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Old 28th Jan 2020, 7:53 pm   #1
cdm1christopher
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Default Making an RF probe for VTVM

Hi all
I want to make an RF probe for my VTVM using the following components but wanted to know where I can get a test lead probe to fit it in as I cannot find any empty prob casings.
Components 2x10nf capacitors 1meg resistor a EA50 and coaxial cable.

Cheers Chris
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Old 28th Jan 2020, 8:57 pm   #2
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Default Re: Making an RF probe for VTVM

35mm film canister?
You can get aluminium ones as well as plastic.

Peter
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Old 28th Jan 2020, 10:16 pm   #3
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Default Re: Making an RF probe for VTVM

Cheers for that so put all components in can and attache a probe from there via coaxial cable?
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Old 28th Jan 2020, 10:24 pm   #4
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Default Re: Making an RF probe for VTVM

Hello Chris,

I have used two unlikely candidates to make probes. Although I don't smoke I have bought a "high end" brand of cigar that comes in a metal cannister. I also have used the small ladies' reading glasses cases which are also very cheap and generally made from very strong plastics. Both can be separated into two halves which makes installing/mounting of parts easy. I've seen some people have used PVC plumbing tubing/pipe but here in the US the diameter of the 1/2-inch is OK but too small to tuck any components inside and the 1-inch diameter readily avails itself to tucking components inside but it is too "fat" to comfortably fit in my hand. For probe tips I use anything that is straight, metal, can be soldered and epoxied to a hole in the end of the cannister. I usually try to "harvest" a tip from an old VOM probe but have also used small metal rods that I had in the junk box for which I have no idea from where they came. Good luck!
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Old 28th Jan 2020, 10:44 pm   #5
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Default Re: Making an RF probe for VTVM

I used Steradent tubes for a couple of my creations. Old PCB solder pin for the probe tip.

Andrew
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Old 28th Jan 2020, 10:56 pm   #6
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Default Re: Making an RF probe for VTVM

Multicore used to sell a small length of solder coiled ip and fed through a tubular canister with a conical end. The solder came out of a hole at the tip of the cone, there was a plastic cap that clicked over the other end. Deep drawn aluminium.... just itching to become a probe for something. If you can find one lurking around somewhere.

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Old 28th Jan 2020, 11:07 pm   #7
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Default Re: Making an RF probe for VTVM

Cheers guys gives me some things to have mess around with.
Much appreciated Chris
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Old 28th Jan 2020, 11:23 pm   #8
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Default Re: Making an RF probe for VTVM

Plumbing and Electrical merchants have a range of uPVC and copper components such as conduits and pipes that can form the basis of a probe housing.
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Old 29th Jan 2020, 7:06 am   #9
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Default Re: Making an RF probe for VTVM

Just fitted a shower and have some 15mm (1/2") copper pipe offcuts if any good, can send you a bit for £3 post max. PM if interested.

Andy.
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Old 29th Jan 2020, 8:54 am   #10
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Default Re: Making an RF probe for VTVM

Quote:
Originally Posted by cdm1christopher View Post
Cheers for that so put all components in can and attache a probe from there via coaxial cable?
I assumed you were trying to emulate the probe in bottom of the attached picture. In which case I would assemble the EA50 in the can and make up a probe to fit on the end with suitable insulation where it pokes out.

Peter
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Old 29th Jan 2020, 10:33 am   #11
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Default Re: Making an RF probe for VTVM

Didn't spot you were using an EA50 sorry, that means the old Steradent tube's out!

This company sell all different sizes of aluminium containers reasonably priced

https://oipps.co.uk/metal-containers/metal-jars

Andrew
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Old 29th Jan 2020, 10:51 am   #12
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Default Re: Making an RF probe for VTVM

Looking at the data for the EA50 valve (TV detector valve), the maximimum diameter of the valve is stated as 12mm and the maximum length from the tip of the heater and cathode pins at the bottom to the tip of the diode pin at the top is 69mm. Hence, the valve and other three components will comfortably fit in a length of 20mm diam plastic conduit, which has an internal diameter of 16mm. Might be a tight squeeze to fit in 15mm copper tube, so you'd probably need to use 22mm diam, which would perhaps be more in keeping with valve probes of the era when they were the norm.

Valve Datasheet: http://www.r-type.org/pdfs/ea50.pdf

Back in 2016 for use with a signal tracer, I designed a PCB to make a compact automatic AF/RF probe which doesn't need a switch. (It could have been made of perf-board on course). I housed it in a piece of plastic 20mm electrical conduit. Iíve also used discarded felt tip marker cases, which are a bit wider in diameter, for AF & RF probes. The last pic below is of an RF probe built into a felt-tip marker.

Though not really relevant to this thread other than the method of housing it, the automatic probe I made will measure both audio signals (AF) and radio signal (RF). If an audio signal is being measured, the path of the signal is via C1. Audio signal canít pass the C2 since the capacitance is very small and having very high impedance for audio frequency signal. If you measure high frequency (RF), C2 has low enough impedance to pass the signal to the rectifier D1 and D2. Not only rectifying the signal, D1 and D2 also double the DC level at C3. You can see the envelope or strength of RF signal with oscilloscope or voltmeter.

I claim no credit for the design of the circuit, which I found here:

http://circuitswiring.com/automatic-af-rf-probe/

The same method of housing the EA50 and other components could perhaps be used by Chris. I'm pretty sure that he has a lathe - the 'bungs' at each end of my probe were turned from acetal rod. I've got plenty of 22mm plastic conduit if Chris would like a length FOC.

Hope that might help a bit.
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Old 29th Jan 2020, 12:49 pm   #13
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Default Re: Making an RF probe for VTVM

Well plenty of info there. I will be using an EA50 as opposed to germanium diodes just feel you'll get a tighter reading.
But David those probes are exactly what my build intention is.
I have a pair of surplus micronta probes solderable which I can sacrifice rated 1000v although metre goes beyond that but I won't be messing around with those sort of voltages
Bungs are a good idea and I can get access to a lathe.
I like the idea of a small piece of breadboard keeping it all neat and tidy.
I'll take you up on the FOC piping.

Cheers Chris
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Old 29th Jan 2020, 12:51 pm   #14
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Default Re: Making an RF probe for VTVM

pic of probes
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Old 30th Jan 2020, 12:04 am   #15
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Arrow Re: Making an RF probe for VTVM

cdm1christopher: you have a P.M.

Al. / Jan. 29th.
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Old 30th Jan 2020, 7:11 am   #16
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Default Re: Making an RF probe for VTVM

"I'll take you up on the FOC piping." You'll need to send me your address in that case.

Andy.
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Old 30th Jan 2020, 10:38 am   #17
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Default Re: Making an RF probe for VTVM

Hi Andy
might have cross wires here I was referring to David's conduit as it's ideal for what I want.
Cheers Chris
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Old 4th Feb 2020, 12:21 pm   #18
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Default Re: Making an RF probe for VTVM

I've attached diagram of RF probe and I'm bit unsure where the filaments connect to. Don't forget this is all going in a tube using coax and will be connected to the DC output on valve voltmeter.
Cheers Chris
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Old 4th Feb 2020, 12:41 pm   #19
David G4EBT
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Default Re: Making an RF probe for VTVM

The pinouts can be found at this link Chris:

http://www.r-type.org/pdfs/ea50.pdf

(The two outer pins of the three at the base are the heater, the centre is the cathode).

Hope that helps.
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Old 4th Feb 2020, 2:01 pm   #20
cdm1christopher
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Default Re: Making an RF probe for VTVM

Thanks David
but still trying to work out where the heaters connect. As it just shows them unconnected on schematic. Obviously the anode and cathode one connected to earth other into the input of the circuit. Or are the heaters just grounded? I think that's only option its showing but they require 6.5v so where would that come from if their grounded unless the cathode is tied to the heaters and the dc output on the metre provides the voltage!
I cannot see having to put a 6.5v battery somewhere as it's all supposed to fit in a can. So no room for that.
Anyhow just thinking out aloud. Unfortunately I've never made a prob so it's a new adventure but hopefully you can make sense of what info is provided.
Couple more pics with info.
Cheers Chris
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