SMR-4 Board No Measured Voltage

Hello,

I got as far as measuring the voltage at points on the board, but there is no p.d. at any of the points marked. Is my first step to replace the VREGs? I think I might have fried them when I had to desolder the +5 (LM7805?) because I put it in the wrong place.

Thanks,

Chris.

Have you checked the polarity of your power supply?

Have you bridged the 2 pads for the main power switch?

Yup, negative is soldered to the ground plane on the bottom of the board. And yeah the switch pads are short circuited.

I’ll have a look at the circuit diagram and see where the voltage is dropping.

Thanks,

Chris.

Did some brief checking earlier, I have like (+/-)11.5V going into my VREGs, but nothing coming out of the other pins.

Dead VREGs?

Thanks,

Chris.

Maybe you have swapped them? Two VReg dead at the same time… weird!

I know it’s really weird, it’s not like I’ve been using a blowtorch on them or anything stupid, just a 15w iron for a few seconds.

The VREGs are definitely in the right places and have the right polarity.

Any other ideas?

Does it matter my supply is at 12.19 volts instead of 12?

There’s 11.46 going into pin1 of the 7805 and (-)10.92 going into pin2 of the 7905.

Are these right?

Side note, what’s the advantage of the tantalum cap I’ve installed?

Also, I doubt it does but does it make a difference that I’ve installed some of the resistors already?

The voltages at the input of the regulators are correct. Having installed the resistors doesn’t matter. The tantalum cap is not necessary on the SMR4 board but might be useful if you use the +/- 5V rails to power more power-hungry devices/extension boards.

Do the regulators get very hot? Have you checked for a short between the power rails and ground?

The VREGs are stone cold. If there were a short wouldn’t my power supply (ATX psu) cut-off?

Am I safe to measure the resistance between the ground plane and the PSU without risking damaging anything?

Also I’ve used different sized capacitors, same capacitance, still electrolytic, just bigger/smaller footprint, should this matter? I’m concerned they are not making contact with the tracks on the top of the PCB. How can a good connection with the top tracks even be secured anyway? (I’ve never worked with double sided boards, I’m a bit confused how they work)

Thanks,

Chris.

You shouldn’t worry about the working of the PCB. Solder flows through the hole until it reaches the pad on the other side. Even with no capacitor you should read something at least on the 7805 regulator.

I’m not using a kit btw, I used your PCB schematics and had the board printed for me through my university; I probably should have mentioned this earlier.

Is it a bad idea to try soldering some of the components on the top side as well? Most of them have no solder on the top side.

Thanks,

Chris.

Then maybe you should worry about the PCB… Have you checked that continuity between pads/traces is consistent with the schematics?

Yeah; it’s completely identical. What do I try next?

Try to change the regulator?

Okay update:

I removed the old regulators and patched in new ones(to avoid soldering damage): replacing the 7805 with a L7805ACV and the 7905 with a 7905CT. Replacement of the 7805 was successful, I now get a nice +5.01V, but the same cannot be said for the 7905, there’s no input, not even any output from pin 5 of the DC-DC converter (which gets blisteringly hot, I actually burnt myself on it).

Any idea to what’s going on?

The DC-DC converter shouldn’t get hot, there’s probably something wrong around it, and you might have burnt it.

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EDIT:

hahahaha, I’ve realised the problem, I was using metal helping hands to hold the regulators, so the plates on the regulators were shorted.

Sorry to waste your time.

Thanks,

Chris

Now that I’ve soldered my regulators in, my DC-DC converter is producing +0.5V.

Have I damaged one of the caps or something?

You would see if you smashed a cap, it would have a bulge at the top/be burnt… You could measure it tho ^^