LF211/311

[DavidB]

I am trying to repair an old Heathkit off resonance metal detector. It uses a LF211 JFET input voltage comparator at the front end. I have not seen any other JFET type comparators and this is an obsolete part. I have been trying to find something to substitute. Someone had already put a LM311 bipolar type in the detector which loads the input signal to much. Does anyone have any ideas about a possible substitute. This is the most critical part in this odd detector design.

[Qiaozhi]

I am trying to repair an old Heathkit off resonance metal detector. It uses a LF211 JFET input voltage comparator at the front end. I have not seen any other JFET type comparators and this is an obsolete part. I have been trying to find something to substitute. Someone had already put a LM311 bipolar type in the detector which loads the input signal to much. Does anyone have any ideas about a possible substitute. This is the most critical part in this odd detector design.

Your best solution would be to make an adaptor board to allow connection of an LM311 to the metal can pin-out of the LF311.
Or (if there's enough room) superglue an LM311 upside down on the detector PCB and use flying leads to make the connections.

[Skippy]

Is the PCB layout for the round can package or a DIL type pack?

[DavidB]

It already has a dip socket and someone tried a LM311. The LM311 will not work because it has bipolar input transistors that load the signal. The LF311 has very high impedance JFETS on the inputs. I tried adapting the leads on a JFET input op amp and I can see on the scope that the input signals are at the level they should be but the JFET op amp does not have the open collector output. I will try adding a complete circuit on a small perf bd with a transistor added to the output to get the full negative swing. I was hoping for an easier solution but it seems the LF211/311 were the only JFET type comparators ever made and now they are obsolete. I guess that it would be possible to use a LM311 by adding JFETS at the input but that seems like even more of a task.

[Altra]

Hi David,

I know of a dual mosfet comparator the CA3290. If you think it might work
I have some and would be happy send you a few. Let me know with a PM.

Its also obsolete
http://www.intersil.com/en/products/...rs/CA3290.html

Edit: ST Micro makes this dual cmos comparator TS3702, maybe they have a single in this product line?
http://www.st.com/internet/com/TECHN...CD00000882.pdf

[DavidB]

Thanks guys for all the info and willingness to help. In the last few hours I put together a LF441 JFET OP AMP on a small perf board with a 1k resistor and a 2N3904 at the output and it is working now. Comparators swing to ground when the + input voltage is higher than the - input voltage and it is just the opposite with an OP Amp but the NPN transistor added to the output of the OP AMP makes it act like the comparator.

[franjasamo]

Hi David, my name is Francisco, and I live in Spain, long ago I came across a detector such as you describe, and I never got it to work. I read your post and see that I have the same problem with my metal detector Heathkit GD-1190, also has mounted the LM311, but does not work. I would like if you could put a scheme like this problem solved really appreciate it, because I have wanted to see how this relic detector. (Heh heh heh). A greeting ...

[Davor]

GD-1190 is a real gem. A real discriminating off-resonance device with external oscillator driving an off-resonance search coil tank.
I'm not certain it really requires a FET comparator. The impedances of the + and - inputs indicate proper biassing of a BJT comparator, not FET comparator. A FET comparator is indicated on the schematic, but with proper tuning it will most probably work just as well with BJT.
GD-1190 has several quite complex switches, so I'd first make sure those work properly.

[franjasamo]

Davor Thanks for your answer, the truth is that I do not have an oscilloscope to see the signals shown in the diagram. What if I have noticed is that if I disconnect the coil cable from its connector, there is a high beep and the meter moves to the bottom of the scale. The same is true, if up from its socket the circuit IC1 (LM311). When everything is connected, you do not hear from the speaker, and the meter needle does not work. When I approached some metal to the coil does not move the needle. I have also tried adjusting the variable capacitor and tried bringing it to different types of metal and does not work. Seen that the voltages on all circuits are as indicated in the diagram. Therefore, and reading this post earlier, I had opted for the LM311 circuit was causing the problem. Well, greetings and try to continue testing ...

[Davor]

Yeah, you'll need a real oscilloscope with real probes to see what's going on. Maybe the first attempt should be turning the switch violently left and right, and applying some tuner spray to it. There are far too many important things going on through that switch.
Next should be level calibration potentiometer - it should give you some change.
And so on...