The errors of my ways... Can you help me find my mistake? (EQ woes!)

[jfrabat]

OK.  If I remember the schematics correctly (cannot see pics from postimage at the office; site is blocked), the idea is to jump R3 and R7 and go directly to the output, right?  So it will tell us if the issue is between these two resistors by effectively bypassing band 4, correct?

Today I was actually thinking of pulling the resistors before and after band 4 and bypassing to be 100% sure the problem is there, and if I understand correctly, this is what you are suggesting...  Just to be sure the short is not in the cap and resistors down the line in the output.

[antonis]

Quite right..!! 

[PRR]

> because I AM a perfectionist.

Then quit using the beepin' "Continuity" function. This is not trailer lights. You are an Electronics Technician. (Honorary, in lieu of pay.) You should know numbers. In this case, *numbers* might suggest why different bands show different "continuity".

[jfrabat]

> because I AM a perfectionist.

Then quit using the beepin' "Continuity" function. This is not trailer lights. You are an Electronics Technician. (Honorary, in lieu of pay.) You should know numbers. In this case, *numbers* might suggest why different bands show different "continuity".

Electronic Technitian seems a bit serious.  Mad Scientist seems a bit more fitting...  especially the mad part (the scientist part, not so much!)   

[Rob Strand]

Now, now... Rob, play nice.

I didn't mean anything bad by it.

Short left leg of R3 and right leg of R7..

OK.  If I remember the schematics correctly (cannot see pics from postimage at the office; site is blocked), the idea is to jump R3 and R7 and go directly to the output, right?  So it will tell us if the issue is between these two resistors by effectively bypassing band 4, correct?

No need as the previous test of pulling the opamp IC1 showed the signal passes through R3 and R7; see reply #2 and reply #3.

Tried this.  Pin 5 is in effect, grounded.  Signal dies right after R3 (nothing comes out of Pin 7 IC1).

That's good because it's narrowed down the problem.

It's normal for no signal at R3 (pin 6) as this is a "virtual ground" .  Acts like ground due to feedback but it's not really ground.

The loss of signal at pin 7 isn't normal.

Is the signal lost for all settings of the boost/cut pot ?

So next would be to check the resistors R3, R7 are connecting to the opamp pins.
Measure at the component side of the board by probing the part leads, not from the bottom on the solder pads.
- R3 to pin 6 IC1
- R7 to pin 6 IC1
- R7 to pin 7 IC1

If there's a problem it will be at the IC.  Maybe you can just re-solder all the pins on IC1.

Another possibility is pin 6 is shorted to ground. previous test confirms that's ok too.

[jfrabat]

Now, now... Rob, play nice.

I didn't mean anything bad by it.

It was supposed to be a joke, no worries.  You have been nothing but helpful to me despite my complete lack of talent OR knowledge in this matters!  So thank you, actually!

Short left leg of R3 and right leg of R7..

No need as the previous test of pulling the opamp IC1 showed the signal passes through R3 and R7.

Actually, I did it before I got to read this.  This is the result:

Ignore the increase in hum (the audio probe already hums A LOT!) when I touch the chassis; that goes away when I connect the back plate (that grounds the whole chassis).

Tried this.  Pin 5 is in effect, grounded.  Signal dies right after R3 (nothing comes out of Pin 7 IC1).

That's good because it's narrowed down the problem.

It's normal for not signal at R3 (pin 6) as this is a "virtual ground" .  Acts like ground due to feedback but it's not really ground.

The loss of signal at pin 7 isn't normal.

Is it loss for all settings of the boost/cut pot ?

So next would be to check the resistors R3, R7 are connecting to the opamp pins.
Measure at the component side of the board by probing the part leads, not from the bottom on the solder pads.
- R3 to pin 6
- R7 to pin 6
- R7 to pin 7

If there's a problem it will be at the IC.  Maybe you can just re-solder all the pins on IC1.

Another possibility is pin 6 is shorted to ground.

Well, when I did the above, I noticed that there is a pad on R7 missing.  I guess I clipped the leg a little TOO close to the PCB...  I redneck fixed that like this:



I really thought that was the issue, as I was getting different voltages at R7 (at the outboard pin, which should be connected to pin 6 of IC1) and pin 6 of IC1, but that was not it.  By the way, I am also getting a different voltage at R3 (10.7mV vs 23mV).

[jfrabat]

FIXED IT!!!!!

THANK YOU ALL!!!

It was a bad link between R3 and R7/Pin 6.  Had to jumper it to make it work, but working it is!!!!

Another piece of good news; this one also does NOT suffer from the oscillation of the original!

[Rob Strand]

FIXED IT!!!!!

THANK YOU ALL!!!

It was a bad link between R3 and R7/Pin 6.  Had to jumper it to make it work, but working it is!!!!

Awesome!

[11-90-an]

Gah! I missed the whole debugging process! I wasn't able to access this forum yesterday because for some reason when I try it says, "server stopped responding" or something like that...

Anyway, nice to see it working..

[Rob Strand]

"server stopped responding" or something like that...

I often get that on this forum.    Mostly it clears in 5 minutes, other times 2 hours, very rarely 1 day.

[jfrabat]

Never had it happen, to be honest.

But don't worry, 11-90-AN.  This is part 1 of many.  I still got 2 preamps on the workbench, and I will make 2 compressors, a De-esser, and probably a switcher to switch the mic to one preamp or the other.  Plenty of opportunities there for me to screw up, so I am 100% certain I will need more help debugging!!!!  LOL!!!