LHG 5 reboots instantly upon wireless association (Tx Load Power Sag)

Hardware MikroTik hardware general
1

Hi everyone,

I'm troubleshooting a MikroTik LHG 5 (RBLHG-5nD) that boots up completely fine, stays accessible via Winbox, and runs idle without any issues. However, as soon as it associates/connects to an AP (Sector) and starts transmitting, the board instantly reboots.

Here is what I have observed during diagnostic testing:

Idle State: System is stable, low current draw, no reboots during scanning.

Association State: The moment the RF front-end engages (Tx Power), the device suffers an immediate reboot/reset.

Software Isolation Test: Lowering the ⁠Tx Power⁠ manually to a minimum level (e.g., 0 to 5 dBm) prevents the instant reboot, confirming that the issue is directly tied to peak current draw under RF load.

Component-Level / Hardware Analysis:

As an electronics engineer analyzing the mainboard, this behavior strongly points to an internal Voltage Sag / Transient Power Drop on the main DC-DC step-down rail (3.3V/5V VDD lines). The supervisor/brown-out reset circuit on the main SOC (AR9344 / QCA9531) triggers a system reset due to the power delivery network failing to supply peak current.

The primary culprits seem to be:

1. High ESR degradation in the main electrolytic filtering capacitors on the POE/Power input stage.

2. Degraded DC-DC Buck Regulator / Feedback circuit under load.

3. Partial short-circuit or breakdown in the RF Power Amplifier (PA) front-end.

Questions for the forum:

1. Has anyone encountered this exact Tx-triggered reboot on LHG/SXT units and successfully resolved it by recapping the Low-ESR capacitors near the POE section?

2. Are there specific SMD capacitors or diodes on the LHG 5 power rail known to fail under current spikes?

Thanks in advance for any insights!

2

Look, people on the forum are experimenting with SMD components every day with that model...

When an item breaks down, my policy is simple: I throw it away and move on.

I refuse to waste my valuable time and energy trying to fix something that has already failed.
In today's world, the math just does not add up because fixing an old item usually costs more than it is actually worth.
You pay for expensive replacement parts, diagnostic fees, and professional labor, which quickly drain your wallet.

Beyond the financial cost, there is a massive issue with reliability and peace of mind.
When a product breaks once, it loses its trustworthiness.
If you fix the main issue, you never truly know if other hidden defects will surface a few days or weeks later.
You end up trapped in a stressful cycle of endless troubleshooting and constant anxiety.

That is why trying to repair a faulty product is completely pointless to me.
The smartest and most efficient choice—both for your budget and your sanity—is to simply invest in a brand-new replacement.
And if the item is still covered under warranty, the choice is even easier: just issue an RMA for a fresh, reliable replacement without the headache.

3

BUT - only for the fun of it and as a DIY job - recapping a board costs very little and can give new life to things otherwise ready for the dumpster.

Unless I am mistaken there is only one electrolytic capacitor on that board, so there isn't even the question about which one is to be replaced.

There is a report on the forum about failed NG2P (which should be a 30V MOSFET, probably a AO3401) but then the board would not power at all.

4

Eh no... non me l'aspettavo...

Please dispose of them according to local regulations,
rather than throwing them in the dumpster with other non-recyclable items...

5

Of course I meant the electronic waste dumpster, according to RAEE norms.

Being below 25x25 cm there is usually for them a free dumpster near large shops, so called "1 against 0".

Otherwise there is usually a public recycling centre in your municipality.

6

"I really appreciate both of your points of view!

As a Network Engineer, I always like to draw from other people’s experiences and forum reports before jumping in. Diagnosing micro-electronics isn't my main day job, but I love tinkering with faulty hardware as a hobby—it's just satisfying to give dead equipment a second chance.

Since it's just a single electrolytic cap, swapping it is a low-cost, quick experiment worth trying for the fun of it. And as noted, if it were the NG2P (AO3401) MOSFET, the board wouldn't power on at all.

If the recap fixes it, great! If not, I'll properly dispose of it at a local WEEE / 1-against-0 recycling point as suggested."ن