Voltages are too high
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Voltages are too high

by kfj on Wed Oct 31, 2012 5:34 pm

Hi, I'm attempting a self sourced build, i'm new to electronics, so i'm a bit out of my depth here. This is my first project aside from repairing an lcd monitor by replacing bad caps on the power board (which was a success btw :) ).

Now i've been following the guide here: http://www.ladyada.net/make/x0xb0x/fab/powersupply.html and everything was going great up until the point where you install the IC22. Instead of getting 5.333v I am getting 11.15v. The trimpot does seem to work, but obviously doesn't have the range to get me down to 5.333v, lol. I continued to the next step in the guide to see what happened and so i've installed the 6562 (IC23) and tested the voltages. Once again, for the '5.333v' I am getting over 11.15v. I then tested the '+12v' supply and get a reading of 24.5v. So i'm getting roughly twice the voltage that I should be. I've checked the resistor ratings and the caps to make sure I used the right ones. I checked the diode orientation, the ic orientation, all seems fine. I've tested my ac 9v power supply and it puts out 11.3v which should be fine right? So I'm low on ideas.

Three things I would like to mention, in case they may be part of the problem:

#1. I had a go at soldering the usb chip, and stuffed it up, it is currently attached to the board, but with several pins bridged, I have been unable to remove it. I have been working on the assumption that its presence on the board won't affect 5.333v and 12v supplies (please tell me if I'm mistaken). I was planning to go back to the usb chip later on to see what I can do with it, but I thought I better mention it in case it's responsible for the problem here.

#2. I placed a 202 trim pot and then later I acquired a different one which I liked more (a Vishay CT6EP202 as per the bill of materials. I was unsure of the suitability of the other pot I had, so I thought i'd play it safe and get the one in the bom), so I desoldered the old one. This proved challenging and I had to mutilate the old one in the process, but I managed to remove it (in retrospect I should have stuck with the first one). I am mentioning this because I guess it is possible that I damaged the pcb in the process of doing this. The new pot does seem to function, it affects the voltage as I'd expect it to, but for all I know, it could still be the source of the problem.

#3. Some of my 5% resistors are blue metal film rresistors rather than the usual carbon film, which I ordered by mistake. From what I can tell, this will make zero difference, but I'm mentioning it just in case.

Thanks in advance for helping.
kfj
 
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Re: Voltages are too high

by MMM on Thu Nov 01, 2012 1:18 pm

I'd remove IC22 (and IC23) and measure the voltage at the junction of R3 and D47. My guess is that either IC22 is defective or something in the "chain" D47 to D44 is wrong.

But before you do anything: Re-check again the orientation of IC22. I recall that mounting this one the wrong way around resulted in roughly twice the voltage.
MMM
 
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Re: Voltages are too high

by kfj on Thu Nov 01, 2012 5:27 pm

Thanks for the suggestions MMM. I have decided to photograph the pcb before proceeding to make things easier for us.

Hopefully these pics will help.

PCB top: http://i50.tinypic.com/9srlf7.jpg
PCB underside: http://i47.tinypic.com/qy69z4.jpg
Ac Adaptor: http://i47.tinypic.com/2j69wud.jpg

Let me know if you spot anything amiss.

If everything looks ok, then I will remove IC22 as you suggest. Hopefully it will prove less challenging to remove than that wretched trim pot.
kfj
 
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Re: Voltages are too high

by MMM on Thu Nov 01, 2012 6:36 pm

Looks ok to me.
MMM
 
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Re: Voltages are too high

by kfj on Fri Nov 02, 2012 2:19 pm

This is quickly turning into a distaster. I've managed to remove IC22, but one of the pins snapped off. I don't have a spare and the only places that sell it online seem to charge £12 to deliver.

Is it possible to repair a snapped pin? Can I just solder the pin back on to the point where it snapped off?

*EDIT* I've just managed to order a couple of replacements for IC22 (free delivery, hurrah!) and they should be arriving on monday, so that's one thing sorted, you can disregard my whining above. :) *EDIT*

About measuring the voltage at the junction you suggested... Could you please tell me precisely where to place the multimeter probes, I'm unclear on what to do. Thanks.
kfj
 
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Re: Voltages are too high

by MMM on Sat Nov 03, 2012 11:46 am

kfj wrote:About measuring the voltage at the junction you suggested... Could you please tell me precisely where to place the multimeter probes, I'm unclear on what to do. Thanks.


Usually a good point for the negative probe is the metal tab of the 7805 (because one can easily clip the probe to it); however this is actually the "digital" ground that is connected to the "analog" ground with just a single soldering point. I've seen at least one case where this connection was missing. So you might want to check if you have continuity between the metal tab and - for example - the middle pin of J4.

Then trace with the other probe the following path: R2, R3, D47, D46, R4, TM6, R5, D45, D44. (this probably will become clear when looking at the schematic).
MMM
 
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Re: Voltages are too high

by kfj on Sat Nov 03, 2012 5:33 pm

MMM wrote:So you might want to check if you have continuity between the metal tab and - for example - the middle pin of J4.


Ok, so with IC22 and IC23 removed, I tried an audible continuity test. There are 3 holes in a line underneath the J4 marking, so I tested all of them, with the negative probe attached to the metal tab of the 7805. The middle one passes the test, the left and right ones fail. Do they all need to pass, or is it just the middle hole that matters?

MMM wrote:Then trace with the other probe the following path: R2, R3, D47, D46, R4, TM6, R5, D45, D44. (this probably will become clear when looking at the schematic).


Excuse my ignorance but I'm still not sure. Do you want me to check the voltages at each component?

If so then I got 31.2V (R2) 29.2V (R3) 18.5V (D47) 17.8V (D46) 17.2 (R4) 8.3V (TM6) [two of the TM6 pins read 8.3V and the one that goes to R5 reads 5V] (R5) 1.2V (D45) 0.6V (D44) Nothing. Repeated attempts caused it to let off a spark. :shock: Hope that didn't damage anything.

Excuse me if I'm misunderstanding your instructions.
kfj
 
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Re: Voltages are too high

by MMM on Sat Nov 03, 2012 6:20 pm

You did everything correctly. The measurements are consistent with the correct (and working) components for this path.
This of course points at IC22 as the culprit; so once you get the replacement you can solder it in and then hopefully everything should work.

One thing to keep in mind: It is crucial that IC22 is a LM336Z-5.0; the "5.0" denotes the voltage it is regulating.
MMM
 
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Re: Voltages are too high

by kfj on Mon Nov 05, 2012 10:13 am

MMM wrote:This of course points at IC22 as the culprit; so once you get the replacement you can solder it in and then hopefully everything should work.


I installed the replacement IC22 and IT WORKS! I'm now getting a perfect 5.33V. Thanks for all your help, I would've been lost on my own.
kfj
 
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