Valiant RF Choke Missing?

[w8khk]

As I was replacing the postage stamp loading caps in my Valiant with transmitter grade micas, I discovered there was no RF choke in parallel with the RF output connector.  I thought it was customary to have a choke (about 2.5mh) from this terminal to ground, to prevent dangerous plate voltage on the antenna circuit in the event of a failure of the plate blocking capacitor. 

I checked the schematic (from BAMA) for the Valiant, and sure enough, none is specified.  Is this common practice for the Johnson rigs, or am I just paranoid?

Think I will add one to mine, and also check the Viker and Ranger to see if they also expose this hazard.

[AB2EZ]

It was common practice of Johnson, Heath, etc. not to include a safety choke in their 1960's series transmitters.

FYI: I use the following approach:

Since there is normally (unless the blocking capacitor shorts out) no DC, and since the output impedance is low (~50 ohms).... I wind several turns of insulated wire around one or more of the 1" OD x 1 1/8 inch long #43 ferrite cores that many of us use for making rf transformers.

For a given number of turns (e.g. 8 turns) the number of cores required depends on the power level (and therefore the rf voltage that will appear across the output connector of the transmitter).

I use 1 core and 8 turns of #22 insulated hookup wire at the output of my Ranger. I didn't put it inside the Ranger. Instead, I use a UHF "T" connector, and I soldered a PL259 plug on to the wires of my choke. That way, I can share the choke among multiple transmitters if I need to.

For a Valiant... I would start with 2 cores and 8 turns. Run it key down into a dummy load for 5 minutes. Switch to standby. If the cores aren't warm, you're all set. If the cores are warm, try 12 turns or try 3 cores.

For my legal limit home brew amplifier I used 8 cores and 6 turns of #14 insulated electrical wire.

Stu

[w8khk]

Hi Stu,

Thanks for the tips, I will apply your approach to all my rigs.  In the past, I used a 2.5mh rf choke, but I often wondered how it would hold up to the DC current available if the blocking capacitor shorted.  Your method would certainly protect the user, rather than allow the choke to act as a fuse and expose the HV on the coax.

I did some research late last night, and I was surprised to find the Adventurer, Ranger 1 and 2, Viking 1 and 2, and Valiant schematics do not include the safety choke.  The Viking 500, Desk KW, and Thunderbolt do include the choke.

I looked and discovered the DX-60 and DX-100 do not have the safety choke.

Out of curiosity, I also looked at the Kenwood TS 530 and 830, and Heath HW 100/101, SB101, and SB401, and they also did not include the choke.  (I just assumed it was included - I have never seen a design in the ARRL or the Orr handbooks that do not have the safety choke.)  Seems the designers assumed the lower voltage in these rigs did not expose a hazard, until you get up into the KW power range.  Or maybe the bean counters figured it was a way to increase profits. 

In any case, I will not use these rigs until I retrofit.  The idea of using the Tee connector is cool, but as I get older and more forgetful, I think I will install the chokes inside.  Time to order some ferrite rods!

Thanks for your advice, and 73, Rick

PS I will also install the ferrite fix in place of the 2.5mh choke in my 4-400 linear and the 4-1000 rig currently under construction.

[AB2EZ]

Rick

I'm pretty sure it was just a typo... but just in case... use ferrite cores, not rods.

Here is where I purchase them from (SB-1020-43)

http://www.cwsbytemark.com/index.php?main_page=index&cPath=206_219_234


I buy them by the dozen because they are useful for making n:m rf transformers, safety chokes, and they are also very useful as common mode chokes on audio cables, telephone cables, and tv cables. Most (but not all) RCA plugs, TRS plugs, F-connector plugs, and RJ11 plugs will fit through the hole in the core.

I consider these to be the rf equivalent of duct tape.

Best regards
Stu

[WQ9E]

Rick,

I don't think the RF choke was common in a lot of the commercial rigs.  My Collins 32V1 is also chokeless. 

Rodger WQ9E

[w8khk]

Stu, thanks for the link to order the cores.  Yes, it was a typo.  Not enough coffee this morning. 

Rodger, I did not think to look at the Collins schematic.  I have two 32V2s, (only one restored) and my brother has the 32V3, we will check.  I think the choke is a small price to pay for safety insurance.  600 or 800 volts can be just as lethal as kilovolts!