Building the Tiny Giant amp

[Jdansti]

Here's a long shot guess.

You have power on pin 2 but not pin 6. Both of these pins are two from the end.  Could you have counted from the wrong side while checking the voltage?

[Jdansti]

[MDK002]

you are correct....i had them reversed.

[MDK002]

TDA7240
pin 7  9.3
pin 6  11.9
pin 5  9.06
pin 4    0
pin 3  8.17
pin 2   0
pin 1   8.70

[Jdansti]

Ok. Well that's good news and bad news. I'm not an expert on this, but your voltages look fine.

Have you made sure neither terminal on your speaker is connected to ground?

If that's ok, then it looks like its photo time. Can you get some clear shots of both sides of the board and the off-board connections?

[MDK002]

neither is connected to ground.

i'll take a few photos and post shortly

[MDK002]

[MDK002]

Here's a couple of photos of the off-board wiring...not sure how much these will help.

[Jdansti]

Is this your speaker jack?



You said that neither speaker wire was grounded, but I have to ask, if this is your speaker jack, do the metallic parts which are in contact with the enclosure have continuity with either of the jack's terminals? You might want to check this with your DVM.

[MDK002]

That is not the speaker jack.   That's the hardware for the power-on LED.   It has thick rubber grommets between the LED, wiring and the hardware.   No grounding issue there.
It was originally meant to house a lamp, but I removed the base for the bulb and replaced it with grommets to accommodate the LED.

[MDK002]

top picture....left to right...   instrument input, standby switch, volume pot, LED (power-on), on-off switch
bottom picture....left to right...   power jack, speaker jack.

[Jdansti]

Ok-thanks.

What are the wires with the arrow pointing to them below connected to?

[MDK002]

those wires just loop back around to the input jack....cannot really see that from the photo

[Jdansti]

Thanks.  Well for now, I'm stumped.  Your wiring and soldering look great.

I was wondering how the two heat sinks are being held in contact with the chips.  I don't see any screws, so is the white stuff a heat-conductive adhesive of some kind?  The reason I ask is that if the TDA7240 gets too hot, it could make some noise.  Can you hold your finger on it without experiencing pain?

The speaker output pins voltages seem high, and the only other thing that I can think of at this time is tha the TDA7240 might be bad.  I've seen at least one other post where someone had a bad chip, although he was getting 0V on the output pins: http://www.diystompboxes.com/smfforum/index.php?topic=89687.msg910767#msg910767

Maybe Taylor or someone else can help us out.

[MDK002]

the heatsinks are made for RAM modules....i just used the adhesive that came with them.
no heat to speak of from TDA7240.   the problem starts as soon as i turn the switch on.     
i'm stumped too....my guess was that either the TL072 or the TDA7240 were bad....or both?   I don't have any spares of these 2 on hand or I'd swap them out.   
I did see where someone else had a bad chip but I could not find any other problem in the forum like i'm having.   
I appreciate you looking at this.   Maybe someone else will have some ideas.

[MDK002]

So, I plugged in a different guitar....popping sound not so prominent today but it's still there.   the TDA7240 is getting pretty hot real fast.  wouldn't want to keep my finger on it...this only after playing about 10 seconds or so.
the sound is clipping really bad too...can't hardly turn the volume up past about 1/3 unless i play VERY gently.   

[PRR]

You have your guitar input wires running with your speaker jack wires. They really must be SEPARATED.

Those heatsinks are much too small for a 20-Watt amplifier. Since you glued them, a fan may help; though it might have to be a LOUD fan to focus huge air on those small sinks.

Is that a key cabinet?

[MDK002]

Hi Paul.   I was wondering if the wiring should be separated.   no problem to move things around.
i can replace the heat sinks with something more substantial..   i'll make sure it's isolated from the chips and doesn't ground them.   I have a handful of those copper heat sinks....maybe i can come up with a way to mount more them on some aluminum sheet and attache to the chips....we'll see.
and yes....that is a key cabinet     i'm building a case to go around it so it will look like a small amp head....once i work out the bugs.
I'll update as soon as i get things moved around...  I'll post more pics as i get closer to finishing the cabinet.
thanks for all the help.....i'll let you know how it works out.

[Jdansti]

>You have your guitar input wires running with your speaker jack wires. They really must be SEPARATED.


Good catch, Paul!  I didn't even consider that. I'll store that fact in my little brain!

[bluebunny]

i can replace the heat sinks with something more substantial..

Just rearrange things so you can bolt the two chip tabs directly to your enclosure (with a mica washer to insulate the regulator).  With the size of box you have, it won't even come close to breaking into a sweat.  I have mine in a small extruded aluminium case and it's barely ever above room temperature.