GHz AC probe connector for Marconi TF2604

[6SN7WGTB]

Just restored a rather delightful Marconi TF2604 'Electronic Voltmeter'.

Aside the >100MΩ DC impedance, another attractive feature is the ability to measure AC up to 1.5GHz.

However, connecting the AC probe to a source at that sort of frequency is a bit challenging.

Without complete shielding (first picture) the probe loses considerable signal to the environment - just reaching across the probe to the frequency control caused the signal to fluctuate by 20-30%.

Some tin foil did the job (picture 2) to allow me to check and calibrate, but didn't feel like a very workmanlike solution.

The correct Marconi T (picture 3) must be completely unobtanium unless one is very lucky.

So, a home made job (picture 4) was produced using a brass 1/2" outdoor tap connector, into which I soldered a thin sheet of brass which I had rolled using soft hammer and steel bar. This was then 'quartered' to give the necessary spring to allow it to hold itself on the probe earth ring. Turned the O-ring end down to tidy up.

A female Belling to BNC female was soldered into the opposite end - the female bit of the Belling being just enough to allow the AC probe end to slide in and connect. Used a female BNC as it was easy to knock the central pin/insulator piece out while soldering.

It was all then polished and silver plated to look more like an RF connector than a piece of garden equipment...!

Indeed I doubt is is 50Ω, but it works as advertised (see picture 5) - you may not be able to read the meter but it's showing 600mV.

(To be fair the probe f response isn't flat - showing a dip of about 3dB between 500 and 1,200MHz, but at 1.5GHz it happens to be back to datum).

[Radio Wrangler]

Unobtainium without crazy luck, I suspect.

I have a set of 50 Ohm Tees for the HP vector voltmeter with all the loads and shorts waiting for when I get a vector voltmeter. So it's possible to be on the other side of the equivalent fence. They have other uses as the little Sealectro passive SMC resistor probes fit inside the fingering for the HP probes, and that lets me get into power meters and spectrum analysers.

Never throw any RF adaptors away!

David

[Sinewave]

Very good, would be interesting to see a full write up on the whole job. I've got one of these and it sort of works, but I've never measured particularly high RF with it.

[cmjones01]

Interesting, thank you for writing this up. I have a TF2604 which I don't use very often but I like having it around. The analogue meter and 100MOhm input resistance are handy for finding drifting CMOS input pins.

Chris

[G0HZU_JMR]

That TF2604 looks to be in nice condition. In my student days, I really wanted to have a proper VTVM like this. In those days stuff like this was hard to find as it would be either expensive, or it would sell very quickly.

As a cheapo alternative, I used a classic diode detector probe and a multimeter for many years. This worked quite well but it wasn't quite the same. The EA52 detector valve used in that probe is an interesting device.

Looking on US ebay there are loads of HP 8405A VVMs for sale these days and the prices seem to be dropping at last. Most of them are being sold as faulty though. I'm not surprised at this as I had to clear numerous faults on mine in order to get it operational.

I made various RF probe tip adaptors and impedance measuring jigs for the HP 8405A VVM. They look ugly but work well. I've also got an 8405A accessory kit with the isolator and attenuator tips. This looks to have never been used. Everything was congealed into the rotting foam inside the HP box when I first opened it.

I think my homebrew tip adaptors are more useful as they are much smaller, and they interface with SMA connectors.