The AM Forum

Guys

Im about to finish my Viking II restoration,  but I found the tiny "vhf filter choke (L21)" in shorth, this choke is in series with the 6146's RF Choke, so I wondering if I can make a jumper an get rid of this vhf filter choke?

any advice after I starts my fireworks show?  ;D

Happy thanksgiving to all

Pedro XE1YZY

Hi Pedro,

I believe that is a necessary part of the circuit-I am going from memory but I vaguely recall reading somewhere in the Johnson literature about it being operational at some point where the main choke has a resonance issue.  Is this choke open-is it repairable?  If not let me check as I may have one of these lying about.  I would be concerned about just jumpering it as Johnson wasn't known for throwing a lot of unnecessary parts in their rigs (you got to keep the accountants happy).

Rodger WQ9E

Hi Rodger

Well this choke looks like a resistor, the transmitter have 5 of this instaled over the circuit, and they are not repairable,  the manual dont say nothing about the inductance value, only says " VHF filter choke".

Tnx!

Pedro,

You are right and I was thinking about a different transmitter.  You can probably safely just jumper over that choke but if you want to replace it then it is probably a 4.7 uh unit like those used in the Johnson Valiant.  Someone who is faster at inductance calculations can probably tell you how many turns are needed over a 1 or 2 watt high value resistor to hit this value.

By the way, if you have not done so already then carefully clean the tube bases of the 5R4's and the sockets they plug into.  Both my Viking 1 and Viking 2 had some issues from dust forming carbon tracks so hopefully you can avoid that problem.

73, Rodger WQ9E

that is he fix to the Viking 1 well known channel 6 problem, where the tx puts out a VERY strong vhf signal on TV channel 6. This was eliminated on the Viking 2 by generally de-TVI'ing the whole transmitter and adding that choke/rx combo to break up the parasitic.

they sold a kit to de tvi the Viking 1. I would see if you can obtain a parts list of that kit. basically any 47K 2 watt resistor wound with 4 turns of bare hookup wire wound medium spacing would probably knock it out, but it could also change the freq of it instead of getting rid of it.

why is it not repairable? too burned to read color codes?

Hi Tim,

Burns to his bones , but you give me an idea, since L14,L16,L19.L20 and L21 are the same chokes I read the code, is yellow violet, 4.7 uH maybe?

does it have any wire wound over the body of it? or just a resistor looking thing?

I think we may be talking about 2 different components. There's only one of the parasitic suppressors I am referring to and it is in the buffer plate / final grid circuit. without it you will be worked all TV sets.

I think you're referring to the chokes they used in various parts of the rig else where.

Pedro,
    I have burned up the very same choke on my JVII.  I used a big fancy
impedance meter here at work.  I measured the original choke (one that
was not burned up!) and measured it at 310 ohms at 10MHz, this worked
out to be 4.7uH, which makes sense as there are yellow and purple
bands.

Rich

Hi guys,

The man is right, it's a 4.7uH molded choke, green body with yellow, violet and silver bands on it. For what it's worth I've seen the plate VHF parasitic chokes smoke up too, the resistors they're wound on are 22 ohm 2W units.

TNX Guys

Im about to finish some little problem with the driver pot, as soon as I finish I test the TX, with and without this filter choke to see what happen, I will let you know

Pedro

Hmmm, "some little problem with the driver pot" sounds familiar, I've replaced several with JS arrangements  of ww pots and resistors to keep the smoke inside.