Disassembling the MP-155 multimeter: step-by-step guide with photos and secrets of the masters

Multimeter MP-155 (or MP-155 in Russian transcription) is one of the most popular budget testers among car enthusiasts and electricians. Its compactness, simplicity and functionality make the device indispensable for diagnosing a car's on-board network, checking the battery or searching for breaks in the wiring. However, over time, even such a reliable device can fail: buttons fail, readings become erroneous, or the display stops working. In most cases, the problem is solved by disassembling and cleaning the contacts, but many are afraid of damaging fragile electronics.

This article will help you safely disassemble MP-155without breaking the latches or losing small parts. We'll look at:

• 🔧 What tools will be needed (and what can replace specialized ones)
• 🔍 Step-by-step disassembly process with photos of each stage
• ⚡ Typical breakdowns and how to diagnose them “by eye”
• 🛠️ How to put the multimeter back together so that there are no extra screws left

Important: if your MP-155 Still under warranty, disassembly will void it. In other cases, self-repair will cost less than buying a new device - the main thing is to follow the instructions and take your time.

1. Preparation: tools and conditions for disassembly

Before you grab a screwdriver, make sure you have everything you need. Unsuitable tools - the main cause of broken latches and scratches on the case.

Here is the minimum set:

• 🔩 Phillips screwdriver PH0 or PH1 (preferably magnetic)
• 📏 Plastic spatula or pick (for separating parts of the body)
• 🧲 Magnetic mat or bowl (to avoid losing screws)
• 📷 Smartphone for photographing the process (useful during assembly)
• 🧴Isopropyl alcohol 90%+ and cotton swabs (for cleaning contacts)

If you don't have a plastic spatula at hand, you can use old bank card — it is flexible enough not to damage the plastic. And instead of a magnetic mat, a piece of double-sided tape glued to the table will do.

⚠️ Attention: Do not use metal spatulas or knives - they leave scratches on the case and can short circuit the contacts on the board if moved carelessly.

Work for clean, well-lit table. A table near the window with a table lamp is ideal. If you are disassembling the multimeter in the garage, make sure that there is no dust or dirt nearby - it can get inside the device and cause new problems.

2. Removing the back cover: the first disassembly step

Housing MP-155 consists of two halves held together four screws and latches. We start by removing the back cover - it usually does not have hidden fasteners.

Algorithm of actions:

1. Turn the multimeter upside down.
2. Remove all the screws on the back panel (usually located in the corners).
3. Using a plastic spatula, carefully pry the cover off, starting at the corner next to the battery compartment.
4. Run the spatula along the seam, gradually releasing the latches.

If the cover does not budge, do not force it - most likely you have missed a screw. In the model MP-155 sometimes occurs fifth screw under the rubber foot (if there is one). Check all holes!

After removing the cover you will see:

• 🔋 Battery compartment 9V (usually 6F22)
• 📊 Printed circuit board with tracks and contacts
• 🔊 Speaker (if your model has a dial-in function)

⚠️ Attention: Do not touch the board with bare hands - static electricity can damage sensitive elements. If you have dry skin, touch a grounded metal object before working.

What to do if the screw does not come out?

If the screw is stuck or has a damaged head, try:

- Drop a little WD-40 and wait 5 minutes.

- Use a screwdriver with a rubber tip for better grip.

- As a last resort, drill out the screw with a drill with a thin drill bit (only if you plan to replace it with a new one!).

3. Removing the circuit board: accuracy is paramount

Fee MP-155 attaches to the front of the case two ways:

1. Screws (usually 2 pieces near the probe connectors).
2. Plastic latches around the perimeter.

Removal order:

1. Remove the screws holding the board in place.
2. Carefully lift the board by the edge where there are no connectors.
3. If the board is tightly seated, check whether you forgot to disconnect speaker wire or display cable.

Pay special attention LCD display cable - it is often glued to the board and can come off with sudden movement. If the cable does come off, don’t panic: you can carefully insert it back, but you need to do this only with tweezers, no pressure.

Board element	Typical problems	How to check
Resistors	Burnt out (blackened)	Visually or with a multimeter in 200Ω
Capacitors	Swollen, leaking	Inspect for deformation
Chip (IC1)	Oxidized legs	Call a break tester
Buttons	Dirty contacts	Cleaning with alcohol, pressing test

If you are disassembling a multimeter due to non-working display, in 80% of cases the problem lies in the oxidized contacts of the cable. Just clean them with an eraser or alcohol, and the screen will work again.

4. Disassembling the front panel and buttons

Front panel MP-155 - the most “fragile” part. Here are located:

• 🔘 Mode switch (a common cause of breakdowns)
• 🔄 Buttons HOLD, BACKLIGHT etc.
• 📺 LCD display (sometimes glued to glass)

To remove the panel:

1. Carefully pry up the plastic frame around the display (it is usually held in place by latches).
2. Disconnect the display cable from the board (if it has not been disconnected previously).
3. Remove the mode switch - it is secured with a nut on the back of the panel.

The buttons on the MP-155 often get stuck due to dust or moisture. They can be washed with alcohol, but under no circumstances should they be lubricated with oil - this will lead to even more dirt sticking to them.

If the mode switch is difficult to rotate, you can disassemble it and clean it:

• 🔧 Unscrew the nut from the back side.
• 🧹 Remove the plastic cap and clean the contacts with a cotton swab.
• 💧 Drip 1 drop of silicone lubricant on the axle (do not overdo it!).

⚠️ Attention: Do not use acetone or solvent for cleaning - they will destroy the plastic and may erase markings from the board.

5. Diagnosis of faults: what to look for inside

Now that the multimeter is disassembled, you can start looking for the problem. Here the most common faults MP-155 and their signs:

Symptom	Probable Cause	How to fix
Doesn't turn on	Dead battery or oxidized contacts	Replace battery, clean contacts
Readings go wrong	Dirty board tracks	Clean with alcohol, check soldering
Backlight doesn't work	Burnt out LED or resistor	Ring the chain, replace the element
The chains don't ring	Problems with transistor continuity	Check Q1 on the board

If the multimeter shows incorrect voltage, first of all check variable resistor (trimmer) next to the probe connector. It could have gone astray from the vibration. Gently turn it with a screwdriver and calibrate the device against a reference source (for example, a new battery 1.5V).

To check track integrity use a magnifying glass and a multimeter in dialing mode. Pay attention to the areas near the probe connectors - the contacts there often come off due to mechanical loads.

6. Cleaning and repair: restoring functionality

When the problem is found, we begin to fix it. Let's start with general cleaning - it won’t be superfluous.

Procedure:

1. Soak a cotton swab in isopropyl alcohol and clean all contacts on the board.
2. For hard to reach places use toothpick, wrapped in cotton wool.
3. Wipe the board with a dry cloth and let it dry for 10-15 minutes.
4. If there are oxidized areas (green deposits), remove them with an eraser or a special contact liquid.

For soldering repair you will need a soldering iron with power 25-40 W and solder. Typical places requiring re-soldering:

• 🔌 Connectors for probes (come off due to frequent use)
• 📟 Chip legs (may come off due to overheating)
• 🔊 Speaker contacts (oxidize over time)

If you find burnt resistor or capacitor, it can be replaced with a similar one. The element values are usually indicated on the board (for example, 100Ω or 100nF). To find a replacement, any radio market or online store like Chip-Dip.

⚠️ Attention: When soldering, do not overheat the board - keep the soldering iron in one place for no longer than 3-4 seconds. Use gumboil (for example, LTI-120) for better solder adhesion.

7. Assembling a multimeter: how to avoid mistakes

Collect MP-155 putting it back is more difficult than taking it apart - accuracy and consistency are important here. Start by checking:

• 🔍 Are all the screws in place (check the photo taken before disassembly).
• 📋 Are all cables and wires connected (display, speaker, battery).
• 🔄 The mode switch is set to the correct position.

Assembly order:

1. Install the board into the front of the case and secure with screws.
2. Connect the display cable (it should go in without effort!).
3. Secure the back cover, starting with the latches near the battery compartment.
4. Tighten all screws, but do not overtighten - the plastic may crack.

After assembly be sure to check your multimeter:

1. Insert the battery and turn on the device.
2. Check all modes: voltmeter, ammeter, dialing.
3. Make sure the buttons work without sticking.

If the multimeter does not turn on after assembly:

• 🔋 Check the polarity of the battery (in MP-155 contacts are sometimes marked as + and – implicitly).
• 📊 Make sure that the display cable is connected correctly (an inverted cable is a common mistake).
• 🔌 Test the power circuit with a multimeter (yes, tester tester!).

8. Common mistakes and how to avoid them

Even experienced craftsmen sometimes make mistakes when disassembling MP-155. Here are the most common:

• 🔨 Too much pressure on the body → cracks or broken latches. Solution: use plastic spatulas and take your time.
• 🔥 Overheating of the board during soldering → tracks peel off. Solution: Solder at minimum temperature with flux.
• 💧 Using unsuitable cleaning fluids → corroded plastic. Solution: only isopropyl alcohol or specialized products.
• 🔌 Incorrect display cable connection → white screen or artifacts. Solution: reconnect the cable, checking the orientation.

Another common problem is loss of screws. B MP-155 They are small and roll easily on the table. To avoid this:

• Work for light surface (a white piece of paper will help you spot the fallen screw).
• Use magnetic mat or stick a piece of tape next to the multimeter.
• Sort screws by size - there may be screws of different lengths in your case!

If after disassembly you find that plastic latch broke off, don't despair. It can be restored using:

• 🔥 Soldering iron and a piece of plastic (melt and “weld”).
• 🧴 Superglue (but this is a temporary solution - the plastic may crack again).
• 📌 Staples from a stapler (if the latch holds a non-critical part of the body).

FAQ: Frequently asked questions about MP-155 disassembly

Is it possible to disassemble the MP-155 without special tools?

Yes, but with reservations. Instead of a plastic spatula, you can use a pick or a bank card, and instead of a magnetic screwdriver, you can use a regular one, wrapping the tip with electrical tape for better grip. However, without tweezers and a magnifying glass, cleaning small parts will be difficult.

What should I do if, after assembly, the multimeter shows incorrect values?

Most likely the calibration has gone wrong. Try:

1. Calibrate the device using a reference source (for example, a new 1.5V battery).
2. Check the integrity of the tracks near the trimming resistor.
3. If the problem persists, the ADC (analog-to-digital converter) chip may be damaged.

How to check if the display is working if the multimeter does not turn on?

Connect the battery and shine the flashlight at an angle onto the screen. If the numbers on the display are barely visible, the problem is in the backlight. If the screen is blank, check the cable or power board.

Is it possible to replace a 9V battery with a rechargeable battery?

Technically yes, but you need to consider:

• 🔋 The battery should deliver exactly 9V (for example, 8.4V from Li-ion will not work).
• 📏 The dimensions of the battery should not exceed the dimensions of a standard “crown”.
• ⚡ When using battery Ni-MH Compartment modification may be required.

Where to buy spare parts for MP-155?

Spare parts for MP-155 can be found:

• 🛒 On AliExpress (search for "MP-155 spare parts").
• 📦 In local radio parts stores (resistors, capacitors, buttons).
• 🔧 At the disassembly of old multimeters (donor boards are often sold).

For an accurate replacement, take a photo of the faulty part on both sides and indicate its markings.