Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter

[schwarz-brot]

It is an 2005 vintage Konica Minolta (screw you, SONY!!!) Dynax 7D, also known as Maxxum 7D or Alpha 7Digital, with its 28-100mm cheapy kit-lense that I only use for the pictures covering a larger area as it is just crap. Every detail I shoot with an early nineties 100mm / 1:2.8 AF Macro lense. This is an absolutely perfect lense and I was very lucky to get it for just 200€. Back then these things went for 500€ to 800€ on Ebay. Canon or Nikon would proudly stick this lense into a white body and paint a golden ring on it. Not well suited for portraits though, as you're going to see every single pore in the skin. An what is inside that pore. Urgh   

I use the built in flash and my daylight-coloured bench-lighting.

Everything in this topic is shot free hand. I do not own any more equipment as I figured out very soon that this is a pricey hobby and I would not carry such a huge and heavy camera on my motorcycle or while hiking. Since then I always looked at smaller cameras but never really wanted to start again, or when I decided to, had no money for that. What I have is perfect for macro-shots as you see, and this is what I especially like. So maybe I will get me a decent tripod some day and build me a ring-flash. We will see. This is something I like, but for sure no major hobby.

[PA0PBZ]

Thank you for the explanation of the temperature switch. Since the pin beloning to the yellow wire is not visible and the 'strange' figure 8 I thought that it maybe was some kind of capacitive detection. Mystery solved!

[Vgkid]

Congratulations on getting it running correctly. I found the explenation on the ovenized dual fets to be very enlightening.

[schwarz-brot]

Finally I come to

Pt. 6: Schematics of the bottom PCB




Pic 67: Partnumber Overlay of Input PCB (Reference 201)

Again I give you first the Overlay with the partnumbers for reference. The bottom PCB has reference number 201.




Pic 68: Roughly indicated Board sections. Everything not marked is Inputselection, Signalconditioning and analog Signalanalysis


Pic 69: Schematics of the input section (Reference 201 FG 1)


Pic 70: Schematics of the rectifying and averaging circuits (Reference 201 FG 2)

Picture 68 gives you an indication of where the single sections can be found. The relating schematics are given in pictures 69 & 70. Note that the functional group FG 2 contains two separate areas on the PCB. Both, rectifying (red) and averaging (green) are for preconditioning the signal for the A/D conversion. The rest of the PCB corresponds to FG1 and does everything related to range and function selection. The signal is routed to the corresponding circuitsections and treated depending on the choices made by the user on the front panel. Every measurement is to be based on a equivalent DC-voltage measurement of a maximum of +/- 2V.




Pic 71: Schematics resistance input protection circuit (Reference 214 FG 6

Picture 71 gives the schematics of an input protection circuit which is missing in my device. I guess this can be found in later units. It is obviously an extra PCB (Reference 214), and FG 6 indicates it was kind of an afterthought / retouch thing. The Fuse number 35 can be found on the Fuse-PCB and relates to the resistor measurement jacks. Maybe this PCB was exchanged in later units. I don't see much room for an extra PCB elsewhere. Whatsoever, this board is solely dedicated to protect the input circuitry from voltages mistakingly applied during resistance measurements.


At this stage the walkthrough is complece, nothing more to mention on this device. I welcome every comment. As I said before, every comment gives lots of motivation and kills the feeling of doing all this work for nothing.

Thank you Sync, SeanB, chickenHeadKnob, Vgkid for pointing out some Details and giving helpfull links. And also thank you to everyone else for your kind words!









Pic 72: Replaced Transistors


Pic 73: 40 wires to solder and bind to a harness


Pic 74: the selfmade wireharness


 The last pictures I give you are just for amusement and show my attempts to bring the display to live again as descriebed in an earlier post. The replaced transistors in Picture 72 and the rebuilt wire harness in progress (Picture 73) and completed (Picture 74). I tried to use the same technique as the original wire harness featured as I wanted to try it and also wanted to restore the original look and feel of the device. Not too easy but I managed to get the binding right somehow. If you get used to it it is not even a slower technique than using cable straps. My wire is flexible 0,25mm² which is way bigger than the original wiring but was the smallest I had on hand. Barely fits in 

As I said before, maybe I will retouch this thing completely, but this can be done later. I need a well working Bench Multimeter and have another candidate waiting right now: The Philips PM 2422 A. Same era, same ranges and accuracy, but nixie fun. Also seems to be working better. I guess this is a nice candidate for the next teardown 


See you,
Jan

[daqq]

Thanks for the very nice teardown so far! I especially liked the temperature switch

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

It's Czechoslovakian Tesla's version of the (LM)725 OP AMP. If needed they SHOULD be interchangeable.

[Vgkid]

Nice job on the wire wrapping.
Daqq: Can i safely assume that most Tesla parts have equivalents to Western companies?

[daqq]

Daqq: Can i safely assume that most Tesla parts have equivalents to Western companies?

Hmm... a lot of the standard stuff (logic, op amps, simple analog, some power (78xx), misc...) has a similar number to an equivalent western device. Transistors had very different numberings, diodes as well AFAIK. They did have special stuff (hybrid devices, precision stuff) that had a special numbering as well.

For instance the 74xx series is just MH74xx (optional extra text). The 8080 has its clone in MHB8080A.

Now Soviet parts had very different numbers, although a lot of them were pin compatible ( K155?A3 = 7400 ). Mechanically, I've heard that some used a 2.5 mm pitch instead of 2.54 mm. Could be wrong though.

[Thilo78]

Hey guys,
this teardown is really great.

I'm looking forward to the delivery of my G-1001.500, which should arrive today or tomorrow.

I paid a bit more, but expect a working meter. 

@schwarz-brot:
I read about the missing power button in these units. Do you plan to add one for future use?
Or do you leave it on stand-by duty as it was originally designed?

Thilo

[mamalala]

I could swear i have seen those 7-segment displays for sale as NOS on eBay some while ago. I remember that due to the uncommon way the segments are built (two short segments instead of a single longer one, for each of the seven segments) as well as the pin arrangement. There are tons of old GDR parts floating around out there on various sales channels. Too bad i never bookmark that stuff..

Greetings,

Chris

[mamalala]

Ahh, found some of these displays:

http://www.ebay.de/sch/i.html?_odkw=vqb+71&_osacat=0&_from=R40&_nkw=vqb+71&_sacat=0

Greetings,

Chris

Edit: Just saw that Oppermann has some of these displays as well:

http://www.oppermann-electronic.de/html/halbleiter.html

(Search for "VQB 71" on that page, 0,95€ per piece)

Edit 2: Damn, scratch that. Just saw that in your display-board you have common-anode displays, while the 71 is common cathode...

[Thilo78]

Chris,
thanks for the info anyway.

It's always helpful to identify parts of that kind.

It seems that VQB71 is the common-cathode numeric display, while VQB73 is the +/- display.

Interesting thing here: The parts list posted by schwarz-brot earlier states the VQB71 and 73 for the display.
And these are, as far as I could figure out, all specified as common cathode. (source: http://www.mikrocontroller.net/topic/215116)

I wonder how this fits together. Going to find out as soon as my G-1001 is here 

Edit: Apparently people are not quite sure about the setup. In some Eagle libraries (http://www.mikrocontroller.net/articles/Eagle-Bibliotheken#RFT-Opto_Bauelemente) the VQB71 are stated as common-anode, while the common-cathode displays are referred to as VQB17.

Interesting...

Does anyone happen to have a copy of the original RFT data sheets?

[mamalala]

Chris,
thanks for the info anyway.

It's always helpful to identify parts of that kind.

It seems that VQB71 is the common-cathode numeric display, while VQB73 is the +/- display.

Interesting thing here: The parts list posted by schwarz-brot earlier states the VQB71 and 73 for the display.
And these are, as far as I could figure out, all specified as common cathode. (source: http://www.mikrocontroller.net/topic/215116)

I wonder how this fits together. Going to find out as soon as my G-1001 is here 

Edit: Apparently people are not quite sure about the setup. In some Eagle libraries (http://www.mikrocontroller.net/articles/Eagle-Bibliotheken#RFT-Opto_Bauelemente) the VQB71 are stated as common-anode, while the common-cathode displays are referred to as VQB17.

Interesting...

Does anyone happen to have a copy of the original RFT data sheets?

Yeah, it's all rather strange. In his post where you can see the schematic, https://www.eevblog.com/forum/testgear/vintage-teardown-and-hopefully-repair-rft-g-1001-500-benchmultimeter/msg366706/#msg366706, it is shown as VQB 84 for the digits, and VQB 85 for the +/- sign. But in the partlist in the same post, it is listed as VQB 71/73. Also strange is that there is nothing to find for VQB 84 on the net, at least i couldn't find anything. Also, all the displays in that style that i have seen so far have always been VQB 71...

But then i guess it might be worth a try to grab some of these displays and try them. If it doesn't work one would still have some rather curious display type for some project... Or maybe the U121D's have enough drive strength to drive the displays directly. In that case just bypass the transistors and change the former base resistors...

Greetings,

Chris

Edit: But then, who knows, maybe they thought "Damn those imperialists, we'll show them and swap the meaning for anode and cathode!" :-D After all, IIRC the pin pitch on old GDR IC's/headers is 2.5mm instead of 2.54mm. At least i had that experience a few times, when the standard 0.1" spacing turned out to be just a tad too much after 10 or so pins...

[Thilo78]

The pin pitch topic is quite common with eastern tech.
Especially the 2.5 mm pitch ist quite common, apparently.

btw: As for the wire bonding I can top that with my today's arrival:
The Meratronik Electronic Multimeter U726, made in Poland in 1976 (thus two years older than me).

I popped it open, as it's missing the power cord, which would attach to a strange DIN-like socket. So I replaced the socket with a nice fabric-clad lead, and added a more solid earth connection to the aluminium case.

They made the internal wire harness from solid wire, bound with blank wire. Looks neat.
I'll see top post the pics as soon as I have downloaded them.

Works fine, but I'll need to do some adjustments. The 30V range seems quite off.

That will certainly help me pass time until the G-1001.500 arrives. 

[FrankenPC]

Too bad you can't apply a patina to the replaced transistors.

[Thilo78]

Aaaand.... here is my part in this game:

My very own G-1001.500 arrived today, only minutes after I harrassed the seller about a maybe lost package 
Shame on me for my impatience 

Although I did not do an extensive teardown like schwarz-brot did, I have some pics for comparison.

I think it's very close to mint condition, except  some scars on the front buttons and a few scratches on the covers.
And: It came with some goodies 

[Thilo78]

The goodies:
Spare VQB71 display units! 
And some more pics...

[Thilo78]

I powered it up and set my power supply to just about 5 V DC. The U729 is hooked up in parallel for comparison.
Please note: The seller informed me, that the G-1001.500 has recently been calibrated. So it shows that the U729 is quite good for its state 

Only problem: One of the fuses is already quite corroded. I'll see to replace it asap.
btw: I just noticed, that it's exactly the same fuse as with schwarz-brot's unit. Interesting...

[mamalala]

Congratulations, it really looks like almost new.

Still wondering about the VBQ 71's... Are they common anode or cathode, and are the ones in the unit really 71's as per the parts list, or 84's, as per the schematic? Another thing occured to me. It's no secret that east Germany had to make do with what they could get/make. Maybe there are indeed different variants of the overall display/drive electronics for this meter, for two different types of displays? Like, ran out of one type? Do the other version with the other type. Just a wild guess.

Greetings,

Chris

[schwarz-brot]

Chris,
the G1001.500 was produced in reasonable volume over a long period of time. The documents I linked in are actually for different versions of this meter. The circuits got updated from time to time. The Service manual gives some hints about differences (kind of errata...) but not enough to tell exactly what was changed when.

[Thilo78]

Congratulations, it really looks like almost new.

Thanks! That was quite a catch, and I'm happy to have it on my desk.

I'm quite sure, that schwarz-brot's unit will look similar when he's finished with his setup 

Still wondering about the VBQ 71's... Are they common anode or cathode, and are the ones in the unit really 71's as per the parts list, or 84's, as per the schematic?

I'm still wondering myself.

There seems to be but one option: Pop it open and measure the voltages at the display unit...
Not too happy with this approach, but maybe I'll find a way to figure it out without shorting the VQBs.

But about one thing I'm quite sure as of my experience with German equipment of the era before 1990:
Those guys knew exactly what they are doing. If they have chosen one way or another, there's a good reason behind it, although we as "modern" engineers might have a hard time understanding it.

Material availability has always been a common issue, from the 50s through to the 90s, and they always have found clever ways for serial production of reliable gear. (that's confirmed by what I've learned from my father, who headed a development department with Philips' Process and Machinery Automation section, and his colleagues)

I'm curious what I'll find while inspecting my new toys tools 

[Thilo78]

OK, I have done some measurements on the G-1001.

I figured out, that the reference point 7 (see schematic) is about ground potential, named Cp.

Given that, the upper rail for the LEDs is supposed to be at +7.5 V, and reads between 7.0 and 8.0 V with my Fluke 1587.
I'll take that as good level.

Further on, this level shows up on the common point of the LEDs.
Interestingly, the LEDs show a voltage drop of close to 3 V, which seems quite a bit from my point of view.

We'll have to investigate that further, I think, unless somebody manages to find data sheet.

As a conclusion I'd say that at least my unit features a proper common anode configuration.

[mamalala]

Interestingly, the LEDs show a voltage drop of close to 3 V, which seems quite a bit from my point of view.

Since each segment is made out of two smaller segments, i guess that these are in series, and  thus you have 2 times Vf as drop.

Have you looked at the spare VQB 71's that you got? Are they common anode as well, or are they common cathode, as the image there in µC.net says?

Greetings,

Chris

[Thilo78]

Have you looked at the spare VQB 71's that you got?
Are they common anode as well, or are they common cathode, as the image there in µC.net says?

Yes, I have in the meantime.
It's a little bit fiddly, as the display are quite fragile without any supporting structure like a PCB.
Modern displays are a bit more sturdy due to the shorter leads.

I hooked one up with 5 V DC over a 380 Ohms resistor to the central contact, which I suspected to be the common anode.
When touching the segment lead with a grounded clip, the segment lights up nice and bright. 

Doing it the other way round showed no result.

So, it has to be common anode.

Trial and error shows that half of the data sheets I found are wrong 
Or (which is possible) they changed the specs over the time, keeping the part number.
Like: "What the heck? The old ones sold out. Let's do it the other way round for the lulz!" 

Edit:

Since each segment is made out of two smaller segments, i guess that these are in series, and  thus you have 2 times Vf as drop.

Yep, seems to be the answer. All segments are actually two LEDs.

Open questions:
>What is the actual difference between VQB71 and VQB84?
Judging from the schematics, they're compatible/exchangable.

>Why are there tables stating the VQB71 to be common cathode?

[Thilo78]

I just found another site, which appears to be rather reliable:
http://www-user.tu-chemnitz.de/~heha/bastelecke/Konsumg%C3%BCter-Bastelei/DDR-Halbleiter/

The page is hosted by Technical University Chemnitz and collects technical data of GDR semiconductors.

For VQB71 it has this data sheet:


It says Common Anode there...

The label "Nicht für Neuentwicklungen" means something like "not suitable for new design".
Maybe the VBQ71 has been discontinued and replaced by the VQB84...

I also just found out what purpose the single VQA12 serves: It's an additional decimal point next to the algebraic sign, used for the 0.2 V/mA/kOhm range 
Interesting UI concept.

[Thilo78]

... The inside is covered with some brown non conductive sheet (Picture 12). Does not feel like plastic, any ideas what we got there? ...

Hey, I completely missed that question which seems to be unanswered. 

I'd think that this is a layer of Pertinax, an insulation material similar to Bakelite.
It's a composite of phenolic resin and paper, and it's a predecessor of current FR4 material (which replaces the paper by glass fibre and the phenolic resin by epoxy resin)
I don't know if there's a proper English name of that, but I found something like "synthetic resin bonded paper" or "SRBP"

It was used for insulation layers like here, for separations within measuring or power equipment, and for circuit boards.