Yokogawa 7534-03 Multimeter

Yokogawa 7534-03 MultimeterJapanese equipment maker Yokogawa made handheld DMMs for HP before HP became Agilent.  This 3200 count 3½ digit Yokogawa-branded model is very similar in appearance to the HP E2378A model.  Unlike the HP, this unit adds a high-impedance ‘ADP’ mode, presumably for current-clamp attachments and the like.  Otherwise, it has a very fairly mundane feature set.  It is also the only multimeter I’ve ever seen with a ‘Data Hold’ button on both the top and the side.

Opening the case reveals an obvious problem.  Something has gotten very hot and burned part of the circuit board.  There are burn marks on the inside of the case as well.  The resistor (R6) that died violently has already been removed.  There is a pencil-written note on the side of the unit that says “4.9 Ω 2 watt wirewound resistor”.  In retrospect, I realize this may have actually said “4.99 Ω”, but since it’s now been cleaned off, we’ll never know.  In any case, it was certainly helpful for someone to note what had been removed from the device.

Some simple testing shows that the DC voltage measurement function is still working and is reasonably well-calibrated.  Resistance measurement also works.  Only the current ranges do not work.   The meter passes current in the μA and A ranges but reads incorrectly.  No current is passed in the mA range, so the conclusion is that the missing/damaged resistor is the mA range current shunt, and is probably part of the voltage sense circuit for the other two ranges.  No schematics or service information can be found, so we’ll have to go on probabilities here.

Closer examination of the burnt PCB reveals that the larger copper traces are intact and relatively undamaged, aside from lifting slightly from the board.  There is one small trace missing that leads to a 1/8W 3.3K resistor, which is in parallel with the missing shunt resistor R6.  That doesn’t make a lot of sense, unless the designers intended to ‘trim’ the shunt a bit for more accurate readings.

Due to the amount of possible heat damage, the resistors surrounding R6 were removed from the board for testing.  All measured correctly, so the damage was limited to the shunt and the PCB itself.

4.9Ω 2W resistors are hard to come by, either in my spare parts inventory or from a parts supplier, so it looks like a non-ideal solution might be called for.  To start off, I tacked in a standard 4.7Ω resistor and checked the operation of the meter.  The current ranges all worked.  In fact, the A and μA ranges were OK.  The mA range was just a bit too low.  I decided it may be possible to substitute two parallel 1W 10.0Ω 1% precision resistors I had on-hand in place of the shunt, and then replace the ‘trim’ resistor with whatever produced accurate readings.  Again by experimentation, 600Ω was determined to work well.  So the 3.3K ‘trim’ resistor was replaced with a ¼W 600Ω 1% metal film resistor.

The resulting combination of the 2 10Ω and 1 600Ω resistors actually measured 4.98Ω.  This is where I decided that the note on the side of the unit might have actually been 4.99 instead of 4.9, because 4.99 in parallel with the original 3.3K trim resistor is 4.98Ω.  Also, I found a picture of the inside of an HP E2378A.  It had a slightly different layout, but R6 was clearly marked “5.020”.

Given the nature of the malfunction and the somewhat um… ‘ghetto’ nature of the repair, a good check of the meter’s ability to measure current is in order.  Side-by-side comparison with another dependable DMM reveals that the repaired meter is reading quite accurately.

So let’s call that success, and make a few more observations.

This meter is quite ruggedly built.  The sturdy case halves are gasketed and fastened together with 4 machine-threaded screws going into big brass threaded inserts.  The input jacks have o-ring seals, and the lens is permanently sealed in.  The buttons are also sealed in with a metal retaining piece.

There are unpopulated footprints near the input jacks for what appears to be two MOVs and a spark gap, components that would provide over-voltage protection.  That, in combination with the use of small fuses, indicates this meter would not be considered appropriate nowadays for use with high-energy circuits.  Perhaps that’s the reason it has a warning sticker on the side that is remarkably at odds with the 750V~/1000VDC rating on the top.

DMMCheck calibration check results for this Yokogawa 7534-03:

Reference Reading Notes
DC 5V 5.00
1mA 1.00
AC 5V 5.55 nominal for non-TRMS is 5.55
1mA 1.09 nominal for non-TRMS is 1.11
Ohms 100Ω 99.9 0.1 with leads shorted
1K 0.998K
10K 9.98K
100K 99.8K

About modemhead

Fixologist and multimeter junkie.
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9 Responses to Yokogawa 7534-03 Multimeter

  1. Philip says:

    Hi,I‘ve got a E2377A and that is almost the same as yours but now all the measurment is not right and I want to know how to calibrate it could you give me some help?

    • modemhead says:

      Hi Philip. I no longer have this multimeter in my possession, but in looking through my notebook I found these notes:
      RV1 (DC)
      RV2 ?
      RV3 (AC)
      RV4 (Temp)

      Highly recommend cleaning the circuit board using isopropyl alcohol (strongest you can find) and allow time to dry. Contamination on the board can cause bad readings. Then using another trustworthy DMM, compare the readings with a stable voltage source at about 3.00VDC. Tweak the DC cal pot only if they don’t agree within a few counts. This is obviously not the most ideal means of calibration, but should suffice for basic home/hobby use.

  2. Philip says:

    Thanks so much for your reply, but I still want to ask you for some circuits of this multimeter,and I had know that soar 4050 also share almost the same circuit. Certainly, If you had and kept that, whatever I would try as you said and share with you if any progress.

    • modemhead says:

      If I understand you correctly, you need the schematic for this meter. I am sorry, I never found one, which is why I had to do some guessing during the repairs. And you’re always welcome to share your progress.

  3. Philip says:

    O,I see, That’s my fault , my English should need to be improved. Now I have solved some problem and the measurement function is able to work but not precious,for example, at the DC voltage measurement when the probe is shorted it donot say ‘0’,but a negative number decreasing, and the positive and negative number it says is not the same absolute value ,the difference was very huge about 500mv until just now when i shocked it ,the difference suddenly fall down to about 3mv““That’s amazing ,and I want to know why.
    Sorry to disturb you all these days ,and let you read this terrible not easy to understand words.

    • modemhead says:

      Your English is better than my Mandarin. I am still not sure exactly what the problem is, but I think what you are describing is most likely caused by tiny leakage currents through circuit board contamination. The input to the A/D convertor on any DMM is very high impedance, on the order of giga-ohms, and is easily disturbed by very very tiny currents. It does not hurt to clean the circuit board multiple times, you may see some improvement.

  4. Newton says:

    Hi , I have a multimeter E2378 model, but isn’t working. It’s an old equipment, but I want to use it . I opened to see what is wrong and there is PTC and I don’t no what is the value of that component . Can someone help me?

  5. Samuel says:

    Hi

    I inherited three Yokogawa 7533-03 DMM’s. What exactly does the ADP setting do?

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