[ARC5] BC-459A Status - oops

[Kenneth G. Gordon]

On 5 Oct 2013 at 12:56, Mark K3MSB wrote:

> Still, the oscillator waveform looks bad. 

Oops. For some reason, when I clicked on the link you gave in your e-mail, the 80 meter 
oscillator waveform showed up.

Yes. The 40 meter oscillator waveform looks TERRIBLE!

The output waveform looks very good.

In fact, from what little I can see of the osc waveform, it looks as though the waveform 
includes several other frequencies, and if your scope controls are set the same as for the 80 
meter osc, output is VERY low.

Are you CERTAIN you're not getting feedback from another source, or that your oscillator 
bypass capacitors are good or that all your "grounds" are good? As strange as this may 
sound, it almost looks like your oscillator has parasitics! It looks as though it is oscillating on 
more than one frequency simultaneously, or that the DC has huge amounts of AC on it.

The 40 meter oscillator should look as good as the 80 meter one does and should have the 
same output voltage.

Also, you say when you remove the scope ground, the waveform gets worse: well, that 
indicates that something does not have a common ground somewhere. The TX ground is 
NOT the same as the scope's and in a shop, they should be. Removing the scope-probe's 
ground should effect the way the signal looks, but should not appreciably change the basic 
wave form.

You also may have on horrendous ground-loop somewhere, or a missing or badly-soldered 
ground connection.

Have you measured the voltage between the TX chassis and the power supply chassis, both 
for AC and DC? There should be none.

I might suggest that you run just the oscillator off batteries and look at its output with a scope. 
A series of 9 volt transistor batteries for B+, and a 12 V lantern battery for filaments would 
allow the oscillator to run for enough time to get a close look at its "native" output. And for this 
purpose, you would not need the full 200 VDC. 90 VDC would probably tell you what you 
need to know.

SOMETHING in the connections to the 459 or in the oscillator circuitry or its connections, or 
in the shielding and bypassing is simply not right.

I might add that the output of the 40 meter osc. appears to me to be too low to drive the 
transmitter to any output at all. In fact, the FINALS may be oscillating which is what is 
providing you with 30 watts output. I would not be a bit surprised to find that you have 
significant output with the oscillator tube completely removed from the TX.

Ken W7EKB