Egodriver IC voltages

[Marv Mod]

Hi guys, I built myself an Egodriver this weekend from one of Brian's boards (v4) and I'm really pleased with the results. Worked first time and the sound is good to my ears.

Looking around the tech forum it seems the general voltages across the IC are 0v on pin 4, 9v on pin 8 and about 4.5v across the rest. Mine tally with this (I'm using a TL082CN on a 8.6v power input) but I've got 2.7v on pin 3. Is this an expected / acceptable voltage? As I say, I'm happy with the sound, but just curious as to this reading.

Cheers for any help or thoughts,
Jeremy

[oldhousescott]

When you measure the voltage at that pin, your multimeter is making a voltage divider to ground with R3 and pulling down the voltage. Your measurement is OK.

[madbean]

When you measure the voltage at that pin, your multimeter is making a voltage divider to ground with R3 and pulling down the voltage. Your measurement is OK.

I'm not sure I understand how you arrive at this conclusion. Where does the extra resistance some from to create a voltage divider? Is it something internal in the DMM? Why wouldn't it pull down voltages everywhere then? Or, is it only in the case where you have a resistor tied to Vb?

I'm trying to think through this one, but it's a head scratcher.

[oldhousescott]

Yes, it's the internal resistance of the DMM. They are usually on the order of 1M or so and that comes into play when the output impedance to the source voltage (in this case R3) is less than an order of magnitude away from that -- that is, greater than 100k.

If you had an old VTVM, you'd not see it since the input impedance is tens of meg-ohm.

[madbean]

Good to know, thanks. I need to look up the value in my DMM.

[Marv Mod]

Cheers Oldhouse, glad to know the value's in the right area! Thanks for your thoughts!
Jeremy