Noise: no 130mV?

Noise-section-crop-01.03.14.jpeg

what a wonderful jigsaw this Yocto is... I have (well, sorta, you'll see) finished my Noise section (after successfully finishing and testing Power).

Power works. there is +15, -15, -5 volts on the mainboard everywhere I checked (the square pads for the potentiometers)

I selected all parts for the Noise section, put them on a piece of paper, doublechecked the components, measured all resistors before building (because the color codes are hard to read in the instructions -- the pictures sometimes have quite a color tint, so a resistor code app is my best friend for those -- and measuring them).

After soldering everything I am taking the measurement from pin 1 to R131 and try to adjust for 130 mV using the trimpot.

I set my multimeter to 2 V AC -- 0.130 is what I should aim for? I am getting a reading of 0.016 which while turning the trimpot changes to 0.015 -- more of a rounding error, it seems -- but nothing else.

I double checked the solder points, all the components' values and used two different multimeters. Power is connected to the power board and +/-15/+5 is present

Can someone suggest where-else to check to narrow down the problem?

thank you!

On comparing to the instructions again, I see that you soldered R127 as well, which isn't part of the list of parts of "Noise". Do I need to put that in too, before checking?

Also: +15 and-15 on the opamp 4558C are still working.

I think I overlooked R127 -- just found that it *is* part of "Noise"... so let me check that, for the time being, please excuse my standing in the corner blushing...

OK, got R127 in there, power is there, and I can (finally) adjust -- but the range is between 250 mV to close to 1000 mV -- so the lowest I can go is 250 mV.

*now* I need advice!
how bad is 250 instead of 130 at this point? I suppose changing one of the fixed resistors would change things? But which one? for what?

Thank you!

Hey Misa,

I removed R127 and replaced it with a 100k trimmer like this:

The middle leg is bent across and soldered to the leg below the trimmer screw, then both the outer legs are soldered in the PCB where R127 came out. Like the image below, although the middle leg on this example has been bent across the opposite way.

I laid the trimmer flat on the PCB with the legs bent at a right angle, this keeps the profile low.

There is more info in this thread regarding the Noise section reference voltage: viewtopic.php?f=16&t=155

Ah, thanks for the link -- I tried using the search function but didn't find anything ("130 mV", "noise"), that's why I posted...

So after replacing the 4K7 for a 47K I am getting a usable range and got it set to 130 mV. Thanks!

But I don't understand why you replaced R127 with a trimpot? I thought you set TM4 once, then forget?

misa wrote:So after replacing the 4K7 for a 47K I am getting a usable range and got it set to 130 mV. Thanks!

But I don't understand why you replaced R127 with a trimpot? I thought you set TM4 once, then forget?

Excellent, glad you have achieved the reference voltage.

I used a trimpot as an alternative to a fixed resistor that is all.

I always get 8.2mV, and if I change the TRIM to any other position nothing happens. Ideas?!

mrgif, I'm afraid you need to check (measure and/or color-check) every resistor in the Noise section, double and triple check...

if it helps at all, maybe you can compare your build to my picture above?

I use this tool a lot to check resistor values:

http://www.digikey.com/us/en/mkt/calcul ... stors.html

Hope this gives you a start...