2nd Surf PI board problem ?????

[dfbowers]

No Vortxrex,

It doubled what I was getting with the faulty board. I really wish I could get my scope going but I'm not having any luck at all. With my good board all it did was give me a constant unadjustable threshold. It was too loud to check any targets. I'm guessing with the right resistors and caps perhaps the LM833 could show some depth gains. I wish I understood electronics better but I do not. I'm probably wrong. If I could just get my 2nd board working correctly at this point I'd be happy.

Bob

Bob,
Try the divide and conquor method and you will eventually figure it out.
I have been fixing computers for almost 30 years. When I am stumped, I just start substituting suspect bad components into a working unit until I figure it out (not using a customers unit of course).

We don't always have that luxury, but in this case I think you do.

* If you socketed your IC's you can rule them out quickly, checking them one at a time by substituting into your working unit.

* Check all your solder connections for cold joints (pull on each component to see if you can pull one of the leads back through the board.

* Check all your connections for an unwanted solder bridge with a small magnifying glass.
Particularly, check for small wire "whiskers" or solder residue.. It doesn't take much!

* Clean the backside of your board with a flux remover.

* Double check diode and capacitor polarization.

* Check resistors (red and orange can sometimes be difficult to tell apart)!

* Lastly, pull each component and measure with DVM. I always check them BEFORE I
put a project together. It really helps troubleshooting of you KNOW that your
components are good when you put them in.

* Before you start your next project, get an electostatic mat. Radio Shack sells them. Can ESD destroy a
project? YES!! Particularly CMOS components or JFETS. It's not as common now adays
as a lot of components have diode input protection. (Used to teach the class at work).

* Lastly, send me your board before you decide to thow it away. I have a working scope
and the same project as yours. No guarantees, but I usually figure things out.

Don

[plumabob]

I've checked and double checked pretty much everything. I pulled all components off the board replaced the transistors, diodes, checked the caps and resistors and swapped the ic's and still no good. I agree that it is probably something simple but I'm having no luck locating it. I'm not giving up yet, but If I do get to that point I just may take you up on your generous offer Don. I had such great success with my first board, it's baffling why all the trouble with this one.

Bob

[Reg]

Hi Bob,

The noise just may be the problem. Have you tried the detector out in the field to see what happens?

I looked at the schematic and there is no adjustment for the pulse rate that I can see. This means that there could be differences in the exact frequency of the two units and that could be just enough to display a variation in the threshold. So, I wouldn't rule out noise as the culprit.

You can make a few measurements to be sure, though. Check the output voltage of U3A as you vary the threshold pot and compare it to what you have on the other detector. If they are the same, then check the noise level with your scope.

Now, you can disconnect the coil and see if the threshold now adjusts smoothly. If you don't have a connector but do have sockets for the IC's, then simply remove the 5534 and see if the threshold now becomes smooth and adjustable. If it does, then noise is the culprit.

If you can't remove the coil or didn't use sockets, then simply jumper D1 and D2 and check the threshold again.

Let me know what you find out.

Reg

[plumabob]

Hi Reg,

So good of you to answer. I'm really am not too savy with a scope so I'm not able to check what you asked me to. I did disconnect the 5534 and nothing changed still no threshold. If I use the lm833 instead of the lm358 I get a threshold that is adjustable but if drifts. In other words it goes louder then lower all by itself or visa-versa. I may just pull everything off and try again.

Regards,

Bob

[Reg]

Bob,

Check your -5V supply. I suspect that is the problem or at least, could be the problem. It should be a very steady -5V and not be wandering voltage wise.

Reg

[plumabob]

Reg,

I have a steady -4.77 volts coming from pin 1 on the lm358. Is that what you were asking?

Bob

[Reg]

Bob,

I am surprised you have -4.77V on pin 1. Are you sure that this voltage isn't what you measure on pin 4? With a -voltage on pin 1, it would be difficult to get any audio at all.

Check the voltages at all the pins with a good voltmeter if you are unsure on how to use the scope. Let me know the voltages.

If the voltage at pin 1 is negative, double check for a solder bridge across something. The voltage at pin 1 should be positive and adjustable as you adjust the threshold control.

Reg

[plumabob]

You were right Reg I did have it reversed. This is what I took

1 +.01-.12 fluxuates
2 -.01
3 -.02
4 -4.77

5 +5.13
6 -.41
7 -.21
8 -.22

[Reg]

Did you have the coil connected when you took the voltage measurements?

The fluctuations at pin 1 on U3A indicate noise that is coming from somewhere. My suggestion of removing the 5534 may have allowed noise to be injected. So, instead of removing the IC, jumper the feedback resistor R9 with a resistor between 100 ohm and a 1K resistor. This should reduce the gain of the preamp to 1 or less. This should minimize noise and the threshold should stabilize. The voltage at pin 1 of U3A should vary smoothly as you adjust the threshold pot, P2.

If you still have an erratic threshold, then we need to look at the next op amp stage and make sure nothing is wrong there. The logical thing would be something in the feedback circuit causing a very high gain. So, checking the 220K (R16) would be in order as well as all associated solder connections.

Reg

[plumabob]

No coil Reg,

I connected the volt meter to my working board and there is still a fluctuation at pin 1. This board works great with an adjustable threshold! I get NO threshold at all on my problem board. I do not think noise is causing me to get no threshold at all. I'm not too smart about these things but it doesn't seem possible to me that this is the problem.

Bob

[Reg]

Bob,

Looking at the schematic, the threshold is set by adjusting P2. This adjusts the voltage to pin 2 which then should change the voltage at pin 1 of U3A.

So, we need to make sure the pot is working fine and to do this, we can measure the center leg of the pot as the pot is varied from min to max. What we should see is something like a voltage change of maybe 0V to something less than -1V, maybe -.6 or .7V when the pot is at max setting. The output of U3A should vary accordingly from approximately 0V to 3 to 4 volts at max pot setting.

It is almost acting like the pot is bad but checking it using my info above should let us know if that is the case. So, the next thing is to measure the center leg of the pot and at each setting, measure pin 1 of the op amp. This will tell us more about what is happening and what can be wrong.

Reg

[plumabob]

Did what you asked Reg,

-2.05 to +1.73 reading at the 1 pin as I'm turning the pot.

[Reg]

I am puzzled how you could get such a large negative voltage at pin 1. Running a quick calculation using a simple form of spice and the voltage at pin 1 should vary between 0v and about 2.6v.

So, I am not sure just where the negative voltage is coming from. So, now we need to find out just what is what and why. To find out, we need to adjust the threshold control again until we have the negative voltage at pin 1 and then measure the voltages at pin 2 and 3. With that info, we should be able to figure out what might be happening.

Reg

[douglas]

I had a similar problem. My solution was to change R20 to a lower resistance value. such as 12k. I had to experiment but got a threshold. Now I am looking to increase the volume on my phones. Is there another npn to use for Q6?

[plumabob]

Hi Reg,

You are gonna laugh at this one. I left the board in the house last night instead of the unheated garage and it's working fine now. Something on this board does not like the cold weather. My first board did the identical thing but I didn't put 2 and 2 together until now. Threshold is normal and fully adjustable. Here are the readings

pin1 +.21 to +2.78 volts
pin2 -.01
pin3 -.01

Hooked up my 9.5 coil with no headphones and can hit a nickel at 12-13 inches with the tiny board speaker. Tuna fish can at 28 inches. So I guess it's working fine now. I learned a few things also so thanks for the lesson professor.

I also want to thank Don and the others who pitched in here as well.

Bob