Shruthi SMR4 power supply testing gives +5 and -0


Beginner DIYist.

Where the assembly instructions show a red dot for the power test, I get +4.9V, but where the green dot is I get 0.

I read through old forum posts regarding this but could not find anything that fixed my problem. Guys, what are your thoughts?

Love and light

ok so if the black probe was connected to GND there should be negative reading at pin 5.
so if everything else is ok (eg 7805 and 7905 not swapped, continuity =close to 0R reading between pin 5 of LT1054 and input pin of 7905 and same on corresponding pins of 7805, stable input voltage, no shorts) then it’s your 1054, like fcd72 pointed out

There are tons of tips, im wondering you dindt find anything…

Heres the SOP:

  • check the direction of the LT1054 chip
  • check everything around the LT1054
  • measure Voltage at the Regulators 7805 and 7905 and report

Look here

Hi and thanks!

I didn’t say I couldn’t find it, only that what I found didn’t fix my problem.

If the link in my post works, you can see the direction of all the components in the pictures.

I have updated with readings of the regulators too, like you suggested. They are at around 0 as well!

Can you please measure at the regulators (7905 especially) directly - it looks like your negative Voltage doesnt work - (obviously), so something in the chain Wallwart > LT1054 > 7905 is not working.

  • can you please measure the Voltage of your Wallwart without anything attached - might be that it is to high without load so the 1054 got damaged when plugging in…

Oh, and check that C3 is really 10P (marking 100) and you didnt scramble it with a 100N (marking 104)

Hmmm sorry for that.

Which voltage do you get on the Vout pin of the LT1054. What is the value of C3?

Yay - i should have spotted this but with a phone its a bit hard…

How about a Troubleshooting Highscore Hall of Fame? :wink:
This one goes to your account . . .

Wallwart without anything attached gives 9.2V

Pins of the 7805 seems fine, but the 7905 pins all give 0


“This round mark must be near the IC1 marking on the PCB” ? Isn’t this what I did?

9.2V is OK so it didnt die of overvoltage.
If the 7905 pins all give 0 then the negative voltage from the 1054 either doesnt get thru (bad soldering) or the 1054 is dead.
Measure PIN 5 (VOut) against any GND Point to see.

Pin 5 says 3.0V

C3 around 7 V, marked 100.

Have you soldered the Tantalum Cap?

The optional one? I haven’t. I found a component that looked similar to what it was supposed to look like in the instruction picture, but I couldn’t decide on its polarity so as it was optional I skipped it.

Optional ? Did i miss something ? oO

Yups, due to the spec the Tantalum is optional. Might be this specific 1054 needs it though…
The marking indicates the negative Side.

there should be a little + symbol on one side (so the marking indicates the positive side)

have you measured directly on pin 5? there should actually be a negative reading with regard to ground.
also, if there’s a reading on the pin itself but there’s nothing at the 7905 there must be a weak solder joint between them.
i’d try to reheat all joints in the psu section again, applying some fresh solder (just a little bit) and not move anything while the joint’s cooling down (the helping hand tools seem always a bit shaky).
to be sure you can do continuity checks with psu unplugged, check from component leg to component leg, not just the pads, they’re connected anyway.

OK, im the dumbest here - ill grab another Coffee and see . . .

@fcd72 i have my doubts about that :wink:
and i’ll grab a coffee too