meltedly, good, hot, tinned tip, about 4 seconds for 1 lead per 1 pad joints.
  I use an old RS iron with a beat down tip for removing solder/reflowing joints, it 'grabs' a blob of solder while reflowing, I can knock that off by whapping the iron on the egde of the table, then repeat until enough is removed.