madbeanpedals::forum

Projects => How Do I? Beginner's Paradise. => Topic started by: RobW on July 10, 2021, 02:49:21 PM

Title: Checking resistor values that are soldered in place - what's the trick?
Post by: RobW on July 10, 2021, 02:49:21 PM
This probably seems very simple to most experienced builders, but I'm having a heck of a time trying to check the values of resistors on a board I'm troubleshooting.
Is there some trick to this? I have 2 different DMM's, and can get readings from the IC chips without problem.
Any suggestions on this? I've tried multiple settings on the DMM's but nothing works.
Any help on this would be appreciated.
thanks !
Title: Re: Checking resistor values that are soldered in place - what's the trick?
Post by: RobW on July 10, 2021, 03:37:57 PM
Just found my own answer.
For other idiot beginners like me out there, you cannot have power going to the board.
That's what I was doing wrong.
Title: Re: Checking resistor values that are soldered in place - what's the trick?
Post by: madbean on July 10, 2021, 05:04:52 PM
A bit of help: you may still be doing in wrong if you are using a multimeter to check resistor values on a PCB, whether powered or unpowered. Remember that resistor values will change when you have them in parallel (or measuring across two in series). So, depending on what that particular resistor is doing in the circuit you may not get the correct reading.

The best way to identify them is by the color bands. It's not hard to learn to read resistors by sight (it takes maybe a week of practice) but if you don't want to put in the time just use resistor color band charts. You can find them on the internet easily. In most cases, you only need to identify the first three bands to get the value.
Title: Re: Checking resistor values that are soldered in place - what's the trick?
Post by: gordo on July 10, 2021, 07:48:34 PM
Not an idiot move by any means.  Two things that will help:  As Bean mentions, you can't really trust in circuit values, but at the same time it's a bit of a sanity check, so take the values with a grain of salt.  Still, it will often point out grossly out of whack errors.

Second, measure values as you stuff the board.  It's a pain in the butt and slows down your workflow but if you test before you install parts you virtually eliminate mistakes.  I have one of the cheap testers from eBay that I wouldn't bet any money on accuracy, but it gives me a backup to reading color or number codes.  When I started using it my failure rate dropped to near zero.  If nothing else it gives you time to think thru common mistakes like 4k7 vs 47k.  Simple stuff like that.  At that point mistakes are reduced to bad solder joints and solder bridges.
Title: Re: Checking resistor values that are soldered in place - what's the trick?
Post by: jimilee on July 10, 2021, 07:59:13 PM
Just to throw my hat into the ring. If you have a harbor freight nearby, you can get a cheap or even free DMM. I also use the eTools pro app on my phone, itís super helpful for troubleshooting. Resistor and nap codes along with lots of other cool nerdy stuff.


Sent from my iPhone using Tapatalk
Title: Re: Checking resistor values that are soldered in place - what's the trick?
Post by: RobW on July 11, 2021, 07:06:06 PM
As always, thanks for the input from those of you that have been down his path once or twice !
I'm currently working on the Touchstone Chorus, and was pretty careful with this one.
Checked values on all caps and resistors before putting them in place, and discarded any part that was really far out of spec.
BTW.. .that was something else I picked up in the forum, and really appreciate!
The problem I'm having is that IC2 ( the MN3007 chip ) values are pretty far off for pins 3, 7, 8.
I started testing values, and this post relates to R24 in the build, which should be 180k.
The after insert/solder value is less than half of that, roughly 94k , so It appears something is wrong.... just not sure what.
Should I just replace R24 at continue troubleshooting at that point?
Or is a different approach better?

Again, thanks for the input   8)

Title: Re: Checking resistor values that are soldered in place - what's the trick?
Post by: Cybercow on July 12, 2021, 07:54:42 PM
Another trick is to lift one leg of a resistor. That way a DMM measure will not be fooled by parallel resistances on the PCB.
Title: Re: Checking resistor values that are soldered in place - what's the trick?
Post by: peAk on July 16, 2021, 08:13:45 AM
I think this is what gordo is talking about and I use it all the time just as he said. I just quickly check mine before putting them in the board. I got my mine from Amazon but I am sure they are probably even cheaper on Ebay

https://www.amazon.com/gp/product/B07V557KXP/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

(https://app.box.com/shared/static/hifn30j03pgfe9vpk8q9jnjgrekk2n2n.jpg)