UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Test Gear and Workshop Equipment

Notices

Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

Closed Thread
 
Thread Tools
Old 13th Feb 2020, 5:27 pm   #101
Luxman1050
Octode
 
Luxman1050's Avatar
 
Join Date: Apr 2019
Location: Margate, Kent, UK.
Posts: 1,723
Default Re: Making an RF probe for VTVM

post 18# how probe to be made.
EUT?
It's become apparent that I cannot use the tap on original VTVM as it will overload the transformer so now I will need to make an external power source for the heaters on EA50.
So I will need to build a small aluminium box to house the tranny in including other components as the main power source for the heaters on the EA50.
The original components etc will still be used for the rf probe but will mean additional leads from power source to the EA50 all components used will include coaxial cable shielded which will be ground to the VTVM. Now looking at most RF probes they are all generally big with quite a few leads running off them.
With ref to 0V earth not quite sure what you mean as Earth is not going to have voltage applied to it.
Like I say post 18# should give you an idea of spec.
But if you could extrapolate on your question be helpful not quite sure where your going with it and not sure what EUT means not come across that abbreviations before.
Cheers Chris
Luxman1050 is offline  
Old 13th Feb 2020, 8:24 pm   #102
Skywave
Rest in Peace
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
Arrow Re: Making an RF probe for VTVM

Chris:

EUT: Equipment Under Test - a common abbreviation used in Test & Measurement.

I looked at post #18, but it does not detail a connection between the probe and the 0v. / earth of the EUT.

I'll expand on my thinking . . .
The probe will connect to a point in the EUT where the magnitude of an RF voltage needs to be measured. That voltage will usually be referenced to ground / 0v. / chassis. Therefore to measure that voltage, there must be a connection from the probe to the ground / 0v. / chassis of the EUT.

I'm simply asking what type of physical arrangement you are intending to use for that necessary connection.

Al.
Skywave is offline  
Old 14th Feb 2020, 12:57 pm   #103
Luxman1050
Octode
 
Luxman1050's Avatar
 
Join Date: Apr 2019
Location: Margate, Kent, UK.
Posts: 1,723
Default Re: Making an RF probe for VTVM

Double screened coax one to smc connector the other to chassis ground using cable with an insulated tiger clip on it which will be housed in probe and come out the side of probe which then connected to EUT. Remember the switching on the vtvm has also been designed for an RF probe.
Is this what your going on about? If not I'm not understanding what it is your trying to relay and assume you have the answer to your query.

Remember I've never built an RF probe neither have I built an external power supply for heater supply.

Cheers Chris
Luxman1050 is offline  
Old 14th Feb 2020, 6:49 pm   #104
Skywave
Rest in Peace
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
Arrow Re: Making an RF probe for VTVM

Quote:
Originally Posted by cdm1christopher View Post
Is this what your going on about? If not I'm not understanding what it is your trying to relay and assume you have the answer to your query.
Chris
Hi! Thanks for your reply.
You seem to have the general idea as to "what I was going on about" - as you put it. But I'm not trying to 'relay' anything & I obviously don't have the answer to my query. If I had, then I wouldn't have asked you the question that I did in the first place!

That aside, I'll now get to the point.
If I have understood you correctly, you will be fitting a flying lead from the earth / 0v. of your probe to the chassis / ground of the EUT when you are using that probe. In other words, the same sort of arrangement that is commonly used with an oscilloscope probe.
So - what range of R.F. frequencies do you expect that you will using your probe for? I ask that Q. since with such a 'flying earth lead' connection, the measurement of voltages at high R.F. frequencies will be substantially affected by that flying lead.

I am trying to help you make a success of your project, using the experience I have gained with 'all things R.F.'

Regards,
Al.
Skywave is offline  
Old 14th Feb 2020, 8:05 pm   #105
Luxman1050
Octode
 
Luxman1050's Avatar
 
Join Date: Apr 2019
Location: Margate, Kent, UK.
Posts: 1,723
Default Re: Making an RF probe for VTVM

Sorry Al
paranoia setting in for some reason not been too well the old cancer been taking its toll.
Anyhow Check post #39 that's the manual for this meter. Now having read it's abilities properly as opposed to scanning through I don't think you need an RF probe for this model as it seems already set up for RF alignment sine wave etc. But I could be wrong.
The only thing which has thrown me is the DC connector. Now the bnc is grounded and it's wired to DC. Now the probe in the manual has no ground lead for the bnc connector instead you use the common probe? Now why install a grounded bnc as opposed to just a normal jack? So according to the manual the DC probe does not need an earth lead coming off it as you use the common lead. All the DC probe has is a 1meg resistor installed.
Maybe I'm getting confused.
Luxman1050 is offline  
Old 15th Feb 2020, 2:05 pm   #106
Luxman1050
Octode
 
Luxman1050's Avatar
 
Join Date: Apr 2019
Location: Margate, Kent, UK.
Posts: 1,723
Default Re: Making an RF probe for VTVM

Joe those retaining clips for valves on your meter did you fit them yourself or where they already installed?
I have some retainers which I could fit but would need to drill some holes in the board.
Cheers Chris
Attached Thumbnails
Click image for larger version

Name:	20200215_125850.jpg
Views:	58
Size:	42.7 KB
ID:	199026  
Luxman1050 is offline  
Old 15th Feb 2020, 3:00 pm   #107
Skywave
Rest in Peace
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
Arrow Re: Making an RF probe for VTVM

Chris - I did not know that you were suffering from cancer: you have my sincere sympathies; hope that you are managing to cope, although I am sure that sometimes things will be a real struggle.

And so to the VOM.

If - in what follows - some of it is obvious to you in places, please bear with me. I just want to make things as clear as I can to avoid misunderstandings - and ideally, state the following only once!

When ever I read USA manuals on test equipment and I meet the word 'Jack', I always scratch my head: it's a technical language problem. Here, in the UK, a 'Jack' refers to a socket that is primarily used for making a low-frequency connection, but in the USA, that word is often used in a broader context. So for me, with this instrument, there is ample room for confusion.

The point I have been trying to make in my posts is that when any attempt is being made to measure a voltage or current with an instrument, two connections must be made between that measuring instrument and the EUT. In the case of a VOM measuring an R.F. voltage, the usual arrangement is for one connection to go to the point where that voltage is to be measured, but that voltage will be referenced to something - nearly always ground / chassis / 0v. of the EUT. Hence, another connection, meter to EUT, is required.

In the case of measuring high frequency RF - e.g. typically 5 MHz or above - to avoid disturbing the circuitry of the EUT at the measurement point and obtaining a reasonable accurate reading, an 'A.C. probe' is required. That probe will - by the above reasoning - require a connection from the ground / chassis / 0v. of the EUT to the 0v. / common connection point of the indicating meter. And that is where problems can arise. The length of that lead to the 0v. point of the meter - at high radio frequencies - is very critical, on account of its inductance. A simple insulated wire from meter to the EUT will simply not do for high frequency R.F.: any measurements thus made will be very much in error. And that arrangement seems to be what this design uses. However, that arrangement will be O.K. for audio freqs.

So, in summary, for use at R.F., the use of an a.c. probe (with a very low input capacity) and a very short ground wire from the EUT to the probe is essential.

Aside.
The same problem arises when using a probe with an oscilloscope. I always solder a BNC socket to the measurement point with the 'earth' of that BNC joined to chassis as close as possible to the measurement point and use an adaptor at the tip of the probe which plugs directly into the BNC socket. That way, the 'earth connection' is a short as is mechanically possible. The 'flying earth' lead of the 'scope probe is simply not used.

This critical point about a suitable 'earth connection' at R.F. - and a few others - seem to have been overlooked in the manual for this VOM.

Al.
Skywave is offline  
Old 15th Feb 2020, 5:42 pm   #108
Luxman1050
Octode
 
Luxman1050's Avatar
 
Join Date: Apr 2019
Location: Margate, Kent, UK.
Posts: 1,723
Default Re: Making an RF probe for VTVM

I kind of understand what you mean Al.
So for the probes AC and common would you be better using rg174 cable which I have and is military grade pic attached. Plus it's only 2.8mm in diameter.
Now how that would work with the probe not sure as you have the sheild to contend with or just leave that unconnected. Thinking of impedance.
What else I'm not sure about is having the AC with as short as possible a ground connection. Now going by the cable above you could connect the shield to ground in the VTVM add small connector on side of the ac probe then use different lengths of cable which can be directly connected to the ac probe. But the manual stipulates using the common probe on unit.
Now the dc probe again not sure about this as I have made a probe with 1meg resistor and 75ohm cable installed plus added a ground clip coming out the side of the probe. Now I'm assuming this is correct as the bnc installed in the VTVM has a ground connection. Now in the manual its saying connect both tips of the DC probe? and the common test lead to place to be measured. Now I'm confused here as to the probe having two tips. I know dc can be used to adjust the FM dector and AC used for everything else with common probe. But as your saying you can't use just any old cable.
Now an RF probe I would assume would get round the problems of measuring low RF frequecies? Without giving false readings. Me just thinking out loud and going by what your saying.
Attached Thumbnails
Click image for larger version

Name:	20200215_162055.jpg
Views:	59
Size:	36.2 KB
ID:	199034  
Luxman1050 is offline  
Old 16th Feb 2020, 11:16 pm   #109
joebog1
Banned
 
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 2,704
Default Re: Making an RF probe for VTVM

Quote:
Originally Posted by cdm1christopher View Post
Joe those retaining clips for valves on your meter did you fit them yourself or where they already installed?
I have some retainers which I could fit but would need to drill some holes in the board.
Cheers Chris
The whole meter is absolutely bog stock standard Chris.

https://www.americanradiohistory.com...i-Clifford.pdf

That may give you some ideas.

Joe
joebog1 is offline  
Old 17th Feb 2020, 1:55 pm   #110
Skywave
Rest in Peace
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
Arrow Re: Making an RF probe for VTVM

Quote:
Originally Posted by cdm1christopher View Post
I kind of understand what you mean Al.
That's good. I've explained the problems that can arise in measuring high frequency voltages to the best of my ability, experience and understanding: without repetition, there is nothing more that I can add.
If I have over-looked any aspect of your proposed approach to your project, or if further advice appears necessary, hopefully others here will pick up the torch.

Regards,
Al.
Skywave is offline  
Old 17th Feb 2020, 2:27 pm   #111
Luxman1050
Octode
 
Luxman1050's Avatar
 
Join Date: Apr 2019
Location: Margate, Kent, UK.
Posts: 1,723
Default Re: Making an RF probe for VTVM

Aw nice one Joe thanks for that did pick up elsewhere. But many thanks again.
Cheers Al input much appreciated.
Many thanks Chris
Luxman1050 is offline  
Old 17th Feb 2020, 3:57 pm   #112
ColinTheAmpMan1
Octode
 
ColinTheAmpMan1's Avatar
 
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 1,465
Default Re: Making an RF probe for VTVM

Quote:
Originally Posted by cdm1christopher View Post
Joe those retaining clips for valves on your meter did you fit them yourself or where they already installed?
I have some retainers which I could fit but would need to drill some holes in the board.
Cheers Chris
If I'm seeing what I think I am, you don't need any extra holes in the circuit board to mount these, you just use the valve-holder screws to retain them. OK, you might need to put a washer onto the screw to better retain the end of the springy wire, but that shouldn't be a problem. What would be more of a problem is if the valveholders are mounted using pop-rivets or similar. They would need to be replaced with screws, nuts and washers.

Colin.
ColinTheAmpMan1 is offline  
Old 17th Feb 2020, 4:17 pm   #113
Luxman1050
Octode
 
Luxman1050's Avatar
 
Join Date: Apr 2019
Location: Margate, Kent, UK.
Posts: 1,723
Default Re: Making an RF probe for VTVM

Cheers colin no there pcb mounted.
Luxman1050 is offline  
Old 18th Feb 2020, 7:51 pm   #114
Luxman1050
Octode
 
Luxman1050's Avatar
 
Join Date: Apr 2019
Location: Margate, Kent, UK.
Posts: 1,723
Default Re: Making an RF probe for VTVM

Again not related to RF probe waiting on some bits and pieces to build chassis and circuit board etc.
Anyhow changed the ceramic caps for similar wow they both gone well high one 7892pf the other 8354pf supposed to be 5000pf.
Anyhow tested today and must say everything spot on ac/dc resistance measurements etc around 1% out but pretty good in my book.
Luxman1050 is offline  
Old 18th Feb 2020, 11:53 pm   #115
joebog1
Banned
 
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 2,704
Default Re: Making an RF probe for VTVM

The spring retaining clips simply fit thru a hole in the PCB that hold the bootlace ferrel type rivets that hold the bases.
The ends of the spring clips are L shaped. They are made from bits of piano wire and very similar to those found in guitar amps.

Joe
joebog1 is offline  
Old 21st Feb 2020, 1:32 am   #116
Luxman1050
Octode
 
Luxman1050's Avatar
 
Join Date: Apr 2019
Location: Margate, Kent, UK.
Posts: 1,723
Default Re: Making an RF probe for VTVM

Cheers Joe
Luxman1050 is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 2:31 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.