Some old, but interesting multi meters


I agree, that a very good hand held meter is probably a Fluke. I have some electronic multi meters too, and they have two main disadvantages for me: 1) The have a battery, and 2) They have no needle.

Time line of battery leakage.

Batteries seem always empty when I do not expect it, and damaged a lot of my equipment stuff over the decades. In the 1960's batteries were a disaster, leaking this rust generating fluid already before they were fully empty. There would be some white powdery substance underneath the paper cover, und from there, they soon would begin to leak so much fluid that you wondered where it all came from.

In the 1970's came the 'leak proof' batteries, and also 'transistor' batteries, with a better discharge behavior, and these cost extra. In the 1980's batteries were at it's best, because the high contents of Mercury and Cadmium was no problem by then. It seemed the leakage problem was solved.

Today, quality of 1.5V batteries is back to the bad level of 1960 again, Free of Cadmium and Mercury, they can hardly be stored longer than a 3 years, and they leak just as unexpected, and already when still half full. Same is in the good old days. This part of nostalgia has come back to us :)

It seems however 1.5V Lithium Batteries are the 'solution'. I am using these now, and so far no problems.

Here you find some multi meters caught my attention for some reasons.


EAW East Germany.

Very boring to look at, and I bought it only to calibrate my AVO tube testers, because it has the same impedance as the AVO-8 multi meter, and I needed one. A good AVO8 is very hard to find. WIth the EAW however I realized what this is, when I received it. It had a tout band meter with mirror scale, and that is usually a very good meter. So a tout bad is friction free, more sensitive, more precise, more linear, and more expensive, and more everything. Later, I finally found a good working AVO8, so I sold this EAW meter so somebody who indeed is working on AVO testers as well. But this meter works as nice as it is bad looking. It has a one calibration pot meter inside for AC and one for DC in general. So it was designed with precision in mind. It was easy to drill two 3mm holes a hole in the deck, and then I could can calibrate the meter from the outside. This gives the option to calibrate for instance a particular DC range, let's say the 30V range. Set it exactly right, and then use this for what I want to do with it. Since these meter looks really ugly, nobody even looks at them in Ebay. I payed only 7 Euro, but when someone puts them for auction beginning at 1 Euro, you may as well be lucky for 1 Euro. What is nice, it has separate banana connectors for voltage and current. (So the four you see at the top) and they did some intelligent switching at the inside. You can hook it up the four terminals to a circuit. Two for the voltage and two for the current. Then change from voltage to current measurement without having to plug cables, as with almost any other multi meter. After playing with this EAW, I realized that they ALL saved on this fourth banana connector. So you can buy a 500 Euro fluke, but you don't have this option.

So with the four banana terminals, it becomes hard for a fool like myself, to swap current and voltage by mistake. Also there is a security button, you need to unlock before changing to another sensitivity, which works simple and good. Also some very intelligent switching was done inside, to short the current terminals when you change the range, and forget to unlock the meter. (So when you change the range, the security button pops out, and the current terminals are shorted and the voltage terminals are opened). I am not sure if it had mechanical overload protection, but the mechanics inside suggest it.

Make sure the case is has no cracks, and the bottom is not scratched as it has printed quick the use guide on the bottom. This is just a very nice tester almost for nothing. They appear every now and then, so you can pick a nice one.

UNI10 by MTM Messtechnik Mellenbach Thueringen VFB.

Build in 1981 in the socialist paradise of the DDR Germany.

It looks also a bit ugly, but it does reflect a little bit the design style of the 1980's This is the best analog multi meter I have ever seen, because of it's incredible sensitivity and precision, and build in electronics, and several ways to use it, with two AA batteries, or with a build in power supply from the mains, or totally passive, so without any electronics. It has also some disadvantages, but in short, I would say, when you want to buy something weird, this is it. Only it doesn't look weird, just very down to earth. A disadvantage to my opinion is, the panel meter is upside down, I find it strange. But indeed it is better, because now the mirror scale is better visible. Another disadvantage is, the scales do not correspond with any of the voltages. So the 2.5V scale you need to read from a '50' scale and that is the only scale one it has. But my complaints list ends already here.

What is the BEST I HAVE EVER SEEN, is a 10uA current scale. This was only possible because MTM made their own tout band meters. Or, use it as a 100mV DC meter. Yes you read it correct. So at the 10uA setting, it has 0.1V drop. Probably the panel meter itself is even more sensitive. This 0.1V drop is theoretical, because when you measure for instance 2uA you have only 20mV drop. Or, in case you really have 10uA but can not use the 0.1V drop, set it to 50uA, and the drop is only 20mV. Mind you, this is for a fully passive meter. Any digital, new made meter which can do the same, you will have to pay a fortune, And it has a mains cable on it. Any new hand held digital meter will badly disappoint when trying to measure 2uA. (Do it!)

Repairs and maintenance needed: There is coupling capacitor at the 800Hz output, to remove the DC from the transistor output. Since that cap was never used under voltage, it was leaky, and needed to be replaced. The switch has some wheel inside with teeth, which are originally not lubricated. The sound and the feel is too harsch. I applied some silicone grease on it, and also oil some other moving parts, because it is quite a complicated mechanism. Instead of raw and primitive, the knob feels now smooth and sounds good.

Ebay. The UNI 10 looks not attractive, and we learn on Ebay quickly, that non-sexy items are low cost. And yes it looks so boring, you can find a UNI 10 anywhere from 5 to 25 Euro, depending on it's condition. You need to know the "UNI" meters were the workhorse in the technical departments of the socialist workers paradise where they were made. So sometimes a German seller has more than one for sale. The ones that are directly from dirty depots, don't buy any of those. There are many other types of UNI around, but these can not compare with UNI10. Buyers don't understand the difference. So a nice looking UNI-7 may score a 5x higher bid than a UNI 10 which looks less nice. Yet UNI-7 is worthless, and UNI-10 is a pearl. People just don't know it.

Here are few of the specs:

Impedance. Amazing! At 100 Volts it has an impedance of 50Mega Ohms. Tested by myself.

DC current: 10uA full scale. Stand still with this. Many digital multi meters have fake specifications. They do show a digit for 0.1uA steps, suggesting this resolution, but it is for the birds. When you try to measure for instance 50.6uA, you will see the "6" is blinking between all kind of numbers, and you may not even be able to guess it's a "6". With the UNI-10 this could be read directly from the 50uA scale. So 50 plus 3 divisions = 50.6. Take your fabulous Fluke and try to measure 50.6uA. It will say 50.B. Where B means blinking.

The only affordable digital meter which does not blink the last digit is the Hewlett Packard 3465A. So again, for this please do not refer to manuals. They are all fake. Try it out, and come back. Stability of the last digit is specified nowhere. For trade purposes, when selling good over the counter, it is not legal for instruments to display an instable digit. Even so, they must have the NEXT digit stabile, and not show this. So when you buy 520 grams of meat from the butcher, all three digits must be stabile, but internally the software must prove to have a stable 4th digit.

Try it with a "so and so many digits" expensive meter, to measure in 5uA with it. You will probably see it fails BADLY. I have a so called 6 digit 34401A. Can it do this? No, it can not! In my collection only the Hewlett Packard 3465A can do this, and.... the UNI 10. But 3465A needs a warm up time, like all digital meters, before the internal reference becomes stabile.

Note: There is also a Siemens "Multizet" with a 10uA tout band, but it has no electronic protection relay

Capacitors: from 1000pF to 2uF. Not a big deal, but it works. For this it needs the mains cable plugged in. In case this cable is lost or broken, it's the same as on any old razor which works on 220V.

Excellent Protection: Electronic or passive. The passive function is slow, and protects against the big mistakes. The active protection needs the battery inserted, and uses to little current in idle mode, it has no switch off. ( I checked, idle current draw from the battery is 4A. So 40mAH per year, which is less than self discharge indeed like MTM writes. The protection is specified to work at 15x overload or at wrong polarity. I checked it, and it still works perfect. Also it features a vacuum device spark gap, to protect against very high voltage, with virtually no delay. The passive relay protection works as an emergency solution, and it works with a reset button. So when you have pressed it, there is energy stored in a spring, which is used for reasonable fast operation of the relay. However, this works only good at higher current. For overload protection of the sensitive 10uA meter you need the electronic protection.

The connections to a capacitor under test are then cut also.

Many of the other specifications are just average, but I use my UNI10 often for tube grid current tests. Which is nasty because you try to measure a few uA at a faulty tube, and when the tube fails during that, it can blow up the meter if there is no good protection.

Resistance measurement. It works good and precise, but there is no advantage over any 7$ Chinese DMM.

Calibration of internal oscillator. The oscillator is what they write a normalized 600 Ohms source. This was kind of normal in the days of analog signal transmission, and everything was always in dB. It has 1.55V unloaded, and drops to 50% if loaded with 600 Ohms. Calibration is needed because the mains is now 230V instead of 230V. The oscillator rejects this difference somewhat, but sure not completely. It would be a good idea to add a voltage stabilizer inside. There is enough room. Anyway I recalibrated it for 230V. I have no calibration instructions, but obviously one pot meter is for the output impedance, which must be 600 Ohms, and one is for the amplitude. So adjust the impedance, load the oscillator with exactly 600 Ohms, and voltage must drop from 1.55V to 0,775V. If that is the case, both pot meters are set right. With enough trying you'll get this done.

Actually this can indeed nicely be used to test a HiFi amplifier. 1.55V is ok, and the 600 Ohms is not significantly loaded by 47k. So you have a separate 800Hz oscillator, with it's own Banana connectors, and moreover, it's output is even floating.

This is the unloaded signal of the 800Hz oscillator.

AVO8 by AVO. Several Millions were build over a period of 85 years, from 1923 to 2008.

No other multi meter was in production that long, and made in such massive quantity.

I think this meter is over hyped, some folks feel they MUST have one. I only use it to calibrate an AVO tube tester, because you need exactly the load of the AVO8 to do it right. Apart from that, the AVO 8 multi meter r is useful in tube land, because it has 2500 Volts range. In case you are interested, is a bit strange meter to use, and there are many AVO8 around with a deformed meter coil. This us very hard to repair, as it gives unlinearity. The different versions are from AVO8-Mk1 to AVO8-Mk7, and definitely beginning with AVO8-Mk5 they look nice. However, the Mk5 is very problematic to repair. They used a plastic foil assembly instead of normal wires, and it is difficult to take that off, as it melts easily. However, you must take it off for most repairs. Next problem is, the Mk5 has those nice looking red collars around the reset button and the "reverse" button. However, that is bad quality rubber, and it cracks. Some users peel it off then, and it looks "normal" again. The problem is, the reset button is physically part of the meter system. So when the red rubber is cracked, or peeled off to sell it on Ebay, you have now an air path directly from the front deck into the meter, and magnetic dirt will come in for sure. Also with AVO8-Mk5 and higher, the resistors inside are laser trimmed, which may have been fine by then, but the laser trimmed assembly is not painted or protected against detoriation. The burn spots are just left open. So precision of the laser trimmed parts is either already a problem now, or will become a problem later. The the highest version AVO8 I would recommend is the AVO8-Mk4. Having said that, I would never take Mk3 or lower, because they are just too old. In short: I recommend only the AV8-Mk4. And because it's probably Ebay where you buy it, let all the "experts" outbid themself for the nicest Mk7, and just buy a dull Mk4 for a low price.

Metrix MX573. Tout band meter with very precise mirror scale

This is just a nice multimeter. Metrix still exists, as the joint company with Chauvin Arnoux. Until 2012 some MX573 stock left over was still normally for sale via the RS electronics website for 275 Euro. I don't know the production data of those, but I guess they must have been 15 years old or more. They were unused, and sold as such. In 2015 I tried calling with Metrix France to get the calibration instruction, but they were very confused by the question, not helpful, and surprised they had such a product for sale only three years ago. That was a waste of time. Later I found the calibration instructions just it in the internet somewhere.

I like using it for it's comfort. For the rest it doesn't excel with brilliant electrical specifications, but specs are not bad also. Very good is remarkable low battery usage, and a full electronic protection of every range. So theoretically you can set it for 200 Ohms, and plug it in the mains by mistake.

Indeed these need calibration. I have three of them, so you can see i like to work with it. They do seem to have some internal noise, meaning at high sensitivity AC, it doesn't go fully to zero, but it's just a tiny little bit, and no problem. I have the schematic and calibration instruction in French, which I can get around with. .

What is interesting, the digital meter is a separate unit with an analog input, just in parallel to the analog panel meter. The LCD display is a standard device, and the digital part it a single chip IC, which original chip you can still buy on Ebay as surplus. Also complete units could be tried, as the original unit is 200mV full scale, and that is still normal standard today. So try to get one digit more, with a newer version digital read out. That MAY work, and you get 5 digits. MX573 was made as an industry product, but also not a toy for the DIY market. It is in between.

The Ohms measurement of the MX573 is done nicely, because it works opposite direction as with low cost analog meters. So at higher resistance the needle moves to the right. Meaning at open connections it moves all to the right, at "endless". Which is more logical as soon as you get used to it, and then you don't want anything else. in Germany sells the second generation of those, under the new brand name Chauvin Arnoux now, called CA5011, for the nice price of 541 Euro. I saw some Chinese imitations for 27 Euro on Ebay too.

Unigor 6e. Tout band meter with very precise mirror scale

This is the best ACTIVE analog multi meter ever made. Period.

It is not impossible to find, but a bit rare still. People do not really know, they just see a clunky meter, and are not interested. Also there are many other Unigors for sale which look just like the 6e, and people see too many of them. The 6e However, features a 1mV AC scale, with a beautiful 100 divisions mirror scale and a friction-free tout band meter. So you can measure AC volts in the uV range at no problem.

Battery consumption is very low. This is best described by the way it arrived to me in 2012. It came from a military surplus store, and inside were the first Batteries still. (Yes!) They were of the type NBA3042. 4Ah 1.5V Type C, "Baby" cells. They were still full.

The Unigor 6e is not just precise. It is specified for 1%, and that's what it really is. All ranges, anything.The electronic protection saved it a few times from my own mistakes, and probably also from previous users mistakes.

The unigor 6e features a FET chopper amplifier, which chops the DC signal into an AC signal, and after amplification it gets rectified, so you have a DC signal again. That way it can be amplified without internal DC offset or drift problems of the internal amplifiers. Also amplifiers can be made to block anything below the chopper frequency, so mains hum doesn't affect it either. Amazingly the internal parts are not even that much, and the PCB material is very old fashioned. Yet it's functioning is really good.

Interesting feature of electronic multi meters is normally, the scales for AC and DC are the same, as they use electronic rectification (not rectification with a diode, which looses the forward voltage). So it has a linear scale for AC too.

About old batteries...

Over the decades, leaking batteries have damaged a lot of me equipment. Batteries were a desaster in the 1960's I can tell you that. They have become substantailly better in the 1980's, but is seems to me the Chinese have succesfully managed to make them as terrible again as they were in the 1960's. But... some good examples exist too. Here are some examples of batteries I saved up, that refused to leak. The oldest one I have is a sharp GII battery, from 1988 and now 32 years old, and used for 28 years in my TV remote control. This makes me think, a small current consumption, irrelevant to the capacity, may perhaps help to prevent leakage. At least this does change the chemistry inside. Sharp Battery,  still not leaking Current draw is only a few mA and only when you push a button. But you don't know my wife and daughter. They zap the remote control continuously. Still, the original batteries were never replaced ever since, until 2016 when we threw the whole TV away. I removed the old batteries, because today we dispose of that separate, and much to my surprise they were the first (with date code 88-10 on the bottom of them) , and still full. I kept them in my curiosity box.

These army batteries were in a used Unigor-6e multimeter I bought, and they have date code 1990. But the Unigor 6e draws almost no current when it is switched on, and moreover it is switched fully off after use, with a real switch. When I bought this Unigor in 2012, the seller wrote the batteries were low. After checking them, one was 0.5V the other three had still 1.45V. So it was running on 5 V which is above minimum 4V specs. In the bottom was date code November 1990. Isn't that crazy? I took them out, and saved them as curiosity. Two more got empty now in 2020 while I revise this text, but one is still good after 30 years.

Lithium Batteries

Lithium Batteries are not new by itself, but today many new variations on their chemistry have occured, and it seems to me we have a real technology change coming up here. As well for rechargeable as for disposable batteries. The final end of the leaking battery decades. (Not talking here about artifial batteries, made from Li-ion batteries with electronics inside to simulate a 1.5V battery). I checked some youtube videos of people taking them apart, to fire down the lithium foil. Most of all, I see no wet stuff inside, just dry foil, or it looks like dry to me, and Energizer advertized them as leak proof. When searching for Lithium battery, you get always showed rechargable batteries, or 3.6V batteries. Wich are not these here. These are disposable batteries, with only 1.5V. I bough a 10-pack of those for 14 Euro (while I write this is Nov 2020). Expiry date is 12-2040. These are now in my UNI-10, to power on the meter protection circuit. This circuit can not be switched off, but the current draw of it is is 4uA. So 40mA/h per year, or 800mA in total over 20 years. They are specified at 3500mA, and moreover they hold the voltage until the very end. Mmm... when this is all true, this is a perfect product. Data Sheet here.


Sharp Battery,  still not leaking The oldest still working battery of the world, the "Oxford Electric Bell" battery from 1840. Also used with small current spikes, but regularly. This is not a joke. Search on youtube for 'Oxford Electric Bell'.

Tektronix Current probes

The Chinese sweep this market empty. They use them massively for designing switched power supplies, which is their specialty. Supply is low and China is large. sometimes you see them offered from China on Ebay, You can say this is device they didn't want to have anymore. It will not make you happy.

This is no insider report, for those who know all about it, and are looking for more. This is rather if you ever thought of using a current probe for DIY projects, and don't know where to begin. At least I had this problem myself. It seemed quickly clear: Tektronix makes and made the best current probes ever. They are smaller and higher bandwidth than any others, and it seems to me the vintage probes are much sought after, though they are 50 years old or more.

A probe should look not like an old tool, but like a carefully treated instrument. The plastics of the head can damage, if forced with violence over too thick cables, and that can chip off that little sleeve inside which is glued on the Hall element. So beware for cracks or chipped off pieces. The typical death cause of a probe is a cracked hall element, which can't be fixed. So there is a reasonable supply of cases upper part, lower part, cabling, connectors. etc etc.

Why use a current probe?

There are direct ways to get the current waveshape of a device on the scope screen, like use a series resistor. That becomes complicated if the device is connected to a high voltage, and you can't ground your scope there. So then you need to make a differential measurement, which myself I find very easy to do, but for some reason most people regard it something difficult. Consider a scope with differential inputs otherwise, such as the good old HP130C (some say it's the last good scope, HP made). Or several of the Tektronix, but these are all a bit of a Dinosaur. Yet, there is no way around opening up the circuit, and put a series resistor in there. This of course is tedious for fault finding, or doing random tests on all parts. So what is the reality? It doesn't get done. So here is where a current probe comes in. There are so many comfortable things you can do with it. Like a bundle of wires coming out of a transformer, but which one is which... That sometimes would be instantaneously clear when you know the current and the waveshape. Or have you ever thought about the AC ripple current through a power supply capacitor? When offering a transformer Ra of 4600 Ohms, where the circuit diagram says 4400 ohms, people react as if this something very wrong. Then, they put a 200uF cap on a position where 30 uF is maximum allowed, and do not even think about what that does to the transformer saturation, to the choke or the poor tube rectifier. When using a simulation program like PSU designer, you get an idea of why that is very important indeed, and why the current or peak current can be out of specs so quickly. What a simulation program can do, but very inaccurate, is check the start up behavior of the power supply. This and many other situations is just asking for a decent current probe. Have you ever heard that deep noise, coming out the amplifier's mains transformer when you switch it on? The excessive current draw sometimes even visible via the room electric lights. Do you remember it? . With a current probe you can an least see the primary current draw. That will shock you, to see so many Ampere go in there for a peak of 0.2 seconds. At the secondary side, you can see which windings are the users of the excessive current, and follow the path from there to the end user. So which part is absorbing this massive energy for 0.2 seconds? Perhaps the choke saturates, because you made the first capacitor too big, and you are slowly killing the rectifier tube with it. No trying to be dramatic here, but so many causes and mistakes, can become visible now. If you build an extension socket, from isolated pieces of 2mm copper wire, you can measure DC plate current of a UX4 based tube, without opening the amplifier. Quite useful actually. There are so many applications for it.

What probe to choose?

The first choice is, there are DC probes and AC probes.

1) AC probes. The types to choose from are several, P6016, P6019, P6020, P6021 and, P6022.

P6016 is the first type, and like all probes, the passive response is too poor for Audio applications, so the 3dB low point is far too high, and sensitivity is too low to display signals of 1mA. Together with it's dedicated amplifier R131 the sensitivity becomes 1mV/mA at a 50mA/div scope, so a 5mA/div scope like the Tektronix SC504 which I am using, results in 10x the sensitivity, or 0.1mA per division which is really very nice. However, the low 3dB point of 50Hz makes I cannot measure power supplies with it. So P6016 is useless for me.

P6019 is better than P6016. With the (somewhat) universal probe amplifier R134 it acts basically like the P6016, just with higher bandwidth. It has a low 3dB point of 12 Hz, and a 1db point of 21Hz.So this covers really the audio range, and AC parts of the power supply too. The high frequency 3dB point of the P6016 was already 20mHz @ 3dB, but P6019 comes even with 40MHz @3dB.

P6020 is the companion of P6019. It is made for higher frequency, and the technical limit is set by the R134 amplifier, not the P6020 itself (which goes up to 200MHz with passive termination). The disadvantage is, the low 3dB point is higher, we are at 50Hz again. So P6020 is useless for me.

P6021 is similar to P6019, but it features an extra contact which connects to the amplifier 714A, and allows the amplifier to identify the probe. The 714A fits only in 7000 series scopes. It can However, also be used on ab R134 amplifier.

P6022 is the high frequency version of the P6021, so we can't use it.

Use the R134 Amplifier with the P6019. This little device changes everything. Now, the P6019 will go down to 21 Hz fully linear, and from there to 12Hz @-3dB, and fully collapses below 7Hz. Also sensitivity is excellent, and you can see signals if 1mA with very good resolution. Amazing is also to see the transformer based device work at such high frequency. I have to admit, my equipment can't hold up with it. So it's specified up to 50mHz? Yes, it sure looks like it, when I can display a square pulse of 30nSec. It really stresses my 80mHz scope to make this visible, and I must say about the R134 amp + P6019 combination, the pulse is displayed free of distortion. When I look at it directly on the scope, I see no difference with the current probe signal. Just a few nano seconds time delay.

So yes, they're only AC, but they go deeper down in frequency than any loudspeaker, any AC signal in an audio your amplifier or power supply.

The R134 Amplifier is good thing. This is a high frequency amplifier with bandwidth up to 70mHz. It is made really nice, with an external power supply. So it is small and light weight. It features a clipping connector, which makes you can hook in up on any scope directly on the input. So to say, you add a current probe input to your scope that way. I am afraid to damage the scope like this, and I stick a cable-to-connector in there, so I can hook up a normal BNC cable on it. The output is 50 Ohms, so it will drive a cable without reflections. At the scope end, you need to use a 50Ohms termination, and then you can make indeed needle impulses visible, in the range of 30nSec, with the probe. For low frequency the 50 Ohms termination is not needed. The good part of the R134 is, there are too many available. So when it damages, you just buy another one for a low price.

A DC probe is made of an AC probe plus a Hall element for the DC and low frequency range. The first one, was the P6042, and it's still in interesting device, provided there is nothing wrong with it. The advantage today is, you use it as a separate unit on the bench, it simply has an output for your scope, and when you are done, you can put the probe inside, in a shaft. The disadvantage However, is, the amplifier is complicated, and they do get defective, and many parts to adjust. The probe itself is not so much different from the later family, but not compatible. Though the transformer assembly is comparable, you always have the probe + amplifier as a combination.

The Hall elements are quite variable. Some have negative offset, some have positive offset. The greater part of the offset is compensated inside the probe with hand selected resistors. So this means you cannot just replace the hall element inside a probe, by another. In that case you also need to re-adjust the balancing Inside. The final DC offset rest is adjusted to zero inside the amplifier itself.

When there is something defective with the probe it's normally the Hall Element itself, as these are the ones that get sold on Ebay. There no replacements around. Some real specialists operate the Hall Element out of a later 6302, but offset of 6302 Hall Elements is larger then P6042 Hall elements, so that is tricky. Also 6302 has two versions, of which you can only use the older version, with circular arranged pins, and not the one with a rectangular arranged pins. So consider a P6042 carefully. These seem to go for prices between 400 and 700 Euro depending on the quality. Myself I would rather pay more for a very nice one. If they studs at the back are cracked, it has been kicked around. Check the probe for cracks. If the probe screws look as if they have been taken out a few times with a non fitting screw driver, that is what it is So taken apart multiple times, but there are no serviceable parts inside, and they did not know how to use a screwdriver. Not the kind of service I am dreaming of.

Probe maintenance (That is for all probes).The magnetic core may not be touched with your fingers, and if so you must clean it with alcohol. Old cables can become stiff, until a point where it becomes very inconvenient, and you need to replace it. There are cables for sale on Ebay, good used ones, and sometimes NOS. I would not play with "any" cable, so you don't have one with the wrong capacitance or whatever. 50mHz is really a lot, and it would be a pity to compromise on that, just because of the wrong cable. The moving parts can be lubricated sparely with silicon oil. Do not use any other oil or grease, as that may crack the old plastics later. For service you can unscrew the BNC connectors, this is not a crimped connector.

The DC probes P6302 and A6302 do not differ so much from each other. The A is the later series. Inside can be a Hall Element assembly with rectangular connector or with a square connector. The square connector is the later version. I do not know how this relates to the A or P series.

These were made to be used with a universal amplifier, or with a passive termination device. Logically with an amplifier sensitivity and bandwidth is a lot better. There are a few amplifier types around. Long ago, these were of course sold in a ratio of 1:1 probes + amplifiers. However, the probes are delicate, and today there simply too many amplifiers for sale. So the amplifiers are cheap and the probes are expensive. Unlike the P6042, when you have a problem with the amplifier for the later 6302 probes, you just buy another amplifier on Ebay at a low price. When the probe is good, it always works on a random, good amplifier.

The amplifier AM503 is a unit which fits into a chassis. These mainframes can be bought with 1, 2, 3, 4 or 6 slots. So when it's a "current probe only" application, you can buy the TM501 chassis. Or, you may want to plug more units into the chassis, and you need the TM502, 503, etc. I prefer the TM504 because I can still carry it, and it has no fan. The next generation chassis is TM5000 series, which has a GPIB interface, 4cm higher size, heavier, they have a fan, and they're not fully compatible with TM500 modules. I expanded the 4 slot TM500 chassis with an 2 channel 80MHz scope, and the low distortion oscillator SG502. The scope can be externally triggered via the back panel with the oscillator, and by itself this set up is a great tool, for working on an amplifier, and it features a 50MHz current probe now. It's all I need :)

Today my Hewlett Packard 6236 Power supply went up in smoke.

Hewlett Packard P6236 power supplyRead why using 220V devices on 230V can catch fire.




Philips GM6004

I have good memories to this one. When I was young, my father brought me this meter as a gift in 1979. Today there is little use for such a tester, I suppose it makes only sense for radio frequency, because it is really good at that. If you ever repaired the HF part of a radio, you will know, a self made diode-test-head from a Germanium diode will somehow do the job, but not if you need reliable numbers. After revising it, I had great respect for the way it was build. Read more.