Probe accessories for the GR 1806-A VTVM

Over on the Antique Radio Forum, Philip Colston was asking about accessories for the RF probe of the General Radio 1806-A VTVM.

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It turns out that the tips are identical to the accessory tips for the Tektronix P6000-series scope probes:

Many of these tips can still be found on eBay, so that’s easy. But the Tek ground lead clip is far too small to fit the GR 1806-A RF probe. Philip posted that he had formed some clips out of spring steel wire, as the phosphor-bronze wire he had purchased on eBay was not spring-tempered.

I purchased some 0.032 inch diameter phosphor-bronze spring-tempered wire from McMaster-Carr and tried my hand at forming some ground clips for the probe. The probe clip ring is exactly 1/2 inch in diameter and a bit wider than 0.032 inches; the 0.032 inch wire fits perfectly in the clip ring channel on the probe.

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I made a mandrel from a length of 3/8 inch diameter aluminum tubing. I drilled through the tubing with a #50 drill bit, bent a 90 degree angle on the end of a few feet of phosphor-bronze wire, and captured the bent end of the wire in the hole in the tubing. I then wound the wire on to the mandrel, using my finger to serve the wire, or sometimes using the workbench top to serve the wire. The wire springs back to a larger diameter when you release the tension. For my 3/8 inch mandrel, the actual diameter of the windings ended up at almost exactly 1/2 inch – perfect!

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I first tried making clip rings with a loop for connecting the wire. I started with two turns of the wound wire, and formed a loop in the middle.

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I used a pair of small wire loop pliers that I think I bought from Micro-Mark many years ago to form the wire. I made two reverse bends to finish the loop.

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I then started forming loops on the ends of the clip.

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I found it hard to keep everything symmetrical,

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but it did fit the probe just fine.

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I tried another method, where I started with just one full circle of wire and didn’t form a loop for attaching the ground wire. This was much easier.

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I stripped and tinned some #22 gauge stranded wire, tinned the phosphor-bronze clip, and soldered the wire to the clip.

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I finished them off with some heat-shrink tubing.

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Here’s the clip with the loop for the wire. I used two sizes of heat-shrink for this one, as the 1/16 inch heat-shrink tubing wouldn’t fit over the loop where the wire was attached.

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Despite the ugly asymmetry, the clip fits the probe very well.

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The simpler single-ring clip without a wire-attachment loop doesn’t fit as well, due to the bump where the wire was attached. But it fits well enough to work.

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Then I had the idea to just bend a little relief into the loop to provide some clearance for the wire. These fit the probe perfectly as well.

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I start with a single turn of wire, form the end-loops, then form a tighter bend in the middle of the clip.

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Then I make two reverse-bends to provide clearance for attaching the wire.

Update:

I ordered some Mueller BU-30 mini-alligator clips and BU-32 insulators from Mouser to finish off the ground clips. 

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I ended up making a small production run of the ground clips to share with other GR 1806-A owners.

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Here’s a close-up.

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While perusing Mouser’s search results, I stumbled upon the Mueller BU-30TBO clips. They are BU-30 mini-alligator clips with a 6-32 threaded barrel. 

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They take a different insulator, the BU-31.

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They mate with the GR 1806-A probe.

The next step is to get some K&S 0.032” brass strip and reproduce the other style of ground clip for the 1806-A probe.


© Steve Byan 2011-2016