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 Post subject: Heathkit VTVM V-6
PostPosted: Feb Thu 22, 2018 6:03 am 
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Location: Cottage Grove Oregon; 97424
Hello all I just scored a near mint condition Heathkit VTVM V-6 at St. Vinnys. looks really nice, clean, no dings or cracks just missing the probes but I think I might have a pair someplace....
Are these good usable units? Easy to service and use?


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 Post subject: Re: Heathkit VTVM V-6
PostPosted: Feb Thu 22, 2018 6:31 am 
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Greetings to mjohansson2 and the Forum:

I would say yes to your question. Most Heathkit VTVM's are easy to work on and quite straightforward in their operation. You probably will want to replace the electrolytic in the power supply and probably replace the selenium rectifier with a suitable silicon diode.

You may wish to replace the two paper caps in the unit, although finding the high voltage AC cap may be a bit problematical.

Cleaning the zero and ohms adjust pots with a good cleaner / lube will probably be necessary and perhaps the rotary switches as well. Be sure to wash out any oil based contact cleaner from the rotary switches afterwards so that they don't linger and become absorbed into the phenolic switch wafers.

In resurrecting a Heathkit IM-28 I found it necessary to lubricate the switch shaft bearings with light oil and the detent mechanisms with spray grease.

Calibration is only as good as the standards you use.

The only show-stoppers are the meter movement itself and the rotary switches, If these are in good shape and work well, anything else is replaceable.

Edit: Of course, replace the ohms battery. Someone here on ARF reported bad corrosion issues with alkaline batteries.... he reported that even when not visibly leaking, they give off a corrosive vapor which enters the meter movement and ruins it.

I have not experienced this myself, but it is better to be safe than sorry.... I am ordering carbon-zinc cells for the four VTVM's that I am currently refurbishing.

Regards,

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Jim T.
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 Post subject: Re: Heathkit VTVM V-6
PostPosted: Feb Thu 22, 2018 8:29 am 
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Location: Cottage Grove Oregon; 97424
It does have an old style Hercules battery in it but it's clean and no sign of leaking. There is a paper condenser that says Vibrator condenser on it, what on earth is that? It is also labeled as 2,000 VDC. Also looks like someone replaced some resistors as two are taped with black electrical tape together and doesn't not look like the rest of the work in the unit.


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 Post subject: Re: Heathkit VTVM V-6
PostPosted: Feb Thu 22, 2018 11:23 am 
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Greetings to mjohansson2 and the Forum:

You asked about the 2KV capacitor. Normally, the only application for such high voltage capacitors in consumer electronics way back when was as "buffer" capacitors. These were connected across the HV secondary of the power transformer in radios with vibrator power supplies designed to make B+ from a low voltage DC supply. Because of the abrupt switching transients associated with mechanical contacts opening, it was necessary to "snub" these transients to prevent them from damaging the transformer and also the vibrator feeding it.

A typical rating for a buffer capacitor was 1600 volts; some went higher. Since the primary application for such high voltage caps was for use in vibrator power supplies, no doubt some manufacturer capitalized on the name.

As far as resistors go, the voltage multiplier resistors are quite critical in value in order to achieve meter accuracy. It is possible that someone needed to replace a multiplier resistor and could not find the correct value in a single part, so he made one up. It is also possible that it is just sloppy workmanship.

You will have to determine where the resistor is in the circuit in order to tell if it is critical or not. There is no reason to ever burn up a multiplier resistor unless the meter is exposed to voltages far in excess of what it was designed for. If a multiplier resistor has been replaced, then you need to examine not only the replacement but the rest of the resistors in the string to ensure that they are of the correct value.

A pdf copy of the manual can be found here: https://www.vintage-radio.info/heathkit/

Regards,

Regards

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 Post subject: Re: Heathkit VTVM V-6
PostPosted: Feb Thu 22, 2018 7:27 pm 
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The 0.01/2-kV cap is in series with the AC input and serves to block DC voltages from affecting the voltage rectifier (6H6 tube). This allows you to measure the AC components of voltages which consist of both AC and DC mixed together.

Since the meter is designed to measure up to 1.5-kV on its top scale, Heath must've felt that standard 1.5-kV or 1.6-kV capacitors were too close for comfort and went with a 2-kV one. I would stick with the same plan for any replacement cap.

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 Post subject: Re: Heathkit VTVM V-6
PostPosted: Feb Fri 23, 2018 12:08 am 
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Hmmm....ok, I will look for a 2,000 DC cap I do have some .01 rated at 1,600 VDC but I can hold out until I find a 2,000.

Thanks!


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 Post subject: Re: Heathkit VTVM V-6
PostPosted: Feb Fri 23, 2018 6:53 am 
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I have never had to replace a selenium rectifier with a diode. There was a fella on Youtube that also stated that a resistor is require as well.

What kind of diode will I need? What is the process I have to use? voltage rating and what not.


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 Post subject: Re: Heathkit VTVM V-6
PostPosted: Feb Fri 23, 2018 12:12 pm 
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Greetings to mjohansson2 and the Forum:

Re your questions about the diode:

Any of the 1N400X series will do the job. 1N4005 is 400 PIV at 1 amp... will work fine. So will 1N4006 (600 PIV). Most common and what I buy in bulk (they are only a few cents apiece) are 1N4007.... 1000 PIV at 1 amp. This is overkill, but for 7 or 8 cents, who cares?

In looking at the later Heathkit VTVM schematics, they all use virtually the same circuit with the same tube and the same resistor values.... only the later ones use a silicon diode instead of a selenium rectifier. Therefore, you can just pop the silicon diode in there in place of the selenium rectifier without any other circuit changes.

One note, however.... the filter cap specified by Heathkit is only 150 volts. This is marginal. Since you are replacing this cap anyway, I would up the rating to at least 250 volts.

Regards,

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 Post subject: Re: Heathkit VTVM V-6
PostPosted: Feb Sat 24, 2018 2:10 am 
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Location: Cottage Grove Oregon; 97424
Thank you so much! I have stopped by our one and only electronics shop in Eugene, and they of course don't have anything cap wise rated at 2,000 VDC so....I do have some at 1,600 that may just have to do or I could order something off the net. They do have diodes.

What about the resistor for the diode?


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 Post subject: Re: Heathkit VTVM V-6
PostPosted: Feb Sat 24, 2018 2:33 am 
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The original Caps were usually 1600v so there are plenty of good options. 2000v is not required. These caps rarely go bad, for what reason I don't know.

Mike

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 Post subject: Re: Heathkit VTVM V-6
PostPosted: Feb Sat 24, 2018 2:52 am 
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Greetings to mjohansson2 and the Forum:

I think 1600 will be fine. In fact, I just came in from my freezing cold garage where I finished up refurbishment on my RCA WV-77C VTVM. It used only a 400 volt paper cap as the AC coupling cap. I replaced that one with a 630 volt film, but I think I will let the 1600 volt caps in my Heathkit meters alone. Mike is probably right.... and since I have added (or will add) 3-wire grounding cords to all the VTVM's I have, measuring AC is problematical anyway.

I am collecting meters for a lab for a class I hope to teach and I will tell the students that the VTVM's are for measuring DC volts and ohms... period.

My next experiment is to see if I can build a practical DC - AC/ohms probe with a switch in it to short the 1 meg resistor so it can do double duty. If it works out well, I may make a few extra and see if there is a market for them.

Have fun with your V-6.

Regards,

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Jim T.
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 Post subject: Re: Heathkit VTVM V-6
PostPosted: Feb Sat 24, 2018 3:07 am 
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Jim-
Good luck with your switchable probe design project. Many have tried before, some succeeded at least partially. The biggest problem is that the switch must withstand the maximum voltage to be measured, between its contacts and its case and actuator.

The PACO VTVM that I assembled as a kid, had leakage between the contacts and body of the slide switch - which put the aluminum switch plate at HV, while the strain relief spring at the back of the probe (only inches away) was at ground.

-Chuck


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 Post subject: Re: Heathkit VTVM V-6
PostPosted: Feb Sat 24, 2018 5:27 am 
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Greetings to Chuck and the Forum:

Thanks for your input. I am aware of the safety issues. It is my intention to carry the shield of the probe coaxial lead up into the probe and connect it to a ground lug underneath the switch frame. If the thing arcs over, it will not hurt the operator.

I think this must have been a common problem. I have a B&K VTVM with a factory probe. I had it hooked up to the plate supply on a Johnson Adventurer transmitter I was working on and as I brought the rig up on a variac, I heard a loud "snap". At first, I assumed that I had done something wrong with one of my new series pair of filter caps. However, the transmitter was fine after I disconnected the probe and now, the meter is very inaccurate on the higher DC volts scale. I will have to dig into it to see if it is just the probe or if something blew up inside the meter as well.

It is my intention to hit the probes that I build with my Biddle Megger and see if they will take 500 volts. If so, I will state clearly that that is a limit. If they can't handle the Biddle, I will have to de-rate from there.

Regards,

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Jim T.
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 Post subject: Re: Heathkit VTVM V-6
PostPosted: Feb Sat 24, 2018 11:14 pm 
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At the risk of sounding like a PITA, what I have done more than one time is to just make a DC probe and an AC probe. You just change probes. Now before we start barking at me, how often do you change from measuring AC and then DC and then AC? Not too often. Small, very small inconvenience and no bloody switch to go bad or become intermittent.

"Your Mileage May Vary..."

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 Post subject: Re: Heathkit VTVM V-6
PostPosted: Feb Sun 25, 2018 12:37 am 
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Jim-
Please understand that I am not warning of an internal short in the probe, but rather a breakdown or leakage between the probe tip circuit and any metallic part of the probe that can be touched by YOUR FINGER !
best regards,
Chuck K7MCG


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 Post subject: Re: Heathkit VTVM V-6
PostPosted: Feb Sun 25, 2018 8:40 am 
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Greetings to Don, Chuck and the Forum:

In answer to Don's comment, I don't use a VTVM for AC measurement because the case is common negative. For RF AC measurements, I use an HP 410B with a dedicated probe.

The complication comes in not from AC measurements, but from ohms measurements, which are done with a straight-through probe. If going back and forth from ohms to volts frequently, switching probes is a pain... hence the desire for a switch.

In answer to Chuck's comment, I did understand what you said. The slide switch slide handle is plastic, of sufficient dimensions to easily insulate against 500 volts. The problem comes in, as you point out, when the operator touches the metal switch frame or one of its mounting screws. An arc-over from the switch contacts to the switch frame could expose the user to high voltage.... except it can't, because the switch frame is grounded. It might be possible, if the short path is low enough resistance, to blow up whatever equipment you are probing, but you can't be shocked by the probe.

The caveat here, of course, is that the probe has to be properly connected to the meter, the meter has to be grounded (or have the negative lead connected to the common of the device under test, which is usually the same thing) and the probe cable has to be in good shape, i.e. no breaks in the shield of the probe cable.

If these conditions are satisfied, one cannot be shocked by the probe as I have designed it.

Regards,

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Jim T.
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 Post subject: Re: Heathkit VTVM V-6
PostPosted: Feb Sun 25, 2018 1:54 pm 
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The biggest barrier aside from no reasonable substitutes for probe bodies is lack of modern switches to handle the voltages. Separate probes or even "Siamese probes" are a great solution. Original Heathkit switchable probes are especially problematic, as I find the switch at the end of the probe rediculously difficult to manipulate and read. My probes and probe kits on eBay use "Siamese probes" in all switchable probe applications.

Mike

P.S. it was the late great Gary Clark who first pointed out to me that the 1600v caps rarely went bad, he too was unsure of why. He was a fount of knowledge when I first became interested in VTVMs and the source for my original ideas regarding Siamese probes

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 Post subject: Re: Heathkit VTVM V-6
PostPosted: Feb Sun 25, 2018 5:51 pm 
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The probe switch should be held in place with nylon screws and mounted in the probe body so that none of it's metal parts are exposed. The switch body should not be connected to anything. With a typical VTVM the maximum voltage across the switch contacts is 1/11 of the voltage at the probe tip.

I have two probes from Knight Kit 625 VTVM's. They were supplied fully assembled as described above and I believe are identical to the probes supplied with B&K 177's.

RRM


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 Post subject: Re: Heathkit VTVM V-6
PostPosted: Feb Mon 26, 2018 7:49 am 
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Greetings to Mike, RRM and the Forum:

Mike: I looked at your probe kits on eBay. The "siamese" configuration was not apparent from your ad.... at least not to me. This is the first I've heard of it. You might want to revise your ad copy to reflect this. I specifically wanted to avoid the nuisance of changing probe connections at the VTVM.... this is bad enough with the Heathkits which use a 1/4" phone jack.... it would be horrible with the RCA which uses a screw thread Amphenol microphone connector. Had I known this was the design, I might have just opted for your probes.

However, since I now have materials and parts in hand or on the way for my switchable probe project, I think I will continue.

One thing I might point out, however, is that over 90% of the voltage being measured with the DC probe appears at the probe tip of the unused AC probe. Although the 1 meg resistor in the DC probe should limit the current, someone touching the AC probe while the DC probe is in use could get quite a tingle.
It just goes to show that there are no ideal solutions; all engineering is trade-offs.

RRM: There are two approaches to safety as evidenced by the construction of power tools. One approach is the so-called "double-insulated" approach, which seems to be what you are advocating. This ensures that there is no conductive part of the apparatus that can come in contact with the user.

The other approach is to ground the exposed parts of the apparatus so that they cannot exhibit a potential that could be applied across the user. I am taking the latter approach.

While it might be possible to mill a narrow enough slot in the probe handle to prevent one's fingertip from extending into the slot far enough to approach the switch frame, there is enough difference in human finger geometry that it cannot be guaranteed. Much better to ground the switch frame... then the switch is surrounded by a ground plane... no energy can leak around this to bite the user.

In this day and age, there are limited applications for measuring high voltages. These probes are intended for student use in a class where the highest voltage present will be 12 volts. Most voltage measurement these days will be in the range of less than 50 volts.

The design that I envision will probably be adequate to three to five hundred volts (to be determined in test) and the probe will be placarded accordingly.

Unfortunately, as I pointed out in my earlier post, having problems with high voltage is not unique to modern attempts to re-create these probes. The factory probe on my B&K model 177 VTVM arced over at less than 1/2 of the full-scale reading of the meter on its highest range.

I was measuring the plate voltage on a Johnson Adventurer transmitter and bringing it up slowly on a Variac after a re-cap. I made this measurement using appropriate safety protocols, as I always do. The probe was connected to the B+ with a clip lead and lying on an insulated bench surface. One hand behind my back and the other on the Variac knob a considerable distance away. Therefore, when the probe went bang, I was startled but otherwise unhurt. Anyone who doesn't use similar precautions when measuring high voltages is going to be killed sooner or later, regardless of what sort of probe he is using.

I retired after 32 years working with broadcast transmitters and I'm still here.... because I observe precautions as detailed above. Betting one’s life on the insulating qualities of a VTVM probe is not something a prudent person would do, in my humble opinion.

Regards,

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Jim T.
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 Post subject: Re: Heathkit VTVM V-6
PostPosted: Feb Mon 26, 2018 7:08 pm 
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The slot in the factory-made probes I described is almost exactly the width of the switch tab and length of its travel. They essentially made the slot in the probe body match the one in the switch itself. This really isn't difficult. You'd have to work really hard to touch the edge of the switch opening.

Floating the switch body keeps it from having a potential between it and the switch contacts. It's the same principal that allows birds to sit on high tension lines. Grounding the switch body puts the entire voltage at the probe tip across the switch.

I don't really like any of the "uniprobes", mainly because the switches are almost never good enough to allow low-ohms measurements. They also make it easy to goof and wipe out some ohms range resistors. I bought two RCA VTVM's from a TV shop and both had this problem.

RRM


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