Voodoo Main Discussion

[Qiaozhi]

Thanks for the notes Waltr. I was able to track down the problem to the coil. The DXF coil has been giving me all sorts of problems. I used George's description for a "small concentric coil" (Pg 73) and it's seem to be working, and I am getting the same numbers as the examples in the book!!! Now I can do proper debugging.

Carl has noticed a small bug in the menu system where it is supposed to restrict the disc sample from going over 100us. Unfortunately it was changing the main sample instead. Software Version 1.2 fixes this issue. The updated project and hex file can be found in post #1.

[dingbatca]

Thanks for the notes Waltr. I was able to track down the problem to the coil. The DXF coil has been giving me all sorts of problems. I used George's description for a "small concentric coil" (Pg 73) and it's seem to be working, and I am getting the same numbers as the examples in the book!!! Now I can do proper debugging.

[waltr]

I did build this board in stages, and tested as I went. Lets just go back to the basics.

TP2 & TP3 wave form. Per page 31 of the book, I should see a drop of about 20us, but I am getting closer to 224us? Pic attached.

The "drop" on TP3 is when the pre-amp is saturated. If it was coming out of saturation in 20us before then check the clamping diodes, D2, D3.
Also recheck the offset adjustment (R30) and look at TP5 to see if it is first or second pre-amp stage. Are the PL7, PL8 jumpers correct?

Also take a look at TX coil and ensure Damping resistor is still doing its job.

[dingbatca]

I did build this board in stages, and tested as I went. Lets just go back to the basics.

TP2 & TP3 wave form. Per page 31 of the book, I should see a drop of about 20us, but I am getting closer to 224us? Pic attached.

[MartinB]

Thanks for the good feed back. Now I have more to check. :-)

If U7 is a crappy... Would it be safe to connect my shop power supply 0V, -5V, +5V, up to TP1, TP14, TP15? Just as a simple test to see if everything powers up? I understand that if I built the board wrong, said test will release the magic smoke.

.
Hi Dingbatca
Taking a simplistic approach, I would pull the IC's and then reinsert them in line with Georges build instructions given in Post #277 (copied below)
You will then stand a chance of finding with part of your build is not working.

1. Install the PIC and program it with the hex code from post #1. You need to have the PIC working, otherwise there will be no +5V supply. This is because the oscillator for the voltage converter comes from the PIC.
2. Install the transmitter circuit and test that it's functional.
3. It would probably be worth connecting the LCD and menu buttons at this stage, and flipping the detector into PI mode.
4. Build the PI preamp and check that the detector is working as a PI with a mono coil.
5. Proceed to build the rest of the PI channel and the audio amp. At this point you should have a working PI detector.
6. Build the DISC preamp.
7. Connect a concentric coil and test the DISC preamp output.
8. Flip detector into Hybrid mode, and build the DISC channel. You should now be able to test the hybrid capability.

Refer to page 31 of the book for the relevant waveforms.

[Qiaozhi]

Thanks for the good feed back. Now I have more to check. :-)

If U7 is a crappy... Would it be safe to connect my shop power supply 0V, -5V, +5V, up to TP1, TP14, TP15? Just as a simple test to see if everything powers up? I understand that if I built the board wrong, said test will release the magic smoke.

You could remove U7, and then connect an external +5V supply between TP15 and TP1. If your power supply has a current limit control, you should turn that down to prevent any magic smoke being released.

The 78L05 regulators are rated at 100mA, which is more than enough to power the Voodoo circuits. It would probably be worth testing one of the devices you have with a variable load to see how much current can be extracted before it drops out.

[dingbatca]

Thanks for the good feed back. Now I have more to check. :-)

If U7 is a crappy... Would it be safe to connect my shop power supply 0V, -5V, +5V, up to TP1, TP14, TP15? Just as a simple test to see if everything powers up? I understand that if I built the board wrong, said test will release the magic smoke.

[waltr]

Sounds like the 78L05 to shutting down with load when all chips are in.
Since you did swap out the 78L05 (U7) this is a mystery.

Maybe try a different brand 78L05, they are not made all the same.

On the pre-amp there are series R's, 27 Ohm, R17 & R32. With the pre-amp in measure the Voltage drop and calculate the current draw. Should be less than 10mA. Too bad the other op-amps don't have a series R on their power to measure current draw.

The other circuits that use +5 are the level shifters, Q7, Q4 & Q8 but these have 10K Ohm to +5. Do check for shorts.

So what every the drain is, it's down stream of U9/U10. I am eyeing R34 or R28, I think. Hard to read Fig 3-12.

Yes, and lift one end of R35 & R33 to disconnect Filter outputs from PIC input.


Last, it could be a bad cap. These are hard to find.

[Qiaozhi]

By the way, the voltage on pin 2 of U8 (LT1054) needs to be measured with reference to the -ve side of the battery. Note that U8 pin 3 is connected to -VB.

I've just done some measurements here on my working unit:
+5.03V at TP15
+9.07V on cathode of D6
+9.01V at pin 2 of U8 (LT1054) referenced to -VB.

This is becoming a mystery.

[dingbatca]

Sorry for filling up the thread with my real time debugging.

I removed U9 and U10. Everything checked out, as expected.
TP20 to Pin 2 of the PIC, 10K
TP17 to Pin 3 of the PIC, 10K
TP20 to TP14 (-5V), 20K
TP20 to TP17 (-5V), 20K

Checked R58, and R63, 1M
Checked R52, and R55, 2.2M

Any other points to check for debugging?

[dingbatca]

Very interesting!

If I remove only:
U3 (PI dual preamp), I get +5V
or
U9 (DISC channel filter), I get +5V
or
U10 (PI channel filter), I get +5V

So what every the drain is, it's down stream of U9/U10. I am eyeing R34 or R28, I think. Hard to read Fig 3-12.

[dingbatca]

Jumpers fixed. Thanks for the note. I should go back and re-read post #1. That was a long time ago. ;-)

Pulled most the IC's and systematically put them back in. Looks like U9 is the problem child. Swapped out a new TL072 and the problem remains. I check the soldering and it all seems clean. I am guessing I have a component level issues. Any advice on debugging it? I will deep dive the chip data sheet and DISC schematic tonight.

[Qiaozhi]

Cathode side of D6 is at 9.51v. The data sheet for the 78L05 say minimum voltage should be 7V, so I think that's good.
Pin 2 of U8 is at -2.54V. Swapped out U8, no change.
Sync pulse on TP9, and U8-pin7 both look kinda correct. Its not the exact same as the book. Scope pics attached.

Looks like something is loading the +5V line.
Pull out U1 (TL071), U9 (TL072), U10 (Tl072), and U5 (4066).
Then test if the +5V comes back.

Just noticed that your jumpers are also reversed on PL7 and PL8.
Check post #1 - Errata #1.

[dingbatca]

Cathode side of D6 is at 9.51v. The data sheet for the 78L05 say minimum voltage should be 7V, so I think that's good.
Pin 2 of U8 is at -2.54V. Swapped out U8, no change.
Sync pulse on TP9, and U8-pin7 both look kinda correct. Its not the exact same as the book. Scope pics attached.

[Qiaozhi]

Correction. TP15 was reading +4.96V. Now it's reading -0.8V. I swapped out the 78L05 (U7) thinking I was having a regulator issue. No luck, still reading -0.8v. Any ideas? Pics attached.

Check the input pin of U7 (measure at the cathode of D6). I suspect the voltage there is too low.
Then check the voltage on pin2 of U8.
U8 might be kaput or there is no sync pulse on pin 7. If sync pulse is missing, check on TP9.