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Author Topic: Fluke 8846A repair  (Read 1026 times)

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Offline mr_DTopic starter

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Fluke 8846A repair
« on: June 30, 2021, 02:47:30 am »
I have troubleshooting the power of Fluke 8846A.
The power supply 5 voltage was broken. I tested Q20 and Q21 (follow the image on attachment section) and they were done.
Now I want to know what are Q20 and Q21 (What is semiconductor type: Transistor, FET... and what are they code: name, code...) because I could not find them in the datasheet.
It would be really awesome if someone in the know can enlighten me !
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Online coromonadalix

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Re: Fluke 8846A repair
« Reply #1 on: June 30, 2021, 03:11:29 am »
seems to act like a diode, the picture seems to show  a third pin unconnected ?  or i did see it wrong ?
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Offline bob91343

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Re: Fluke 8846A repair
« Reply #2 on: June 30, 2021, 05:21:36 am »
Since it's in the power supply, I would guess NPN transistor.  But without a diagram, it's just conjecture.

Can you measure them with an Ohmmeter?
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Offline picburner

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Re: Fluke 8846A repair
« Reply #3 on: June 30, 2021, 06:39:53 am »
The dot "Q" indicates that the devices are transistors (or jfet/mosfet) with an magnified image or by reading the code on the top one could trace the device.
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Offline TMM

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Re: Fluke 8846A repair
« Reply #4 on: June 30, 2021, 03:36:57 pm »
Quote from: coromonadalix on June 30, 2021, 03:11:29 am
seems to act like a diode, the picture seems to show  a third pin unconnected ?  or i did see it wrong ?
It's probably connected to a via underneath the transistor. On the LM393 nearby, pins 3 and 4 appear to be unconnected, but they definitely have to be as they are one of the comparator inputs and ground.

The pins on the right side of J7 go to the AC secondary of a mains transformer, right? If so I would guess that the two transistors in question are MOSFETs to disconnect the secondary winding(s) under certain conditions (line overvoltage, delayed startup etc). The LM393 comparator nearby might be operating them.  If this is the case then a short in the powersupply would explain them being blown, and you can work out if they should be NMOS or PMOS by the polarity of the connection and if the LM393 is pulling the gate high or low. Usually there will be a high value resistor connected from gate to source, and the comparator pulls the gate towards drain to turn it on. If it's just operating as a switch as I suspect then you can choose any MOSFET which is rated for the voltage, current and doesn't have an Rds-on which will drop too much voltage.
« Last Edit: June 30, 2021, 03:44:31 pm by TMM »
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Offline gamalot

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Re: Fluke 8846A repair
« Reply #5 on: July 02, 2021, 08:04:17 am »
« Last Edit: July 02, 2021, 08:11:04 am by gamalot »
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