Voltage problem?

[montechristo]

Hello!
A couple of days I made the 3 function generator-high current version. The problem is that I have different voltages between the pins of the sockets, from the values that is referred on your PDF file:
pins 1 & 8 : 4,98 volts/OK
All other pins & 8 : approximately 1,7 volts
Also the voltage between the vertical mounted components and the 0V stripe is about 0,63 volts. Are these normal values also? Of course I have not still mount the 629 PIC before I will verify that everything is nominal. I forgot to mention that I have already double & triple checked the positions and the values of the components on the board - I made a second version of the circuit on a breadboard that I had the exactly same values...
Any ideas would help me a lot!

Thank you in advance
Chris

[Paul Harman]

Referencing all voltages from pin 8 the most important are pins 1, 6 and 7. The voltage on the top of the vertical components is OK, it should be the forward voltage of the diode that you have used so can vary a little depending on the diode manufacturer. The voltage on pins 6 and 7 will vary a bit with your supply voltage so they are OK too.

The voltage on the other pins is probably down to your meter or the transistors and resistors. It is well worth checking that they have been fitted OK. These pins have no connection to anything other than pin 8 so should all read zero when the processor is not installed. They may read higher if your meter reads random values when the probes are connected to a high impedence point. This is where an old fashioned meter like an AVO will give more meaningful results. As a workaround try putting a high value resistor (like 10M) between pin 8 and the pin you are measuring and see what results you get. If you are still seeing a high voltage investigate a construction problem.

[montechristo]

First of all, thanx Paul for your quick reply!
OK, as I understand the problem has to do with the voltage in the BC182 part of the circuit. There is no problem with the connections (I've done a lot of checks - except I am missing something obvious ). I used 2 multimeters and they both gave me the same reading:

Voltage between pin 8 and pin 2=-1.7V
Voltage between pin 8 and pin 3=-1.7V
Voltage between pin 8 and pin 4(&5)=-1.7V

I followed your recommendation and by checking different resistor values, I found that putting a 750K resistor between pin 8 and pin 2(or pin 3 or pin 4) I took finally a more logical voltage of -0.05V. I also took the other measurements and they were nominal(4.98V on the pin 1 and 0.63V on the vertical components - all relative to pin 8).

So to summarize all, will be a satisfactory solution to put 3 resistors (750K) between the 8 pin and the 3 other pins in order to proceed - insert the PIC?

I forgot to mention that I used BC182 transistors not the "L" version(I had 6 of them lying around), changing of course the emitter position(easy on the breadboard - little difficult on the PCB) according the schematics...

Thanx again
Chris

[Paul Harman]

You don't need the resistors - the resistor is just to help the meter give a more meaningful reading. If 750K is enough to pull the voltage down you have nothing to worry about, pop the pic in and all should be fine.

I just specified the BC182L because it was cheap, and the base is conveniently on an end pin to make installation easier. Any equivelent transistor will be fine as long as you can fit it OK.

[montechristo]

Thanx a lot Paul! Everything worked as expected, great decoder!