Saving a Holmes SHV500v3

[Mchaz]

My friend recently rolled his TRX4 with an SHV500v3 servo into a hole full of nasty water. The front end along with the servo were completely submerged.

Later on, thinking the servo was dried out, he powered his rig on, and smoke poured out from the servo. After confirming with Holmes that there was nothing they could do (no fault to them), I decided to attempt a rework to spare my buddy another $100 for a servo.

On closer examination one of the DTM4606 MOSFET array chips driving one of the motor phases had a burn hole indicating the point of failure. After checking with my multimeter I confirmed a drain/gate short on both FETs.



The DTM4605 doesn't appear to be available domestically. However, it appears to be a dupe of the AO4606. Searching for subs to the AO4606 I finally found one that was in stock: Vishay SI4532CDY.

The burned chip quickly popped off with some chip removal alloy, and the pads cleaned up nicely.



With some tacky flux I got the chip oriented and stuck to the board. 8 dabs of solder later and the new chip was on quickly.



After a thorough clean and dry with alcohol and compressed air I reassembled the servo and powered it up. Success! I was afraid the drivers for the MOSFETS, or even worse the control chip, may have been collateral damage, but fortunately the servo seems to function fine.

Anyway, thought this might be interesting or useful to someone with a similar issue.

[GA Dawg]

Nice work!

[Mchaz]

Originally Posted by GA Dawg

Nice work!

Thanks! These kind of reworks can go sideways pretty quick if you're not careful. Not my first time working at this scale by far, but it had been a while and I only had my trusty Hakko 888 and a chisel tip. No fancy hot air or other luxuries on my home workbench.

[ferp420]

flipping amazing job keeping it all so clean

[Mchaz]

Originally Posted by ferp420

flipping amazing job keeping it all so clean

I appreciate it! Flux is your friend.

[twade984]

This is very cool, well done.


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[themountain]

Wow..?you are my new hero SMDs to solder is some kind of art form! This is why I conformal coat all my servos before they get installed.

[meatmonkey]

You make that look easy. Meanwhile seems like every time I touch an iron to a circuit board it gets instantly destroyed. Why they gotta make all the contacts so close together

[Browneye]

Uhm...way above my pay grade.

I just got one of these servos for my new SSD build.

I did change out all the stepper motors in my Chevy truck dash though - and guess who makes them? Yes, SWATCH, the watch company. There are eight of them - for all of the indicator gauges, what the needles are attached to that move them.

Disassemble, desolder off board, solder new ones in. I had never used a solder wick before - was pretty impressed.

But someone told me what to buy, and how to disassemble and replace them. Identifying a bad chip is not in my realm, nor identifying a substitute. My hat is off to you.

[heyok]

Beautiful work there!

[Mchaz]

Thanks, all.

Frankly, there are a lot of people who put my skills to shame, but I do have about 5 years of electronics tech work under my belt. That includes a lot of SMD work with similar component packages (they get way smaller than this!). 10 years later and I can already tell the difference in my eyesight!

Tacky flux and a pick are my main tools when working with SMD (other than that the iron itself). A pair of really pointy precision tweezers can come in handy, but not necessary for an 8-pin SOIC chip like this. The first pin is key. Once that is tacked down the rest falls into place. Did I mention tacky flux is your friend? With enough flux the surface tension of the wetted solder helps prevent bridged pins. The solder wants to flow to the pads/legs. Anyway, if you want to practice SMD soldering save those old circuit boards from broken electronics and solder away.