Unexpected behaviour from LM399 board when drawing it's schematic

[systemCorrµPti0n]

Hi

I recently acquired a cheap LM399 board and when drawing it's schematic I don't understand why it is working correctly if the resistors markings are to be "followed"

More specifically I don't understand why the resistor R2 says 18k. In the non inverting opamp configuration R2 and R7 would have a gain of `1.9` $$1 + \frac{18}{20} = 1.9$$

If I were to short the first 3 switches, J1-J2, J3-J4 & J5-J6 I would get the 6.95v from the reference directly into the non inverting input of the opamp and the output should be $$6.95 * 1.9 = 13.205$$

But it is in fact 10v, and when measuring R2 (in circuit tho) it reads 9k, which would make sense with the non inverting gain calculation, but the reading of the resistor still says `1802`

$$1 + \frac{9}{20} = 1.45$$
$$6.95 * 1.45 = 10.0775$$


So I made a list of stuff it could be, but idk how I can tell for sure without applying heat to the board (from what I know that would mess the stability of the ref and the resistors)

• The marking is wrong
• I drawed something wrong from the board to the schematic. This and: knowing that when measuring in circuit the reading shall not be trusted as internally the opamps output "leaks" into other pins, hence puting the resistor in parallel with the rest of the circuit.
  (Although this doesn't explain why the gain is `1.45` and not `1.9`)
• Something else completely that has just gone past me

The board in question:




The schematic I came up with:



The jumpers table:


Regards

[Alex Nikitin]

The resistor is made out of two 18K resistors stacked in parallel one on the top of another, from the look of it …

Cheers

Alex

[systemCorrµPti0n]

Damn.

I was so not expecting that. I thought I could be missing another resistor in parallel and even checked under the opamp with a thin wire!

It was something so obvious and simple tho 

Thank you, I was going nuts re-checking everything trying to find what was missing or wrong!


Best regards