Author Topic: Hioki 3540 milli-ohm meter via repair  (Read 1119 times)

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Offline pigrew

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Hioki 3540 milli-ohm meter via repair
« on: March 11, 2017, 02:36:04 am »
Hi,

I've recently fixed up a pair of fried Hioki 3540 meters that I got off of eBay. As received, they had a few dead Schottky diodes, inductor, and power regulator. After replacing them, the meter is seemingly fixed.

See the other thread at https://www.eevblog.com/forum/testgear/hioki-3540-milliohmmeter-teardown/ for more details on this meter (and some photos I posted.

But, what's really nagging me, is a via which was burnt up (lost its connection to the top ground plane). This seems to be a four layer board, and I can't figure out why only the one via would be damaged while the other hundred vias are perfectly OK (unless it was thermal damage from close-by overheating components?). Maybe the DC input track on an inner layer was too close to that one via, and that arced?

Do I just call it fixed, and sell one on eBay? (I don't need two). Are there any suggestions other than X-Ray to figure out if that via needs any sort of repair work done to it?

I'll followup in a few days (once I receive a power adapter or find enough AA batteries) with test results as to its accuracy.
 

Offline SeanB

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Re: Hioki 3540 milli-ohm meter via repair
« Reply #1 on: March 11, 2017, 11:57:44 am »
R133 is a polyfuse, a PTC resistor that will go high resistance when it heats up enough, either from current flow through it, or from high ambient temperature. When it cools down it allows current flow again, and is probably what helped protect the rest from whatever killed those diodes and the main switcing supply.

My bet is somebody used an AC supply ( or a failed DC power pack) on the unit, and this had a high reverse voltage which blew all those parts.  I would replace CP as well, it would have been damaged with the reverse voltage applied, and check D104 is not damaged either.

As to the filed via, I guess it was connected to an internal layer, and was carrying the return current path of CP, which I guess is the Panasonic cap next to it,, which would have resulted in this damage. PCB designers might have not wanted a connection to the ground plane on the top, as this can be a poor connection with the large plane removing heat, so put a small stub trace and a non soldered via as thermal isolation.
 

Offline pigrew

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Re: Hioki 3540 milli-ohm meter via repair
« Reply #2 on: March 29, 2017, 08:02:49 pm »
I discovered that there was also a fried buck converter IC on the -01 interface board.

The interface board provides fully isolated I/O. Most of its logic is powered by the motherboard's 5V rail. The board has an additional 5V buck regulator (same model as the others). This extra regulator powers an isolated switching regulator module (Burr Brown DCP010505BP).

Logic powered by the main board's 5V drives a series of opto-isolators. The isolated supply powers the input/output drivers.

After replacing the LT1174-5, the interface board started working. The DCP010505BP has fairly bad regulation, and outputs too high of a voltage when lightly loaded. In this circuit, it outputs 5.5V.  Note that the 5V connector at the back of the unit is a power OUT, not a power in.

Another note is that the interface board is quite power hungry. At 9V input, the unit uses 55 mA without the interface board. After connecting the interface, 178 mA is used. If I were to use the unit under battery power, I'd definitely want to leave the interface board disconnected.
 

Offline pigrew

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Re: Hioki 3540 milli-ohm meter via repair
« Reply #3 on: March 30, 2017, 01:36:47 am »
And now, I'm running into weird (ground-loop?) issues.

When powered using a no-name "Upbright 24W0920025" AC adapter (9V, 2A, but really 9.5V), I'm reading a 12ohm offset in the 1k and a 120ohm offset in the 10k range when I connect it to a decade resistance box.

When powered via battery, there is no offset, and everything is just fine. When I earth the GND of the board, the offset also goes away. Weird.

EDIT: Upon more thinking, this may be due to the capacitance to earth of the decade capacitance box. My guess is that  the AC adapter's output/secondary has Y capacitors to some point between L & N, so the output is floating, but pulled towards 60VAC by the capacitor (half of the primary voltage). This is interfering with the series resistors of the sense terminals. I've tried flipping the AC adapter around (swap L&N), but it does not help. I'm not able to think of any other solutions at the moment.
« Last Edit: March 30, 2017, 02:15:55 am by pigrew »
 


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