Nooelec noise source , low voltage ?  - Page 1
Forum section: electronics / repair

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: July 18, 2015, 01:58:26 pm
content: I am using this for ham radio , and general RF monitoring .After assembling the Noise source kit & doing the testing on Alan's page W2AEW .I find that the places that are suppose to voltage , have 4.72v and the places that are suppose to have 12v - only have 4.72 ?This is the info on this and diagram , sorry the 1st link did not work , try again .Thanks John

author: codeboy2k
author_rank: Super Contributor
timestamp: July 18, 2015, 03:55:16 pm
content: You have VBUS on the schematic, are you powering it from a PC or laptop USB port? Lots of USB ports are current limited to 100mA and your schematic has two RF amplifiers on it that can each draw 100mA typical and can reach peaks of 300 mA according to the datasheet. That's for each one. So you should have at least 1A @ 5V available, and should try it from a USB charger or a bench power supply to rule out USB current limiting.Also, even if it's not a USB 100mA current limit issue, some PC USB ports are just bad at getting the 5V right, and are often on the low side, so check that too.As for 12V being 4.72V , it's clear to me that your sc4503 switcher is not switching. Vin @ 4.72 volts is just passing straight through the inductor and diode to the 12V rail, so check your switcher, maybe it's blown (triple check your soldering there too ... )

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: July 18, 2015, 05:30:12 pm
content: Thanks for the reply , about 95% of the time I was using one of my charger wall-warts marked at 1 amp .I also retouch all solder points .Now checking the switcher , how would I proceed ?I used my multi-meter up to this point , but I do have a Rigol 1152E .

author: codeboy2k
author_rank: Super Contributor
timestamp: July 18, 2015, 08:20:01 pm
content: The wall wart USB charger should work fine.1) remove the JP1 jumper (NoiseSource_EN). Then with your DMM check the voltage from the wall wart USB charger at the jumper pin to make sure it's 5V, not 4.72V. If it's 5V with the jumper removed and 4.72V with the jumper in place, then you've got a 280mV voltage drop that is caused by the load and the wall wart can't maintain regulation. This would mean either a poorly performing wall wart or too much current is being drawn by your circuit (and the wall wart is good but can't maintain regulation). So you can then try to see what the actual current draw is by using your DMM in amps mode across the pins where JP1 would normally be (i.e. your DMM in amps mode will be the jumper JP1 instead). If the current draw seems unreasonable (i.e. 300-400 mA or even more) then something is wrong on the board.2) With the jumper JP1 back on, check that the 5V supply (or 4.72V) is present at the switcher IC pins Vin(5) and EN(4).3) With the DMM in continuity mode, check that your switcher IC has a good connection to GROUND at pin 2.4) Finally with your Rigol o'scope set at either 1us/div or 500ns/div check the SW pin 1 on the switcher IC. I don't expect you will find anything there, but it should be switching at that node, around 1.3 MHz. Likely it's not. If all the voltages and ground continuity checks out on your IC pins then I think the IC is blown and you should replace it.5) as a final test of your circuit's functionality, you can remove that switcher IC completely and use a bench supply to put 12V directly into the circuit at the cathode of the switcher's diode. It's going to be big enough that you can solder a wire to it's pad there (at the cathode). Then you can at least test the circuit functionality and look at the noise output with your scope. (removing the IC might also bring your 4.72V back to 5V, if the switcher was the cause)