Author Topic: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter  (Read 20971 times)

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Offline schwarz-brot

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Hello Forum,

I am on to a teardown again. This time it is going to be a digital Benchmultimeter. Again I will do this in Parts, and here we go:


Pt. 1: The Outside

Click here for
Pt. 2.1: Topless miss Piggy
Pt. 2.2: Topless part porn
Pt. 3: Flipping out the PCB
Pt. 4: Schematics of the top PCB
Pt. 5.1: Lifting the skirt
Pt. 5.2: I want her naked!

We are talking about the "RFT DC AC R I - Digitalvoltmeter", model RFT G-1001.500. The roots of this beauty go back to the early eighties as far as I know. I got it as an untested, non working unit from ebay for - hold your breath: 1,00€. Make this 7,90€ delivered - sweeeeeeet. It turns out the device is actually working. Not sure about if it is in spec, but I will find out. What am I talking about?


Pic 1: Front


Pic 2: Side


Pic 3: Front


Pic 4: Front

Pictures 1 to 4 give you an overview what unit I am talking about. This is an classical bench multimeter, Ranges 200mv to 2000V AC and DC, 200µA to 2A AC and DC, 200Ohm to 2MOhm. The Accuracy is pretty impressive. I have to look up the exact numbers, but I think it was about a maximum of 0,5% +/- 1 Digit at all ranges over a common temperatur range.
You will immediatly point out the missing knobs and the shabby overall look of this thing, but hey, not even 10€ - what do you expect? I knew that, when I bought it. I think, I will call 'er Miss Piggy.

This device was made in the GDR, as you can tell by the shot of the back. One trimpot to reach from the outside and a shielding connector, that is all on the back. On the front only a few knobs and four terminals - what else do you need? Maybe an on-off-switch? You won't find any. We will for sure find out later, why this is the case.
There was a hole range of devices made in the same form factor like this unit to form a nice looking, stackable laboratory system.




Pic 5: Inventory Sticker


Pic 6: Calibration Sticker

My unit was not only build in the former GDR, it was also used there, as you can tell by the calibration and inventorystickers on the sides (Pictures 5 & 6).



Pic 7: Detail Handle


Pic 8: Just for stacking?

The handle can be used as standing foot as always. It locks in many positions. Look at the nice metalwork on the spring-mechanism (Picture 7).

The feet (can be seen in Picture 3) and plasticpieces on top (Picture 8 ) are not only for stacking, they also alow to open the housing very easily: By giving the screw about one or two turns they loosen up a bit and can be pushed back. Then the top and bottom halves of the case can easily be taken off. A very nice and well working solution.



Pic 9: Display

Of course my beautyful miss Piggy was fired up, and there is a problem. The two last digits have dark elements (Picture 9). The third digit comes on completely after a while, the last one won't. I of course suspect some bad solderjoints there.



Pic 10: Manual


Pic 11: Fold-out-schematics

The last pictures for today (Pictures 10 & 11) show the manual, a specification sheet and the service manual. I got them as .pdf on a different forum and made them into that little book. This is of course diy (another hobby of mine). I still have the (crappy scanned) .pdfs. If you want them, drop me a line (I was allowed to give them away). And yeeees, there are fold-out-schematics! I like it!


If you are interested you can join in when I lift miss Piggys' skirt the next time. Maybe this leads to some nice crusty sovjet electronics porn.
Comments are appreciated as always.

See you!

Click here for
Pt. 2.1: Topless miss Piggy
« Last Edit: January 13, 2014, 06:31:33 pm by schwarz-brot »
 

Offline Vgkid

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Re: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter
« Reply #1 on: January 10, 2014, 09:25:05 pm »
I actually like the detail that was spent on making the tilting handle. I really want so see the insides.
If you own any North Hills Electronics gear, message me. L&N Fan
 

Offline brabus

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Re: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter
« Reply #2 on: January 10, 2014, 09:34:41 pm »
Undress her slowly... ;D

Great bargain!  :clap:
 

Offline schwarz-brot

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Re: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter
« Reply #3 on: January 11, 2014, 02:00:53 pm »
I told miss Piggy we were going to do some more shootings, and of course - tell a lady she is beautiful and sure enough she is all in.  O0

Pt. 2.1: Topless miss Piggy

Before you go any further make sure you understand we are going to undress a grandma - maybe a bit odd perhaps. I spent a while browsing through the service manual and found some interesting details: My Manual dates back to 1977! And for sure it was not the first version. It also got some extra sheets with errata. By looking at it I found some for units made starting from fabrication Number 5405, dated back to october 1979. Look at Picture 4 in my last post and see: my model is number 3285, therefore I guess it has to be dated back to around 1978 maybe.  :o I did not expect that...

Well, we wanted some vintage porn, right? I will start slowly as suggested with her top, no gentleman will head straight to the panties?  :blah:



Pic 12: Overview with top lid opened


Pic 13: Detail of clips

The top cover comes off very easy by turning the four screws in the plastic clips a few times and pushing them to the back. The piece is made of aluminium by the looks and weight of it. The inside is covered with some brown non conductive sheet (Picture 12). Does not feel like plastic, any ideas what we got there?
The clips are some cleverly designed and nicely molded plastic parts. They can not rotate out of direction and stay where they belong when the lid is taken of. Nice Detail. Appart from that they seem to be pretty robust and the plastic does not look aged (which goes for any plastic part as far as I can tell).



Pic 14: Overview of top PCB

I had bad luck with my camera, the batteries are empty, therefore you get only a part of the top PCB and lots of detail shots which I managed to get before it turned off. But take a peak at the full package in Picture 14. You see the parts nice and clearly laid out and can get a look at some sweet details. The dimensions of this board are 22x23 centimeters by the way. I think I will go into the details of this board later. Let us first look at some random detail shots.


Pic 15: Detail of beautifully tied down wiring


Pic 16: Large Heatsinks


Pic 17: large crystal (or maybe condensator?) It is some kind of Diode!


Pic 18: some axial electrolytic caps


The first thing I noticed when I first opened this device was the beautiful work on the wires. No cable straps or metal tabs to keep them in place - they are all tied together neatly and also secured with some cord (Picture 15). As you will see in the following pictures any large part was tied down in the same manner. No gunk, but anything is for sure in its place, no matter how hard you vibrate the lady.
Sure enough the large heatsinks are tied down, too (Picture 16). I always liked those round ones, I think they look beautiful.
Picture 17 shows what seems to be a crystal oscillator by the looks of it. It may also be a large capacitor or something else. There is no visible marking on it. The partlist lists it as reference element (no word on what kind of reference). There is an overview of connection diagrams, that finally gives a hint. The device is marked with a colordot on the side, indicating a kathode. Type number in the part list is "SZY 23", therefore I guess it is some kind of zenerdiode or something like this. In a metal can? Never saw something like this before.
Of course the electrolytics are large in this thing and therefore tied down, too (Picture 18). The insulating foil around them looks a bit odd because it came loose in several spots, but the devices look good. No leaking, no warping so far.



Pic 19: Detail of trimpot


Pic 20: precision thin film resistor in the background and artistically mounted one in front

On to some resistor pornography. Have you ever had a trimpot with indicator on the side? (Picture 19). Or a resistor mounted like the one in the front in picture 20? It is soldered to some connectors on the pcb with the leads bown into a accurate S-shape. I guess for heatspreading and possibly easy desoldering. Also, the S-shape may help taking out some vibrations, I dont think they really needed the extra lenght in the leads there because they chose larger types for higher power applications whenever needed.
In the background is a precision thin film resistor, whatever that meant back in the days. Well it is a 0,5% precision type, not bad at all. And look at its leads: Nice little plastic spacers on them! You find these on many standing parts. There are also Transistors that have nice moulded plastic spacers under them. Attention to detail...



Pic 21: Spacer

Remember the case is made of aluminium? Not the strongest metal yet. But it surely will not warp in this unit! Look at these nice, non-conductive spacers. (Picture 21) There are two on the top PCB and I know there are two more in the same places on the bottom one.  They mount onto an internal structure that helps supporting the PCBs. Therefore they double to screw the PCB in place. You have to know this, else you won't be able to ever take any PCB out of this thing.


This is it for today, next on I will show you some detail-shots of some obscure sovjet ICs and the topology of the top PCB. Maybe I will also start to take it out, we will see. Hope you like it! Keep your comments coming, motivates me every time  :-+

Click here for
Pt. 2.2: Topless part porn
« Last Edit: January 12, 2014, 01:30:36 pm by schwarz-brot »
 

Offline sync

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Re: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter
« Reply #4 on: January 11, 2014, 02:18:14 pm »
Picture 17 shows what seems to be a crystal oscillator by the looks of it. It may also be a large capacitor or something else. There is no visible marking on it. The partlist lists it as reference element (no word on what kind of reference). There is an overview of connection diagrams, that finally gives a hint. The device is marked with a colordot on the side, indicating a kathode. Type number in the part list is "SZY 23", therefore I guess it is some kind of zenerdiode or something like this. In a metal can? Never saw something like this before.
It's a temperature compensated zener. The metal can is for hermetical sealing.
www.oppermann-electronic.de/assets/applets/szx23a.pdf
 

Offline SeanB

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Re: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter
« Reply #5 on: January 11, 2014, 02:27:33 pm »
The brown is a phenolic paper resin sheet, used for insulation and damping. Same with the spacers, though those are resin impregnated cotton sheet wound on a former.

Sad to say those capacitors, though made in January 1978, are likely to be now in need of replacement. You will have to cut the nice lacing cord used on them and replace them, though for now you could use them but at some point they will fail. Measure ripple voltage across them and it will likely be very high.

 

Offline schwarz-brot

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Re: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter
« Reply #6 on: January 12, 2014, 01:23:32 pm »
Pt. 2.2: Topless part-porn

Just some random shots of the beautiful parts in this device. before I lift out the board in the next post. Too many pictures to get it done in one. If you don't want that many pictures, just say it.





Picture 22: electrolytic cap


Picture 23: more ancient capacitor pornography


Picture 24: Moneyshot of two beatifully lined up rectifier bridges


Picture 25: This fuse is a bit corroded 


Picture 26: as promised, some more spacers underneath some transistor cans this time


Picture 27: Crude sovjet IC Packages 1: Flip-Flop Type U103D


Picture 28: Crude sovjet IC Packages 2: Operational Amplifier Type A109D


Picture 29: Crude sovjet IC Packages 3: Silicon Diode Type SA403


Next part:
Pt. 3: Flipping out the PCB
« Last Edit: January 12, 2014, 02:03:50 pm by schwarz-brot »
 

Offline schwarz-brot

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Re: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter
« Reply #7 on: January 12, 2014, 01:55:37 pm »
Pt. 3: Flipping out the PCB




Pic 30: Little metal tabs hold the board in place


Pic 31: rotary joints allow to flip up the board


Pic 32: And there it is


Pic 33: Display PCB directly mounted onto the large PCB

In part 2.1 i mentioned the spacers had to be unscrewed to take out the board. This was wrong, they are directly mounted on the PCB and can stay in place. There are four metal tabs (Picture 30) that hold the board down. I dislike this solution as these tabs tend to brake if you wiggle them too often. But hey, this shouldn't have to be done that many times.
When the tabs are open the board rotates in some nice joints in the housing (Picture 31). You have to flip it up 90 degrees to be able to take it out completely, else it will stay in place. This solution is well engineered.
To be able to tilt the board to the angle seen in picture 32 some cord has to be cut to give the wires more room. They must be renewed before using this unit again as this ensures the insulation of the metal housing in case of a brogen wire. Now we know, why everything is tied down so neatly.
Ontop the large PCB you see the display board (Picture 33). It is mounted with some brackets. No connectors anywhere! Every signal comes to this board with an individual wire...




Pic 34: dirty or corroded solderjoints


Pic 35: eeeew. Dislike! Yes, it is large.


Pic 36: possibly some joints were retouched?

What does 35 year old sovjet soldering look like? Not so nice... Some joints look really crusty, maybe the ones in Picture 34 are corroded? At least they are far from nice looking. Lots of gunk on the backside of the board. Don't even want to know what that blob in Picture 35 is!
Picture 36 shows something that is quite remarkable: There are lots of solderjoints that look a bit greasy, as you would expect after 35 years. But there are nearly as many that look nice and clean. My guess would be this board was reworked some time. Haven't checked the other PCBs yet.
But what can I say? All the joints relating to the faulty display element do look like they were not reworked while all the other display-related joints look nice and fresh. I for sure will point my iron there!




Pic 37: just for pure amusement


Next post will give the schematics of the top board and and overview of the functions.
Pt. 4: Schematics of the top PCB
« Last Edit: January 12, 2014, 06:01:53 pm by schwarz-brot »
 

Offline Vgkid

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Re: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter
« Reply #8 on: January 12, 2014, 03:55:32 pm »
Thanks for posting pics of the internals, I'm surprised to see so much flux on the board.
If you own any North Hills Electronics gear, message me. L&N Fan
 

Offline schwarz-brot

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Re: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter
« Reply #9 on: January 12, 2014, 05:55:51 pm »
I've got a few spare minutes, so on to

Pt. 4: Schematics of the top PCB


Pic 38: Partnumber overlay of top PCB (Reference 202)


Pic 39: Partnumber overlay of Display PCB (Reference 203)

Pictures 38 and 39 give you the partnumbers of any device on the boards, if you want to find them when looking at the schematics. The reference numbers 202 and 203 are the ones given in all documents relating to this unit.


Pic 39: roughly indicated sections of the board


Pic 40: Schematic power supply (reference: 202 FG5)


Pic 41: Schematic A/D Conversion (reference: 202 FG3)


Pic 42: Schematic digital output circuitry (reference: 202 FG4) and display board (reference: 203, no FG)

Picture 39 gives you again a top view of the top PCB. I roughly indicated the areas dedicated to FG3 to FG5, only for quick reference, don't be too harsh on any part sitting in the wrong section. Pictures 40 to 42 are the schematics of those indicated sections. Please note that the display board seems to be considered as part of the digital output section, although it has an own reference number (but not Section Number!).

After that, if you really want to dive in, I give you two .pdfs: The first one being the Parts-list, which is somehow incomplete as it only gives the items that could be ordered to repair this thing. Second is kind of a quality chart ("Qualitaetspass" ==> "quality passport") which gives all the numbers on rated accuracy. May be interesting, but be warned: everything is in German!

1: Part Numbers.pdf
2: Qualitätspass.pdf


That is it for today with miss Piggy, I don't want any trouble with my real miss  :scared:

Next:
Pt. 5.1: Lifting the skirt
« Last Edit: January 13, 2014, 06:34:04 pm by schwarz-brot »
 

Offline iDevice

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Re: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter
« Reply #10 on: January 12, 2014, 07:15:22 pm »
Thanks for posting pics of the internals, I'm surprised to see so much flux on the board.

Looks like standard tropicalization for that era to me.
 

Offline chickenHeadKnob

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Re: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter
« Reply #11 on: January 13, 2014, 03:54:09 am »

PIC 20: the S curve in the resistor leads allow it to accommodate thermal expansion from self-heating and move up or down accordion style without inducing as much pressure on the solder joints or resistor body. That would be my guess anyway.
 
 

Offline schwarz-brot

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Re: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter
« Reply #12 on: January 13, 2014, 03:00:09 pm »
Hang on guys, miss Piggy is about to get her skirt lifted!  >:D

Pt. 5.1: Lifting the skirt


Pic 43: Shielding between top and bottom PCB


Pic 44: again metal tabs to hold the Board in place


Pic 45: Shielding removed

When the top PCB is flipped up you can see on a large sheet of phenolic board underneath (Picture 43). This is obviously not the second PCB but a large shielding board. The copper side is directly connected to ground on the bottom PCB via a short wire. This board is mounted again with some metal tabs (Picture 44). There are six pairs of them. It is easy to remove the board but a pain in the a** to put it back on - trust me! Underneath you won't find anything interesting: Just the backside of the bottom PCB (Picture 45). It is remarkable this side is in way better condition then the top PCB was. The board looks quite clean, even though the mounting side looks as awful as the one on the top PCB did.
The Shielding is obviously needed to shield the bottom board against the digital circuitry above it.
So, nothing fancy here, time to lift miss Piggys' skirt and take off the bottom half of the case!




Pic 46: More Shielding on the mounting side of the bottom PCB


Pic 47: Again wired to ground


Pic 48: Bottom PCB without shielding

So underneath the skirt you will find...

... Panties. Of course no lady would leave the house without  ;)
Yepp, I am talking of shielding. Picture 46 shows you, what to expect after you open the case. Note the holes in the shield, whit a nice label beside every one. Attention to detail again. The sheet is again wired to ground (Picture 47). I am glad on this side it was mounted with screws. Makes it easy to take off and see inside the can (Picture 48). You get three seperate compartments in there.




Pic 49: in the can 1: precision resistors and caps


Pic 50: in the can 2: laaaaarge cap

One compartment takes the extended backs of two range switches ("0,2" and "2"). The next takes some thin film precision resistors and a few small capacitors (Picture 49). In the last one you only find one huge styroflex (is it everywhere called like this?) condensator. Never saw such a big one before (Picture 50)




Pic 51: Trimpot for zero compensation


Pic 52: precision resistive network

Some more random shots. Picture 51 shows the trimpot for zeroing the device if needed. Note the large head they put on there. You won't be able to miss it with your screwdriver. Again, attention to detail. Picture 52 is just a resistive network I found beautiful enough to take a picture of.

In the next post I will reveal the secret of the missing powerswitch. Or can you guess by now? Also I will show the main chips on this board.
Pt. 5.2: I want her naked!
« Last Edit: January 13, 2014, 06:32:17 pm by schwarz-brot »
 

Offline Vgkid

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Re: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter
« Reply #13 on: January 13, 2014, 03:38:30 pm »
What is the component in pic 47 marked 28?, it is beside the top smaller metal can. It has what appears 6 leads, dual fet/bjt.
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Online PA0PBZ

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Re: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter
« Reply #14 on: January 13, 2014, 03:42:06 pm »
What is the component in pic 47 marked 28?, it is beside the top smaller metal can. It has what appears 6 leads, dual fet/bjt.

If you look closer.... you've been fooled ;)

btw, I love the detailed pictures!
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Offline schwarz-brot

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Re: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter
« Reply #15 on: January 13, 2014, 04:01:57 pm »
I will give the schematics and the part overlay in one of my next posts when I bring this series to an end. These parts are actually numbers 289 and 290, you cannot see the 9 of the first number. It is two SC239 (german) in what I think is a current mirror configuration by a quick look at the schematics. It is low noise npn-Transistors. I will take an extra shot of these for my upcoming post.
 

Offline schwarz-brot

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Re: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter
« Reply #16 on: January 13, 2014, 04:20:56 pm »

Pic 53: Detail transistors

Picture 53 shows the requested transistors. Sorry, I was not allowed to edit and attach this picture to the last post. I hate doubleposting.
 

Offline schwarz-brot

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Re: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter
« Reply #17 on: January 13, 2014, 06:23:59 pm »
Pt. 5.2: I want her naked!





Pic 54: A wire with a partnumber?


Pic 55: These are no inductors!


Pic 56: More beautiful input networks


Pic 57: Trimcaps

Let me show you some input circuitry. Can you spot the resistor in Picture 54? Yes, this wire has a partnumber, it is actually a 0R1 shunt. This isn't the only shunt, it is actually a ladder construction, adding more shunts when switching through the input ranges. The three large coils in picture 55 are actually resistors! From left to right: 0R9, 9R and 90R, finally a standard thin film resistor with 900R which can be seen on the right.
Next we have some reference resistors (partnumber 211 to 218, horizontally lined up) for resistance measurements and a part of the voltage divider (numbers 219 to 231, group of 3x3) in picture 56. Once again only because it looks beautiful.
Picture 57 shows some trimcaps. These are the ones to be reached through the holes in the shielding. They are part of a frequency compensated AC voltage divider. This thing measures in the AC ranges accurate over a given frequency range of 45Hz up to 20kHz. Well, only sine, of course.




Pic 58: AC Amplification with OPamp Type A109D


Pic 59: DC Amplification with OPamp Type MAA725C

Some fun with operational amplifiers... Most of them are the A109D or B109D types we have seen before. AC amplification is done with a A109D (Picture 58). But take a look at the DC amplification: I rarely saw a metal can with so many legs. Beautiful! This one is Type MAA 725C (Picture 59). I could not find a datasheet for it.




Pic 60: Diodes coupled to share the heat


Pic 61: some more Diodes coupled together


Pic 62: Zenerdiodes bodged onto some caps


Pic 63: diagram of picture 62

All around the board you will find Diodes mounted in a way that allows for temperature coupling. And yes, these are actually diodes in picture 60.
In Picture 61 it is a whole bunch of them keeping each other warm.
Finally I found a bodge! These Diodes in picture 62 are soldered directly to the legs of the coupling caps. They are mounted in the same holes, but their second pin is soldered together on the fly, there is a hole missing in the PCB. I don't know if there is a pad on the other side. The caps are for coupling the output of the AC-amplifier into the rectifying circuit and bridged by those diodes (Picture 63). I am not sure what is going on there, but it looks like the diodes are purposefully mounted in a way that ensures their case to touch the lead of the capacitor. Maybe this is also some obscure temperature coupling? I don't know what to think of this.
But I can tell you they were really serious about anything relating to temperature drift. Follow me through the next pictures to see with your own eyes.



Pic 64: One more can to open


Pic 65: Some foam. What's underneath?


Pic 66: one glass tube, two cans, many contacts. Hmmm.

Remember this can in Picture 64? I did not forget to open it up. Inside you find mostly some foam (Pictures 65 & 66). And some not so obvious solutions. This is a oven that keeps two FETs at a constant temperature. The glass tube you see is mercury filled, it is a temperature sensor. Better not drop this thing! The FETs are integrated in a metal cube, brass by the looks of it. This cube is heated by a coil that is switched on and off via the mercury tube. The foam insulates this hole package, of course. But I think it also is used for shock absorbing purposes.
According to the manual the FETs form a temperature compensated (obviously!) differential amplifier stage that sits in front of the DC amplifier you saw in picture 59. This is supposed to give a high input impedance and minimal offset currents.

I am pretty sure, this is the reason for the missing powerswitch. Most of the parts in this device are chosen or tweaked for low drift over temperature. With an oven in a device without any ventholes you would get constantly changing temperatures if you switched it on and off regularly. But if it is always on, it will reach a stable point and stay there while being pretty immune to short-term ambient temperature changes. Not sure if this is exactly the reason, but that is what I suspect is going on there.


I will do one final part, hopefully tomorrow, to give you the schematics of this second PCB. Stay tuned.
« Last Edit: January 13, 2014, 06:30:03 pm by schwarz-brot »
 

Online PA0PBZ

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Re: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter
« Reply #18 on: January 13, 2014, 08:21:00 pm »
How does that mercury tube work? Maybe capacitive, because the mercury will not be able to move freely like in a mercury switch.
« Last Edit: January 13, 2014, 09:59:05 pm by PA0PBZ »
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Offline david77

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Re: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter
« Reply #19 on: January 13, 2014, 09:10:12 pm »
I suspect it is a kind of mechanical switch. As the mercury column expands it closes a contact and the heater is turned on off. I reckon the tiny red and yellow wires go to the switch contacts inside the glass tube.

Could be entirely wrong, though ;)
« Last Edit: January 14, 2014, 02:55:39 am by david77 »
 

Offline Alana

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Re: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter
« Reply #20 on: January 13, 2014, 09:37:36 pm »
Basically everything that is in modern multimeter but in discrete parts. I'm looking for repair results.
 

Offline Vgkid

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Re: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter
« Reply #21 on: January 13, 2014, 10:10:37 pm »
Thanks for the clarification with the dual bjt's
here is some info on the MAA725
http://www.amapro.cz/datove_zdroje/katalogy/katalog_tesla/katalog_tesla_57.php
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Online anotherlin

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Re: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter
« Reply #22 on: January 14, 2014, 09:13:36 am »
Really nice !
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Offline schwarz-brot

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Re: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter
« Reply #23 on: January 16, 2014, 10:14:31 am »
Sorry for not posting, folks. I had some very nice hours with my miss Piggy. We even came to the point where the hole Display was working.

As I mentioned before I suspected some bad solderjoints to be the problem. I then resoldered everything that is somehow connected to any element in the display: The driver ICs, every resistor, the transistors, every wire and the 7-segment elements themselves.


Pic 67: What the display should look like

Sure enough, it all worked after that. Look at Picture 67: The only thing photoshopped is the size. Every Element worked, trust me!
I then built everything back into its right place, only to find the same elements as before to be not working. WTF?! I grabbed my DMM and started measuring. Every voltage was where it should be, but three of the four related transistors hat a somehow lower BE-Voltage. I ripped them out and replaced them with BC550's I had at hand. All the elements lit up. A bit dim, but they are there. Built in - elements dead. AAAAARGH  :scared:

Ok, take a breath. Measure again. Everything is fine, these suckers just do not work as they are intended to do. Checking again, one element came on, another one on another 7-seg died. Came on again a little later. WHY?!

Again a soldering-party. Retouched every joint again, resolderd even every single track on the display PCB (No soldermask on there). Turn it on, some random elements dead.
I then decided to rebuild the whole wire harness connecting PCB 201 with the Display board. I had seen it was built with solid wires, maybe there were some broken ones? Percussive maintenance would sometimes help to turn on or off a random element. Seemed to be a possible problem. This took me the whole afternoon yesterday. Yes, I tried to bind my wires in the same way it was originally done. Does not even look too bad (pictures will follow later). Good luck I beeped every wire before I ripped them out - the display module is not original! It is not connected in the same way the manual said. The 7-seg-displays are not the ones listed in the part-list. So maybe this is the reason why so many solderjoints seemed to be retouched? Maybe someone had this issue before? But I am sure this was done by someone who knew what he was doing. Well, 40 Wires later - it worked!





At least for about a hour. Now its the same as before.
 :-BROKE I then went a bit crazy and again, with some percussion  :box: some more elements went dead. But this time I pressed directly onto a 7-seg and this element went completely dead after that. But these were resoldered several times by now. I can only suspect the elements being bad themselves or possibly their leads to be corroded and not taking the solder as they should.

At the moment I think of rebuilding the hole display-PCB on stripboard. Maybe I should get me a desoldering gun first.

That's where I am right now. I cannot simply bodge in some modern 7-segs as the original ones have a very uncommon footprint. They are not only wider than normal while not being very high but also have an unusual pin-arangement. Also, I want to keep the device as original as possible on one hand. On the other, if I start to go crazy with this repair, I am going to replace every electrolytic as well. Hmmmm.



For the temperature switch: As far as I could find out it works like a usual merury thermometer. Inside is the mercury on one side, some other compressible stuff on the other. The hotter it gets the more the mercury will expand, taking more room in the tube and eventually reach the switching contact which than gets closed. Please refer to Picture 66:
The small red wire goes to the lower contact ring on the glass tube, the yellow one to the upper. Both contacts can be seen on the outside of the tube, the lower as a ring, the other in form of an eight. These Contacts also have a Pin that is embedded in the glass and reaches to the inside of that tube to allow contact with the mercury. On my photo these inner contacts cannot be seen, sorry for that. On the right side one can see a larger cave inside the tube. This is a kind of blast protection: It simply gives more room for the materials inside the tube to compress. This information on general function is taken from an Wikipedia article (german only!)
In the G-1001.500 this tube is part of a two-point-temperature regulator. The mercury tube is obviously the temperature sensor. It is on one end embedded in a metal block. This has holes for the transistor cans (they can be seen on the left). Also there is a coil on the round part of the metal, connected to the big red and clear wires. All the other wires are for the FETs of course. If the mercury does not reach the contact (that means it is open), the coil will heat up the construction. The large piece of metal gives some thermal mass for speeding down the effects of heating up and cooling down a bit (giving some kind of hysteresis). Then, when the mercury closes the contact, the coil is shut down. The device starts to cool down, eventually open the contact again to restart the cycle. This Switch is built to hold the construction at about 55°C.

I especially like the fact that this construction cannot wear out. Very reliably as long as you don't breake the tube. Which is nearly impossible. Just see how it is held in place by the flexible wiring, a drop of glue and completely surrounded with nice soft foam. Even if the PCB brakes it sits safe in its own can with the leads mechanical uncoupled.
« Last Edit: January 16, 2014, 10:48:46 am by schwarz-brot »
 

Offline PA4TIM

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Re: Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter
« Reply #24 on: January 16, 2014, 11:00:38 am »
That are very good pictures. Some are pure art like the line of rectifiers. What do you use for that (camara, lense and light)
www.pa4tim.nl my collection measurement gear and experiments Also lots of info about network analyse
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