Teardown and repair of a Gossen Metrawatt Metratop 51 (Model M2510) benchtop DMM

[Stefan .]

The Metratop 51 is one of three Models  build into this kind of case.
Other models are the Metratop 52 with a second channel for voltage measurements and the Metratop 53 with a calibration generator build in. But these seem to be very rare, close to non existing on the market.


The Device unpowered. Look at this german baroque beauty of the early 2000s.


The backside with unfilled slots for options.

It features a 5 3/4 Digit Display (updated 2/sec at full resolution), a RS232 Port, 2 and 4-wire Ohms, temperature, frequency and capacity.

Build in are a real-time clock, one can use it as counter, stop-watch and store measuring in the internal memory.
Unfortunately, the stored measurings are only accessible via Computer.

The Display is a graphical one with 128*64 pixel resolution (containing a KS0108 Chipset) and originally yellow/green illuminated. The light can be switched on and off separately.

The instrument can be upgraded with a batterypack (NI/CD or NIM/H) which is charged when the meter is plugged into mains.
Also available ist a scanner option, a modem and/or fax connection. It is also possible to retrofit a IEEE-488 Controller.


Options one could buy.

Inside the instrument are 2 multi-layer circuit boards:

• the mainboard containing the powersupply and mainly analog (measuring) circuits for the frontend
• the display/keypad board with the main processor

Mainboard, Processor part of display board, more display board logic, display board with pushbutton traces.

The power supply is an interesting build.
The mains transformer (capable 120 and 240 volts) has 2 separate windings on the secondary (6v) wich are connected in parallel. A bridge rectifier and a cap do their job perfectly to make a stable 9v.
Afterwards is some kind of battery charging circuit and a switch-mode DC to DC converter.

From separate taps of the transformer are the voltages for the logic and the analog fed to the circuits.

The device is always on hence the power cord is plugged in; it only does a soft-power off.
Weird enough, the realtime clock (M6242B) is not battery buffered so every time you pull the plug the clock needs to be set.

The digital controls on the display board are done via a 80C552 8-bit 8051-like controller. This controller drives the display, scans the keyboard, does the RS232 and communicates with the analog frontend. It runs at 11.05MHz.
The firmware is stored in a flash (!) memory 29F010, a 128k static RAM is used for data. As for the clock, there is no battery buffer for the RAM. (More than weird again, because these two chips are powered separately from the supply.)
Three temperature sensors are around the processor, one is used as internal reference for the external probe. I can’t really imagine what for the others are.

The analog (measuring) and digital circuits are completely separated on the board, commands from the front panel controller to the measurement controller are transfered via a two induction loops near the front of the instrument.


induction loop for coupling the two functional parts.

The analog (measuring) part of the board is controlled by a 8032 controller running at 11.05MHz. Software is stored again in a flash 29F010, 8k static RAM is attached.


controller on the analog part of the mainboard. In the top/middle of the picture is a GMC custom chip. All other parts used are pretty standard.

The whole instrument is bodged all over. Several desolderings have been done, new components put in place, there is one bodge-wire on the display board feeding some components with +5V.

Overall, the whole instrument ist a very nice an accurate one. The size and weight is comparable to a Keithley 2000 and tons of features for a former, very reasonable low price of about 500,- Euros makes it a bargain nowadays.


more pictures. The underside does not contain any parts.

The Instrument I own has had a defect and was sold as defective.
Some investigations, starting with the power supply and display-board gave at first no clue what could be wrong with the device.
I could power it on, push buttons and switch the modes, but the display didn’t show anything except the backlight.
Digging further on I got aware of the fact, that the power-button ist not only scanned by the processor but also works as a push-button for the power supply.
When pressing the power button, the logic for the power supply puts 5V to the display/processor board, the processor boots up and holds the logic for the power supply up.

So, my conclusion was, that the processor did work properly.

A short support call to Gossen gave me the answer I expected: the display should be defective. A repair at Gossen would cost 250,- €, the display only would be around 130,- €.
After some digging in google I found various compatible displays from 13,- to 19,- € to purchase. I got 3 variants and they all worked well with the instrument.
Except the dimension (thickness) of the displays differ from the original, I found the grey/blue display with the white backlight fits the best and does not bulge the foil in the front of the display.


Defective display. Does not generate the -9V contrast voltage any more.

Some fiddling with the contrast voltage lead to a now very fine working instrument. 


Working fine and spot-on compared to my Fluke 289 and Keithley 2000

Feel free to browse through my Flickr Gallery (http://www.flickr.com/photos/uferst/sets/72157642914867723/) to see more photos and the specs of the instrument.

Sorry for my bad english, I'm not a native speaker and may use wrong words or grammar during this text. Please feel free to comment. 

[GreyWoolfe]

Good job on the repair, that is a nice meter.  don't worry about your English, it is just fine.  Your English is better than some native speakers that I have had contact with.

[electronics man]

it loks a bit ugly and cheep from the front, im sure it is very good and must cost a fortune but they could have chosen a better design, but mabe it just the the "style" of the time, how old is it?

[Stefan .]

It looks very ugly, indeed.

But it is build up properly and reliable, the outer case is made of steel and can be removed by unscrewing only one screw in the lower back, the inner frame is made of steel and very well made.
The board is perfectly made except for the lack of any silkscreen. All components on the board are perfectly soldered and of high quality as far as I can tell.

If you have a closer look at the keypad you will be a bit confused because the keys themselves only have numbers written on it.
In fact, these number don't have any fuction, the functinallity of the key is written above on the front bezel.

I'm not sure how long and since when these instruments were build, some chips have datecode from 1999 on it, the firmware is a release from end of 2001.

One can hardly find any information on the internet about this instrument, luckily I've got the original usermanual for it which I assume is revision 5 from february 2000.
Unfortunately I didn't find any kind of service information; Gossen themselves do not provide some. But, even on their website, there is no mention of these instruments!

More information on the microprocessor used in the Display/Controlboard can be found here: www.nxp.com/documents/data_sheet/80C552_83C552.pdf?
It's a Single-chip 8-bit microcontroller with 10-bit A/D, capture/compare timer, high-speed outputs, PWM. For what ever reason these features are usefull in this instrument.
Maybe for the optional 8-port scannercard.

There are also two different 1.2V Voltage references used in the design, one presumably for the Controlprocessor on the display board, the other one for the analog/measuring part on the mainboard.
One is located near the custom GMC-Chip, the other one is located in the powersuply part of the mainboard.


GMC custom Chip and voltage reference (middle-left, silver case, below the blue caps)

For the fun of it, here is a closeup of one of the bodges on the board.
This is located near the powersupply and has to do with some of the power-on logic.


And no, it wasn't definately not me doing this!

Keep in mind, that many things on the instrument are just there to support some of the options. I'm sure that there are tons of unused routines in the firmware because of the lack of options in my instrument.
The count of 4 unused connectors and the presence of two multicolor LEDS on the front panel (bare visible when on through the bezel) speak for their own.

[retiredcaps]

Welcome and thanks for sharing your repair efforts and excellent photos. 

[patrik]

hi Stefan - I've got such an Meter a few weeks ago and I guess we got it from the same source 

The bodging is in mine as well, but missing the Tantal - luckily it works fine, so I don't bother to play around fixing that..


Unfortunately I din't get the manual, so I had to do some trial and error to get more insight. 
Is there by chance a list in the manual showing the SCPI/UART commands included ?

FYI: By going tru the SCPI specs I did find some working ones, still the device doesn't seem to support the full specification.
(given its age its not too bad that it understands some of them..):


*IDN? -> gives you the Device ID + FW versions

SYST:TIME? -> Current time

SYST:TIME H,M,S  -> sets time to H:M.S
Example SYST:TIME 12,30,30  -> sets time to 12:30.30

SYST:DATE? -> Current Date

SYST:DATE D,M,Y -> sets date to D.M.Y
Example: SYST:DATE 23,03,14 -> sets date to 23.03.14

SYSTEM:BATT  -> ? YES  => There was maybe a battery option available in the past
SYST:LOC -> YES => would set the control back to the local control, haven't found how to get it to remote..


Any command which may give access to the logging feature is not understood by the Device, so I'm guessing that these are using a proprietary format.


As you write the existence of the Device isn't visible by any means from Gossens Homepage and the offered software is too new to have still support for the Meter. Also the only document I found is a spec sheet for the entire Metratop 5x line. So also no possibility of further reverse engineering - I do not want to go that far and try to read out the controller.


Nevertheless so far I'm quite happy with it and it does a decent job on my bench 
 

[Vgkid]

Thanks for the teardown, it is a rather peculiar looking meter.

[Stefan .]

@Patrik: I will do a copy of the manual asap and post it here. The commands are all listed in the manual, examples are given.

@all: thank you for the warm welcome.
I've read in the forum for a while and gathered some information and hints for some of my other test gear. So I thought to give back something to the community.
As I am not a professional EE (but willing to understand everything!) one might forgive me false descriptions and/or wording.

[rsjsouza]

Stefan, thanks for sharing the teardown and the repair. This may encourage/inspire others to dig further before assuming a test equipment is doomed to be trashed.

To me the look resembles HMI-PLC modules, and it was probably designed with this idea in mind. But at least the new white backlight LCD has a more 2010s look...

[Vgkid]

Hopefully the manual will have specs in it.

[Stefan .]

Well, OK, I mentioned it in my first post; on my Flickr set I already posted the specs of the instrument.

Nevermind I do post them here:

 

[Stefan .]

Okay... I'd love to upload the instruction manual .PDF here, but with around 4MB it's too big to upload.

Please could someone be so kind and tell me a proper way to share it with you?

Thanks!

I uploaded the manual to KO4BB, as soon as it is sorted into the correct directory, it is searchable and I will post the link to the manual here.

[patrik]

The PDFs are available since today, all the supported commands are completely described in chapter 16:
http://www.ko4bb.com/manuals/index.php?dir=09)_Misc_Test_Equipment/Gossen

My Meter is accepting configuration and spilling out the values via the serial port - online and stored via the MEM option.


That was more I dared to ask and really helpful - Big thank you to Stefan ! 

[Stefan .]

I've been asked via PM which display might fit. As the answer seems to be interesting for everyone, I will post it here.

There are many displays out there which will fit electrically but may not fit in size.

Basically any Display with 128*64 and KS0108 controller (20 pin header) will work, but there are restrictions regarding the size of the PCB and display area and the position of the connector.
The PCB must have the size of 93mm widht to 70mm height and shall not exceed the thickness of 13mm overall.

The german based company Pollin Electronic sells some Displays which will fit.

I found, that this one: http://www.pollin.de/shop/dt/NDc1OTc4OTk-/Bauelemente_Bauteile/Aktive_Bauelemente/Displays/LCD_Modul_TG12864B_13B.html fits the best.
Although the display is a different colour (see my pictures above) than the original one, there are only little things to do to make it a perfect replacement.

There is another one with the original colour (yellow/green), but it is slightly thicker and will bulge the front bezel if you put everything together. The bulge will also effect the keypad so the keys will get a bit wobbly.
I do not recommend this: http://www.pollin.de/shop/dt/Njc1OTc4OTk-/Bauelemente_Bauteile/Aktive_Bauelemente/Displays/LCD_Modul_TG12864B_05.html

I found a seller at Ebay who sells compatible displays for an even lower price than Pollin. The Display has the correct color and thickness but I didn't bother to implement it to my instrument because the grey/blue/white one looks much nicer than the green/yellow. 

You can just search at ebay for LCD 12864 and will get a lot of results; take a close view at the display I showed in my thread and compare to the ones sold from china or somewhere else; most likely they will fit electrically. The thickness is the point, make sure not to exceed 13mm! Measure the thickness of the original one, be aware of the stickyness of the front bezel and that it's kind of glued to the display at the corners.

Once the display is connected to the instrument you might see that the contrast ist much too high.
The instrument controlls the contrast via the microcontroller (see manual for setting contrast), but the circuitry does not match the requirements of the new display.
I simply bodged two resitors directly to the connector of the display althought the contrast can no longer fully be set via menu.

I hope this helps everyone and I wish you good luck repairing your instrument. Show us some pictures!

This thread will be continued for sure as there are more traps to be found while repairing this thing. 

[EEVblog]

That's a river of isolation!

[Bored@Work]

That's a river of isolation!

Yes, calls for reenacting a scene from Crocodile Dundee:

You call that isolation <showing a 5V wall wart>?! This is isolation <showing the Metratop 51>!

[Stefan .]

One advice for users trying to replace the Display.

As I mentioned earlier, there might be a problem with the new display and the processor controlled contrast voltage.
This can easily be fixed by just adding a resistor between two pins on the display forcing the contrast voltage down (or up, depending on your display) to zero volts.

The second fail is, that if you buy the white/grey display I used to repair, the backlight connections are configured wrong.

The Datasheet of the display is wrong, + and - backlight are the other way round! There are a couple of solderjumpers on the board to configure the correct polarity.

Be aware of these things, don't trust the datasheet and good luck with the repair!

[experience2080]

Hi,

what resistor value and pins did you use for the contrast? Got the same problem, but the contrast is not perfect yet.

Thanks.

[Chipguy]

Nicely done, I like that article 

[experience2080]

Finally i fixed my Metratop 51 today.

I also dumped the Flash from the Device, if anybody needs them for repair.

Heres the main multimeter firmware: http://dump.elektroda.pl/download15069.html

Eeprom Date: 06.11.1998 (!)

Firmware from the keypad PCB coming soon.

[Stefan .]

There is one Metratop for sale on ebay germany. Looks like it has the usual (?) faulty display.

eBay auction: #221697562265

And no, its not mine! 

[v1t0r]

I bought the Metratop of the auction mentioned in the previous post, but the problem is not related to the display it seems is related to the power supply
which causes the device to be constantly restarting.



Should I try to replace the transformer?

[v1t0r]

Bought and replaces the original display for one that got on ebay (http://www.artronic.com.pl/o_produkcie.php?id=630), but the problem in question remains the same.
when I plug in the ac cable the device turns on immediately and the display freezes, checked the switching power supply at first glance appears to be functioning properly.




Anyone know if there is a service manual for this unit?

[coromonadalix]

boy i would like to posses this one, i love rough looks and feels  loll

Having the same custom ic gmcad30 than the MetraHit 25s 26s and 28s teardowns

[coromonadalix]

Have you sorted out the problem it's been a while ...

[slurry]

I think it is beautiful, and as it is a  Gossen Metrawatt, you gotta love it!

[coromonadalix]

Took me many years to get one from germany in 2023, and another was sold in 2024 in germany ...

very rare indeed

[5U4GB]

it loks a bit ugly and cheep from the front, im sure it is very good and must cost a fortune but they could have chosen a better design,

From another thread on here, can't find the original post any more:

[coromonadalix]

the MetraTop series come / inspired from the 28s 29s  series,  mostly from their same asic and the resolution at 310,000 counts


as for the price  loll     

My new 28s  costed 30% less than the TOP 51,  but costed a lot  ... with an good import fee  ...  Euro to Cad   outch outch outch

Gossen are a niche and very expensive, even the accessories as i wrote in the past are ridiculously expensive(s)

Metrix are not far away too

i would love to get my hand on a Gossen 30M   .....  anyone  loll