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OK, ok... the "sniffer" RF Probe is back

Where Are The Watts?

Some of you - well, quite a few - let me know about a "got'cha" that comes into play soon after you replace the crystals and begin the task of tuning the commercial standard transceiver to its new operating frequency:
"Buck, when I tested the radio on the old frequency on which it was crystalled (up in the 160 MHz portion), it showed good power output. But when I put the new rocks in and attempted to tune the radio on the new frequency, there was no (watts) output."

Yep, that is a fact of life when you move a commercial radio down the band, over 10 MHz. In a few cases the commercial radio will have a broad enough band pass to handle a change of this magnitude. In most cases, though, we have to do a little "snoop'n" to find the watts.

Enter The "Sniffer"

Often referred to as an RF probe, the "sniffer" is a simple RF demodulator or diode detector built into a small cylinder (the one I use resembles the metal casing of a pocket flashlite I saw at a nearby "dollar store"), or metal tube. In the drawing above I've illustrated the component values and the simplicity of its construction.

This valuable tool can be built in less time than it takes to think about it, and as you will soon discover, it will become one of your most valued service tools. The "sniffer" is plugged into your digital voltmeter, or if you are lucky, into an old- fashioned vacuum tube volt meter (VTVM).

If you happen to have a D'arsonval (meter with the pointer/needle), try the sniffer with it. If you have only the digital volt meter, then you are on your own, as here you will have to interpolate the readings high and peak to meter low or signal digress. I picked up a RadioShack meter, an AutoRange VOM model 22-216, for under $35. Not only is this meter useful as an all-purpose bench VOM, it also responds very well with the sniffer I have.

Some considerations, But NOT "show-stoppers:"

The RF probe I use is one that my son Glynn, WB4RHO, built about 20 years ago. It uses the old (TV video detector) 1N34a diode. However, the sniffer can be built with just about any of the 1N914's or 1N4148's from your local RadioShack or Tech America store. If you are fortunate enough to get your hands on the old 1N34, it seems to be more sensitive to RF detection than "fast-switching" diodes such as the 1N4148's.

The lead from the meter into the RF sniffer is piece of small coaxial cable. I think Glynn used a piece of small phono shielded cable at both the goes-into and the comes-out-of the probe. The comes-out-of, part of the shield is only the shield. The tip is made from a one inch piece of number 14 tinned (solid) wire. The shield is bonded to the inside of the metal case of the sniffer.

Your probe needs a low impedance ground path for the RF. I use a five inch piece of braid from some small coax (RG-58 or the smaller RG-178). Strip the insulation and slide the inner conductor out of the braid. Flatten the braid and attach a small alligator clip for easy connection to a ground near the area where we're tuning. The other end is attached to the circuit as shown in the drawing above.

The next time you begin tuning a radio into which you've just installed new rocks, or moved the frequency across most of the visible light spectrum, you can find those missing watts. Whoops! Better make that the missing "milliwatts," for in the beginning, when you begin looking for the power that is not there, you'll need to begin looking around the first stages of RF. This is where the RF probe comes into play.

Always start on the lowest level stage you can when tuning a radio. Look over the schematic of the commercial radio to find a point that has a capacitor decoupling it from the tuned circuit. If you can't get RF from the first stage, there is no point in tweaking the slugs of the succeeding stages! Always check to make sure the oscillator is working before anything else. The sniffer will also prevent lots of cracked ferrite bobbins and coil slugs by helping you find RF before you've cracked every bead in the exciter.

Moving the tuning slugs clockwise into the coil increases the frequency, thus lowering the frequency of the tuned coil. Moving the slug out, or counterclockwise raises the frequency of the tuned coil.

One good place to begin looking for signs of RF is at the output of the crystal oscillator (yes, you must key the transmitter while tuning the exciter). Once you have power there, move to the next stage, whether it is a buffer stage or a stage in the FM multiplier chain. It won't take long to find that you have the perfect tool to begin the tune-up of both the transmitter and receiver crystal oscillator stages. Tune for optimum power out of each stage and then move to the next stage.

Moving always away from the crystal oscillator stage towards the PA section, it won't be long before you see RF power showing up on your watt meter. You know, the watt meter that you have connected to the coax connector at the radio output. Oh, you know, the one that connects the radio into the "dummy" load.