EEVblog #634 - Analog Multimeter Teardowns

[pgross]

Hi Forum,

Just want to chime in with some photos of my Old - but still perfectly fine Goerz (Metrawatt, BBC) Unigor 6e multimeter.

This model will NOT measure voltage or anything without 4 pcs. C size alkalines installed to supply power to the input amplifier.
In order to obtain the impressive (1973 vintage) input resistance of 10 MOhm in the 10-1000 V range and capacitance measurements from 50 pF up to 2000 uF,
no less than one dozen bipolar transistors and two fets are incorporated into the meter. 

I guess that some old heavy bearded guy from Austria not Australia, did have some  on the trimmers inside. 

Anyway - here follows the pictures of its guts:






   

[retiredcaps]

Ebay Triplett 630 teardown. 

http://www.ebay.com/itm/251569828882

[elliott]

Ebay Triplett 630 teardown. 

http://www.ebay.com/itm/251569828882

Good deal if you need some resistors to repair another one, it costs about $12 just to ship one of the beasts complete in the US. I ended up buying 3 of them basically for the cost of shipping and made 2 working ones out of them.

[Lefuneste]

I think I might have a good solution to the 30V battery for the Tripletts. An 18650 Li-Ion cell fits perfectly into the original battery holder. Plenty of room for a small DC-DC converter in these things too.

I was thinking about this modification too as these 30V batteries are a real PITA to find, but won't the step-up stage introduce some HF noise into the meter ? These small step-up boards are all switching voltage converters. Obviously it's going to be in the hundreds of kHz range so maybe the good old Triplett won't move an eyebrow about it. Any idea about that ?

[Lefuneste]

Hi Guys. Just a quick follow-up... question...
I have scored a Triplett 630 (APL version) out of the Bay yesterday as these VOM are so damn sexy. I was wondering about the probe connectors on them. I'd like to get some probe master probes for it (absolutely amazing probes by the way, Thanks for the mailbag review Dave !!). I am wondering what type of diameter the 630 accepts. I understand that shrouded connectors are not an option on these ancient meters, but the diameter and length seem also weird. Maybe it's just my imagination... Does anyone know if it would accept non-shrouded 4mm connectors ? Do they have to be in specific shorter length as well ? Any clue about that ? Thanks for the info !!

[elliott]

I was thinking about this modification too as these 30V batteries are a real PITA to find, but won't the step-up stage introduce some HF noise into the meter ? These small step-up boards are all switching voltage converters. Obviously it's going to be in the hundreds of kHz range so maybe the good old Triplett won't move an eyebrow about it. Any idea about that ?

Haven't got that far into it yet, I've only run mine off my linear bench supply. I'm encouraged by the fact that the battery fits nicely, I'll probably get one of those cheap eBay boards and try it out. Since I have two I can power one from the bench supply and one from the lithium with the step-up and compare. I can use one of those spare 0.5% 4.5Mohm resistors I have from a parts meter as a reference.
 

I was wondering about the probe connectors on them. I'd like to get some probe master probes for it (absolutely amazing probes by the way, Thanks for the mailbag review Dave !!). I am wondering what type of diameter the 630 accepts.

Every 630 has 4mm jacks as far as I know, the Triplett 310 had 2mm. I haven't had problems with anything I've stuck in them, including some cheap shrouded probes I cut the shrouding off of.

Triplett made some awesome probes, I have an original set that is still serviceable. They are needle sharp and have screw on alligator clips. They are so nice I am going to disassemble them and replace the wires, which is easy to do. It is a shame that I'll lose the original "T" branding on the plugs.

[Lefuneste]

A big thanks Elliott for the information. I'll get some retractable shroud probes from probe master as they really have outstanding quality products.I highly recommend them!! I ordered a probe kit from them last month, and it was very well priced and frankly speaking, the best probes I've ever encountered. I had been searching for top quality probes for a very long time (2 years roughly speaking) without finding any reasonably priced ones. These were a BIG surprise as they are not only superbly built (material, sharpness...), but have a tremendous ergonomics too... A big thank to Dave and to the guy who sent them to the mailbag!!
I intend to use this pretty VOM for mobile phone debugging, measuring amp consumption on startup, as it seems to be the best way to find out what is going on without opening the device.
Now for the battery step-up conversion, I've looked around on ebay in order to find out what is the current trend in voltage boost modules. From what I see, most modules are based around two different chips, the ubiquitous LM2577 which runs at 50kHz, and a new XL6009 which seems to have a 400kHz switching frequency. I will try this latter one as it may fall outside the meter BW. The ripple out of theses modules seem to be a potential issue though, as it is roughly rated between 40 and 120 mV (Chinese standards). The good news is that we should be on the lower end as the meter probably don't take much current to operate, but we may need to add a filtering stage to get a better ripple figure. With a freshly charged quality 18650 (taken out of my e-cigarette stock), you can get 4.2V, with a discharge plateau between 3.8 and 3.6. It should be plenty enough to provide a primary voltage to be cranked up to 28-30V on the secondary without getting mad ripples out of the Chinese module. http://www.ebay.com/itm/DC-DC-Adjustable-Step-up-boost-Power-Converter-Module-XL6009-Replace-LM2577-/310717070508?pt=LH_DefaultDomain_0&hash=item48582e3cac
The funny thing about all this is that I still don't understand what the VOM is doing of these two batteries (30V and 1.5V) as it should be operating passively... Maybe some diode biasing ?

[WattSekunde]

If you're really into old and very old T&M gear have a look at this site

http://www.historische-messgeraete.de/index.php

Now there's somebody with a real fetish!

As far as I can tell it's only available in German but the pictures are good enough  .

Thank you very much for the link! Brilliant Pictures and I always hear it in my head: "a bobby-dazzler", "a thing of beauty", "a work of art", "a joy forever"... ;-)

[elliott]

The funny thing about all this is that I still don't understand what the VOM is doing of these two batteries (30V and 1.5V) as it should be operating passively... Maybe some diode biasing ?

It is only for resistance measurement, the lower ranges run off the 1.5V, the high ranges run off the 30V. On the APL (and most others) it is just the 100,000X range for the 30V, the NA uses it for 10,000X and 100,000X. Voltage and current measurement are completely passive and if you don't need the high resistance range you only need the 1.5V D cell.

That is the same step-up converter that I got, at that price I won't be heart broken if it doesn't work out. The only thing I'm not sure about is how I'm going to turn it off so the step-up converter it isn't draining the battery. I might just take one of my rough condition rear shells and put in a switch.

[switcher]

Here are some pics of the AVO 8 MK III, not only the best analogue meter ever made, but the best model of AVO 8. From the MK IV onwards, they were value engineered. The model pictured was made in Nov 1965. It always surprises me that later, inferior, models go for so much on ebay.

http://koti.mbnet.fi/~ijl/avo_8_mk3.html

[linux-works]

that looks amazing.

its cool that there is this much interest in the analog meters.  I started collecting a few of this brand or that brand of analog meter (simpson and triplett, for the most part) over the last few years, after leaving them behind for many decades.  I admit they are now more for collection and appreciation than real use, though, in my lab.

[AlphZeta]

Here are some pictures of a Chinese-made MF-10A multimeter (they are still being made). It has a 10uA range and x100K resistance range. The input impedance for DC voltages (1V to 100V) is 100K/V. Can't figure out how to re-size pictures when linking in... anyway, you can take a look at the full teardown here: http://www.kerrywong.com/2014/06/28/mf-10a-analog-multimeter-teardown/

[Lefuneste]

It is only for resistance measurement, the lower ranges run off the 1.5V, the high ranges run off the 30V. On the APL (and most others) it is just the 100,000X range for the 30V, the NA uses it for 10,000X and 100,000X. Voltage and current measurement are completely passive and if you don't need the high resistance range you only need the 1.5V D cell.

That is the same step-up converter that I got, at that price I won't be heart broken if it doesn't work out. The only thing I'm not sure about is how I'm going to turn it off so the step-up converter it isn't draining the battery. I might just take one of my rough condition rear shells and put in a switch.

Thanks Elliott for the excellent information. As I said I only intend to use it for voltage and current measurements, so I'll be fine without batteries. Still I will be looking forward after our "little experiment" !!   

[elliott]

Thanks Elliott for the excellent information. As I said I only intend to use it for voltage and current measurements, so I'll be fine without batteries. Still I will be looking forward after our "little experiment" !!   

I don't use mine for resistance measurement often, I have DMMs and an LCR meter better suited to that, but they are great for checking for jitter in old potentiometers. The 1000X range will work well for up to 50K, 100K if you check them in two parts. Of course you could just connect the pot to a power supply and watch the voltage on the wiper.

The 1X range is good for checking intermittent faults in cables, less maddening than listening to a DMM continuity tone for extended periods of time.

[ivaylo]

There were some completely different looking Unigors, these black ones
Judging by the looks and the model numbers (1, 2, 3, 4,...) they must be older than the A43 or the 6e, but no idea how old. Remember salivating after them in 70s in the wrong part of Europe...

[MJR]

You call that a multimeter? NOW this is a multimeter! Straight from 1958. Capable of testing your ICBM launch circuits or getting your damaged B-52 bomb doors back in the game!

The TS-505D/U Electronic Multimeter.

Trouble with your TS-505D? No worries! There is a schematic inside the gas and moisture sealed case. And the technical manual has a nice fold out centerfold of Ms. Schematic 1958. WOW! What a gal.

Wait! TUBES? Yup. None of that silly solid-state stuff here. Well, except for those sexy old-time resistors, that is.

... and yes, I do believe that was  nice wiff of 1958 I detected when I opened the case....

Enjoy!

[mattinx]

Loved the meter teardown, and felt inspired to share photos of my AVO Model 8 - it's clearly Mk II era, although there's nothing to identify it as a Mk II unit. The back panel identifies it as a High Sensitivity unit, and when compared to the Mk II manual, the settings for using the optional external resistance unit (for extended high and low range measurement) are missing - you can see the empty markings either side of the Ohms ranges.

Still has the original probes in the case, although I use newer ones instead This thing is really built like a tank.

I cracked it open and snapped a few photos for you fine folks to peruse - it's another former RAF panclimatic unit, and much like the Model 7 G7PSK posted earlier - you can see the O-ring to seal the unit and the panclimatic markings on some of the resistors.

More photos (including the hires originals) at http://pkl.net/~matt/photos/AVO/

[tzok]

This is not exactly a teardown, but just a gallery of a bunch of Polish analog and digital multimeters produced since 1960-1990:
http://multimetry.tzok.eu/

One of my favorite meters from this collection is UM-110. It's a meter from late '70s/early '80s. This obviously is an electronic multimeter - it has an opamp at the input, and voltmeter input resistance of 10MOhm. What is unique for it is a ohmmeter solution. It works in the opposite way than usual analog ohmmeter, there is an 'infinity Ohm' knob, instead of '0 Ohm'. Please take a look at my video (sorry for a poor quality):


Another great meter from this collection is a V640 made by Meratronik, it was also sold as a Conway Masteranger model 639 and Marconi TF2650. It's unique features are input resistance of 100MOhm and an effective bandwidth of 100kHz (documentation claims 20kHz, but most units were fine up to 80-100kHz) WITHOUT using a dedicated active HF probe (up to 1GHz).

[atkelar]

Very old thread, I know... 
...but it seems nobody has mentioned it so far either on YouTube or here so I made an account to bring it to attention:

Those A43s were aparently very populare with technical schools even in the early 90s. It was basically lab equipment 101 for all new students to learn about the volts, amps and ohms. Since I could not afford anything remotely like that, this was the first real multimeter I had my hands on and I am very sure lots of other people have similar nostalgic feelings about it.

Seeing it on that particular mailbag episode and finally having a model number to go by, I found a reasonably priced one on ebay - it did come with a "scratched in" marker that it did also belong to a school of sorts, so it clearly has seen some abuse, but it seems to do very well still. The wiper contacts might need some cleaning but at several decades everyone needs a scrubbing