Re: Two QCX kits (80m and 40) - both with shorts on Si5351A - fixed

geoff M0ORE

I think this has been mentioned before but be careful when checking for shorts around IC's. Some multimeters feed out up to 15 Volts across the test probes which can blow a sensitive IC before you even start to assemble your kit.

I am amazed that a stray resistor leg shorted the 12 Volt line to the 5 Volt line. Does the manual not mention that the surplus length is cut off.

I use a bleeper designed by W4ZCB ( now sadly a SK) to check for problems which has a very low voltage across the test probes on an open circuit and only a few micro-amps on a short circuit.

Hans has mentioned in the past that the number of QCX's sold is around 4000 but given the number of builders who have purchased several units, the true number of purchasers must be only a few hundred!

On 11/12/2019 14:20, Nick Austen wrote:
Summary: check for shorts between legs of the surface mounted devices before you start (additional cautionary note to add to manual?)

Background: I first bought and built one 40m QCX - all worked first try - then accidentally blew it up with a stray remnant resistor leg shorting 12v to 5v - now fixed after replacing the processor, and other digital chips as well as voltage dropping diodes (first replacement Si5351A replaces twice as first replacement did not oscillate at crystal frequency). All perfect now.

Then bought two more (one to give as a gift). I built them in parallel. Neither worked (initially).

After searching and checking, I looked at the quadrature outputs of the Si5351A’s. One unit had an odd looking three-level signal on the oscilloscope- which I eventually realised was the sum of the two quadrature outputs - they were shorted.
After careful magnified viewing I saw a short right on the pins of this synthesizer (not put there by me).
Solder wick fixed that quickly.

The second unit had only one of the quad outputs oscillating- the other was at 0 vdc. Found another short - this time from the second quad  output to the ground pin. This was also fixed by quick application of solder wick.

All working now - phew.

Conclusion - don’t let it put you off - just check the legs of the Si5351A on the bare board before you start - it may save you some disappointment:

Still highly recommend - the best kit I have seen.

Thanks again,

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