Addition to Debugging: what to do when it doesn't work

[R.G.]

This has come up repeatedly, and I think it makes a good addition for people who have graduated to doing their own debug thinking.

If there's no signal going in, just DC on the power pins, then **all capacitors can be replaced with open circuits and nothing should change**.
For DC conditions as read by a meter, all the places on the PCB that are shown as connected by lines in the schematic should be within millivolts of each other.

So we can do the following.
- hook your meter minus/black lead to the ground connection on the input jack; all places on the PCB which have the ground symbol on them simply must have a voltage that's nearly 0Vdc, within a few millivolts, as measured by the red/positive lead, or there is an open/high resistance connection between them. That has to be found and fixed or it's not going to work right.
- with the meter black/minus still hooked to the ground connection on the input jack, all places on the schematic that are connected to the power supply simply must be the same voltage plus or minus a cat's whisker, or it's not going to work right.
- you can extend this: for the whole board, places connected by a wire on the schematic have to be at the same DC voltage, or they are not actually connected on the board, no matter what the solder joints and components look like.
- it's best to do the measurements on the actual pins of the components, not the solder pad on the bottom of the PCB, because the solder joint to the pin itself may be the problem.
- if you have a place where the voltages are suspicious on either side of a capacitor, then *pull the capacitor out*. This will not affect the DC voltages on either side of the cap at all if the cap was good and was installed the right way around (if polarized). But it will change things if the cap was conducting DC when it wasn't supposed to.
- tone/volume controls are often isolated from DC voltages in well designed circuits; DC on the controls where there is a capacitor which ought to be blocking DC is a read flag; some designs have put DC onto the audio controls either by intent or omission, but it's in general a poor practice, as it promotes pot wearout and audible crackling

[mr_deadmaxxx]

hi

I built the equinox reverb before(still not working now) and some of the electro caps I used were salvaged from an old project a few years back. I looked at them and there were no signs of being busted, so I desoldered them and used them on the equinox.  Problem was, one electro cap(used as 5V dc filter) was accidentally soldered in reverse polarity while I was powering it with dc power. All ICs connected to the 5V supply started to began hot (not very hot but was beyond normal). I tried debugging it, replacing the reversed cap with a new cap with now correct polarity. When I powered it with 9Vdc, the 5V ICs and 5V regulator IC still turns hot. I tested the 5V ICs (PT2399) to check if they were still working by using them on one delay pedal and they turned out okay.

my question:

1. could it be that the electro caps that i salvaged were not usable anymore and were the cause of the heating? (note: no visible signs of being busted on these caps)

or

2. the reversed cap that was installed, damaged the rest of the caps?

i would really appreciate an answer from you. thanks..

[Perrow]

If there's no signal going in, just DC on the power pins, then **all capacitors can be replaced with open circuits and nothing should change**.
For DC conditions as read by a meter, all the places on the PCB that are shown as connected by lines in the schematic should be within millivolts of each other.

LFO's and timers excluded, but you're gonna tell us that those aren't strictly DC

[cthulhudarren]

Thanks R.G.

I actually already do this when I'm debugging. So at least I'm doing one thing right.