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 Post subject: Triplett 603 FET-VOM
PostPosted: Aug Sun 11, 2019 8:18 pm 
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I recently obtained a Triplett 603 FET-VOM. It's really nice. The price was right. And it almost worked right out of the box.

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Many years ago I purchased an earlier model, the Triplett 601 Type 2. It served me well but it requires 10x AA batteries and they don't last very long. It also had issues with its range switch. I was able to fix that, but I like these old Triplett FET analog multimeters enough that I decided two would be better than one. The 603 is a newer model. It is said that a set of batteries will last at least a full year in the 603, even if left switched on most of the time. The 603 battery complement is different than the older 601. The 603 requires 2x 9V and 1x D. The 603 has 2 useful mA ranges (100mA and 1A) that aren't on the 601. Note: The 603 lacks 3 voltage ranges that are present on the 601: 100mV AC/DC, and 30mV and 10mV for AC only. But then, I still have my 601 for when I need those ranges.

When I found a clean 603 on ebay that had its original probe and "the right price," I jumped on it.

There was only one problem: The DC voltage readings were somewhat intermittent, and were sensitive to tapping on the face of the meter. I suspected a dirty switch contact, a dirty calibration pot, or a bad solder connection.

So I looked inside:
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Triplett_603_inside_01B.jpg
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This meter is much "tidier" on the inside compared to my older Triplett 601 Type 2. There are 2 PC boards and a whole lot less internal connecting wires. The calibration pots are neatly located in a line behind the pushbutton switch assembly. Within a few minutes of experimentation I was able to identify that the four pots calibrate the "0-10" DC scales, the "0-3" DC scales, “LP Ohms adjust” (for low power ohms button), and "coarse" zero ("fine" zero adjustment is on front panel).

Within a few more minutes I located the bad solder connection:
Attachment:
Triplett_603_range-switch_bad-solder_01B.jpg
Triplett_603_range-switch_bad-solder_01B.jpg [ 179.34 KiB | Viewed 394 times ]

Can you see it?

Follow the yellow wire with white stripe down to the range switch lug it connects with. There is a short bare wire coming up from the PC board to this same lug. It isn't soldered. From the looks of it, the Triplett factory might have failed to put enough solder on this particular lug. That said, a thorough visual inspection revealed that all other soldering was performed with excellent quality control.

After resoldering, this meter has been rock-solid for me.

-EB


Last edited by electricboyo on Aug Thu 15, 2019 2:27 am, edited 2 times in total.

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 Post subject: Re: Triplett 603 FET-VOM
PostPosted: Aug Sun 11, 2019 8:33 pm 
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FYI, here is some more info about the Triplett 603 FET-VOM that I wrote in a reply to another thread about FET-VOMs:

electricboyo wrote:
For decades I’ve owned a Triplett 601 type 2 analog FET VOM. I purchased it new in 1973.

The Triplett 601 type 2 is battery powered, using 10x standard alkaline AA cells. I must switch it off when I finish using it in order for the batteries to last a full year. Over the years I had problems with intermittent leakage on one wafer of its rotary range switch. When it failed it was impossible to zero the DC ranges. That rotary switch wafer was brown phenolic and it might have gotten a bit carbonized in one spot from an overvoltage event that happened many years ago. Eventually I was able to scrape and clean the phenolic wafer sufficiently to fix that problem permanently. There were also some faults with its pushbutton mode selector switches. But I finally got those issues taken care of too. It’s worked very well for the past several years.

Today I obtained a Triplett 603 FET VOM. It was an eBay purchase. It’s in good shape and the original factory probes came with it. It only needed a set of batteries and now it is working and appears to be within about 3% of accurate calibration. It uses 2x standard 9V batteries plus 1 D cell. The manufacturer’s literature claims the batteries will last a full year even if the meter is kept switched on all time. The internal design and construction is a lot more “tidy” than my 601 type 2. It also appears that they improved the range switch design, using blue plastic instead of brown phenolic for one of the switch wafers. I believe the 603 is a somewhat newer model, perhaps from the late 1970’s.

Both models, Triplett 601 and 603, are battery powered analog meters with DC and AC volts ranges, DC and AC current ranges, and Ohms ranges. The input resistance is 11 megohms for DC voltage ranges, and 10 megohms for AC volts. They can measure resistance up to about 100 megohms (compare to most DVMs which only measure up to 20 megohms). Both Triplett models have a “low power” Ohms feature that keeps the applied voltage below the turn-on threshold voltage of solid-state diodes and transistors.

An unusual feature of both Triplett models is their factory probe design: The + probe contains a 1 megohm series resistor. The probe also has a slide switch that shorts out the resistor for measuring AC volts, Ohms, and current. The 1 meg series resistor is used only for the DC voltage ranges.

For those of you with other brands/models of electronic multimeters:
Does your + probe have a switch on it?
If so, what is the DC resistance of the probe for both positions of its switch?

I want to learn more about meters that have the 1 megohm series resistor inside the probe. I’ve observed this feature to be extremely useful for vintage radio troubleshooting. In contrast, many digital meters, although they claim to have 10 megohm input resistance, tend to upset the performance of the radio when probing. Sometimes the volume will get much weaker or the sound will totally drop out when probing with a digital meter. But this doesn’t happen when using a meter that has the 1 megohm resistor inside the + probe.

-EB


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 Post subject: Re: Triplett 603 FET-VOM
PostPosted: Aug Sun 11, 2019 9:00 pm 
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About these special probes that have switchable 1 megohm series resistors for the DC volts ranges:

When I bought my Triplett 603 FET-VOM, it came with an original Triplett probe. The probe needed some work. At first it didn't work at all in the "DC volts" position of its slide switch.

Attachment:
Triplett_603_probe_01B.jpg
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These Triplett probes can be disassembled (with some effort). There is a tiny hex grub screw at the cable end. Removing it releases the smaller red tube at the cable end. Then the "insides" can be gently pulled out. There is a tiny phenolic tag board inside with eyelets which form the switch contacts. The 1 megohm series resistor is soldered to the tag board (more about that later). The probe needle at the business end unscrews from the probe body. It makes contact via a tiny coil spring which is soldered to the tag board. Therefore, the needle can be replaced without soldering. However, I haven't ever seen a replacement needle available anywhere. But I suppose they did exist when Triplett was still in operation?

When I inspected the tag board, the switch contacts seemed to be OK. But the series resistor measured about 30% too high in ohms, even though it is labeled as a 1% resistor. I temporarily reassembled the probe with the old resistor still in place. It began to work much better. I suspect the resistor has an intermittent "open circuit" failure, but when I disassembled/reassembled the probe, I wiggled the resistor leads enough to temporarily "fix" the intermittent.
Attachment:
Triplett_602-603_probe-resistor_02B.jpg
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The value of this resistor seems a bit odd. I don't understand why Triplett used 1.12 megohm instead of 1.0 megohm. My experience is that most VTVMs have a 1.0 megohm probe resistor. I decided to standardize on using 1.0 megohm probe resistors. I already had some on hand that are actually 0.1%. So I changed the resistor from 1.12 to 1.0. Only a very slight adjustment of the Triplett 603 DC calibration was needed to accomodate such a slight change in probe resistance.

I also found another Triplett probe on ebay that was said to work with the model 602 FET-VOM. So I purchased it.
Attachment:
Triplett_602-603_probe_01B.jpg
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This probe also contained a 1.12 megohm series resistor.
Attachment:
Triplett_602-603_probe-resistor_01B.jpg
Triplett_602-603_probe-resistor_01B.jpg [ 76.36 KiB | Viewed 386 times ]

This resistor tested good. And it worked. But because I want all of my "resistor probes" to be interchangeable, I replaced it with a 1.0 megohm resistor.

So, now I have a perfectly functional Triplett 603 FET-VOM along with 2 probes that both work with it. I'm happy!

-EB


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 Post subject: Re: Triplett 603 FET-VOM
PostPosted: Aug Sun 11, 2019 10:06 pm 
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The value chosen for the resistor is probably an attempt at making the input resistance of the meter a standard value, perhaps 10 Megohm or 100 Megohm, whatever the spec is. And may result in a slight error when changing ranges, with a 1 Megohm. But this is mostly nit picking.

My analog voltmeter of choice for dc is the HP 412A. For ac I have the HP 400D. I only use those for casual measurements; for more precision or wider bandwidth etc. I have other instruments.


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 Post subject: Re: Triplett 603 FET-VOM
PostPosted: Aug Sun 11, 2019 11:19 pm 
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A majority of "radio/tv service grade" VTVMs and their solid-state companions, such as my Triplett 603 FET-VOM, have an 11 megohm input resistance for their DC ranges. This includes the 1 megohm probe resistor, which is only switched in for the DC ranges. Their DC ranges are calibrated with the 1 megohm probe resistor in place. The input resistance of the meter itself (with probe resistor switched out) is usually 10 megohms.

Evidently the 1 megohm probe resistor is rarely a feature of vintage "lab grade" multimeters. Most of them have a straight "10 megohm" input resistance, and that's how they are calibrated.

The main reason for the 1 megohm series resistor at the probe tip is that it permits measuring DC voltages in tube/valve grid circuits that also have RF signals present. The 1 megohm resistor provides sufficient isolation to prevent detuning oscillators and other tuned circuits. This is a valuable feature for tube/valve radio/TV repair.

A majority of today's DVM multimeters have a 10 megohm input resistance. The input capacitance of most DVMs is far too large to permit probing tube/valve radio circuitry directly when RF signals are present. It might be feasible to use a rather small probe resistor (perhaps 100k ohms) which will cause only a -1% error in the meter reading. I've also thought of using a precision 90 megohm series resistor to create a "divide by 10" DC voltage probe for DVMs.

I have observed vintage multimeters that require probe resistors with values other than 1 megohm.

For example I have a B&K model 290 solid-state FET multimeter where the meter's internal input resistance is 15 megohms (rather than 10 megohms). B&K specifies a 100k ohm probe resistor for this meter. Although I don't own an original B&K factory-made DC voltage probe for this model 290 multimeter, I did find that its original calibration is more accurate with a 100k resistor in series with the + input. Without the 100k ohm series resistor it reads a bit on the high side.

I'm researching this subject of "DC probes with internal series resistors" and I'll probably start a thread about it soon.

-EB


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 Post subject: Re: Triplett 603 FET-VOM
PostPosted: Aug Mon 12, 2019 1:19 am 
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electricboyo wrote:

I'm researching this subject of "DC probes with internal series resistors" and I'll probably start a thread about it soon.

-EB


A Start:

https://archive.org/details/Probes_Brun ... +the+probe

https://archive.org/details/How_to_Use_ ... +the+probe

It's pretty much the same for all VOM/VTVMs - the series resistor in the probe is part of the input voltage divider and it's value is chosen for the specific meter's input. On some meters with active input, it may also be part of an impedance matching configuration. Most however, provide isolation of the meter from the voltages measured.

1.12M, 1 Meg, 9.09Meg, 9.79Meg and 22Meg are in some of my OEM DC or isolation probes - for Sencore, HP, Triplett, Keithley, and Ballentine products. My HV probes (RCA, two Flukes, and an B&K) have from 191 Megs to 1090 megohms series resistors in them, to establish the HV range of the meter.

There is no "standard value" of input resistor. You use what the instrument was designed for.

_________________
Brian
"Capacitor Cosmetologist since 1979"
USN Retired 1984-2006 (Avionics/Cal)


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 Post subject: Re: Triplett 603 FET-VOM
PostPosted: Aug Mon 12, 2019 1:46 am 
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Findm-Keepm wrote:
A Start:
https://archive.org/details/Probes_Brun ... +the+probe
https://archive.org/details/How_to_Use_ ... +the+probe

It's pretty much the same for all VOM/VTVMs - the series resistor in the probe is part of the input voltage divider and it's value is chosen for the specific meter's input. On some meters with active input, it may also be part of an impedance matching configuration. Most however, provide isolation of the meter from the voltages measured.

1.12M, 1 Meg, 9.09Meg, 9.79Meg and 22Meg are in some of my OEM DC or isolation probes - for Sencore, HP, Triplett, Keithley, and Ballentine products. My HV probes (RCA, two Flukes, and an B&K) have from 191 Megs to 1090 megohms series resistors in them, to establish the HV range of the meter.

There is no "standard value" of input resistor. You use what the instrument was designed for.
That book on probes is incredibly good. Thank you very much for posting those links. I will read the entire book from cover to cover.

I’m aware that the probe resistor is a fully functional part of the input voltage divider. What I hope to learn by comparing schematics and user guides is to understand why one model of DC voltmeter specifies 1.12 megohms for the probe resistor vs. 1.0 megohm for a different model. That’s not a very significant difference for an analog meter. It will be interesting to do the math on this.

I agree that, for most purposes, the original factory probe is the best choice. My decision to “standardize” probe resistors to 1.0 megohm applies only to the meters I own and use frequently in my own workshop.

-EB


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 Post subject: Re: Triplett 603 FET-VOM
PostPosted: Aug Mon 12, 2019 1:57 am 
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electricboyo wrote:

I agree that, for most purposes, the original factory probe is the best choice. My decision to “standardize” probe resistors to 1.0 megohm applies only to the meters I own and use frequently in my own workshop.

-EB


Not quite sure why you'd sacrifice accuracy that way, but to each his own, i guess.


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Just be sure to divulge that tidbit should you ever loan or sell any instrument you've modified - would save some grief.

_________________
Brian
"Capacitor Cosmetologist since 1979"
USN Retired 1984-2006 (Avionics/Cal)


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 Post subject: Re: Triplett 603 FET-VOM
PostPosted: Aug Mon 12, 2019 2:53 am 
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And don't forget the HP410B with input resistance of 122 megohms.


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 Post subject: Re: Triplett 603 FET-VOM
PostPosted: Aug Mon 12, 2019 12:38 pm 
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bob91343 wrote:
And don't forget the HP410B with input resistance of 122 megohms.


Another great example.

DC/AC Input impedance, yes, 122 Megohms. Resistor in the DC probe is 22 Megohms. The remaining 100 Megohms is internal, in the input resistor divider chain (R3 through R13),



Attachment:
410B.jpg
410B.jpg [ 180.02 KiB | Viewed 335 times ]


Edit: ....and not one that anyone would dare "standardize to 1 Megohm" :shock:

Attachment:
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Pct chg.jpg [ 64.07 KiB | Viewed 333 times ]

_________________
Brian
"Capacitor Cosmetologist since 1979"
USN Retired 1984-2006 (Avionics/Cal)


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 Post subject: Re: Triplett 603 FET-VOM
PostPosted: Aug Mon 12, 2019 2:59 pm 
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I should stop using that word “standardize” in regards to probe resistors.

What I want to achieve, in my own workshop, is to be able to grab a probe that will be accurate with two or three of the meters that I use everyday. I want to have several probes that can interchange between my meters without any loss of accuracy.

Last night my favorite 10:1 scope probe broke in the middle of some delicate RF measurements. Although scope probes are standardized and easy to calibrate, that incident spoiled my train of thought. So now I need to sort through my scope probe collection too!

As I work through this information about probe resistors for DC voltmeters I will post the math that describes the relationship between the probe resistor and the multimeters internal voltage divider.

Also, thanks very much for commenting!
There’s a great group of people here on ARF.

-EB


Last edited by electricboyo on Aug Mon 12, 2019 3:17 pm, edited 1 time in total.

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 Post subject: Re: Triplett 603 FET-VOM
PostPosted: Aug Mon 12, 2019 3:15 pm 
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bob91343 wrote:
My analog voltmeter of choice for dc is the HP 412A. For ac I have the HP 400D. I only use those for casual measurements; for more precision or wider bandwidth etc. I have other instruments.
It’s fascinating that you mentioned the legendary HP412A.

I have one.

More accurately, I have the carcass of an HP412A that was junked many years ago by a nearby University. I doubt that it is restorable. However, the analog meter movement is in good working condition and the range switch looks intact and undamaged.

How difficult is it to keep an HP412A in working condition?
That opto-mechanical chopper with 4 incandescent lamps seems a bit daunting.

This may sound heretical to some, but I was thinking of replacing the chopper, amplifier, and voltage reference sections with a modern IC chopper-stabilized precision op-amp and a modern IC voltage reference. If I do this I will carefully preserve all of the unneeded original HP parts and offer them to other HP412A users.

-EB


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 Post subject: Re: Triplett 603 FET-VOM
PostPosted: Aug Mon 12, 2019 5:28 pm 
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electricboyo wrote:
bob91343 wrote:
My analog voltmeter of choice for dc is the HP 412A. For ac I have the HP 400D. I only use those for casual measurements; for more precision or wider bandwidth etc. I have other instruments.
It’s fascinating that you mentioned the legendary HP412A.

I have one.

More accurately, I have the carcass of an HP412A that was junked many years ago by a nearby University. I doubt that it is restorable. However, the analog meter movement is in good working condition and the range switch looks intact and undamaged.

How difficult is it to keep an HP412A in working condition?
That opto-mechanical chopper with 4 incandescent lamps seems a bit daunting.

This may sound heretical to some, but I was thinking of replacing the chopper, amplifier, and voltage reference sections with a modern IC chopper-stabilized precision op-amp and a modern IC voltage reference. If I do this I will carefully preserve all of the unneeded original HP parts and offer them to other HP412A users.

-EB


This is my HP 412A here:
Attachment:
HP_412A_DC-VTVM_01B.jpg
HP_412A_DC-VTVM_01B.jpg [ 175.21 KiB | Viewed 304 times ]

In addition to its test leads having been chopped off, the power cord was also cut off and the fuse was missing when I got it. I haven't checked it out functionally yet, but now I'm getting interested in it...
If the power supply works OK (using Variac, dim-bulb to bring it up), then it will be fascinating to see if the lamps light up and the chopper motor starts to spin...

-EB


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 Post subject: Re: Triplett 603 FET-VOM
PostPosted: Aug Tue 13, 2019 1:16 am 
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The B&K model 277 Electronic Solid State Multimeter has an input resistance of 15Mohms, inclusive of the probe resistor, which is 100Kohms.

Vince


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 Post subject: Re: Triplett 603 FET-VOM
PostPosted: Aug Tue 13, 2019 3:54 am 
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Well I had plenty to do to get my 412A working. I rebuilt the chopper section and the hard part had to do with the light pipes. Fortunately I had three meters to draw from and I was able to get a good working one from it all. Amazing that they used el cheapo #12 bulbs.

I still have plenty of parts. I have meters (they had some variation in style) and power transformers. One of them was a 412AR and that has quite a different chopper section. I still don't like my dc probe but it's the only one I got.

I also have an HP bench DMM that gets plenty of use. I love the autoranging feature. But the 412A has very interesting ranges that make it attractive for some measurements.

My 3456A is very nice as well. One of these days I am going to give another shot at getting the 3455A working. I have too many projects.


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 Post subject: Re: Triplett 603 FET-VOM
PostPosted: Aug Tue 13, 2019 4:16 am 
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Findm-Keepm wrote:
I downloaded the entire book on probes. It is totally and completely terrific!
I may wind up printing the whole book. It is that good.

Thanks again for providing the link.

-EB


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 Post subject: Re: Triplett 603 FET-VOM
PostPosted: Aug Wed 14, 2019 5:29 am 
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Slightly off-topic but fascinating:

After reading more about the HP 412A, I decided to install a new power cord on my derelict HP 412A and see if it would power up.

It did!

The only internal issue is that the lamps for the optical chopper required cleaning their contact pins. After doing that, this ancient VTVM actually WORKS. All of the power supply voltages are within spec. It zeroes adequately (not perfectly). But of course this is the first time it has been powered up in 20+ years.

A limited test of the DC voltage ranges shows that it is reasonably well in calibration on the ranges from 10V and up.

So it looks like the next step for my HP 412A is to construct a set of probes for it. I’ll need to get some triaxial cable for the Ohms/mA probe cable. I think I have some thin coax wire already that will work for the +DC and common test lead.

It amazes me that a 50+ year old VTVM is able to work without needing major repairs. The previous owners apparently scrapped it because it was old. Not because it was broken.

-EB


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 Post subject: Re: Triplett 603 FET-VOM
PostPosted: Aug Wed 14, 2019 3:26 pm 
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electricboyo wrote:
So it looks like the next step for my HP 412A is to construct a set of probes for it. I’ll need to get some triaxial cable for the Ohms/mA probe cable. I think I have some thin coax wire already that will work for the +DC and common test lead.

Be sure to read section 2-1 of the manual. It's entitled "Low-level electrical phenomena".

I believe the 412A used cable selected for a low triboelectric effect. I know the 425A did. And the material of the probes matters due to the thermoelectric effect. You want to use Mueller unplated copper test clips, not plated steel, and the DC probe needs special consideration. I don't know what material HP used for their DC probe tip. Gold-plated tellurium-copper would be a good choice, but I don't know of anyone that makes a probe out of that.

Also be sure to observe the test lead dress shown in section 4-36.

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


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 Post subject: Re: Triplett 603 FET-VOM
PostPosted: Aug Fri 16, 2019 9:29 pm 
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After making improvised probes, I can report that my HP412A DC VTVM is no longer “scrap equipment.”

It functions on all ranges with decent accuracy.

I have not yet attempted any calibration or adjustment other than “bias adjust,” “zero adjust,” and “ohms adjust.”

When I tried measuring a 0.05 ohm resistor, the indicated value was very close to 0.05. So the built-in “4-wire” ohms measurement circuit works quite well.

I shall start a new thread soon with the title “HP412A saved from scrapyard.”

-EB


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 Post subject: Re: Triplett 603 FET-VOM
PostPosted: Aug Mon 19, 2019 4:22 pm 
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Both my Triplett 603 FET multimeter and my HP 412A DC VTVM are restored and reassembled.

After I use both of them on my test bench for a period of time I will post a review of each multimeter.

-EB


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