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Teardown of a rebadged Yokogawa (?) multimeter

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LazyJack:

--- Quote from: Hydrawerk on July 05, 2016, 10:08:46 pm ---https://www.eevblog.com/forum/testgear/soar-4020-teardown-from-mid-80's/
http://mrmodemhead.com/blog/yokogawa-7534-03-multimeter/

--- End quote ---

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:

--- Quote from: LazyJack on July 05, 2016, 05:43:36 pm ---Yeah, looks like this model was sold under a number of brands. One website mentions that it wad even sold by HP.

--- End quote ---
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   :-)

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