Cobra II

[dbanner]

Nice circuit. But I can't find a user manual.

I like how the tone ID is acomplished. With ratiometric discrim. post demods.

[kt315]

file Viper_manual.htm

[dbanner]

Please can somebody explain how does U109A and associated circuitry work? What is marked as POWER appears to be the sensitivity pot, but the feedback loop of 109A has number of transistors in strange arrangement. Some sort of compression?

[dbanner]

3Hz supplied by 4060 I assume is for low battery beeping, as LP2951 put out error from pin five, chopped by switch U113C, is this correct?

[dbanner]

R107 in the preamp should be connected to pin 2. Yes?

[Carl-NC]

R107 in the preamp should be connected to pin 2. Yes?

R106 is wrong, it should be in series with C108.

[dbanner]

R106 is wrong, it should be in series with C108.

Yes, ok.

The core of this circuit is more or less the same as discovery treasure baron. So it is a Payne design.
discovery base unit.pdf

The error was copied over.

[dbanner]

These schematic drawings are not so easy to follow. It's messy stuff. Lines all over the place.

[kt315]

So it is a Payne design.

Payne had own site with great info, but it had died. I just did keep some info from it, but not all.

[dbanner]

The bare bones tone ID circuit (full range).

Power supply should be +4V and -4V and GND.

R136 and R138 combine for single resistor 24k. Q105,106,109,110 replace with anti parallel diodes.

[dbanner]

R106 is wrong, it should be in series with C108.

like this?

[Carl-NC]

Correct.

[Altra]

I noticed on the factory schematic for the base unit. The pre-amp has the same weird wiring. as the cobra schematic.

The Cobra was another Kellyco creation. Discovery Electronics would send them cases of unlabeled Baron's where they would do a re-brand. Discovery refused to sell Kellyco the upgrade modules for competitive reasons. At that time I had a small contract manufacturing business. Kellyco contacted me to develop and retrofit the Cobra's with a LED bar graph. The LED bar graph was a big selling point at the time. I probably did 150 units. The Baron/Cobra was an excellent detector functionally, but a big design flaw was the cast aluminum control box. It must of weighed two pounds alone.

Here is picture of my test coil, which I later opened. I believe Jim Karbowski was making the coils

[dbanner]

I noticed on the factory schematic for the base unit. The pre-amp has the same weird wiring. as the cobra schematic.

An error in any "factory" schematic is more prevalent than not, it would seem.

Diodes instead of transistors. Not quite the same p-p obviously.

[dbanner]

same sort of arrangement with transistors with the Tek ST schematic. Preamp drawn correctly this time.