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 Post subject: HP 412A Probes and Cables
PostPosted: Jun Sat 15, 2019 8:57 pm 
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Joined: Dec Sat 01, 2012 6:30 pm
Posts: 44
I have a HP 412A VTVM and the original voltage probe is gone, someone simply attached an alligator clip to the lead. I'd like to find or build a suitable replacement. It seems the original simply had a 1M resistor in it. I see on ebay there are some probe shells, but they're more of heat kit quality, is there something better?

Also I'd like to replace the leads as well. I was thinking RG174 or something similar to oscilloscope probe leads for the voltage and the common, something shielded and flexible, I suspect the specs aren't too critical since it's carrying DC. The ohms is a double shielded cable - a normal single conductor with shield and then another shied around that, is there a suitable replacement?


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 Post subject: Re: HP 412A Probes and Cables
PostPosted: Jun Sat 15, 2019 9:11 pm 
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Joined: May Wed 18, 2011 2:40 am
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Location: Littleton, MA
monx wrote:
The ohms is a double shielded cable - a normal single conductor with shield and then another shied around that, is there a suitable replacement?

You might try the flexible triax cable sold by Kiss Electronics, normally for use in HP 410B and 410C AC probes:
http://kiss-electronics.com/products.htm

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Steve Byan http://www.byan-roper.org/steve/steve-at-play/


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 Post subject: Re: HP 412A Probes and Cables
PostPosted: Jun Sat 15, 2019 9:17 pm 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 210
Don't be too casual or try to reengineer the unit. The cables are not simply test leads.

There are three cables. The ground and resistance cables are coaxial because they only connect together at the probe clip end. That's how they get such good 4 wire Ohms measurement, with 1 Ohm center scale.

Get the manual and see the details. The manual is available online.

I have one of these meters. Actually, I have three of them but it took that many to make a working unit. I still am not satisfied with the DC probe, as it was damaged and doesn't clip that securely.

It's a very nice unit and I use it frequently. It's one of only about three devices I have that use tubes. Inside is a sophisticated optically coupled system that is motor driven, using light pipes and lamps. It is isolated, meaning there is no connection between the 'ground' lead and the meter case. There is a small circuit setup inside just for termination of the test leads.

My main complaint is that it only measures dc. But it does an excellent job of it, with about 1% maximum error. It also measures very small currents, and can test diodes for leakage.

Further, there is an unmarked position on the range switch; read the book for how to use that.


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 Post subject: Re: HP 412A Probes and Cables
PostPosted: Jun Tue 18, 2019 8:16 am 
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Joined: Nov Mon 02, 2009 7:01 am
Posts: 3471
Location: Lincoln City, OR
Greetings to the Forum:

If it is a mechanical chopper-stabilized instrument, don't bother. Get rid of it and find something more modern. I have had two of HP's chopper-stabilized meters... both died due to chopper problems and they cannot be repaired... no parts available. Everyone is entitled to one mistake... the mechanical chopper system was HP's. In fairness to them, they didn't anticipate their instruments lasting 40 years or more nor did they anticipate the company disintegrating and being unable to provide old-time spares. However, the system is a kludge.... best to sell it off while it still works.

Just my $.02 worth; your mileage may differ.

Regards,

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Jim T.
KB6GM


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 Post subject: Re: HP 412A Probes and Cables
PostPosted: Jun Wed 19, 2019 5:46 am 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 210
Jim, that feels a bit heavy handed, your comments. I have, as I mentioned, three of these and cobbled together a working one and still have a bunch of spare parts. I had to get into the chopper section and change out the light pipes. The pipes are different for the rack mounted version and one of my spare units is that configuration.

Another weak point is the range switch. They are made of some kind of odd plastic that disintegrates. One of my switches has fallen apart.

Having said all that, I now have a perfectly working unit and use it frequently enough to have developed an affection for it. It's accurate and has some very useful ranges. And it's isolated so I can be casual with how I connect the COM lead. With 1 Microampere and 1 Millivolt on the sensitive range, and 1 Ohm center scale for resistance, this baby gets used plenty for one-off measurements.


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 Post subject: Re: HP 412A Probes and Cables
PostPosted: Jun Sun 23, 2019 9:58 pm 
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Joined: Dec Sat 01, 2012 6:30 pm
Posts: 44
Thanks for the info. I'll probably end up using the existing ohms triax cable for now, it seems good enough. Someone had previously connected the leads incorrectly. The 4 wire resistance built right into the leads is a nice touch, I've not see it done this way before.

I'm aware of the mechanical chopper, but it's working fine. I have modern equipment, I wanted a vtvm that's nicer than the heathkit ones I currently have. A 555 timer and some LEDs could likely replicate it's function without too much trouble.

I'm going to use a Pomona 5432-2 probe for the voltage probe. This a do-it-yourself prob that seems to be of good quality and a good fit - matches the diameter of the lead well, I'll likely epoxy the lead in place to avoid stress on the resistor and its connection inside.

I've since replaced the electrolytic and paper capacitors. All of the capacitors were in good shape - low leakage (just testing with ohm meter and on value). Some capacitor's terminals were used to to support some resistors so I made a PCB to hold the modern replacements and the resistors they supported. No other modifications other than longer screws were needed.

I've completed calibration and it's working well with a couple minor issues and observations.

There's quit a bit of noise at the last ac coupled amplifier tube plate which is what the meter is reacting to. As a result the needle wiggles very slightly, about the width of the needle back and forth, it's very slight. This may be normal, especially with the cover off. It did initially jump around a lot, I noticed the waveform of the noise was somewhat regular, not random, then my cell phone rang and I knew what was, this thing is very sensitive to external noise.

At full scale while measuring 1V, it indicates 1V exactly on the 1V range (as calibrated), however, 1V shows about 0.95 or so on higher ranges, about a needle's width below the mark. A little accuracy is lost below full scale.

Finally the 1x resistance range does not go to infinity when open, about an 1/8" below, making all readings a little low. Suspect a resister in the divider has drifted. This is a bit disappointing since this does a 4 wire measurement and this is the range where it'll matter most. This will be tough to track down.

I also noticed that care was taken to not put any excessive stress on resistors and capacitors. Their leads are simply passed through terminal lugs straight and soldered, not wrapped around or mechanically tight. I used technique was used for replacement components.

Image of new PCB to support new capacitors.


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 Post subject: Re: HP 412A Probes and Cables
PostPosted: Jun Sun 23, 2019 11:29 pm 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 210
On the rear panel there is a pot to adjust infinity Ohms. It's at the bottom on the full scale side of the unit.

Make sure the mechanical zero of the meter is properly set.

My meter seems to meet original specifications. I think 1%.

Yes it's a great meter and certainly head and shoulders better than any Heathkit. The large meter with mirror scale looks good and works well. Since I put this together from three meters, I have spare meters, power transformers, and other things.

As for the instability of the pointer, I don't have that. Does it go away when you put it into the case?


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 Post subject: Re: HP 412A Probes and Cables
PostPosted: Jun Mon 24, 2019 1:22 am 
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Joined: Dec Sat 01, 2012 6:30 pm
Posts: 44
The mechanical zero was already set perfectly, so I didn’t need to touch that. The ohms adjustment was set to infinity at the 1k range, it stays throughout all of the other ranges except x1.

With the case back on the instability is still there, very minimal but still there. Has to be something in the amplifier section. Unfortunately the PCB that makes up most of it has fused itself to the mounts, the 4 screws come out, but the board won’t budge.


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 Post subject: Re: HP 412A Probes and Cables
PostPosted: Jun Mon 24, 2019 5:40 am 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 210
I don't remember enough about this unit to answer your problem but it probably pays to examine the schematic diagram to see where the supply for the x1 Ohms range originates. There may be a separate adjustment for that. After all, it's still a voltmeter so with nothing connected to the leads it should read the open circuit value, at full scale, unless the source has changed.

Some of your instability may indeed be due to corrosion. Try lifting some of the circuit from the board. It could be some sort of beat frequency related to the power line and the motor speed. One corroded joint and you can have annoying results.

Incidentally there is no freedom from error no matter how exalted you are. I have repaired unsoldered connections, from the factory, in gear made by Collins, Measurements, and Tektronix. And we thought those guys were untouchable. In their defense, the problems didn't show up for a few decades.


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 Post subject: Re: HP 412A Probes and Cables
PostPosted: Jun Tue 25, 2019 3:32 am 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 210
I took a closer look at my unit and yes there is some wavering of the pointer. It's too small to be of concern but it's there.

I also checked calibration and it's not entirely within specification. At this point I am not going to look into that but I am not going to trust the readings totally.


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 Post subject: Re: HP 412A Probes and Cables
PostPosted: Jun Tue 25, 2019 4:42 am 
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Joined: Dec Sat 01, 2012 6:30 pm
Posts: 44
I found the problem with the x1 resistance range. It outputs 10mV in this range and the meter simply measures it, it is also in a 10mV range. The service manual shows the equivalent circuit for each range, making it much easier to follow.

The 10mV was present and correct. So the related resistors and switches supplying that must be ok.

The voltage divider for voltage measurement in this range is made up of an 8.1M and 900k resistor. The 900k is used in many ranges, all that are ok, the 8.1M is only used in this this range (and the 0.1V) so it was suspected. Sure enough, it had drifted out of tolerance, around 8.25M.

Stringing together a bunch of resistors to get close to 8.1M to test fixed it. I’ll probably use a 4.7M 3.3M and a 100k in series to make the 8.1M since that doesn’t seem to be obtainable.

Its nice how the 1x mode goes all the way to 0 when shorting the leads due to the 4 wire measurement, I don’t have that on my digital meters.

Incidentally there is one pin on a tube socket that isn’t well soldered, at lease the solder didn’t wick up in the rivet on the board as the others, can’t see under it yet to check, continuity is good.

The needle movement is minimal, but I’d rather any movement come from what’s being measured, sounds normal.


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 Post subject: Re: HP 412A Probes and Cables
PostPosted: Jun Tue 25, 2019 5:29 am 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 210
Good hunting! I am glad you found the trouble.

The meter is great for low resistance, although I can do 4 wire Ohms with other instruments.

My 3456A has a math feature so that with a cheap thermistor I can measure minute temperature variations, down to one millidegree. While it's not accurate, the resolution allows me to track down minute temperature variations such as from current faults.

I also built a low range Ohmmeter with an LM317, a wall wart, and a cheap Harbor DMM. It can resolve 0.01 Ohm I think. I have other stuff to make those measurements so I don't use it.

Thermal effects can louse up a resistance measurement when testing at dc. My GR 1658 uses ac so it's not a problem. The 3456A has thermal compensation that works most of the time. A few millivolts of thermal emf can spoil a resistance measurement. If the test probes are made of various materials like copper and cadmium plate or steel or even more than one kind of copper you will get spurious readings unless all parts are at the same temperature.


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