Music Man Rp-2100 212 (GP-1 Board)

[Mr_bibbles]

AH woops, sorry I meant pin 8, not 3. There should only be two rails on any of these IC's, + on 8 and - on 4...

[g1]

I don't see any connection to DC for those diodes.
I do see that you have a slight negative DC voltage at the inverting input (pin2) which would come out as positive DC at pin1.
What happens to the DC on pin1 if you short pin2 to ground?

[Mr_bibbles]

You are definitely onto something. When pin 2 is grounded, the voltage on pin 1 raises 2v up to -11, and the problem is improved. A lot more signal is let through, but it's still distorted.

I think I understand the issue better now too- An inverted input, that's not supposed to have any voltage on it will create the opposite voltage on it's output. Is this correct? lm1458: http://physics.gac.edu/~huber/classe...ets/LM1458.pdf


So, first step is to get that -vdc off of pin 2.

I already checked the 2uf cap, but I'll go back and solder in another good one just to be sure. I also still don't really understand what those two diodes are doing... does anyone have an idea or can point me to a site that might explain it?

[g1]

As far as I know, the 2 diodes just limit the max. signal that can appear between the inputs (clamping at around .7V), most likely for spike suppression.
I would try lifting one leg of each for test purposes.

[The Dude]

........I would try lifting one leg of each for test purposes.

^^^^^^ That!

[Mr_bibbles]

Thanks G1, that's a very succinct explanation. So clamping as a sort of 'limiter' for the audio signal to prevent spiking audio from creating too large of a signal on the other side of the IC?

Lifted both legs, both test fine at .538/OL....

I feel like I must be missing something very obvious because we're dealing with a limited number of components here.

Is anyone able to briefly explain the functions of C1-C3? C1 is an input coupler? C3 some sort of isolation cap between input and output?? IC's can be... confusing to say the least.

[The Dude]

Did you try the amp with the diodes lifted or measure the voltage on pin 1?

[Enzo]

Preventing too large signal from pushing the IC input past its power rail voltage.

[Mr_bibbles]

Dude: I did not! I will lift them and check the voltage presently. Thanks for your continuing support guys.

[Mr_bibbles]

SOLVED

So I flipped the board again, cleaned the underside with de-oxit and scraped in between the IC pins. When I flipped it back up with the diodes pulled, it was working! Then, I reconnected the diodes and.... STILL WORKING! Must have been some a short on the board of some sort. Chasing my tail over here!

I really appreciate y'all taking the time to help with the diagnostics anyways, I feel like I learned a lot more about how IC's work in practice and now I understand what clamping diodes are! Thanks guys!

[g1]

That's a good lesson for us all. When there doesn't seem to be a logical explanation, don't forget to clean the applicable area of the board. Stray resistance/capacitance from flux or contaminants is not always visibly obvious.

[nosaj]

That's a good lesson for us all. When there doesn't seem to be a logical explanation, don't forget to clean the applicable area of the board. Stray resistance/capacitance from flux or contaminants is not always visibly obvious.

I always try to scrap the flux residue away more or less and OCD thing than anything else.

nosaj

[Helmholtz]

and OCD thing than anything else.

Sorry, what does this mean?

[Mr_bibbles]

Ocular Cathode Distortion - It's when you can shoot hard-clipped sin waves out of your eyes!


Either that or Obsessive Compulsive Disorder.

[g1]

Sorry, what does this mean?

zwanghafte Verhaltensstörung