Tesoro Cibola

[Merc]

Does the firmware posted earlier in this thread support VCO pinpoint?

Thanks,
Merc

[Merc]

Is the circled capacitor 2200pF?

[Qiaozhi]

Is the circled capacitor 2200pF?

That would be my assumption, as it sets the cutoff frequency of the preamp to 15.4kHz.

[Merc]

Thank you Qiaozhi. I hate to be a bother, but what about VD7(circled). This appears to just be a reverse biased diode from -5V to ground. It makes me wonder if there is a mistake there and it is needed in some way for the CD4053 switch. Maybe it's just where the author decided to place it. I don't know. It just seems out of place.

[Qiaozhi]

Thank you Qiaozhi. I hate to be a bother, but what about VD7(circled). This appears to just be a reverse biased diode from -5V to ground. It makes me wonder if there is a mistake there and it is needed in some way for the CD4053 switch. Maybe it's just where the author decided to place it. I don't know. It just seems out of place.

I'm not sure why it's there. I've looked at other versions of the Tesoro Cibola schematics, and one version doesn't have the PIC, but it has the same reversed biased diode between -5V and ground of the 4053. It's probably not needed, so you can choose to include it or simply leave it out.

[Gwil]

Maybe to protect the CD4053 against some sort of transient reverse voltage on those pins? CMOS ICs can be damaged if input pins are driven far outside the supply pins.

[Merc]

I found a picture of the PCB online. I have marked the diode in question. The schematic appears correct.

Merc

[technos]

Maybe to protect the CD4053 against some sort of transient reverse voltage on those pins? CMOS ICs can be damaged if input pins are driven far outside the supply pins.

Pretty insightful there Gwil. However only one datasheet mentions that (as far as I could find) that there should be blocking diodes on both V+ and V- to protect the device, as the 4053 needs a specific sequential "turn on" sequence when using a negative supply. However it is not hooked up as shown in the schematic, which leads me to the conclusion that whoever transcribed the schematic got that part wrong. They also missed the diode for V+.

However, when working from a single supply Vee is connected straight to ground. See page 12 of the Maxim datasheet discussing this.

It would be interesting to investigate this by looking at an original board. But indications are that it should be removed and other diodes put inline for V- and V+ as per datasheet.

Edit: I just saw merc's picture after I made this post, which case the diode is in the circuit as described. Which case this diode could be working to clip negative excursions of the signals on the inputs to the 4053. I would love to see an o'scope picture of what this is doing!

[Qiaozhi]

Maybe to protect the CD4053 against some sort of transient reverse voltage on those pins? CMOS ICs can be damaged if input pins are driven far outside the supply pins.

That was my initial thought as well, but then where is the protection diode for the positive supply? As Technos mentioned, maybe there's a glitch on the negative supply line during power on. If it's in the original design, then I see no need to remove it.

[Derx]

Without this diode is in such a scheme no appears -5V
scheme - the imperfection, like everything else in Tesoro

[kt315]

I asked ***** and he wrote me the diode for what. it is for connection of TX generator to ground at first moment of power supply turn on.
-5V will not come on if TX generator would not generate. really the diode is standing up in a place I did draw at pics.

he did ask to add (c) to his comment

[Merc]

This makes sense. It is needed at startup. This is the first Tesoro I've seen that the transmit oscillator returns to -5V instead of ground.

Merc

[Merc]

What is the value of the circled capacitor?

Merc

[GSL]

3,3 mkF

[Merc]

Would this be the same as 3.3 microfarad?