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 Post subject: HP 412A DC VTVM rescued from scrapyard
PostPosted: Aug Thu 22, 2019 8:33 pm 
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Location: Urbana, Illinois
Here are the results of inspecting the HP 412A DC VTVM I picked up 25 years ago from a local junkyard/recycling center.

I originally grabbed this meter because it looked cool and it was free. A local scrapyard processed huge amounts of discarded electronic gear and computers from a nearby University. This scrapyard had a giant warehouse packed with items similar to this HP VTVM. I never expected to restore or use this HP 412A. All of its cables had been chopped off, both the power cable and the permanently attached factory probes:
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I didn't even take a look inside the HP 412A case until about 3 weeks ago. At that time I was posting about another analog multimeter recently added to my workshop (a Triplett 603 FET multimeter). See this URL for my Triplett 603 FET multimeter ARF thread: https://antiqueradios.com/forums/viewtopic.php?f=8&t=363234

We were discussing "DC probes with internal resistors" on that other thread. It turns out the many older electronic DC multimeters, both VTVM and FET, have a resistor located at the very tip of their DC probe. These DC probe resistors cover a rather wide range (100K up to more than 20 megohms). Our discussion concluded with a statement that "one must identify the proper probe resistor for their particular meter if they wish to get accurate readings from it."

More than one person in that other thread mentioned the HP 412A. I responded by writing "I have an HP 412A."

So that got me interested in checking it out!
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HP_412A_no-probes_02C.jpg
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But my poor HP 412A had no probes at all.

It hadn't been powered up for 20+ years either.

Fortunately the HP manual has been scanned and is readily available.

So I decided to install a new 3-wire power cable and then bench check my HP 412A. It powered up nicely with my Variac and dim bulb tester. I wasn't expecting that.

2 of the 4 incandescent lamps that are part of the "opto-mechanical chopper" lit up. And the other 2 lamps lit up after I cleaned their pins and the spring clips that hold them. The clock motor that drives the opto-mechanical chopper was rotating. That was a good sign!

After gradually bringing up the AC power, all of the DC voltages settled into specification, matching the values on the original HP schematic.

Other than needing to clean the lamp pins and the lever switch contacts (Volts, mA, Ohms, and +/-), there didn't seem to be any urgently needed repairs.

Next I checked the uF and ESR of the electrolytic capacitors. All good. And the electrolytic capacitors weren't getting warm to the touch.
I checked the tubular capacitors for DC leakage and out of spec uF. Also all good.
I tested the tubes in my B&K 700 tube tester. All read close to 100%.
I didn't find any damaged or missing components (other than the missing external cables).
The range switch looked fine. I didn't touch it at all. The HP manual says "be very cautious when working on the range switch." Fortunately my range switch didn't need any work.

So at this point I had a functioning HP 412A but no probes for it.

After more studying of the HP 412A manual, I found that the the value of the original DC probe resistor is 1 megohm.

While restoring my other electronic multimeters (2 different Triplett FET multimeter models, and a B&K FET multimeter), I obtained several original Triplett factory probes. These Triplett probes have a switch on them for AC or DC. In the DC position the 1 megohm resistor is switched in. In the AC position the resistor is shorted out. Actually the original Triplett probe resistor was a slightly different value (See my Triplett 603 thread for details). In short, I discovered that the Triplett probe resistor could be changed to 1 megohm, and with only a slight modification and a small tweak of the calibration inside the meter itself, then the Triplett 603 was perfectly accurate with the new 1 megohm probe resistor.

So, since I now have several working DC probes with 1 megohm series resistors in them, I decided to temporarily rig up the HP 412A with a pig-tailed female RCA jack so that I could plug in a Triplett probe that has the 1 megohm resistor in it.

After further study of the HP manual, I discovered that the common (-) probe is simply a shielded coax. There is one critically important detail for wiring this “common” probe cable: Its center conductor and shield are connected together only at the very END of the cable for the common probe, where it attaches to the alligator clip. This connection allows very small resistances to be measured accurately because it implements the “4-wire” technique for resistance measurement. So I constructed one of those.

The probe for DC mA and ohms is a bit more complicated. The factory mA/ohms probe used "triaxial" cable, which I don't have. But I did have some stereo headphone cables with 2 wires inside covered by a shield. So I used that and it works. The 2 internal wires are connected together only at the very end of the mA/ohms probe cable. The shield floats at the alligator clip end. As with the “common” probe, this probe is also wired to enable “4-wire” ohms measurement. This is why a cable with two wires and a shield is required for the “ohms” probe.

One might ask: “Why should the probe cables for a DC multimeter be shielded?”

My thinking is that shielding reduces noise and RF interference that could interfere with accurate measurements on the more sensitive ranges. The HP 412A offers a 1mV DC full-scale range. It wouldn’t take very much stray RF to interfere with that.

Here is a photo of my HP 412A with my improvised cables and probes:
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HP_412A_with-probes_01C.jpg
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The "Volts" cable has the RCA jack with Triplett probe plugged into it. The "common" cable has the black alligator clip. "mA/ohms" cable has the red alligator clip.

And... It works!

I only had to make minor adjustments of the "BIAS ADJ" and "ZERO ADJ potentiometers," which I did by following the instructions in the HP manual. The resulting "zero" indication looks like this:
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HP_412A_zero_01C.jpg
HP_412A_zero_01C.jpg [ 108.52 KiB | Viewed 774 times ]

I have a Geller labs 10V DC reference voltage module which reads at 10.0006V on several of my 5.5 digit DVMs, and it reads 10.00300V on my HP 3456A (6.5 digits). This is a 300 ppm error (in percent this is .03%). At some point I need to find somebody with a HP3458A to test my Geller labs reference module. But I think this reference is good enough when checking/calibrating analog meters.

The HP 412A is specified to be within +/- of one small graduation on the 0-1 scale. There are exactly 100 of these marks on the 0-1 scale (not counting the "0" mark at the left end of the scale)

So, here is my HP 412A, on its 10V scale, receiving 10V from my Geller 10V reference:
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HP_412A_10V-input_01C.jpg
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There's a bit of parallax in the above photo, taken from my smartphone while holding it in my hands. But, as you can see, it's within 1 small division on the 0-1 scale, which means it is "within calibration."

Next test: What does 10V read when the HP 412A is on the 30V scale:
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HP_412A_10Vin-30Vscale_01C.jpg
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Not bad at all!

Next range: 100V. Result:
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HP_412A_10Vin-100Vscale_01C.jpg
HP_412A_10Vin-100Vscale_01C.jpg [ 119.32 KiB | Viewed 774 times ]
Still within spec.
Continuing on, here is 10V applied to the 300V range:
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HP_412A_10Vin-300Vscale_01C.jpg
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Again surprisingly accurate.
The last test is applying 10V while on the 1000V scale. This should read exactly one "minor" division up from zero:
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HP_412A_10Vin-1000Vscale_01C.jpg
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And... It does!

What do you think of that?

-EB

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Last edited by electricboyo on Sep Sun 22, 2019 3:38 pm, edited 2 times in total.

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 Post subject: Re: HP-412A DC VTVM rescued from scrapyard
PostPosted: Aug Thu 22, 2019 9:26 pm 
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Good testing but what about Ohms and milliamperes?

I have one of these that I made out of three broken ones and I like it very much. The fact that the Common lead is floating from ground makes it useful for things like measuring bias on audio amplifiers. And the very sensitive current range is nice.

But the Ohms function is especially great, since it's almost a 4-wire instrument, with the wires connected at the probes, so they can provide a very low Ohms scale. With 1 Ohm center scale you can measure low resistance units and even track down shorts on a PC board. It will give you a usable reading down to maybe 0.02 Ohms, which no other common meter can do. I suggest that you make sure your Ohms test clips have solidly soldered connections so that your readings will be useful. A good test is to short the leads and see what reading you get while wiggling the wires going to the clips. You might even see a difference when shorting the leads in different directions, as one side of the clip is connected via an eyelet and the other via solder.


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 Post subject: Re: HP-412A DC VTVM rescued from scrapyard
PostPosted: Aug Thu 22, 2019 10:38 pm 
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My next post will explore the “Ohms” ranges of the HP 412A. They are functional, and very useful, both for very low Ohms values ( < 1 Ohm) and very large ( > 100 megohms ).

-EB

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 Post subject: Re: HP-412A DC VTVM rescued from scrapyard
PostPosted: Aug Fri 23, 2019 3:19 am 
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There you go. A real good meter as compared to the service grade units. Glad you saved it. Was there anything else that you were able to save? Stuff like that, usable test equipment, not carcasses and useless bits. The only advantage to my service grade jobs is they are notably smaller. A nice vintage lab grade instrument like that is a joy. Best high quality stuff around at the time.

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 Post subject: Re: HP-412A DC VTVM rescued from scrapyard
PostPosted: Aug Fri 23, 2019 4:18 am 
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With current range down to 1 microampere full scale you can use it to measure semiconductor leakage.

I would not use it for capacitor leakage with high voltage for fear of damaging the meter in case of short.

But it's so cool to just clip the Common lead on to anywhere in a circuit without fear of the usual problems doing such with ordinary meters.

One of my parts units is a rack mount version which has a different arrangement for the light pipes. The other one has broken light pipes so I can't really create a second working unit. I don't think I have the vacuum tubes either. But I have a couple of spare meters and power transformers and some knobs.


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 Post subject: Re: HP-412A DC VTVM rescued from scrapyard
PostPosted: Aug Fri 23, 2019 8:13 pm 
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I'm glad to see your 412A brought back to life.

The cretins that cut off or otherwise separate probes from their companion test instrument should be condemned to a lifetime of using rusty nails as their VOM probes.

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 Post subject: Re: HP-412A DC VTVM rescued from scrapyard
PostPosted: Aug Fri 23, 2019 11:30 pm 
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[quote=

"One of my parts units is a rack mount version which has a different arrangement for the light pipes. The other one has broken light pipes so I can't really create a second working unit. I don't think I have the vacuum tubes either. But I have a couple of spare meters and power transformers and some knobs".[/quote]

I repaired a chopper with a broken plastic rod on my h/p 412A years ago. I went to a plastic supply house and bought a few rods , of the same diameter. I cut one rod to slightly over length. I polished the ends with a variety of emery cloths, etc., with the rod chucked in my drill press. The final polish was with a pool of Brasso on a rubber pad.

It worked with no problem. I wonder if those pesky lamps (#12) could be replaced with some LED's??

Charlie


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 Post subject: Re: HP-412A DC VTVM rescued from scrapyard
PostPosted: Aug Sat 24, 2019 12:13 am 
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What are the "light pipes" that must be fit to the meter? Are they something that could be replaced with fiber optics cable to allow the same task without having to fit inflexible plastic pipes?

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 Post subject: Re: HP-412A DC VTVM rescued from scrapyard
PostPosted: Aug Sat 24, 2019 3:13 am 
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Joined: Feb Sun 17, 2008 11:36 pm
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Two of the plastic "light pipes" or rods are about 4-1/4" long. They transmit light from two associated lamps to two photo conductors. I don't remember if there are two short rods for the other pair, or, if the light shines directly on the photo conductors.

I believe fiber optic cables have to be processed (polished) at the ends. Making replacement rods isn't that hard, or, I was lucky. I did this several decades ago, so some of the details may have been forgotten.

A lens maker would have all of the stuff needed for polishing the rod's ends, probably better than Brasso.

Alignment between light source, motor driven chopper disc and photo conductors is important. I don't think fiber optic cable would make anything any easier. Making and replacing the rods isn't that big of a deal.

Pray that the PHOTO-CONDUCTORS and the chopper motor are ok.

Charlie


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 Post subject: Re: HP-412A DC VTVM rescued from scrapyard
PostPosted: Sep Fri 06, 2019 6:18 pm 
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Thank you for posting. I'm watching this with interest; I have one of these units and did cursory testing, and so far so good. I bought some precision resistors to test out the ranges, and haven't gotten to it yet.

Looking forward to continued posts!

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 Post subject: Re: HP-412A DC VTVM rescued from scrapyard
PostPosted: Sep Sat 07, 2019 2:08 am 
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Super Cool!



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 Post subject: Re: HP-412A DC VTVM rescued from scrapyard
PostPosted: Sep Sat 07, 2019 2:17 am 
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This model is widely underappreciated. The first time I saw one I had to put it on my want list, but it was many years before I finally got one. It is accurate and does everything it is supposed to do.

The light pipes, as was said, aren't that hard to repair. I don't think careful polishing is really needed. The #12 lamps last a long time but their main problem is flaky contact with the very cheap sockets. I have given thought to soldering the lamps in, if the leads will take solder, since I have some spare sockets in case of problems. But it pays to clean and tighten the socket pins.

Other than that, it seems to require no maintenance. I readjusted the Ohms infinity setting once, but I don't know if it drifted or if I was just too casual in adjusting it previously.

The manual mentions an unmarked switch position for adjustment. HP likes to do things like that; there is an unmarked switch position on my 5328A counter too.

I have a variac clone that has a voltage scale permanently marked. But the scale is crowded against something so that the markings between two voltage settings are missing. On closer inspection, it seems that they knew what they were doing, as the winding has no voltage change over that part of the range.

Little nuances that nobody notices.


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 Post subject: Re: HP-412A DC VTVM rescued from scrapyard
PostPosted: Sep Sat 07, 2019 6:19 pm 
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bob91343 wrote:
This model is widely underappreciated. The first time I saw one I had to put it on my want list, but it was many years before I finally got one. It is accurate and does everything it is supposed to do.
Other than that, it seems to require no maintenance. I readjusted the Ohms infinity setting once, but I don't know if it drifted or if I was just too casual in adjusting it previously.
I was skeptical about this multimeter until I actually took a couple of hours to clean it up and check it out. Now, after using it heavily on my bench for several weeks, I am really impressed with it. It performs perfectly in every way!

There is nothing wrong with old technology when it works this well!

-EB

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