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[AMPS] Re: Power Supply problem

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Subject: [AMPS] Re: Power Supply problem
From: (John Hill)
Date: Thu, 13 Jul 2000 04:07:28 +0000
Greetings again;

Thanks to the multitudes who responded to my post. I learned
a few things and appreciated the anecdotal as well as the
technical feedback.

In retrospect, I believe something else is contributing to my
transmitted signal having what appears to be AC components
on it. I say this because, as I mentioned, there is about 180
volts of AC on the B+ of 3800V when idling at 100ma. That's
4.7% ripple. This is where it gets interesting. At 250 ma of
plate current (drive applied) the ripple stays the same. At
500 ma of plate current, ditto. If a component were failing,
I'd expect the ripple to increase as the current increased.

Now, my curiousity has increased. I dug out my trusty
Tektronix 454 oscilloscope and looked at the component
using the metering take-off from the B+. Lo and behold,
this is 60 Hz with some minor commutation in it. Why
60 Hz? This power supply is a full wave bridge, or at
least it's supposed to be. It only gets odder from here.
This AC component was only 50mv p-p through the
metering divider network. Given the division factor
of the network, that equates to 44VAC of ripple, or
approximately 1.2% ripple on a 3800V line.

The only means I have to evaluate the two rectifier
arrays was to take a 30V power supply with a
current limiting resistor and look at the voltage
drop across each leg. They were 14.5V, 14.4V,
14.53V and 14.36V. Not even a junction difference
between any of them assuming they're not Schotky
devices. Allowing for silicon, they must have 20
or so junctions in each leg.

Several people asked similar questions. The exciter,
an FT-1000D is clean by itself. The hum is there
on CW with a continuous carrier. The final filter
cap checked at 16uf (per spec) and almost no
leakage on the Sencore. The B+ drops from a
resting value of 4040V to 3820V under load. It
does not drop much more than that from 100ma
up to 800ma. The bleeder resistors (4x20k ohm)
all get hot, so I assume they are working.

I am seriously thinking along the lines of an AC
loop somewhere.

Thanks again to all of you.
John, NJ0M

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