# SMR-4 mkII Step 5: Power supply testing

I’m really sorry to be such a noob and ask question so early on in the build, but I’m seriously stuck!

I’ve followed the steps closely, but instead of getting readings of 5V and -5V on the specified connections, I’m getting 3V and -3V.

I’m using a 9V power supply and the supplied components.

thanks

What is the voltage on both sides of the diode by the power connector?

Is your meter set to AC or DC measurement?

meter is set to DC.

I’ll have to get back to you regarding the diode readings. If i remember rightly it was 9V before it and approx 8.5V after… but I could be way off.

When i first plugged in a power supply, I used one that was center-pin negative. will this have blown something? I am now using center positive

The diode protects from reverse polarity.

Okay so Lindeborg… Into the diode I am reading 9.02V. Out of the diode I read 8.33V

What are the voltages on the 3 pins of each regulator?

IC 5 is reading: “+” = 0.00
"I" = -8.08
"0" = -3.09

IC 9 is reading “I” = 8.33
"" 0.0
"0" = 3.3

I’ve taken reading on every other component in case they become handy. Also, I switched my DC supply to 12V and the original /-3V readings became/-6V

Not really sure what I can do with this information… thanks for your help so far!

Try temporarily putting a 10k resistor between the +5V and GND pins to see if slightly loading the +5V regulator changes the reading. Can you trust your meter?

Cool thanks, I’ll let you know how it goes! I think it’s a pretty good meter… its a UNI T UT70D

Are you 100% sure that you didn’t swap the regulators, or put them in the wrong polarity?

the regulators are definitely the right polarity but I thought they were identical components!
I’m such an IDIOT. I’ve just checked and they are indeed the wrong way round! Thanks so much for pointing this out. Will this have a damaging effect on any of the other components? I’m swapping them back now…

I don’t think that it could damage any other component but themselve, and maybe the lt1054 if it was overheated. But i suppose you’d notice that, and anyway your input voltages are correct for both regulators.
You might want to declare them dead and cut them out because they are far from easy to unsolder without damaging them or the board. Given the price of those ics…
You can easily find the 7805 and 7905 at any local electronics shop. Actually, not finding them in an electronics retail shop is a good indication that they’re bad at it, and that you should never come back!

Luckily at work we have a desoldering gun, which made swapping these back really easy. I’m now getting the correct readings and am still using the original regulators.

Thanks so much for your help! I’m sorry it was a useless mistake - I can only hope someone else makes the same error and finds this post

hehehe i’ve made that mistake too. but i took new regulators just to be sure.

actually, this is a common error. This and the 165 frying. So in advance : beware that your regulators do not enter in contact with the upper control board

thanks MicMicMan!

nor put the bottom plate of the enclosure the wrong way around or mix up the 8-pin ICs,

Just that you know that from now on its a hard way to Nirvana…

At what level do the banana or bandana charts start to apply?