To: <amps@contesting.com>
> Date: Sat, 31 Jul 1999 05:18:46 -0400
> From: "Tom Rauch" <w8ji@contesting.com>
> Subject: Re: [AMPS] explain this one
>
> Hi Bill,
>
> > I am at a loss for an explanation on this one. When switched to 10
> > meters/AUX, the 160 meter capacitor in question is out of the circuit.
I
> > checked the bandswitch wafer(s) and there is not a short or problem,
> > everything checked out like it should have (or at least I think).
> >
> > The 160 capacitor is connected to a beefy wire that is connected
between
> > the wafer and a side wall. This lead is long and goes across the top of
> > the variable Load capacitor in the PI/Tank circuit. I might also
mention
> > that the tube is running much hotter at 10 than on the other bands, the
> > outer case above the tube is uncomfortably hot.
>
> Is the switch in your LK-500 a pick-up and hold type, where as it
> moves along the unused taps are tied to the common terminal??
The following post is long and I'm trying to answer all the questions that
were asked from various emails that I received (quit a few I might add). I
apologize for the long length.
The bandswitch has 4 wafers.
The 1st wafer switches between the various tuned circuits on the input
tuned circuit board.
The 2nd wafer is used to switch in the 160 meter padding capacitors into
the Plate (Tune) side of the PI network. Only one lug/terminal is used and
when switched to 160 meters the padding capacitors are connected to ground.
The 3rd wafer switches padding capacitors into the Load side of the PI/tank
circuit on the 40, 80 and 160 meter bands. Only the 40, 80 and 160 meter
lugs/terminals are used and the rest are unused. This wafer has a shorting
strip that connects the 40, 80, 160 in a progressive manner. On 40 only the
40 meter capacitor(s) are connected, on 80 both the 40 and 80 meters
capacitors are connected and 160 all are connected into the circuit. I
might add these padding capacitors are disk, yes disk capacitors and there
is quit a few and come in all values, sizes and shapes. I'm guessing that
the designer used a lot of different disk capacitors to share the load and
take the stress off them in order to save money. It is one of the 160 meter
capacitor that is burning and it happens to be the largest one at 1000 pf.
It starts burning when the amp is tuned up on 10 meters, and as previously
mentioned in another post, they are not connected to the pi circuit since
the shorting strip on this wafer does not make contact to these terminals.
The 4th wafer switches the various taps of the two coils in the tank/PI
circuit. The 1st coil handles the 10-20 meter bands and the second coil
handles the 40-160 meter bands. This wafer also has a shorting strip that
connects unused taps together. On 10 meters this strip connects 15-160
meters and on 160 none are connected and the entire two coils are used.
I'm going to stick my foot in my mouth again and now I'm wondering if I
have an old Johnson Matchbox inside my tank circuit? For those of you who
are not familiar with the old Matchboxes they use link-coupling or
inductively coupling. When any two coils are positioned such that the
magnetic lines of force of the one cut across the turns of the other coil,
energy is coupled to the second coil. The expanding and collapsing field of
the 1st provides the changing magnetic field necessary to induce a voltage
across the second with a consequential current flow through the secondary
circuit.
The physical layout of the two coils is not a classic text book case on how
to construct an amp. Nothing is shielded in the vicinity of the tank
circuit, no toroid inductors, long leads to all the components that are
running all over the place. It happens that the capacitor that is burning
is connected to two long heavy gauge wires that run parallel to the turns
of the 40-160 meter coil. I'm having trouble believing that it is inductive
coupling, and if it is indeed due to this why doesn't the shorting strip
doing its job?
73s
Bill, W6QD
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