Friday, November 25, 2016

Iffy STM L7805CV devices from Farnell

This week I started work on my my triband 28V 4/6/2m amplifier using a Kalmus module I got off ebay. It needs a PIC controller which runs off 5V so I ordered twelve STM made L7805CV from Farnell which should give 5V output with up to 35V input.

 I connected up the PCB and LCD and turned it on. The LCD got very bright and the pic refused to work. Remembering the addage "99% of all problems are power supply related" I measured the voltage coming out of the L7805. It was 7.88V!  Replaced the regulator with another one from the same batch. 7.90V. As an experiment I ran the regulator off 13.8V. It gave 5.05V which is fine.
Looking in the component collection i found an old STM L7805. Soldered that in and it gave 5V with both 13.8V and 28V input. The only difference(apart from the date code) between the old and new regulators was that the new one said "STM CHN" the old one said just "STM". Enough said!

I contacted Farnell, who apologised and were persuaded to replace the iffy regulators with the Fairchild LM7805CT. These have now arrived and work fine off 28V

Moral: Measure the output of regulators before applying to circuits!

Sunday, November 6, 2016

LF season approaching

As the winter is approaching it was time to think LF again. When I bought the main parts for the MF amplifier from Dave G0MRF at the HF convention I also ordered one of his preamp kits. I managed to squeeze it into a small pomona box, time to try it out

Initially I connected it to my Alpha Delta DX-EE and ANAN-10 and left it receiving WSPR using K1JT's new1.7.0RC2 WSJT-X software. It seemed to do well:

Time Call dB Loc Power km
18:22 G4FTC -21 IO91pi 2 137
18:36 PA0A -24 JO33de 1 591
18:38 LA8AV -28 JO59cs 5 1151
19:56 G3WCE -29 JO02pt 1 255
23:52 F1AFJ -27 JN06ht 1 625
00:46 DJ0ABR -29 JN68nt 0.2 1148
00:54 G8HUH -19 IO81mg 1 103
01:16 PA3ABK/2 -25 JO21it 0.5 481

The preamp reduced the LOS droitwich 198kHz on my system.

During the evening  I wondered what was on 136kHz. So I dug out my old G0MRF 136khz preamp and let it run for a short period, while, again connected it to my Alpha Delta DX-EE and ANAN-10 receiving WSPR decoded 2 stations

Time Call dB Loc Power km
22:00 G8HUH -20 IO81mg 0.1 103
22:04 2E0ILY -12 IO82qv 0.01 91