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#40m #supply #test short in the PS output line #supply #40m #test

Syd
 

I'm putting in 10V with a 250ma limit from a PS. It drops to 4.3 V when powered up, current limited at 250ma so there is a loading problem. Input to the regulator is OK, but output is approximately 2.4 V. So there has to be a short/load somewhere.
  Removed LCD display, uprocessor, and checked the PCB very carefully several times using a magnifier. Inspected top side for shorts, OK.
  Back in the old days I would have looked at the schematic and found where the pathways went carrying the current and cut the PCB pathways to eliminate the source of the problem. Worked every time!!! But today with these extremely small paths on the PCB, this method doesn't seem to be a possibility.
  The solid state devices are put in properly. The 5VDC line measures about 19K to grnd so that measurement doesn't shed any light on the problem. I'm thinking that  one of the electrolytics is in backwards, but looking at the stripe on these caps, they are all aligned properly assuming the bar is at the negative end.
  Signal tracing with a scope and a DVM is pretty simple, but this problem is a little difficult to solve!  If I had a current sensing probe the problem would be simple to solve, but that device is very, very expensive. ($600 from the UK)

Alan G4ZFQ
 

Removed LCD display, uprocessor, and checked the PCB very carefully several times using a magnifier.
Syd,

Back in the old days I would have looked at the schematic and found where the pathways went carrying the current and cut the PCB pathways to eliminate the source of the problem.
In some cases this might be possible although requiring great care.
Maybe lift one end of D1, could the Si5351 3.3V be grounded? Very carefully lift pin 16 of IC4 with a pin.
But employing the recommended procedure of not blaming components until the very last check all traces very carefully. In a few cases it is not a whisker of solder but a whisker of unetched copper.

The 5VDC line measures about 19K to grnd
I have never tried but does a high resolution voltmeter give any clue? Can a very small drop be seen between the regulator and component feed?

73 Alan G4ZFQ

Syd
 

I did find a short to gnd around D1 going to the IC3. Looking at pins 7 and 8 under a magnifier. it didn't look that bad. But what else could it be? So I used my dental pick and scraped out between pins 7 and 8 and suddenly the short was gone! Back to the testing and alignment.
73 Syd wt1v

James Daldry W4JED
 

You'll generally find zinc-carbon cells labeled "heavy duty". Usually found in dollar stores.

73

Jim W4JED

On 6/1/20 5:03 PM, Syd via groups.io wrote:
I did find a short to gnd around D1 going to the IC3. Looking at pins 7 and 8 under a magnifier. it didn't look that bad. But what else could it be? So I used my dental pick and scraped out between pins 7 and 8 and suddenly the short was gone! Back to the testing and alignment.
73 Syd wt1v