LM394 on my Minelab PCB

[Mechanic]

Hi Mario,

Fet 11 is used for the -5v regulator, I'd leave it alone at this stage. Maybe later
The input n fet could be changed for a tn2404kl, just make sure you have the pinout correct. You could also have a look on farnells (element14) for a 200v n channel fet, low resistance low capacitance use the tn2404kl datasheet for the benchmark and don't discount the surface mount components as you can hook them up with old resistor leads.

Anyways getting nagged to go shopping so I will put a bit more down later.

Cheers Mick

[gef12]

BINGO! I tested your circuit and there's continuity between all components, AND pin 3 of IC 8. Hence we found the elusive AD797!

Thank you!

After more testing, I found the choke to fet 11, and it seems like it's a really important fet. I also found the p fet to fet 11.

Question, what is the purpose to fet 11? And what are those chips: 1,2,5, and 7? Are they 5534s like you suggested? Pin 3 of those chips hooks up to the P fet of fet 11, while fet 11 also hooks up to pin 4 of ICs 3,6,9

It seems to me like fet 11 is an important fet, an older part worth considering upgrading.

Should I also upgrade the p fet/n fet of AD797, if so which part numbers should I replace them with?

Last question, is there only 1 AD797?

thanks in advance

What u reckon Mick .. chips 1 ? and 2 ? could be LT1057 ?

[Mechanic]

Sounds good to me! We would have to do some probing to find out the power pins, inputs and outputs.

Now that no7 chip has me scratching my head. Is it a second ad797 for a dual preamp or just an intermediate stage? Check to see if there is any connection between chip 8 pin 6 and ic7 pin 3. It may go through a resistor first.

If you want to remove that freakin white paint use some good contact cleaner and a small flat blade screwdriver. Keep applying contact cleaner and the paint starts to get softer and easier to scratch off, but don't scratch too hard! Before you take it out detecting spray some clear circuit board lacquer on the board otherwise the humidity will cause grief!

Cheers Mick

[mario]

Seems like a Taiwanese supplier has the lt1028 on ebay. I saw them in DIP configuration, and surface mount, like curent ad797 setup.

Is there a special adaptor socket that i can solder on board so that i can plugin the slightly larger DIP chips? that way i can upgrade later without desoldering directly from the pcb

[Mechanic]

You could use one of those 8 pin dip sockets and just bend the pins in to match the smd footprint. Be warned that this could cause extra noise in the front end, but it might not either! A scope would be a good tool to confirm this. To find the internal noise I have the coil tx running and have the rx wire to the front end disconnected from the coil and hooked up to the coil ground wire. Then look at the signal at the test points on the audio board with a sweep speed on the scope at about 0.2s/div and about 10mv/div. You will be seeing the noise floor of the detector and then you have a benchmark to compare improvement against.


Why not get yourself one of those hot air rework station? I just use a normal old temperature controlled soldering iron and run a bead of solder down each side of the ic and quickly heat up both sides by alternating from one to the other and when it is hot enough pick the piece up with a pair of tweezers. Practice on something else!

Cheers Mick

[mario]

...Now that no7 chip has me scratching my head. Is it a second ad797 for a dual preamp or just an intermediate stage? Check to see if there is any connection between chip 8 pin 6 and ic7 pin 3. It may go through a resistor first.
...

It took me a while, but I did discover a circuit between chip 8 pin 6 and chip 7 pin 3. The only way I found this link was through a resistor, as you said. I wasn't getting continuity until I decided to test the terminals of nearby resistors.

So what's IC 7? Another AD797 chip?

Also, out of pin 6, IC 8, I found continuity between it, and several of the surrounding orange square components (for instance, the orange squares next to IC 4)...I'm thinking they're resistors?

[Mechanic]

Hi Mario,

The orange parts beside the opamps are the power supply bypass capacitors. I should have said pin 2 of ic7 not pin 3, as it appears to go to ground.

Now the signal flow, I think it comes into ic8, then goes to 7
then to 5 then to 4, or 4 to 5. I have attached a schematic of how the cct looks so far. There will also be some diodes in the feedback loop on the first opamp and second and probably some caps too. Just need to find out the resistance values.... Hopefully this makes it a bit easier

Cheers Mick

[mario]

Hi, I had some time to probe my board for resistor/capacitor values (High res image is found here).

Notice the values: .14,.16, and .2 (12 OHM values are correct!). I noticed my LCR was sampling at the wrong frequency. Hence it wasn't giving me correct figures. Please substitute .14,.16, and .2 with a resistance value of 5 OHM.

My probe was giving me an erratic number between 3.x and 7.x OHM. Hence I'm forced to guesstimate. I did have a 10 OHM test resistor, which i was able to confirm. Maybe I need a new probe?

Anyways, I will study your schem sometime later. I need to finish assembly a Lipo battery pack that died on me last week. One of the cells leaked and made a mess of things.

[Mechanic]

Hi Mario,

You will find that all of the little parts directly beside the opamps and other ic's are the bypass capacitors. The small ones will be 100nf, not sure what the large ones are. Just remember testing capacitors in cct will not give accurate results as they are also hooked up in parallel with the main supply bypass cap, which would be about 1000uf or more. You can easily confirm that they are bypass caps by looking at the ad797 or ne5534 datasheets and you will see that they hook up directly to the ic power pin. Also have a look at the 4066 datasheet and you will see that their bypass caps also hook up to the power pins. The bypass capacitor gives the component a local source of power and reduces noise on the power rails seen by the ic.

The parts you have marked 17k and 48k are actually diodes, the ones on the input path to the preamp seen on the schematic earlier.

Cheers Mick

[mario]

Ok, I don't know if this makes any sense to you, but pin 6 of IC 8 goes through two resistors, and hooks up to the Yellow wire next to N fet # 5. The Yellow wire goes through a big toroid and eventually hooks up to pin 1 of the connector hooking up the detector's coil.

Pin 6 does not hook up to any of the other chips

[gef12]

Mick / Mario
Found this in my arsenle ... maybe of some help .. can't garantee it ..but looks good

[Mechanic]

Nice find Gef! So they use the 4053 for the demods, very nice!

Mario, that yellow wire that goes to pin 1 of the coil plug must be for the analog ground.

Cheers Mick

[mario]

OK, What do we know so far?

Hi,

That schematic is spot on!!!

I tried getting capacitor sizes to enter them on the photo, but I can't really measure them directly unless I physically remove them, which I won't do. Unfortunately, that white gunk, which I can not remove with alcohol or scrape off (I wasn't able to find that bug repelant at my store) is all over the capacitors, so I have no way of knowing their sizes.

Also, I tried obtaining values for the diodes, and that's kind of hard to do as well...

--------------

I noticed that IC 7 feeds IC 8, not IC 5.
------------

Should we go on and Id the remaining chips? do you have any ideas what they might be (I don't mind continuing )?

What other chips could we replace to obtain an increase in performance?


Any ways, based on your work (thanks btw), here's what we have so far:


High res photo is here

[gef12]

The diode up front between the fets is probably a 4148 and same for the one hanging of the AD797

[mario]

Nice find Gef! So they use the 4053 for the demods, very nice!

...

Cheers Mick

Hi, I took a look at the 74hc4066 and 74hc4053, and the number of pins is incorrect for that ic. In other words, since there were no 7 pin packages made for 74hc4053s, the chip in question must be a 74hc4066.

I found a good replacement: max4066, which is direct replacement with a lower on-resistance when compared to the 74hc4066 .

Another issue I have with the schem, is that it shows pin 1 of the ad797 feeding the ne5534, which is also incorrect. Pin 6 of the 797 feeds the ne5534s, not pin 1.

I plan on eventually replacing the ne5534s also with the LT1028 chips as well.