Teardown of a rebadged Yokogawa (?) multimeter

[LazyJack]

Hi,

This was my first multimeter. I got it from (then) West Germany nearly 30 years ago. It is branded "Universum" which was the brand of the German mail order retailer, Quelle, obviously rebadged. What is interesting is that it seems to be a quite high quality instrument. For a long time, I was unable to find anything on it, when I came upon this page http://mrmodemhead.com/blog/yokogawa-7534-03-multimeter/. It features a very similar model, nearly identical inside and outside (except for the colour), made by Yokogawa. So I assume this one was also manufactured by them.

Unfortunately, I lost the user’s manual, so I don't have any specs, but here is what I know.
It is 4000 counts, has automatic and manual ranges. Measures mV DC, V AC/DC, A AC/DC (not truerms), resistance, diode, and continuity. One interesting mode is the ADP mode, which is basically a direct, high impedance (30Mohm) connection to the AD, provides 400mV range. It is useful to connect external instruments, like clamp meters or whatever.
It also has a bargraph display, measurement memory, hold (manual) and display illumination. So it is a pretty capable meter given its age.

Let’s have a look inside.  The case is quite sturdy; the screws go into metal posts. Both halves of the case have shielding inside.
The input jacks are solid machined sockets. One big drawback is that the 10 amp input is not fused! But it did survive the stupid step of measuring mains voltage when the leads were plugged in the 10 amp range, the circuit breaker tripped before any damage! Well done. I’m not sure about the input protection, probably not suited to high power work. Obviously it predates the CAT ratings.

The mode selector switch is a real rotary switch, none of these PCB track switches.  There is some flux residue on the switch and the backlight leads, looks like they are hand soldered.
There is a secondary board under the battery holder, and the LCD, but it is not possible to access them without desoldering stuff, so I don’t know what is on it.
As you see on the last picture, this is pretty much spot on. I did check other ranges and modes, they are also good. Given that this is nearly 30 years old meter, which has never been adjusted, I think It’s not bad at all.
The only problem I had is the fainting of the LCD, but it only required to clean the contacts to bring it back to life.

If you happen to have the specs on this or the history of this model, please post.

[LazyJack]

More pics.

[LazyJack]

Even more pics.

[LazyJack]

The last pic.

[dxl]

Looks similar to the Koenig DM908 i had 15 years ago, which was probably also just a rebadged version of some other Multimeter:

http://www.radiomuseum.org/r/koenig_e_digital_analog_multimete.html

[LazyJack]

Yeah, looks like this model was sold under a number of brands. One website mentions that it wad even sold by HP.

[Hydrawerk]

What is the ADP position for? Is it a frequency counter?

[Hydrawerk]

https://www.eevblog.com/forum/testgear/soar-4020-teardown-from-mid-80's/
http://mrmodemhead.com/blog/yokogawa-7534-03-multimeter/

[Mr.B]

Second to last picture, bottom right.
What is the component in the glass envelope?

[tautech]

Second to last picture, bottom right.
What is the component in the glass envelope?

GDT
http://www.bourns.com/products/circuit-protection/gas-discharge-tube-(gdt)-surge-arrestors

[Mr.B]

Thanks tautech.
I was not familiar with what looks like an old metal film resistor inside it.

[tautech]

Thanks tautech.
I was not familiar with what looks like an old metal film resistor inside it.

It's best guess and I might be wrong. 

[helius]

What is the ADP position for? Is it a frequency counter?

ADP is for sensors matched to the unit, for temperature or current clamp for example. The sensors would use 1mV per whatever unit is being sensed. The DMM converts the mV level to the appropriate quantity and displays it; the Range button changes the conversion factor used.

edit: I see that the question was already answered in the original post.

[LazyJack]

What is the ADP position for? Is it a frequency counter?

I mentioned it in the text. It's a direct connection to the AD. 30 Mohms impedance. Pretty much same as in the old analogue meters, when you had direct access to the actual meter, wihtout any dividers. Here it is 400mV range, you can connect a calmp meter or anything, like a frequency-voltage converter.

[LazyJack]

Second to last picture, bottom right.
What is the component in the glass envelope?

It's a resistor. According to the marking on it, it's 156 Mohms. Probably sealed in a glass tube to protect from the environment, as it is such a high value.

[LazyJack]

https://www.eevblog.com/forum/testgear/soar-4020-teardown-from-mid-80's/
http://mrmodemhead.com/blog/yokogawa-7534-03-multimeter/

Wow, Thanks for the first link, I didn't know about that.

[wings515]

I dropped my 2378 DVM and a part labeled PTC became disconnected.  The markings are TDK 9309.  I purchased a non working 2378 and removed the TDK park.  This one had markings TDK 9009.  I installed the 9009 into my working 2378 but it is not installed correctly. One of the lead ends became unsoldered and the repair is not very well done.
The fault I had with the missing part was in the Resistance function.  It would only read OL or 0.00.  Now it reads correct resistance values, I tried a 500 ohm resistor. 
I would like to install correctly a new PTC but I am unable to find a replacement.
Anyone have a part number and a supplier for this part.  I think it is a temperature controller resistance.
Regards,
Dan Kahn

[wasedadoc]

Yeah, looks like this model was sold under a number of brands. One website mentions that it wad even sold by HP.

Yokogawa-HP. https://www.hpmuseum.net/divisions.php?did=39

[wings515]

I purchased a non working 2378 from an ebay seller.  The broken part in my DVM is a 900 ohm PTC resistor.  Since this is in the Resistance function controlling the stability of the current source, I replaced the PTC with a combination of an 820 and 100 ohm fixed resistor to obtain 920. Measuring a 500 ohm resistor displayed 502 on this meter and another DVM.  I think this is close enough.  I suspect the function of the PTC is to control the current source over a wide temperature range.
Going on to the non working DVM, I noticed a large amount of corrosion from the batteries.  I cleaned the battery holder and then noticed under the holder a portion of the PC board also had corrosion remnants.  I removed the battery holder and cleaned all the corrosion from around the resistor and capacitor associated with the crystal.  Putting the holder and batteries back into the DVM and succuss.  The display came up with all segments lit and the 500 ohm resistor again read 502.
Now I have an extra DVM that I think I'll try and sell, no need for two of the same type.
Regards,
Dan Kahn

[robert.rozee]

hi wings515,
   the PTC is there as a safety device, in case of you accidentally connecting the meter to a voltage source while it is set to the ohms range.

the meter will contain some sort of clamp arrangement, quite often consisting of a pair of back-to-back transistors, connected across the ohms current source. under normal circumstances the clamping voltage is high enough (8v or thereabouts) to not affect ohms accuracy. however, if a voltage source (such as 230v AC) is accidentally applied, the clamp will hold back this voltage from doing damage to the current source.

at this point the PTC becomes important. it is arranged to sit between the resistor being tested and the current source/clamp circuit. under such a fault condition it starts to heat up, and its resistance increases significantly. this serves to limit the fault current to a level that can not blow out the clamp, and hence the current source is protected.

the resistance of the PTC is never a part of the ohms value read by the meter, as while the measurement current is injected through the PTC, the voltage measurement across the resistor being tested is fed off the the ADC directly from the top of this resistor via a high impedance (usually 1M ohm) series resistor.

ie, you should put a PTC back into the circuit. one designed for the job, although the exact value is not critical to ohms accuracy. one from, for instance, an otherwise dead Fluke 70 series meter would probably be ok.

btw, the service manual for early Fluke 77 meters contains diagrams showing how the PTC is arranged. well worth a read.


cheers,
rob   :-)