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Author Topic: Question on probe for VTVM  (Read 20062 times)
K1ZJH
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Posts: 974




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« Reply #15 on: March 08, 2012, 07:01:04 AM »

My limited experience with the RF probe which consists of a capacitor, resistor and diode were used for low level RF in receivers and transmitters.  They were used mostly because the person troubleshooting a piece of gear didn't have a scope. 

I've seen them used only with high impedance meters, like 10M and up.

If the probe had a switchable 1M resistor in it, the probe was used to measure voltages above the highest scale on the meter.  Switch in the resistor and you extend the voltage measuring range.  You also decrease the loading of the circuit being tested.

Yes, there is a lot to learn .... when it comes to test gear.  This is why it's best to stay with the correct probes.  Make changes to a probe and you now have a piece of test gear that may give unreliable measurements.

One is a DC probe, the other is a RF detector probe which converts a RF signal to a DC level.

The purpose of the resistor is to isolate the meter and probe cable from the circuit being measured.
If you wish to measure grid 1 voltage on an oscillator, which would be a minus voltage due to self biasing,
you would use a DC probe with the built in resistor to prevent loading the circuit  and perhaps even killing
the oscillator.  Going between 11 meg and 10 meg input resistance wouldn't be very effective for setting
a higher voltage range,  since we're only talking roughly ten percent change on the input resistance.

There were a few "mods" posted
that showed how to add the one meg resistor on existing Heathkit range switches, but these mods
totally defeated the reason why resistor is at the probe tip. Note that in AC the probe does not use
or need the resistor.

Pete k1zjh
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WB4SNU
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Posts: 78




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« Reply #16 on: March 08, 2012, 08:06:22 AM »

I want to thank all that has replied to my question. Most of the time when I don't understand something, I have to ask about it. I have learned a lot from this and learning is what it is all about for me. I hope someday there might be something I can help with and reply to all of you.
Now, if someone can download more knowledge to my brain, I would be the first in line.

Thanks

Richard
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Digital and CW spoken here.
KB1WSY
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Posts: 774




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« Reply #17 on: March 19, 2012, 07:01:35 PM »

>>The VTVM is an Eico 232. I went through it and replaced the paper caps with polypropylene caps and so far it works good.<<

Coincidentally, I also just picked up an Eico 232 for $12 but it was only partially working (no response on the resistance scales). When I opened it up it became clear what the problem was: the 1.5V EverReady flashlight battery which is used for the resistance current was dead. Not only dead, but by all appearances it was the same age as the meter! (The date stamp on the meter says June 19, 1962.) The battery had leaked all over the chassis, so there were dark orange rust stains all over the place. Somehow, the leak didn't touch any of the actual components, just the chassis.

So this forced me to take the meter completely apart, removing the major components but leaving the subassemblies (especially the switches and their resistors) alone. I then treated all the rusted metal parts with anti-rust chemicals and sanded off the more stubborn corrosion. All the metal pieces now look "as new." Phew.

Now that the thing is all in pieces, I am taking the opportunity to do a full restoration. This is quite easy because this meter was also sold as a kit, and the BAMA archive has the full construction manual. So, I have a very detailed resource to rely on when rebuilding the disassembled meter. Dear readers, what do you think is wise to do? I am thinking of the following:

(1) Replace all five paper caps, as they are literally dripping off onto my fingers when I touch them.

(2) Make a Uniprobe (or two probes, one for AC and one for DC). The meter came without usable probes.

(3) Replace the "solder drop" probe socket with a BNC.

(4) Replace the AC power cord, which is in fairly poor condition. Question: when installing the modern three-prong cord, should I connect the AC ground wire to the EICO chassis? I am almost sure that the answer to that is "absolutely not" since the EICO ground is also the negative connection for the probes. On the other hand if I leave the chassis ungrounded, I will have to remember not to touch the meter case when measuring high voltages, right? Keep one hand on the insulated probe, and the other hand behind my back?

(5) Replace the power supply rectifier with a modern silicon diode rectifier? I am not sure whether this is needed. The existing rectifier is a black plastic casing about 1/4" square with the words "Model 50 Rectifier 2A" on it. The modern rectifiers cost almost nothing, so perhaps it is worth changing the old one as a precaution?

I wasn't sure whether to post this here, or on the Boat Anchor forum but then I noticed this very informative thread about VTVM probes so I think it belongs right here.

73 DE Martin, KB1WSY
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G3RZP
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Posts: 4565




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« Reply #18 on: March 20, 2012, 03:52:57 AM »

Grounding the case to the AC line ground is desirable from a safety viewpoint. You could get ground loops though through the ground return of the probe leads. I'd be inclined to ground teh case, and for the few occasions that ground loops cause trouble, use an isolation transformer and so effectively lift the ground. But watch the leakage through line bypass caps - they should be restricted to less than 0.005 microfarad. They shpould also be Class Y.
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KB1WSY
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Posts: 774




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« Reply #19 on: March 23, 2012, 05:01:56 AM »

Trying to open the "solder dot" connector that came with the broken probes on my Eico VTVM, so I can install a Uniprobe. I believe it's a Switchcraft 2501F, which connects to many voltmeters and signal generators from the 1950s, and was also used on vintage ham microphone leads.

There is a central part of the connector, and a rotating, threaded sleeve around it. Probably the sleeve needs to be removed before I can open up the connector but it's not clear how to do this. Tried to get the info on the Web but no dice.

TNX ES 73 DE Martin, KB1WSY
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N2EY
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Posts: 3879




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« Reply #20 on: March 23, 2012, 05:58:38 AM »

when installing the modern three-prong cord, should I connect the AC ground wire to the EICO chassis? I am almost sure that the answer to that is "absolutely not" since the EICO ground is also the negative connection for the probes. On the other hand if I leave the chassis ungrounded, I will have to remember not to touch the meter case when measuring high voltages, right? Keep one hand on the insulated probe, and the other hand behind my back?

(5) Replace the power supply rectifier with a modern silicon diode rectifier? I am not sure whether this is needed. The existing rectifier is a black plastic casing about 1/4" square with the words "Model 50 Rectifier 2A" on it. The modern rectifiers cost almost nothing, so perhaps it is worth changing the old one as a precaution?

Ground the case. VTVM is not meant to be used with the common lead above ground; that's what VOMs are for.

The rectifier may or may not be good. It's probably selenium. If you replace it, add a series resistor, because seleniums had higher voltage drop and you'll wind up with higher-than-normal B+. Series resistor value is determined empirically.

Also consider eliminating the ohmmeter battery. What you do is to rectify and filter some of the heater voltage and use two silicon diodes in series as a zener (each has forward drop of 0.7 volts). If you do it right it can be totally reversible. Only works if the battery and heater circuits have one side grounded.

Besides the obvious appeal of vintage tools for vintage jobs, there are many things that an analog meter does better than a digital one.

73 de Jim, N2EY
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KB1WSY
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Posts: 774




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« Reply #21 on: March 23, 2012, 06:36:33 AM »

Quote
... consider eliminating the ohmmeter battery. What you do is to rectify and filter some of the heater voltage and use two silicon diodes in series as a zener (each has forward drop of 0.7 volts). If you do it right it can be totally reversible. Only works if the battery and heater circuits have one side grounded.

It's a 1.5V battery, but 6.3 minus (.7x2) is still 4.9 or have I missed something? Anyway I worked quite hard to restore the existing highly corroded battery holder and it's fine now. But yes, one side goes to ground, so eliminating the battery is possible and as you said, the modification would be totally reversible.

By coincidence, another piece of test equipment I got hold of is also Eico (the Model 710 grid dip meter) and I may even complete the suite by adding an Eico 315 RF signal generator. The practical reason for choosing the latter is that unlike many other vintage models, it goes down to 75 kHz which takes it into the range of the BC-453 IF.

The three units even look the same, with the plain metal front panel/dial with red and black lettering. The documentation for the Eico range is excellent because they were also available as kits, making them easy to fix up even if they are in battered condition.

73 DE Martin, KB1WSY
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W9GB
Member

Posts: 2623




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« Reply #22 on: March 24, 2012, 04:51:48 PM »

Quote from: kb1wsy
Anyway I worked quite hard to restore the existing highly corroded battery holder and it's fine now.
But yes, one side goes to ground, so eliminating the battery is possible and as you said, the modification would be totally reversible.
Martin -

Many of these VTVM (Eico, RCA, Heathkit) used Keystone Electronics parts for the OEM battery holders.  
Today, Keystone still makes many of the battery holders or updated verisons.
http://www.keyelco.com/

Available from your normal electronic distributors.

Keystone Electronics Corp.
31-07 20th Road
Astoria, NY 11105
Phone: 718-956-8900
Toll-free: 800-221-5510
FAX: 718-956-9040
e-Mail: kec@keyelco.com
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KB1WSY
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Posts: 774




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« Reply #23 on: March 24, 2012, 06:58:46 PM »

Quote
Many of these VTVM (Eico, RCA, Heathkit) used Keystone Electronics parts for the OEM battery holders. 
Today, Keystone still makes many of the battery holders or updated versions.
http://www.keyelco.com/

Thank you very much, I downloaded their full catalogue from the website you provided. It is an amazing resource! Actually, after half an hour of work with de-rusting sprays, emery paper and a fine-grained file, I restored the Eico's original battery holder to its original condition. The only component that is irretrievably corroded is the phenolic washer that surrounds the screw that the "plus" battery end rests on. The battery is above the chassis in a vertical orientation, "upside down" (positive end down) and is pressed onto this screw by the spring clip. The washer insulates the screw as it goes through the chassis and connects to the wiring underneath. I believe nowadays this washer component would be called a "feed-thru terminal" or something like that. There are plenty of "feed-thru" items in the Keystone catalogue but none of them quite fit the bill.

We have a great resource here in the Boston area, a store called "You Do It Electronics" that I finally discovered only a few weeks ago even though it is only a 12-minute drive from my home. I will be checking there to see if I can replace that phenolic washer with a plastic or Teflon version. It's not exactly a washer, more like a washer with a short tube leading into it (with the tube portion going through the chassis), if you see what I mean. It doesn't have to be exactly the right part or exactly the right size, I am sure I can improvise. If I cannot find this, it's no big deal, I can replace the entire battery holder, either with a simple D-cell plastic version from You-Do-It or a more classic one from Keystone.

While I am "here" I wonder whether anyone has advice on my earlier question concerning the "solder point" (Switchcraft) connectors? Or maybe it's too much bother and I should just swap it for a newer BNC, which was my original plan anyway.

Also while I am "here" does anyone know a source for what used to be called "miniature panel meters." These came in a standard size of about 1.7" square and usually had a clear plastic face. Asking because I have two projects that call for these, one of which cannot use a larger substitute because it's a restoration job and a larger meter wouldn't fit. I have found a handful of NOS ones available but none of them were quite right and it is very hit and miss. Apparently, the flood of modern Asian analogue panel meters all start at sizes that are larger than 2 inches square.

Finally, I've noticed that there is no provision to upload graphics or photos to eham forums. If I am right about this, I presume it is deliberate. That is a pity because many words could be replaced with pictures in cases like these (the feed-thru washer, the "solder dot" connector that I cannot pry open, the miniature meter....).

TU ES 73 DE Martin, KB1WSY
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VK2TIL
Member

Posts: 327




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« Reply #24 on: March 24, 2012, 08:35:22 PM »

The "solder-dot" connector is usually a "Motorola" type (Amphenol 80-75-MC1F) on vintage instruments.

It is much the same as a "UHF" connector; the outer thread is 5/8" - 24 (UNEF).

I think the simplest mod is to get a BNC/UHF adaptor and shorten the centre conductor to mate with the socket; a blob of solder on the shortened conductor helps.

http://imageshack.us/photo/my-images/33/uhfbncshortadaptor1.jpg/

BNCs are the standard test-equipment connector and are easier to change than screw-on types.

I hadn't seen the Switchcraft designation for these so I consulted their website;

http://www.switchcraft.com/productsummary.aspx?Parent=810

These are an odd thread; 5/8" - 27 according to the site.  I have no experience with EICO but my experience with other US & JA makes of this era is that the thread is 5/8" UNEF.

Photos?

The easiest way is to use a photo host site; I used to use ImageShack but now use TinyURL since ImageShack wanted me to sign-up.

It's simple once you get the hang of it; just follow the prompts and, at the end, copy & paste the URL into your forum reply.
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W9GB
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Posts: 2623




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« Reply #25 on: March 25, 2012, 10:54:50 AM »

Quote from: KB1WSY
The only component that is irretrievably corroded is the phenolic washer that surrounds the screw that the "plus" battery end rests on.
The washer insulates the screw as it goes through the chassis and connects to the wiring underneath.
I believe nowadays this washer component would be called a "feed-thru terminal" or something like that.
There are plenty of "feed-thru" items in the Keystone catalogue but none of them quite fit the bill.

That part is called SHOULDER Washer, you will ALSO find this part in many well-stocked hardware stores (industrial Ace, True-Value, etc.)
The Electroncis distributors will have this part in their catalogs as well as McMaster-Carr.
Shoulder washers are used extensively with screws passing through heat-sinks (Solid-state amplifiers); Vibroplex Semi-Automatic Bugs; AND
insulating driven elements for outdoor VHF/UHF yagi antennas !

Keystone product page
http://www.keyelco.com/products/prod23.asp?SubCategoryID=43

Aavid Thermalloy (with Motorola part number X-ref) -- DigiKey catalog web page
http://parts.digikey.com/1/parts-datasheet/insulating-shoulder-washer-7721-series-aavid-thermalloy-datasheet

Custom Thermoelectric -- Bottom of this page
http://www.customthermoelectric.com/accessories.html
« Last Edit: March 25, 2012, 11:05:11 AM by W9GB » Logged
KB1WSY
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Posts: 774




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« Reply #26 on: March 26, 2012, 04:59:14 AM »

Quote
That part is called SHOULDER Washer, you will ALSO find this part in many well-stocked hardware stores (industrial Ace, True-Value, etc.)

Thank you very much Greg/W9GB, that is exactly what I needed to know. So often problems boil down to, "what is that thing called???"

I'm excited because I now have a complete "beginner's test equipment suite" from the 1960s: Eico 232 VTVM, Eico 710 Grid-Dip Oscillator, Eico 315 RF Signal Generator. All three were in rough working order (sort-of) but need some repair work to bring them up to new specs. The VTVM you already know about. The GDO is good overall and I have already used it for some "real work" but it has a constant calibration error i.e. it is always the same physical offset on the dial (about 4 millimeters) so I suspect a mechanical misalignment between the capacitor shaft and the dial mechanism rather than a component issue. Looks like the signal generator is fine apart from some minor corrosion to clean off, although I haven't done a full check yet.

And now, Dr. Faust has got in on the act and is whispering in my ear that I should sell my soul to the Digital Devil and also buy a cheap digital VOM and a digital frequency counter just so that I can calibrate my vintage equipment perfectly! Dr. Faust's assistant, "Silicon Sam," is adding that of course, I can get these items in kit form and "build them myself" and therefore this would be OK!  I am very tempted but worried that this may be the start of a slippery slope away from "hollow state" purity! The compromise: join the local Ham Club and use someone else's equipment for calibration!

73 DE Martin, KB1WSY
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VK2TIL
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Posts: 327




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« Reply #27 on: March 26, 2012, 07:34:31 PM »

Look at AADE for a neat, accurate and not-expensive frequency counter in kit or built form.

Otherwise, the standard way to check frequency in that era was a crystal calibrator; very simple to build.

Almost any crystal F would do; harmonics will be at 2F, 3F etc.  1 MHz saves some mental arithmetic.

You may even find one of the precision 100 kHz units made for this service.

A receiver is also a frequency counter.  Calibrate your GDO and signal generator against a receiver; in the olden days receivers often had a crystal calibrator that could, if desired, be precisely adjusted against WWV or a similar service.  Old signal generators like my Marconi TF144 had a built-in crystal calibrator.

Voltage standards are easily done with one of the myriad voltage reference ICs available today; usually a two component circuit.  A silver-oxide battery is a good reference too.

You have some nice old instruments.
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KB1WSY
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« Reply #28 on: March 26, 2012, 08:15:16 PM »

Quote
Otherwise, the standard way to check frequency in that era was a crystal calibrator; very simple to build.

Yes, I am going to be building a 1-tube crystal calibrator. I wish I had kept the one I built 40 years ago but never mind. It will have a male octal plug on the bottom so that it can be plugged into the accessory socket I am planning to build include in the receivers I am building. What I have now is a "chicken and egg" problem: I am right at the beginning of building my station, entirely homebrew, so I think it does probably make sense to go and find a helpful Elmer at the local ham club so I can take my vintage test gear to his/her shack and calibrate it against known standards!

The other interesting thing is that, if I understand the vintage literature correctly, in the 1950s and 1960s U.S. hams were not actually required by the FCC to know what exact frequency they were transmitting on! They were only required to have, somewhere in their shack, a frequency standard that would allow them to know that they were transmitting *inside the band.* This could be a crystal calibrator or, for instance, a BC-221 surplus signal generator because it was very accurate.

Quote
You may even find one of the precision 100 kHz units made for this service.

These vintage units are abundantly available for not very much money (Heathkit, Knight, Allied) but it's more fun to build one from scratch, plus it's a great project for a relative beginner like myself. I also know a source for 100kHz crystals, so there's no problem finding the components.

Quote
A receiver is also a frequency counter.

That's the "chicken and egg" problem again. I have only a single HF receiver, and it has no frequency readout (analogue or digital) of any kind. It's a Ramsey 20 meter direct-conversion ham receiver kit. I have aligned the coil in the RF section entirely "by ear" by first, finding the 20m band edges from the types of transmissions, and then adjusting the component mix (capacitors and potentiometers across the varactor) to give me the bandspread I want -- the bottom 100kHz where nearly all of the 20m CW is found. Then, "peaking" the other coil by ear. Finally I swapped out the original stock 10K pot with a 10-turn one and replaced the tiny tuning knob with a much larger one, both of which made a huge difference in ease of tuning. Because this calibration was entirely "by ear" it is not usable as a frequency standard, except that it was enough for me to test my Eico GDO and find out that in that part of the HF band it has a calibration error of about 800kHz. I also tested the GDO on a consumer AM radio and tuned it to beat with a local station transmitting on a known frequency of 850kHz and the GDO dial was about 50 kHz off. In both cases, that was about 4mm of dial length on the GDO, leading me to believe that there is a mechanical misalignment that needs to be corrected.

Understandably, I am starting my homebrew shack with test equipment ... but I would like it to run "true" so I do think it's time to find an Elmer with calibrated gear, even if s/he just lets me beat my oscillators with his/her transceiver and check my VTVM with his/her digital VOM! That should be more than good enough. Then, I will have a reliable and fun test suite as I get to the nitty-gritty of building my own receivers!! (This would be in the Boston area, in case any Elmer O'Samaritans are reading this.)

Thank you very much for your suggestions.

73 DE Martin, KB1WSY
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W9GB
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« Reply #29 on: March 27, 2012, 08:00:32 AM »

Plenty of schematics for the "plug-in" 100 kHz crystal calibrators used in Heathkit, Hallicrafters receivers of the 1950s-1970s.
The Tube was usually a 6AU6 with standard 100 kHz crystal and variable capacitor adjustment.

In that era, WWV was often used (5, 10, 15, 20 MHz) for "off-air" freqeuncy calibration. 

Today, in the 21st century we have GPS deciplined oscillators and realiable surplus 10 MHz standards all
over the place.  Much of this is due to the cellular/mobile telephone infrastructure build-out since the 1980s
and replacement (surplus) of that 1st and 2nd generation equipment ... for 3G and 4G/LTE systems.
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