Bringing a PINT to life... yeah you read that correctly ;)

[Mullet]

I want to tackle and finally finish this PINT amp I recently acquired. It's old school and I scored the board for next to nothing, so what the heck. I know the project ended up ceasing because of the crankiness of the 8397. There’s got to be a way to tame this thing. At this point, I’m not sure if I’ve fried the opamps, but I can say that the 8397 gets very very hot. I’m using the LM6172 as my GND channel opamp and this is getting quite warm as well. This is when I power the amp with at least 18v or 24v. I'm getting an unusual amount of DC Offset -- basically the rail supply for each reading that I've done. At first the amp seemed to work but music sounded hollow and not quite right. So I decided to reflow some of the resistors, particularly on the left output side. This is when the problems started. I swapped out both 8397s that I had on hand and both have issues. I’ve removed the LM6172 and put an 8397 in its place. Currently, it’s back to one of the 8397s and the 6172 in the U3 position. I started the build with these values…

R2 - 100k
R3 - 120k
R4 - 240k

…for a gain of 3, which should be fine for the AD8397. Then I switched to the default values of...

R2 100k
R3 - 120k
R4 - 620k

…to see if that would help things out by balancing the input bias current. No dice.

Here are some readings with a 9v battery to prevent the opamps from getting too hot:

DC Offset

IG to OG 3.93v
IG to R -3.83v
IG to L  3.83v

U1
IG to Pin 1  2.88v
IG to Pin 2  .426v
IG to Pin 4  -3.81v
IG to Pin 6 .551
IG to Pin 7 3.74v
IG to Pin 8 3.74v

U3
IG to Pin 4 -3.78v
IG to Pin 6 -3.77v
IG to Pin 7 -3.77v
IG to Pin 8 3.72 

So it looks like I’ll order a few more 8397s and maybe another 6172. I recently bought an oscilloscope, but have yet to learn how to use it. I figure this would be the perfect opportunity to see what oscillations look like. Perhaps I’ll even figure out what ferrite smds are needed, if any at all.

Any ideas? Don’t want to give up. I could pull a bunch more parts out and do the alt. version without C1.

[Mister X]

http://www.mb3k.com/hosted/mmmpint.pdf

[Mullet]

Got it. Do not pass go.. and go straight to the minified PINT.

[Mister X]

Before you go ordering more parts....

What's the PSU voltage?

What's the voltage between pins 8 and 4 of U3.

What's the voltage between pins 3 and 8, between pins 4 and 8, between pins 3 and 1 of U3?

And also... do you have the C7 capacitors installed the right way around (the positive lead is marked on tantalum capacitors, which is opposite of electrolytic caps).

[n_maher]

Detailed pictures would probably help.

[Mullet]

Most current readings:

 

PSU voltage unloaded is 24.16v coming from a 24v Jameco linear regulated supply.

 

23.33v between pins 4 and 8 on U3.

11.64v between 3 and 8 on U3.

Nothing between 1 and 3 on U3.

11.5v between 3 and 8 on U1.

23.36v between 4 and 8 on U1.

9.92v between 1 and 3 on U1.

 

Red lead on MM is going to pin 8. Red lead is going to pin 1 as well.

 

Thanks for the help guys.

 

Pictures to come.

[Mister X]

U3 pin 3 should be 1/2 the supply voltage give or take a few mV.

U3 pin 1 should be 1/2 the supply voltage give or take a few mV depending on the DC offset (which is the voltage reading between pins 1 and 3).

The first half of U3 checks out....

 

Are the ferrite beads (L1, L2, L3) installed?

[Mullet]

Nope no ferrites. I have tried installing them to no avail. So I put the jumpers back in place.

 

So basically only thing that doesn't check out is the "no reading" on pins 1 to 3 on U3. I'll try re-flowing those joints. So are you saying that 1 to 3 should be slightly higher than 8 to 3? Does U1 look right?

 

Here are some pics for posterity...