Steve's Web Junkyard

The question of housings for RF probes came up recently on both the Heathkit Yahoo list and the Antique Radio Forum's test equipment forum. Here's a couple of approaches that I've tried.

The classic method is to use an aluminum cigar tube. I'd been saving these for forty years before finally putting one to use. I scarfed them off my dad, though he didn't smoke cigars so I've no idea how he came by them. He probably scarfed them off a buddy and saved them to make RF probes :-)

This probe is a RF probe for my IM-11 VTVM, using the circuit that's been in the ARRL Handbook for umpty-ump years.

I use a vinyl grommet where the coax enters the tube.

The probe tip is mounted in the plastic cap.

I use H. H. Smith model 200 insulated phone tip plugs as the tips for my homebrew probes.

For cigar tubes, I mount them using the plastic handle of the plug as a nut.

For another approach, I use K & S brass tubing from my local model railroad hobby shop. This probe is for my Knight signal tracer, so it's switchable between a straight-through probe and a demodulating RF probe using a subminiature slide switch mounted on the side of the probe housing.

I used a female BNC connector rather than hard-wiring coax to the probe.

Again, I used a H. H. Smith model 200 insulated phone tip plug as the tip for the probe.

You could mount the probe tip plug and the BNC jack using styrene or ABS tubing from the hobby shop to shim down the diameter of the brass tube, but I elected to fabricate the ends from telescoping sections of brass tubing soldered together. If I had a lathe, I'd have turned them from brass stock; it would be cheaper and easier.

I glued the tip plug in using Walther's Goo. I tapped the other end-piece to match the threads on the BNC jack.

And that's it.

I had been asking on the Antique Radio Test Equipment forum for a manual for my Krohn-Hite 3202 Variable Filter. Someone later came across my post and asked for copies of any manuals that I had turned up, so I posted them in my Liberated Manuals section.

I plan to monitor the AC power line voltage in my shack with a Simpson model 59 AC panel meter that I picked up as surplus from someplace or other. 

But it's got an oddity - a external 0.0025 µF capacitor hanging off the back of the meter.

The capacitor looks like it's a molded paper type. The Simpson data-sheet for model 59 panel meters mentions that the higher range AC voltmeters (500 volts and above) have an external multiplier, while the lower-range voltmeters have internal multipliers. I wonder if my meter was modified to use a capacitor in place of an internal multiplier resistor.

Inspection of the internals reveals that the capacitor is shunting the multiplier resistance coils. The meter is labeled for "60 & 400 CYCLES". I wonder if originally there was an internal capacitor to compensate for the inductance of the multiplier resistance coils, which failed and was replaced by this external capacitor?

I cut the cap out, leaving a bit of the leads behind in case I need to replace it to make the meter function correctly. I don't want to mess with the solder joints on the precision multiplier resistance coils.

I don't want to have a suicide cord in my shop, so I'll have to rig up a safe way to test out the meter. Some time ago I built a little rig with a line cord, fuse, and power switch feeding a 25.2 volt center-tapped transformer connected to binding posts; I used it for checking the calibration on the low ranges of my VTVMs and VOMs. I think I'll add a couple of insulated pin jacks to access the 117 VAC side.

I recently discovered the Antique Radio Forum. I hadn't cared much for web-forums; too insular and too painful to browse to keep up with the new posts. But ARF has two things going for it: a very knowledgeable community, especially in the test equipment area, and a nice mix of photos and text in the posts. The photos really add a lot; they're what's missing from the technical email lists like the qth-boatanchors list.

While reading the ARF test equipment forum, I stumbled on a post from Paul Koby looking for a manual for a Cushman CE-21. I've got one too, also sans manual. I bought it at the Deerchester hamfest back in the '90's for $125. Works fine, but someday it's going to need servicing. So I approached Paul and we agreed to split the cost of purchasing a manual from Manuals Plus; I'd scan it and send him on the original. So I'm liberating the manual; I've placed my scan of it in my local archive here and will pass it on to the other manual sites. 

(Hmm, BAMA doesn't take test equipment any more; where's the current canonical repository for liberated test equipment manuals?

Answer from ARF: Ebaman and Didier's site.)