Vintage Teardown: Philips PM 2422 A digital bench Multimeter

[schwarz-brot]

Hi there,
I am at it again. This time I am going to tear down a Philips PM 2422 A digital multimeter. It is seventies equipment again. If you remember my last Teardown of the RFT G-1001.500 you will find quite a few differences between kapitalist- and communist- produced technology. As I already took a peak inside my unit I suspect it to be made in the early seventies, maybe 1973.

The device is in absolutely perfect condition. Nearly no signs of usage anywhere. I did not check the readings against a calibrated device, but they seem to be well in spec. I found this beauty on Ebay and got it for about 20€ delivered. I only did buy the RFT because the seller of the philips played dead and I head to threat her to take legal steps only to find the device delivered out of a sudden via express mail... I guess she was not too happy with my winning bid.

What am I talking about?


Pic 1: Overview front


Pic 2: All nixies work. Model number on front


Pic 3: Typeplate on back

So this is again a bench multimeter. It is about five years older than the previous torn down RFT device. But it looks much more like the devices we are used to see in the western countries. It somehow looks cleaner - not only because it actually is but constructionwise. Somehow more professional and shiny. What you would expect in a laboratory, while the RFT was more looking like it was made with a production environment in mind.



Pic 4: Overview back


Pic 5: Overview bottom


Pic 6: handle can be used as stand

The device looks nice and well constructed on all sides and angles. The back sports some massive spacers to protect the cable and other parts (Picture 4). They double as holder for the powercord. I always like that construction when the cable is built in. The handle can be used as stand as usual (Picture 6). But it then feels a bit flimsy. On the other hand it works reliably and is very easy to tilt.


Pic 7: Detail range and function switch

The PM 2422 A is completely operated with one rotary switch that selects function and range. It can be turned all around and goes with a nice click (Picture 7). AC / DC can than be chosen with two separate pushbuttons.



Pic 8: fuse


Pic 9: Holder for mains-plug shows some wear

One fuse can be reached from the outside. Note the custom printed paper with the rating in the fuseholder (Picture 8 ). Nice Detail.
And there is finally something that shows this device was actually used: One of the rings of the plugholder on the back is broken out (Picture 9). But it still works.

rsjsouza provided the link to a manual for the PM 2422 (without A). It seems to be a slightly older unit but likewise constructed. You can download it here (German!)

PA0PBZ gave a link to a servicemanual in another thread. If you are interested it can be downloaded here.

The manual speaks of quite impressive accuracy. As with the RFT we are speaking of about 0,5% worst case in most ranges. Many are way better. I guess these things were quite expensive back in the days.




Pic 10: Made in Holland by Philips.

Last shot for your amusement: The beautiful Philips-Logo (Picture 10). This device is made in Holland and it is proudly written all over the thing. On the front, on the back and of course on the inside, too.



Stay tuned, next part will give you the insides.

[robrenz]

Add a ";image" to the end of your picture links in the [img][img] to get your pics to show in your text.

Sorry, some of my old posts had that same effect and I had to go back and add that to make the pictures show.

[schwarz-brot]

you were too fast, I have to upload them first to get the links. I then edit them into my text. What you saw were place holders. Please reload and find the pictures embedded

[Vgkid]

I await more pictures.

[don.r]

I love nixie tube equipment. This is one beautiful example too. Looking forward to some retro through-hole p0rn.

[schwarz-brot]

robrenz: Never mind. Your helping comments are always welcome.

Vgkid: Tomorrow! But you shall not be disappointed.

don.r: I hope you get what you luster for. This thing is all through hole stuff but not too much in it at all.

Right now I am working on the service manual. I am going to print it and bind it into a little book as I did before with the RFT-documentation. This helped a lot while trying to understand what's going on in the thing. I love to have a physical book in my hands instead of scrolling around on a screen. I am much faster and more likely to understand if I can skip through real pages. Maybe I am the guy to print out the internet.

[retiredcaps]

I am at it again.

Another great teardown, commentary and photography.  I hope you continue finding bargains on ebay and with the teardowns.

[Maxlor]

Pictures of the inside, please! 

[schwarz-brot]

Thank you for your kind words, retiredcaps! I'm glad you like what I do here. Maxlor, they are coming.

I somehow get the feeling the most important part is the pictures. Maybe I should start a Topic for beautiful random electronics-pornographic pictures in the Off-Topic Area? I enjoy taking pictures of random stuff but have no reason to post them anywhere.

For this teardown: I am going to post random pictures with random comments this time. If you really want me to dive into the sections of this thing let me know it. Else I will not go more into the details. You can dissect the mysteries of this unit for yourself with the service manual I linked in in my first post. It gives very detailed information on any detail of the circuit and its function. Also the circuit diagrams are all in there so no need for me to put them up here. This is much work for me. I would do it if you really want to see pictures of the single sections but I don't think this is necessary.



Pic 11: Wire mesh for shielding

To open the PM 2422 A you have to undo two crossheaded screws for the top lid and two more for the bottom one. They can easily be lifted out then. On the inside of the lids you will find wire mesh for shielding and stopping dirt do get into the venting holes (Picture 11). The mesh is painted black with the sides left blank to allow for contact with a large metal tab. I think this is a bit expensive to produce, but it for sure looks good. The venting holes are surrounded by an extruded frame which allows for stable stacking of several units. The plastic seems to be quite sturdy but after all I would trust the RFT all-metal-construction more. Again there are standoffs to support the case.


Pic 12: Overview of the PCB

Picture 12 gives an overview of the whole PCB. As I said, not too much on there. But some things are for sure worth mentioning.


Pic 13: Inside view onto the display tubes.

The backside of the display (Picture 13). I am not sure, maybe this is even more beautiful than the view on the front panel. I LOVE tubes.



Pic 14: The large custom rotary switch


Pic 15: back of the switch

The large switch is a beautiful piece of engineering. All in all it has four layers. Some components are mounted directly on there (Picture 15). Looks like a bodge but seems to be no afterthought. I guess they needed the shortest connection possible.


Pic 16: fuses

Three additional fuses on the inside. The upper one in Picture 16 is actually a spark gap. Never got to see one in real life before. Another huge one sits besides the transformer in its on little enclosure.


Pic 17: what does this connector connect to?


Pic 18: To the bottom side of the PCB. Straight on to the front connectors and pushbuttons for AC/DC measurements.


Pic 19: wire harness Western European style.


Pic 20: the wires lead to the pushbuttons and connectors on the front

They decided to put their parts only on one side of the board. I am ok with that but I cannot really get why they would want a custom connector to go through the PCB (Pictures 17 & 18). While there are easier ways to go to the other side all around. One reason of course would be to get the shortest connection possible. But I am not sure this is the cheapest possible solution. But was the price really a concern back then for such equipment?
However, the wires go to the frontplate connecting the pushbuttons and input jacks (Picture 20). Note the completely different style of binding them together with some kind of rubber cord. Also beautiful but lightyears ahead the technique used in the sovjet unit. Remember, the philips was even build five years before the RFT multimeter!


Pic 21: western heatsinks are somehow boring


Pic 22: finally a bodge


Pic 23: and a real fail.

I don't like the little heatsinks on the cans in this device as much as I loved the star-like ones in the RFT (Picture 21). But sure, they will work.
Picture 22 shows what I think is a bodge. I don't know whats inside the tube. It feels like a resistor or a diode by its package size. Did not look at the schematics but could not find it in the partnumber overlay. I don't mind this but I DO mind what can be seen in picture 23: That cap gets constant pressure via the tilting handle. Someone has really failed there when designing this thing.  I wonder why they did not bodge that to a better place or even the bottom side of the board. When you look at the service manual you will find this section redesigned. The service manual is actually for model PM 2422/A5. My unit is PM 2422/A4. So I guess this is just one iteration before they noticed the problem. Also I am sure the part with the bodged in device is retouched for Revision A5.

More fun in the next post.

[don.r]

Thanks for the beautiful pictures. A real mixed bag with some exquisitely overengineered parts and some real stupid mistakes. I find tubes to be terribly romantic. Every lab should have one piece of nixie tube or other vacuum tube equipment in it for its esthetic beauty alone.

[schwarz-brot]

Hi there!

I haven't had the time to go on with this topic in the last days but finally I can continue.


Pic 24: Spindle of a trimcap


Pic 25: unusual trimcap

When inspecting the bottom of the PCB I found this little thing (Picture 24). It looked like some kind of fixture to me but there was nothing special on the upper side of the PCB but this strange little tube (Picture 25). Referring to the service manual this unusual construction is a kind of trimmer capacitor. Inside the tube is nothing but the spindle which has a slotted screwhead. I never saw such a "through hole" construction before and I am not sure why they could not use a standart type. There are several usual trimcaps on the PCB which clearly were not suitable for this job. Any ideas? My only guess would be they need a very special range of capacity at this place.


Pic 26: a bit of shielding

Maybe you realized it already: This device sports no real cans, only some very basic shielding of the hole unit. On the PCB there is only a small area "shielded" against or from the neighbour parts. There is no lid, only that "wall" (Picture 26). Obviously this is enough to do the job. Therefore I would call this a very good and stable circuit design. Inside is not much - some caps, some resistors, and transistors (BC107, BFQ12, BFW11). All the circuitry in the can is part of the x1/x10 input amplifier.


Pic 27: large diodes for input rectification


Pic 28: a big zener diode with colourful marking


Pic 29: Looking like torpedos, behaving like diodes


Pic 30: some shunt resistors


The next pictures are just some component fun: There are some real big ass diodes for input rectification (Picture 27, Type BYX42-600). These cannot be found in the linked service manual! For the next revision which this document is for (PM 2422/A5) this section was rearranged and built with type BYX72-500.
Next is just a beautiful Zener Diode in the output section (Picture 28).
And some torpedos. Anyone else thinking of Super Mario Land? These are again some beefy Zener Diodes (Type BZX70-C12), I think for input protection as they are arranged head to head and connected between the active part of the shunt resistor ladder and ground.
The shunt resistors can be seen in picture 30. Beautiful, but by far not as impressive as the sovjet ones which were all but one in form of large coils.




Pic 31: only three real ICs


Pic 32: Overview of the tube driving section

There are only a few ICs in this device (Picture 31). They look much more like the ones we are used to today than the ones in the RFT unit did. Only the sockets look a bit crude by todays standards. But at least they are socketed! The ICs are:
- ADC (right, reference IC501) type FEY101B / S1. Could not find a datasheet for this thing.
- Quad Decade Counter/Register (middle, reference IC701), type FEJ271B.
- BCD to Decimal Decoder and Driver (left, reference IC702), type N74141AN.

Picture 32 gives an overview of the additional display driving circuitry for the tubes and decimalpoint, overrange and polarity handling.




Pic 33: A blob on a lead?


Pic 34: more blobs without leads. Jumpers!

[img]https://www.eevblog.com/forum/testgear/vintage-teardown-philips-pm-2422-a-digital-bench-multimeter/?action=dlattach;attach=78732;image]/img]
Pic 35: production testpoints

When I first saw the solderblobs on the board (Pictures 33 & 34) I did not recognize what they were good for. I thought they might some bad spots from producing the PCB. They are not - actually they are testpoints! Very common to use those solderbridges in todays SMD-circuits but I never saw this before in such an old device. They are labeled as jumpers and allow to get into or cut off parts of the circuit for ease of troubleshooting. Very nice Detail. The service manual shows a lot of testpoints and jumpers on the top of the PCB. They are ment for actual troubleshooting. But they also mention that there are testpoints on the bottom of the PCB. These are only ment for production testing but all clearly labeled and with reference in the service manual. I think that's how it schould be done: help anyone trying to repair a thing as much as possible by giving any information available. Would love to see that behaviour more often nowadays.

Well, I think that is enough for this device. I am amazed by the overall quality of this unit. There are some little problems and the obvious fail with the cap. Compared to the RFT G-1001.500 the distance between sovjet devices and those of the rest of the world at that time becomes quite obvious. The units give nearly exactly the same functions, ranges and accuracy. But remember, the Philips is five years older but yet looks way more modern. I would suspect the RFT to be of much more robust construction, even with it's mercury tube in it. And there are lots of places where there must have been brilliant engineers to make this thing work with the things available (think of the ADC built completely with basic OPamps and discrete circuitry) while in the philips state of the art components were available (ADC is just a single IC) and used. Like it was a different world...

But in the end - both work fine until today. What more can you expect? I love them both.

As always, comments, positive and negative criticism, and more information is very welcome. If anyone knows of the full service manual (without the other languages cut out) or one for the PM 2422/A4, please post a link.

[david77]

Very nice teardown, I've often toyed with the idea to get one of these meters off ebay. But then the brand name (I've not had much luck with anything made by Philips) allways stopped me.

Be carefull if you ever have to solder on this thing, in my experience it is VERY easy to lift pads on Philips gear from that era. I've never seen it that bad on any other PCB's.

[Vgkid]

Thanks for the update and pictures. I found the Soviet one to be more interesting, but this one looks to be more useful.

[schwarz-brot]

david77: I can highly recommend this puppy. Chances are good to find one for under 30,- € which is a real bargain in my opinion. It is not built like a tank but I can see no real shortcuts. It outperforms easily any new handheld multimeter in terms of accuracy in that priceclass. Most parts inside should be easy to replace if needed. Not even necessary to take out the PCB as it is well accessible from both sides. The service manual is quite good and easy to understand. It even tells you which parts of the circuit need to be recalibrated / re-trimmed after switching components. I absolutely like that.

I got a 10V precision reference and checked the meter against it. Although they tell you to let this puppy heat up for about 30 minutes prior to measuring I found it to be stable after about three minutes. Before that it simply would jump the digits wildly which is a good indicator for when it is ready to use.

Vgkid: I think they are both the same to use, only minus for the RFT is the bad display. After repairing that there should be no difference in usabillity.

[lowimpedance]

Lovely old 'BOX' and with a nice glowing display!.
Lots of nice clear up close shots  .

Every self respecting collector should have at least one nixie tubed gear on the shelf.
Mine are the Data technologies DT-360 which was my first DMM as an apprentice !!.
photo with some other appropriate vintage gear here..

https://www.eevblog.com/forum/testgear/%27stacpac%27-inside-a-vintage-power-supply-warning-images-contain-%27tubes%27/msg251266/#msg251266

[schwarz-brot]

The DMM is kind of weird looking to me with its indented Pushbuttons. Looks somehow like eighties design. But sure enough a useful tool.

I totally love the somehow romantic light of shining tubes. Just beautiful. Especially in a livingroom environment when exposed in hifi stuff. Sounds better just because it looks cooler  O0