Hi Steve,
You can't measure many of the square waves created in this
radio with a 100MHz scope, and expect them to look at all
like a square wave.
A square wave is comprised of odd harmonics of the fundamental
frequency, at say, 30 MHz, your scope could see the fundamental,
and the 3rd harmonic, but would be pretty blind to the 5th.
Also, the usual 6 inch ground wire on the usual 100MHz probe
will be adding in a whole lot of distortion to any signal you
are measuring.
As a good rule of thumb, your scope should have at least 10x
the bandwidth of the non sinusoidal signal you wish to measure.
Your probe's ground wire should be at most 1 inch long.
-Chuck Harris, WA3UQV
On Wed, 26 Mar 2025 12:12:32 -0700 "Steve N2IC via groups.io"
<n2icarrl@...> wrote:
Thanks, Stan,
I'm going through all this effort before ordering parts because it
was quite a destructive event. One of the BS170's was physically
broken, and the board under it charred.
My scope is an old HP 1740A, 100 MHz. I changed probes, and now the
input side of IC502 looks like a good square wave. However, the
inputs and outputs of IC503C/D are not clean.? IC503A/B look okay.
I'm betting that IC503 was damaged.
Now I'm looking at Q507. I? have read Ludwig's postings on this. With
all the BS170's removed, the Diagnostics screen shows the transmitter
voltage at 10.9 volts. Ludwig says it should be close to 0 volts, but
maybe that is with good BS170's. The two SMPS look fine.
73,
Steve, N2IC
