Topband: The 160m 4SQR Control Note

Harold Smith w0rihps at
Mon Oct 22 22:45:45 EDT 2007

> over 200m from 4SQR ) were both suddenly full of noise in level of S7.
> I was thinking about a cause of the problem and finally found it.  Due to
> voltage on a long cable feeding 24V relays I had to use higher voltage 34V
> and to regulate it by 7824 regulator: of course they were blocked by
> appropriate capacitors on both input and output sides.  Probably due to
> sudden loss of capacitance of some of them the regulators have started a
> circulate what course a noise. So I replaced 7824 by classic resistors - 
> an
> old fashioned method - set up 24V for relays and QRM totally disappeared.
> My personals instruction is that simple is better and nobody would believe
> that 7824 circuits could make so huge QRM by re-radiation from 4SQR to far
> away  Rxing antennas.
> I hope that this sort of my experience could help somebody if meets 
> similar
> problem in future.
> 73 Jarda OK1RD / OK7XX from remote controlled QTH

Hello Jarda,
The 7800 series of voltage regulators WILL oscillate ! If you have 100 to 
500 mfd on the input
and  0.1 mfd on the output, it will be stable. You can have as much 
capacitance on the input
but not on the output. They do NOT like high capacitance on the output, but 
they need a small bypass capacitor.
If there is a bit of ripple on the input, and the input voltage should be 
about 25 to 30% higher, they clip the
ripple. I have used these for many years in C/KU Band LNBs with no problems. 
This information
was sent out by Motorola about 15 or 20 years ago. By the way, they are good 
for 100mA
without a heat sink and 1 Amp with a small heat sink.

Good luck and they should not creat any noise.

CU on topband

73 Price W0RI

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