Author Topic: It's a short, right? Rookie question...  (Read 265 times)

0 Members and 1 Guest are viewing this topic.

jfrabat

It's a short, right? Rookie question...
« on: December 06, 2017, 10:46:04 PM »
Hi, guys.  I am troubleshooting a delay pedal I recently built from JMKPCBs.  Its his Modular Delay (http://jmkpcbs.com/wp-content/uploads/2014/03/Delay.pdf) with the Mini TapTation Board (http://jmkpcbs.com/wp-content/uploads/2014/03/Mini-Taptation.pdf)  I should probably point out that I built the delay using the Sea Urchin values as per the build doc (so there are some missing caps and resistors, as well as some jumpers as per the instructions).  The issue I am having is that if I connect this board to any 9V circuit, it completely shuts down the circuit.  I tried putting it in a multi-effect pedal I made, but it shuts it down. I tried it on its own, in a circuit that only has the input and output jacks, the batery termina, and an LED indicator, and as soon as I plug this in, the LED shuts off and the battery starts getting hot.  What should I look for?  This symptom is a first for me...
Newb without skills!  Have some patience!

Transmogrifox

Re: It's a short, right? Rookie question...
« Reply #1 on: December 07, 2017, 12:16:52 AM »
Check the polarity of D1 would be my first guess.
trans·mog·ri·fy
tr.v. trans·mog·ri·fied, trans·mog·ri·fy·ing, trans·mog·ri·fies To change into a different shape or form, especially one that is fantastic or bizarre.

bluebunny

Re: It's a short, right? Rookie question...
« Reply #2 on: December 07, 2017, 02:45:15 AM »
hot battery = short

Get that DMM out!
Ohm's Law - much like Coles Law, but with less cabbage...

antonis

Re: It's a short, right? Rookie question...
« Reply #3 on: December 07, 2017, 05:58:31 AM »
If D1 is OK, check for shorted 5V regulator..

With Power off, check for very low resistance between Vcc & GND..
Tired of updating my account to enable 3rd party hosting..

jfrabat

Re: It's a short, right? Rookie question...
« Reply #4 on: December 07, 2017, 10:38:00 AM »
Check the polarity of D1 would be my first guess.

That makes complete sense. OK, quick question (the board is not so clear); the negative (stripe side) goes up according to the build docs, right?  The square pad SHOULD be the positive, right?



I tested for continuity from 9V to GND yesterday, conecting the + of the DMM to the 9V and the - to the GND.  The DMM did not beep.  Theoretically, if the diode is backwards, there should have been continuity, right?

If D1 is OK, check for shorted 5V regulator..

With Power off, check for very low resistance between Vcc & GND..

My first thought was that I had inverted the 5V regulator, so I checked for that (again, the daughter board is not so clear, but I followed the traces, and compared them to the diagram and they are the right way).  I did not check for shorts there, though...  So, if the resistance is low, there is a short, right?
Newb without skills!  Have some patience!

antonis

Re: It's a short, right? Rookie question...
« Reply #5 on: December 07, 2017, 11:07:39 AM »
The square pad SHOULD be the positive, right?
In accordance to electros positive leg pad, yes..

I tested for continuity from 9V to GND yesterday, conecting the + of the DMM to the 9V and the - to the GND.  The DMM did not beep.  Theoretically, if the diode is backwards, there should have been continuity, right?
True & Correct..!!  :icon_wink:

So, if the resistance is low, there is a short, right?
The lower the resistance the shorter the short..
« Last Edit: December 07, 2017, 11:09:31 AM by antonis »
Tired of updating my account to enable 3rd party hosting..

jfrabat

Re: It's a short, right? Rookie question...
« Reply #6 on: December 07, 2017, 11:13:22 AM »
OK, so it seems D1 is correct.  I will check the regulator (both, actually) when I get home.  I will use a new (not installed) one as a base comparison point.
Newb without skills!  Have some patience!

jfrabat

Re: It's a short, right? Rookie question...
« Reply #7 on: December 07, 2017, 09:09:08 PM »
Here's where I am at (not so different from when I started the thread!):

First, the build materials:



I built it using these values (found in the instruction manual):





And used this daughter board:



And this is my build:



Main Board:





As you can see in the image, D1 has the stripe pointing to the pots, as it should according to the board.

Daughter Board:





Now, a 78L05 from my pile (not installed) has about 17.3M Ohm of resistance.  The main board's 78L05 is reading 19.46K and the daughter board is reading 18.84K.

So, whats my next step?  Any recommendations?
Newb without skills!  Have some patience!

Slowpoke101

Re: It's a short, right? Rookie question...
« Reply #8 on: December 07, 2017, 11:06:36 PM »
Looking at the board and the doco...Reverse D1 as it appears to be installed backwards.
Also solder the PT2399's pins 7 and 8. Looks like no solder there.
Good luck.

jfrabat

Re: It's a short, right? Rookie question...
« Reply #9 on: December 08, 2017, 01:09:40 AM »
Looking at the board and the doco...Reverse D1 as it appears to be installed backwards.
Also solder the PT2399's pins 7 and 8. Looks like no solder there.
Good luck.


Did both.  The pedal is working!

Now I have a question and a new issue;

QUESTION: Correct me i I am wrong, but the band side of the diode is the negative (cathode), right?  And typically, the square pad is positive, right?  If so, why does in this case the negative (cathode) go in the square pad?  This is for my own learning advancement...

ISSUE: The board now works, BUT, if I tighten the nuts on the pots, it stops working (even if I tighten them by hand).  If I leave them loose, we are good to go, but if I tighten them, it quits.  I thought maybe something was shorting, so I covered almost everything with electrical tape, but the issue persist (mind you, it is now a little less sensitive to tightening).
Newb without skills!  Have some patience!

Slowpoke101

Re: It's a short, right? Rookie question...
« Reply #10 on: December 08, 2017, 01:49:50 AM »
Oh dear, a question that is really related to conventional current flow.

With a diode it will pass current (conventional) from anode to cathode, so you could think of the cathode being "negative". LEDs are a good example; if the anode is more positive than the cathode, it will emit light. The diode schematic symbol has an arrow that points towards the cathode. This indicates current flow direction, that is conventional current flow (current flows from positive to negative) and not true electron current flow, which is the opposite (negative to positive). Don't worry about this.

On the diode body, the cathode end is marked by the band painted on the body. Usually. Russian germanium diodes can be marked differently.

In this delay unit D1 is placed as a reverse power applied protection device. Connect power backwards and the diode will conduct and "short" out the applied power source (hopefully protecting the rest of the circuit). In this case the cathode is connected to the positive supply rail and the anode to power ground. As a protection system it is better than nothing but other protection systems are available.

The component overlay in the build doco has a square pad for to indicate the positive leg of polarised components. Usually the designers show the cathode of a diode with a square pad. But this is not always the case, you must check against the schematic. Never assume, always verify.

Your problems with the unit failing when the pots are tightened, etc would be a mechanical problem. Shorts are the most likely culprit. Or the board is being twisted slightly and there exists a poor solder joint which opens when stressed. The same applies to IC sockets. The legs of the pots could have poor solder joints or a loose leg where the leg is riveted to the pots' resistance track. Have a good close look, you will find the problem.
 
« Last Edit: December 08, 2017, 01:54:30 AM by Slowpoke101 »

antonis

Re: It's a short, right? Rookie question...
« Reply #11 on: December 08, 2017, 08:22:10 AM »
IF D1 should be reversely placed then it's clearly a fault on PCB marking..!!

Square pads should indicate ITEM's polarity - not circuit's one..  :icon_wink:
Tired of updating my account to enable 3rd party hosting..

jfrabat

Re: It's a short, right? Rookie question...
« Reply #12 on: December 08, 2017, 08:46:19 AM »
Usually the designers show the cathode of a diode with a square pad. But this is not always the case, you must check against the schematic. Never assume, always verify.

I always figured it was the other way around...  I was just checking the other boards I have built, and most of them do have the square pad on the cathode, so I guess you are right.  Of course, they also have the drawing of the diode showing the band, which is how I have always guided myself when building them.  This one threw me off because there is not enough space in the board to see the diode drawing.

Your problems with the unit failing when the pots are tightened, etc would be a mechanical problem. Shorts are the most likely culprit. Or the board is being twisted slightly and there exists a poor solder joint which opens when stressed. The same applies to IC sockets. The legs of the pots could have poor solder joints or a loose leg where the leg is riveted to the pots' resistance track. Have a good close look, you will find the problem.

I tightened it by hand as much as I could.  I will check solder joints tonight, but worst case scenario, it actually works the way it is (its just not tight).  If it is the rivets, not much I can do there...

IF D1 should be reversely placed then it's clearly a fault on PCB marking..!!

Square pads should indicate ITEM's polarity - not circuit's one..  :icon_wink:

That was my thought, but looking at other PCB's, the cathode is usually the square pad.  So I guess it was my misunderstanding...

Thanks for all the help, everyone!
Newb without skills!  Have some patience!