Hot potato

[fishdds]

I'm VERY new at this, but I have a homemade effect I've been working on.  The problem is, when I connect the 9V battery to it, the battery starts to get REALLY HOT.  What can I do to keep this from happening?

[smoguzbenjamin]

If the battery is hot, you're shorting to ground directly = not a good thing. Check your ckt for a direct connection to ground with a DMM.

I had this problem once, burnt the skin offa my fingers  

[Peter Snowberg]

Welcome fishdds,

:shock: That's not good.... (just being obvious)

I had a shorted 9V alkaline battery explode on me once. I'm glad I wasn't near it because it was literally as loud as an M-80 sized firecracker. It blew the case it was in apart. Be caureful!

You must have a short somewhere. Can you provide more info on your circuit?

As a safety measure, you might want to send the power through a 10 ohm 1/4 watt resistor in front of your circuit. That will limit the current to the point where the battery should stay in one piece at least.

Take care,
-Peter

[smoguzbenjamin]

I read somewhere that a guy wired his ckt in series with a small lightbulb, (was it you, peter?) so when the ckt shorted, the light'd come on, limiting current, and otherwise there wouldn't be enough current to work the light so it'd act like a small resistor.

Welcome to the forum!  :wink:

[Peter Snowberg]

Thanks Smoguz, I forgot about mentioning that. It's an old trick from the tube amp world where it's fairly common.

Fishdds, if you get a 6v or better a 9v lamp rated at about 100-250mA and use that in-line with your hot lead, it will work like a current limiter if you have a short and it will show you at the same time. Once the lamp comes on, you'll need to power it down and let it cool a second to reset it. Most pedals draw less than 30ma so you might see the filament glow red, but it will only light up bright with a short.

For 120V amp debugging, I have a 300W lamp in series with a plug and socket.

Good luck,
-Peter

[petemoore]

Negative of battery clip to neg AND pos
 Positive of battery clip to NEG and POS
    if you do these continuity tests you can tell that if you have continuit from neg-ground to pos = short

[Marek]

Hallo!

Take a DMM ( Digital Multi-Meter ), put it  on 'continuity test' ( or audio cont. test, if you have one ).

Touch the +V  trace on the PCB (circuit board )  with one probe, and touch -V ( or Ground, depends on the circuit ) with the another. If your DMM makes  "peeeeep!", then you have a short somewhere in the circuit.  That's bad... Doublecheck your PCB again against the schematic.

Hope you'll solve the prob. Otherwise give us some more info on the circuit you're working on!

Marek

[smoguzbenjamin]

petemoore: The way you describe your ckt you seem to be running the lightbulb in parralel to the rest of the ckt... You should wire the light in series, asin lamp negative to ground and lamp positive to the ground of the ckt board...

[fishdds]

:oops: I'm sorry, when I said I was new to this, I meant like a total beginner.  I do have some basic knowledge of electronics, (transistors, resistors, capacitors, etc.) but I didn't really build this circuit myself.  I took it out of a toy I played with years ago.  It was basically a microphone that would change your voice (sounded like it was adding reverb, distortion, and some pitch mod or something else)  In high school, I thought "hey, that thing would be cool for guitar" so I took it apart and wired an input jack to the mic wires on the PCB, and an output jack to the speaker wires (had to keep the little speaker attached or it wouldn't work) I'm sure there's a reason for that, too that I just don't know about.  Amazingly, it sounded pretty good, as long as you didn't try to sustain a note for a really long time.  Anyway, I recently dug it out, and I thought that if I just knew how to wire it properly with the intput leads, output leads and battery leads, it would probably work better. It now resides in a gutted Ibanez soundtank pedal housing.  I guess I'm in need of some remediation.  :roll:  I'm fascinated by this site, and along with a friend, plan on building a real stompbox once we've done some more research.  Thanks for all your help.

[Peter Snowberg]

Is there a way you could scan the board (top and bottom?), if so, I'm sure somebody can probably draw up something.

Dead shorts are usually pretty easy to find.

Don't worry about being a beginner at all. If you get answers that you don't understand, just say so and we'll all try to mush the words around and provide more info until it all makes sense.

It sounds like a neat gadget. This Halloween there were some voice changer units at the local drugstore and they had a pile left on Oct. 29th. I was hoping I could pick one up for 1/2 price on Nov. 1st, but they sold out. C'est la vie.

Take care,
-Peter

[fishdds]

I have two images of the circuit.  How do I post them here?  I'm also not so sure that the problem isn't just how I have it connected vs. a problem on the board itself.  Thanks

[Peter Snowberg]

Unfortunately there is no way to post an image here, but you can get a free account at any number of free hosting places. I have a free site at http://www.freewebs.com/ ...other ones include http://www.tripod.com/ and http://www.geocities.com/ as well as a bunch of others. I like freewebs because there are no pop-ups and the advertising is limited to a one line link to their home page.

Do you have an adapter jack as well as the battery snap?

Take care,
-Peter

[fishdds]

Here are the images.  Thanks for the freewebz suggestion.  No, the only power is the battery snap.  It's looks like a rather simple circuit.  Also, why do I have to keep the little speaker wired in line with the output jack for the sound to come through?  It's probably just a matter of connecting the wires differently, like a real stompbox (but I've never done that, so...)
Thanks,
David        http://www.freewebs.com/fishers5/circuits.htm

[Peter Snowberg]

Good pictures.

I've been pretty busy lately so give me another day or two to trace it out.

In the mean time could you please post the markings on the top of the chip? They're just a little too small for me to be able to read.

I'm sure there's an answer.

Take care,
-Peter

[smoguzbenjamin]

I think the chip is marked b4809... That's what it looks like to me anyway...  :?

[fishdds]

The chip reads:
TBA820M
SGS 88409
SINGAPORE

Thanks for the reply.
-David

[Peter Snowberg]

Hmmmm... apparently that's just an amp chip. You could sure get distortion but I don't understand the reverb part. It looks like it has a small input cap, so think of it as a treble booster.

http://eu.st.com/stonline/books/pdf/docs/1440.pdf

That datasheet explains why you don't get sound unless the speaker is in there. Pin 7 supplies the power to the preamp stages while pin 6 supplies the power amp power.

I'll trace the whole board in a day or two, but it looks like it's figure 1 from the datasheet with a normal condenser mic power supply attached.

Try replacing the speaker with a 470 ohm resistor (or smaller, down to 10 ohms minimum). That will get you sound without a local speaker.

The output is across the power positive and pin 7, so if you're running this into another effect or external amp, you'll need to use a coupling capacitor from pin 7 to your output jack.

More later.

-Peter

[fishdds]

Thanks for the info.  What type and value capacitor would be good to use as a coupling capacitor from pin 7 to output jack.  Also, what would the practical effect on the sound be from using a 470ohm resistor vs. a lesser value down to 10 ohms instead of the speaker.  With a lower value resistor, is the output hotter?
-David

[Peter Snowberg]

If you take a look at figure 2 in the datasheet above, you can see a circuit that it set up a little more effect friendly.

They connect the speaker from ground to the output coupling cap, and pin 7 is connected to power through a 56 ohm resistor. A 100uF cap routes the output back into the preamp supply to provide negative feedback. I suggested 470 ohms before looking at the schematic in figure 2 and seeing what they had done.

You actually want to take the output from pin 6, but with the speaker to positive design they used, the place where pin 7 ties in is a good place to tap the signal. I would re-wire the speaker to look like figure 2 and then replace the speaker with a 500 ohm pot. That way you don't need an additional coupling cap.

I don't have this chip, so I really can't say what it would do to the sound if you vary the resistor from pin 7 to +. Sorry. It may whine, it may just get louder.... hard to say exactly without a lot of analysis or easier... trying it out.

If you need step-by-step instructions for changing it to a figure 2 circuit, just ask. If you want to just add an output jack, I would put a 56 ohm resistor from pin 7 to the + supply line, remove the existing pin 7 connection, remove the speaker, and attack the output that used to go to the speaker to a 500 ohm pot. A 100uF coupling cap may be required between pins 6 and 7.

Take care,
-Peter

[fishdds]

I think I just want to add an output jack to the existing circuit, but I'm having a little difficulty.  Every time I look at the chip and think about trying to disconnect one of the pins, (not sure which is which, either) I think "that's going to be impossible"  It's just so tiny, and there's a lot of stuff right near it.  I think one problem is my soldering iron.  It's really big and doesn't seem finesse enough to do the job here.  Also, you suggest connecting the speaker ouput to a pot; what about the other 2 pot connections?  Remember, I'm a circuit newbie. :?   Another question:  Radioshack didn't have a 56ohm resistor.  How about 46ohm?  Does it matter 1/4watt or 10watt?  I think I'm close to getting this thing working, I just don't like the idea of messing with that chip directly.  Can I tap in somewhere else?  Thanks again for your help.