SMR-4 v0.7 Power Supply Problem

Hi folks,
Just tested my Shruthi-1 power supply. Instead of +5V I get +3.4V and
instead of -5V I get -3.2V. I got the polarity wrong on C2 and had to re-solder.
Could I have fried the cap? Any other ideas?

Thanks,
Jon

What kind of power supply are you using?

Hi, thanks for getting back to me.
It’s 9V 650mA, 5.5 x 2.1 mm Plug center positive.

It looks similar to the old AdaFruit unit and It tests right at 9V.

Jon

Try removing the LT1054. Does this fix the +5V rail?

Nope the rail stays at +/- 3.49V

In which mode is your meter set? 5 / 3.54 = sqrt(2), this looks suspicious…

This is weird - I’m using a new auto-ranging digital meter.
It reads the 9v power supply and assorted other known DC
sources right. However my old analog meter reads +/- 5V
from the board where the fancy new one is reading +/- 3.5!

???

The old meter is unreliable with other DC sources.

Looks like I really do have a +/- 3.49V rail.

:frowning:

I’m sorry I don’t see what could be done. I would have say a bad regulator but you have 2 of them, this is too strange. Also, if the input DC voltage was too low or if the LT1054 was damaged, you wouldn’t see these values.

Is there a scenario where a problem with C2 and/or the polarity of C31 could explain this?
It was the only part of the build that didn’t go smoothly.

Jon

C2 and C31 are in the charge pump circuit generating the negative circuit. If you remove the LT1054 they are just connected to nothing, so with the LT1054 you shouldn’t see any problem with the positive rail.

I checked the board on a friend’s multimeter and it confirms +/- 3.9V
Everything on the board looks fine.
In the absence of any explanation I guess I need to get some new parts and start again.

:frowning:

What is the voltage at the “I” pin of both regulators?

Thanks for your continued help:

IC2 I pin = -8.2V
IC6 I pin = 8.4V

At this stage, it could only be a bad regulator; but I’m still very puzzled by this multimeter issue…

Me too but I now have two independent readings at around +/- 3.5V
The outlier was an ancient unit with a leaking battery that I should
have checked out more (or thrown out!)

Do the I pin voltages sound right?

Yes, your input voltage is 8.4V (more than enough) and the LT1054 and the caps around it are doing their job at generating a negative rail. What seems to be broken are the regulators.

That’s a bummer - what would your recommendation be for a next step?
Should I re-order a board and power supply components from you?

Thanks,
Jon

If its the regulators you should get a 7805 + 7905 at your local electronics store. Cut the legs of the old ones and desolder the rest, then solder the new ones.

Hi,

Are your Regulator getting very hot ? because if a cap is shorting, an you consume a lot of current the regulator could not regulate the voltage correctly.

it is strange that both regulator are failling … are you sure about the measure ? where did you connect your negative plug (black) of your multimeter ?

it can be also that the gnd is not correctly connected to the power plug GND … can you check the GND pin solder of your power plug ?