Wiring consistency check

I got a pcb-layout from a magazine, but since I've only got veroboards at home I had to relayout it to fit a veroboard.

I've now completed the circuit and construction, but obviously there's an error somewhere that I cannot find. I've double double checked my layout but still cannot find it.

The original, flawless, pcb-layout from the magazine can be found here:

And my veroboard-version for the same layout can be found here:

I would be very grateful if there is anyone that's got some spare time (around 5 minutes) to "proofread" my layout and see if there are any errors.

Thank you.

And have you cut the tracks under IC1?

Cheers.

Ken

The wiring on the veroboard are obviously correct so I can remove it as an error source.

There is however one thing that I find tricky for the wiring in the case. Have a look at this image (schematic from the magazine):

And this case wiring schematic (also from the mag):

P2 is supposed to change the output voltage over the pole terminals (+ and

- just under P2). The schematic is for a lab power supply and is rated to supply between 3 volts and 25 volts, but what happens when I turn P2 is that the voltage does not change at all except for a few millivolts.

Using my DMM for the pole screws (+ and -), it reads 3.350 V. If I turn P2 to its extents, the voltage goes from 3.350 to 3.380, and that's it.

What I'm unsure is about the consistency between fig1.jpg and fig2.jpg. In fig1.jpg, P2 should be in series with the amp-meter, but in fig2.jpg, it isn't (I think).

Other people I've talked to that have built this supply have had no problems, so there is probably nothing wrong in the schematic, but I cannot understand what's wrong. All the wiring (both for the veroboard and for the case) are entirely to the number according to the schematics in the magazine, but still it won't work.

In the magazine, there's a table of "testpoints" for measuring with the DMM. Pins 6 and 5 on the 723, for example, should be around 7 and 3.5 volts respectively, and that's what I can read on my multimeter. The voltage over the base and collector on T1 are also correct, as well as every other testpoint in the table.

Could anyone shed some light on me for this problem? Are there for example any obvious tests I can do to try to find the error source?

P2, R5 and R6 form a voltage divider that must reduce the desired output voltage to about 3 volts to be compared to the divided down 7.5 volt reference voltage. You don't provide the resistance values for Either divider, so I can't check if they are right.

You may have P2 and R6 interchanged.

I would have preferred a different arrangement for the divider that used the wiper as the tap that connects to IN-. That wouldn't pass as much current through the wiper contact for a longer life, and I think, better range.

That means that the reference voltage is 7.5*R2/(R1+R2) or about 3.375 volts.

So with R6 at maximum resistance (1k), The range for the output should be between 3.375 minimum to 3.375*(P2+R5+R6)/(R5+R6) = 15.3 volts.

With R6 set to zero, changes to 3.375 to 3.375*(P2+R5)/R5=51.4volts, which would allow only a small part of the rotation of P2 before the output saturates high.

So no setting explains the low output voltage. I would ohm out the resistance from IN- to the negative rail, to make sure something is not open in the R5 R6 pair.

I'm sorry, I just now realized that I should have written the values since they are hard to read in the pdf in the OP.

R1 - 2.2k R2 - 1.8k R3 - 1.5k R4 - 1k trimmer R5 - 330 R6 - 1k trimmer P1 - 1k lin pot P2 - 4.7k lin pot

P1 and P2 are the ones on the case, R1-R6 are all on the board.

Excellent. Congratulations.

I have now followed your steps and couldn't find any problems.

During the ohming, I did however check the copper tracks once more just in case I had cut the wrong tracks or similiar. And guess what, I found a microscopic size of solder bridging between two tracks. Removing it with a small screwdriver helped, because now the power supply works!

Thank you John for your time and valuable pieces of information. Without your posts I would have assumed, wrongly, that the copper strips and the board itself was no source of an error, and would have never looked there. Your tip about checking R5/R6 made me find the solder bridge.

I forgot, here's a pic of the ultimate and homebrewed power supply in action!

-- Sincerely, |

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