Problem with J4 Voltage drop

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Krippe
 
Posts: 7
Joined: Wed Nov 02, 2005 7:39 am

Problem with J4 Voltage drop

Post by Krippe »

Hello x0xb0x and electronic wizards I have a problem.

I have soldered my PSU, VCO and VCF.

All tests for my PSU went ok so I soldered my VCO. When this was done I tried to perform the listening test with cheap headphones on my waveform switch. It was hard to hear anything but noise and since I had read on the forum that this test often was very noisy I continued to solder my VCF.

When this was done I tried to probe my x0xb0x with a software oscilloscope but all I could see was some very noisy low signals with what looked like a sine wave. My resonance and cut off pots did not have the desired effect on the signal, perhaps no effect at all, it was hard to tell.

This made me go back and check the voltages from my PSU once again and the values had changed to something like 1.2 and 2.5 V instead of 5.33 and 11.8 V. When I disconnect J4 I get the correct voltages again. Also I have seen that my IC23 gets very hot when I have my PSU connected to the main board.

I suspect that I have made something wrong that makes the main board draw to much current. Do you think that this is correct? Do you have any ideas for how I should look for the error? I have checked the direction and placement of my components and looked for solder bridges.

I am not very experienced with making stuff like this and I would be very grateful for any input.

Guest
 

Post by Guest »

there is definitely a short someplace
take a third look at all your components
if that doesnt turn anything up
get a multimeter and measure the resistance to ground
from the 5.333v and from the 12v points on j4

you should do this check with the power supply disconnected
this will help you determine wether the short
is on the 5.333v line or the 12v line

look for traces that were etched poorly and conduct to ground

MJSemtex
 
Posts: 44
Joined: Mon Sep 26, 2005 6:35 pm

Post by MJSemtex »

i had exactly the same problem with mine...

searched and searched the boards to find a short... but, just couldn't sort it out. i ended up sending it off to someone else to have them take a look at it and am still waiting on it.

let me know if you do ever figure it out as i'd love to have a better idea what my problem was / is...

Krippe
 
Posts: 7
Joined: Wed Nov 02, 2005 7:39 am

Post by Krippe »

Thanks for the replies.
I have already tried to measure with my PSU disconnected. The resistance between the 5.33 pin and ground is about 45 ohm and a lot higher, perhaps 10K ohm (I am at work now and don't remember exactly) between the 12V pin and ground.

This makes me think that there is a short somewhere between the 5.33 line and 0, but since I do not understand how the circuit is supposed to work I am not sure.

Any ideas of how I should continue my tests?

And MJSemtex, I promise to make a post here if I find the error. Please let me know if you get to know what your error was/is.

Guest
 

Post by Guest »

unfortunately the easiset way to debug this
is to do some modifications to your mainboard

5.333v comes in on pin1 of j4
the voltage splits from this point and goes to three
different areas

you can cut the traces as they leave pin1 of j4
and then measure the resistance to ground
from the point where you cut
this will isolate a section so you can see if the short
is there or in another area

once youve isolated the area
you can start looking at individual components

when you are done
you can scrape a bit of the solder mask off of
the traces you cut
and then solder a short piece of wire to reconnect it

Krippe
 
Posts: 7
Joined: Wed Nov 02, 2005 7:39 am

Post by Krippe »

Thank you very much for your help.
Now I have cut two of the traces from pin 1 and performed some more measurements with my PSU disconnected.

Two of the traces give an infinite resitance to ground and one gives about 40 ohm. The one with about 40 ohm is the connection to IC9. When i measure the resistance over IC9 i get the following results from pin1:

pin1 to pin8 (5.33 to GND) 44 ohm
pin1 to pin9 (5.33 to CLK) 46 ohm
All other (excpet for pin1 to pin16 that are connected) are a lot higher.

Are these values expected or is there something wrong with my IC9 (74AC174)?

I have studied the area for solder bridges etc without finding anything.

/Kristian

Guest
 

Post by Guest »

i think someone else had a bad ic9
id suggest pulling the chip
and checking all the traces
before putting a new chip in

its possible that its just a bad trace
but you cant check that until you pull the chip anyways

Krippe
 
Posts: 7
Joined: Wed Nov 02, 2005 7:39 am

Post by Krippe »

Thank you for your help.

I think I solved it or partially anyway. I bought a new 74AC174 and replaced my IC 9. After this I was able to hear the characteristic saw and square wave through a pair of cheap headphones and the tune pot affected the frequency.

I also tried to scope the waves with a software oscilloscope with less satisfying results. The square wave looked ok but not exactly as the one from the fab manual.

The saw wave on the other hand looked very strange. It looked like a straight line with some fast peaks occuring at a regular frequency. I am not sure if this comes from the software oscilloscope or computer microphone input or if the signal actually looks this strange. When I had the oscilloscope connected I could not hear the saw signal through my headphones.

Is it possible that the microphone input affects the signal so that I can not hear it? Should I continue to build or should i keep measuring?

guest
 
Posts: 3155
Joined: Fri Feb 17, 2006 5:35 am

Post by guest »

its very possible that the microphone input
is loading down the circuit
making the waveform distorted

if you have sound coming out of the vco
id suggest continuing with the build
and see how it sounds in the next stage

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