The AM Forum

So I was up late last night installing a DPDT 120V relay into my Viking Valiant to mute my Hammarlund HQ-180A. I also used the SSB SO-239 on the Valiant to connect to the HQ-180. Here's the problem. I soldered the RF out and receiver SO-239s backwards to the relay. When I transmitted, it muted the receiver, but it sent RF straight into the SO-239 of the receiver! I think I have narrowed it down to a fried antenna transformer (T16, 7.85-15.25 Mcs, PN: 26459-1). It smells like toast and is the first thing the RF should see when it enters the rig. Is it possible to rewind these transformers, or do I need to find another one? Also, is there anything else downstream I should be worried about? Thanks for the help and I have learned my lesson about working on these radios when I should be in bed!

John
KJ7USA

John,

You probably just burned out the primary of the antenna coil for that range.  It should be an easy repair since there won't be many turns for the primary winding.  If you didn't vaporize the turns just count them and rewind with similar size enameled wire, otherwise hopefully someone has one outside of the case now that can count the turns for you.  Usually this is the only damage that happens.

This is why in all my tube receivers there is a #47 lamp in series with the antenna lead. Mounted it under the chassis right next to the antenna terminal. Makes a great fuse.

Thanks Rodger. I will extract it tomorrow to investigate. It looks like a real charm to take out. The transformer contacts are right below the switch in a tight spot.  The bulb is a good idea Pete, thanks!

If you need some #47s, I happen to have about 10 47s right next to the computer in front of me.

Fred

I appreciate that Fred, but I just bought a bunch of them at the store yesterday.

I pulled the transformer out last night and luckily I could still count the turns on the coil. I had to order some enameled wire to rewind it. Hope all goes well.

That's not enamel wire,  looks like Litz wire.  If you have an old AM radio you could get some off the loop antenna.

Fred

I agree it appears to be Fred and that is odd given the frequency range of that coil, Litz wire is normally used at much lower frequencies where its ability to reduce skin effect is important.  It really has little effectiveness at the frequency of this coil.  Using regular solid copper (enameled of course) should be fine for this primary.

I read after a google search there was an old QST article for this protection and mod. I am interested in trying this. Do you have anymore info?
Thanks

???

Rodger

I'm sure you meant to say: "much higher frequencies".

As you know, skin effect increases with increasing frequency... so, using small diameter, individually insulated wires in parallel (more surface area for the same total cross sectional area) would be more useful at higher frequencies (to lower the RF resistance).

In any event, I agree that solid, enamel insulated wire will probably work fine because there aren't a lot of turns in this winding.

Stu

Hi Stu,

Terman covers this in his Radio Engineering book and states that Litz is primarily useful below 1 Mhz.  Above that the irregularities of stranding and the capacitance between strands negates the effectiveness of Litz.

By the way I am no expert on Terman but I was stuck in an airport several years ago for 17 hours due to an ice storm and Terman was all I had with me to read :)

Rodger

Interesting....

I guess there is a narrow range of frequencies and associated applications, where using multiple insulated wires in parallel is advantageous.

Above the lower end of this frequency range, using separately insulated, smaller diameter wires in parallel (rather than one solid conductor) reduces the RF resistance associated with the skin effect (for a fixed total cross sectional area).

Each smaller wire, of radius r, has a circumference that is 2 x pi x r. If r is greater than the skin depth, then the conductance of a given length of this smaller wire is: 2 x pi x r x the skin depth / (the length of the wire x the resistivity of the material). The resistance per unit length through one smaller wire is, therefore, proportional to: 1/r.

The number of (parallel) smaller wires, N, that can fit in a given cross sectional area is proportional to: 1/ (the cross sectional area of a single smaller wire). The cross sectional area of a single smaller wire is: pi x r x r. Therefore the number of smaller wires, N, that can fit in a given cross sectional area is proportional to: 1/(r x r).

The resistance of all of the smaller wires in parallel, per unit length, is: the resistance per unit length of one smaller wire, divided by the number of wires in parallel; which is proportional to: (1/r) x (r x r) = r

In summary: for a given total cross sectional area of separately insulated parallel wires, the smaller the radius of a single wire, the smaller the RF resistance per unit length of the parallel combination. [This assumes that the radius of each smaller wire is larger than the skin depth; and that the thickness of each smaller wire's insulation is negligible compared to the radius of one smaller wire.]

But, beyond the upper end of this narrow frequency range, the effects that you mentioned (which include increased dielectric losses, and increased capacitive coupling between adjacent wires) will negate the benefits of the reduced RF resistance associated with the skin effect.

Stu

Here's a simple circuit below that was used by contest stations.

On many older receivers, the antenna input connection is an antenna coil with one side of the antenna coil to the antenna input terminal and the other side of the coil to ground so, in effect, the receiver antenna input is at ground potential.  For my older tube receivers as RF front-end protection(accidentally transmitting into it), I just use a 5 or 6 volt lamp with wire leads and wire it in series with the antenna connection to act as a fuse. Cold resistance of the lamp is very low so under normal conditions it will not affect reception.

(http://www.bulbs.com/images/productsmall/1/11558.jpg)

Here is a picture of the newly wound transformer. I used solid enameled wire and the receiver is working great again. This is one mistake I hope to never make again!

Any mistake that can be righted so easily isn't bad :)  Well done!

My Brother owns a coil winding company....

My HQ 170-A had a smoked 75 meter coil.
Turned out pretty well...

https://www.youtube.com/watch?v=ghPoNEHn-1A

I'm glad I didn't smoke that coil. The one I damaged looks a lot simpler!

Is there a question here?

Pete, I built the receiver guard with the #47 bulb due to availability as the specs are the same as the #12. I could not install the circuit in the R390A due to major surgery so I built in a small project box.
de k7iou

  That "Receiver Guard" in a box looks neat. This circuit has been shown here on AMFONE before. One caution to consider is what happens if your antenna is not a DC short? Lets say the antenna is a coax fed dipole, and no tuner is used. Due to storm cloud interaction with the antenna, the side of the antenna going to the coax center conductor will sometimes get charged up, and this charge can get quite high, far higher then the rating of those little capacitors in that "Receiver Guard" box. Perhaps you can add a resistor, such as a 1 meg-ohm from the boxes input to ground. Better yet, use a 2.5 mh pi wound choke from input to ground.

Jim
Wd5JKO

Thanks Jim, talked it over with my elmer wd7f, retired USAF electronics teacher  ;) and he agreed so I added it. See picture.
Cost was $9.99 for the build @ our local electronics supply excluding SO-239 chassis mounts, scrap piece 12 gauge electrical wire & 1 Meg resistor as I had them in my junk box. Cheap insurance for a restored R-390A or whatever receiver your using.
73 de k7iou

One more suggestion when wiring up new antenna relays, if not already suggested, is to ohm out the connections with an analog ohmmeter before you put RF to the relay.   Confirm the goesintos and goesoutofs are wired correctly when keyed/unkeyed.  Then there is no need for adding light bulbs to RF inputs, which just IMHO, adds another future failure point.  I do like the add-on box however.  Also I believe a 47 lamp would be fine for a boatanchor receiver, however I would use a low current fuse between 1-10ma for a solid state receiver. 

I have never TXed into a receiver before but I once smoked an antenna analyzer front end when I forgot to disconnect it from an antenna and then transmitted into an adjacent antenna.  Another lesson learned in the "Stupid Things Done With Transmitters" book ::) ::) ::)

Peter

From what I read, this receiver guard circuit was designed for high power multiple contesting station setups in close proximity to each other. Obviously, I doubt any of the stations were using boatanchor equipment so I have no idea if it was ever tested using boatanchor equipment. I also have no idea if anyone actually put "X" amount of continuous watts into the input of this thing and monitored the output (receiver side) with a dummy load and wattmeter. Might be an interesting experiment.

The purpose seems to be to add an inexpensive failure point that is easy to repair. I may try it myself. Would that circuit attentuate receive signals to any degree?

Ken, the circuit didn't reduce my receiver sensitivity one bit. I built and put it in line due to using a dowkey relay and wanted a safety device to protect the receiver in case of malfunction or operator error. I chose the #47 bulb over the #12 due to cost and availability. The specs of the two bulbs are identical.
73 de k7iou