|[AMRadio] GK500 OTA almost|
n4tua at aol.com
Tue Jan 3 08:33:35 EST 2017
Well, I have had many great responses. Thank you all.
First of all some more info. I am testing using a cantenna dummy load with only the RF deck and the Power Supply deck lashed together with a home-brew HV power supply made from parts of an old BC-610 PS. Rectifiers are SS and all of the rest of the PS is BC-610 parts. I am using a variac to dial down the HV to about 1800 vdc. I am monitoring HV as I go.
I am using a crystal and it is on 3883 kcs.
The final tank is stock. There is some problem with the coarse load switch but the problem contact finger is not being used.
Parasitic oscillations sound possible. Now that I think about it there is NO parasitic choke. I will have to look at the schematic when I get back home to see if it is shown. It did not have one when I got it. My 2 meter rig nearby does occasionally pick up hash when key down on GK500.
I hope everyone who replied to my post can see all of the responses here. I will send my comments to the reflector so all can see.
Thank you, Collin
> On Jan 3, 2017, at 7:22 AM, Jim Candela <jcandela at prodigy.net> wrote:
> It's been while since I have owned a GK 500B. The RF PA stage is not neutralized, and therefore it tends to be slightly unstable from some regeneration. On the upper bands, plate dip and screen current max does not coincide with maximum output. That said, it is not prone to parasitics either. With many class C RF amplifiers, opening up the loading capacitor in the Pi net allows a greater amount of harmonic energy to reach the RF output, and also might push the final RF amp closer to instability.
> I would try to go simplistic and use a crystal instead of the VFO. This eliminates one form of RF ingress.
> Many GK owners use a copper braid chassis to chassis, one piece, straight up and down.
> If the RF PA tube has been flashing inside, the tube might be toast...but not necessarily the cause of the problem
> A neon bulb fastened to a popsicle stick, and placed near the PA tube plate will glow orange when transmitting. Whenever there is a parasitic, the bulb color will shift to violet. Don't hold that stick...just place it nearby, hot melt it in place.
> Is the instability there when transmitting into a good 50 ohm dummy load?
> Also see the post from K1JJ here:
> "on air tuning causing stability issues"
> On Tuesday, January 3, 2017 4:57 AM, Rob Atkinson <ranchorobbo at gmail.com> wrote:
> Don't give up--I almost did a few times with a rig here but when it
> finally got on the air it was like when I was 5 years old at
> I've never owned a Globe King but it sounds like the final PA breaks
> into parasitic oscillation. Get a cheap crummy battery powered
> transistor FM broadcast receiver, something with a telescoping whip
> and 9 v. or 2 AA batteries and put it between stations and next to or
> on top if the rig. Don't know the parasitic frequency for the tube
> but my experience has been if the final oscillates you hear a lot of
> hash on the cheap FM set.
> The parasitic choke can be fine but if there is a grounding problem,
> rusty rivets, poor bonding between chassis and panel, tube socket and
> chassis, and most important, the bonding between the output pi net
> caps and ground (chassis) that can set things off.
> If the output pi net caps are connected to the chassis via the front
> panel that could be your problem. Keep in mind the problem could be
> something else entirely--I'm just an armchair troubleshooter.
> On 1/2/17, Collin via AMRadio <amradio at mailman.qth.net> wrote:
> > Dear AMers,
> > I am having a time with this Globe King 500B. I have it ready to make RF but
> > no modulation yet.
> > Something is just not right and I can't quite put my finger on it yet. Maybe
> > some of you have had an experience like this and might be able to lend me a
> > hand.
> > Tuning the final goes fairly well until I take it out of tune and the full
> > screen voltage is applied to the final. Tuning is very touchy acting and
> > reaches a point where plate current suddenly pegs the meter and some
> > arcing/flashing happens inside of the tube. I can tame this and get it to
> > stay stabilized but increasing loading and the plate current pegs. Any
> > ideas?
> > I have cleaned and secured grounding of tune and load air variable
> > capacitors and also disassembled each air variable and cleaned and buffed
> > all contact surfaces. I have cleaned the final tank band switch. I have
> > inspected every connection and contact between the antenna connector and the
> > plate cap on the final amp.
> > I will not give up!
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