Otter Z5 Kettle Thermostat Repair

Had one of these on a limited edition Hello Kitty Tefal Mini (BF8120 is the model I believe). SWMBO decided to dig it out of storage and found that it kept boiling the water without ever tripping the thermostat normally.

Externally about the only thing that felt wrong was there was no tacile (clicky) feeling when the kettle’s boil switch is actuated. Suspecting the thermostat to have internally welded itself together, I took it apart.

The handle section disassembles with 2 screws on the inside under the lid, once those are removed, there is just enough flex in the plastic to remove the lid and the internal steam manifold section to access the thermostat. A single screw holds the thermostat in place, once that is loose, disconnect the 2 wires taking note which wire goes where (this is just so during assembly, the wire’s don’t twist around and cause issues with the plastic going back together, electrically I’m sure it’ll work just fine).

The entire thermostat looks like this.

Filpping it over, the bimetallic disc is visible, using a sharp tool to dislodge it off the catch, the disc slides off.

This is the pin that disengages the contacts internally when the steam is hot enough to bend the bimetallic disc, once the disc is off, the pin can be shaken/gently persuaded out.

Next the 2 catches on the outer edge, near the electrical connections, can be pushed in and the assembly slides apart. I broke the catches during removal but that’s not a big deal, the securing screw will still hold everything in place when the thermostat is screwed back into the kettle.

Here I’ve gently wire brushed and polished up the contacts with a fibreglass pen, mine were welded closed and the this contact had lost the ability to let the actuating lever cam over and hold the contacts closed, some adjustment later on the hump section of this contact fixed that (I had to bend the hump section up slightly as well as pull on the tab that engages the lever, essentially lengthening the hump section, for a more tactile response). Test often during adjustment and go slow, it’s easier to correct a small error/wrong adjustment.

Once satisfied with the feel and after cleaning the contacts up, I pieced it all together and gave the kettle a spin, which confirms it works good again. Hopefully it’ll stay that way for good long time.

Kenwood KA-691 repair

Unit received in fair condition, front looks good, no deep scratches or missing paint, rear and bottom has very slight corrosion.

Unit will not come out of standby mode, cut bottom chassis for service access, found no issues with main amp board.

Powering up microcontroller (front panel assembly, MCU is a M50941-597SP), was able to get some response from VFD with CD DIR depressed. Leaving it powered up for a couple minutes does eventually result in normal amplifier operation.

Took the chance to check output offset (negligible) and re-adjusted bias for 18mV per service manual.

Pulling power off and allowing amplifier to cool, will mean the next round of power up will take minutes for amplifier to come out of standby, verified clock to MCU is correct and started, Vcc and AVcc were correct and present, tried external 5V regulator, changing input filter caps, trying a quartz 4Mhz crystal instead of original resonator, to no avail, slipped DMM probe and shorted MCU pin from -35V (VP/VFD bias voltage) to adjacent input pin (38 shorted to 39 or 37). MCU now real and truly dead (heard a small pop and immediately shut off the amp by pulling power cord out).

 

Opps, killed this little sucker.

 

Was unable to located MCU (PN M50941-597SP or M50941-589SP) even through obsolete component brokers/traders. Decided to cannibalize from another unit (got off eBay USA).

Rear inputs slightly polished to assure good shield contact, input board was removed for visual inspection.

Re-soldered power input board where joints showed signs of crack formation, re-soldered main amp board as well.

 

Cracked and oxidised joints on main PCB.

Reworked these

More inadequate joints

Main PCB cleaned up after rework.

Signs of cracking on transformer ribbon

Joints reworked

Reworked one joint on transformer input.

 

Service access panel flipped and re-attached with M3 washers and screws.

Speaker A & B switches cleaned and lubricated.

Ebay amplifier arrived in broken condition (likely due shipment damage, tone control’s smashed, power button smashed, will not turn on), returned.

Seller agreeable to sell just the front panel PCB for parts, procured front panel PCB alone.

PCB arrived, wrapped in aluminum foil and plenty of bubble wrap for ESD and shock damage resistance, front power switch PCB area found cracked and power switch pushed in.

Crack stop drilled and PCB trace patched.

Epoxy patch on front of PCB.

 

ESR measured on all front panel PCB capacitors. Qty 4 were replaced as follows

Designation	Original values	Installed values
C80	220uF 6.3V Low Profile	220uF 10V bent over
C92	220uF 6.3V	220uF 10V
C89	10uF 50V	10uF 50V
C83	10uF 35V	10uF 50V

Power switch desoldered, new power switch soldered, cracked PCB trace patched over with enameled wire, crack stop-drilled with 1/16” drill bit. Solder side PCB lightly sanded and epoxy applied over cracks, top side sanded and a small patch applied with epoxy as well.

Power ribbon pads on front panel PCB slightly damaged during removal (qty 3), post soldering power ribbon connection, extra 3 enameled wires were used to jumper over to the ribbon connections and held down with Dow Corning 3145.

Jumper wires on front PCB

 

Unit tested and verified to power up instantly upon application of wall power.

O2 Headamp output board, OPA1688s Part II

It’s been awhile since I’ve had an update on this project, I actually completed it, but never got around to documenting everything.

Drew up a schematic and layed out a board, JLCPCB fabricated it with no issues. I used some existing board images that AGDR made available, so mechanically it’ll fit and clear whatever the O2 has existing on the board.

 

A set of 2 mil-max pin headers are used to interface to the existing opamp sockets, and then a couple of jumpers are required to tap onto the existing O2 board (ground, signal ground and lastly for the battery voltage comparator).

Each channel again is a bunch of OPA1688s paralleled (see previous post here for details) The Danger Zone: O2 Headamp output board, OPA1688s Part I (fillwithcoolblogname.blogspot.com)

Here it’s partially assembled, I use liquid flux (that’s kinda messy). Everything gets dunked in my ultrasonic cleaner afterwards though so I’m not worried.

View of the rear, there’s an output relay that latches and prevents that annoying O2 issue of dropping in and out when battery levels get low and creates that popping noise (which is probably not good for your headphones).

From the front assembled, here the signal ground jumper is visible.

Top assembled, here the comparator jumper is visible.

Last jumper for power ground, straight to the battery holder.

Offset voltages measure pretty good.

I stacked a whole bunch of capacitors on my breadboard to do some random capacitive loading tests (with a ~33ohm) load, it performed admirably (I can’t exactly remember the numbers, I think it was 2nF or so load capacitance with 33Ohms, typical headphone cables are maybe 30pF per foot, so this is plenty stable). 

 

Finally this is it all assembled and cleaned up.

I haven’t gotten around to putting the schematics and board diagrams anywhere. If somebody is interested I’ll get off my ass and do it. Myself I’m fairly interested in what this measures properly in terms of THD+N, but I don’t have access to any cool equipment to do it.

Spectra S1+ Not Charging Battery

Had another unit to mess with. This will not charge it’s battery, upon power on with the charger/adapter, the battery icon blinks empty continuously for abit, before it gives up trying to charge the battery.

Turning the unit on without the adapter, the LCD will come up for 1 sec, then shutoff.

Took it apart, battery looked fine, voltage measured ~11.1V and stable (I thought it was dropping out when the pump was commanded to run), pulling the pump and solenoid connector off and trying to run the unit with the battery, exact same behaviour (shut itself off after 1 sec).

I then turned my attention to figuring out how the microcontroller on board measures battery voltage, found a resistor divider and a little output capacitor just to the right of the microcontroller (R36, R38 and C35R).

Measuring the top (input from battery) and turning on the unit from the front panel (fiddly yeah), I only measured ~8V (when it should be ~11V), somehow full battery voltage was not making it through. The input is not applied to the divider all the time, only when the micro commands the unit on, does input voltage appear thru the action of Q12, Q11 and Q9.

Probing around Q12, Q11 and Q9, I couldn’t find anything wrong with them. I ran a jumper from battery voltage to the input of the voltage divider and verified the unit works as it should, next I shifted the jumper to the input of Q11 and once again verified that the unit works (will accept charge, will turn on and stay on with battery power).

This was when I realised there might be some broken trace or via from battery voltage to Q11.

Flipped the PCB around and removed the LCD+plastic support.

Looks like some fluid damage under the Down switch.

I removed the switch, cleaned the mess up and found that a via leading to the other side had broken, there’s no good way to patch that via (since it’s so small) and I opted to keep the jumper on the component side. Didn’t take a picture of the completed patch, but it looked like the original jumper I put on, except tucked neatly away and the battery voltage tapped from closer to the battery connector (after the ferrite bead from the charging section).

Reinstalled everything, charged the battery and tested it out, I can confirm everything is working as it should.

Spectra S1+ Repair

Received one that wouldn’t turn on even with the adapter plugged in. Unit opens up with 4 screws removed from base.

Quick check found the main power rail shorted, initially I thought it was a shorted TVS on the input, removal of the input TVS didn’t fix that, so I started looking around, that’s when I found a small burnt area on the PCB.

Towards the left is the burnt out component, nearby is a NCP3063, as far as I can make out it’s a boost converter (I didn’t know initially what it does until later). I couldn’t de-solder the FET normally as it had fused itself to the PCB track when it failed. Applying heat and leveraging it off the board the copper it was soldered onto de-laminated from the PCB itself. Luckily the source and gate pins came off the PCB ok (just the drain pins were affected).

As far as I can tell, the SMD markings point to the FET being a NTGS4141N (S4 marking code, 7 date code).

With the FET removed (I cut the dangling bit of the PCB off with a scalpel), I could actually power up the unit with the adapter plugged in (yeah!).

I then decided to check if the FET driver and NCP3063 were at least still operational.

Hooked up some insulated magnet wire to the drive to my oscilloscope.

That verified it was still working, also during this time I saw the drive only come on for a couple secs during initial power up, my guess is it’s used only during battery charging (the pack is a 3S 11.1V nominal battery, so it needs ~12.5V to charge it fully when the adapter only outputs 12V, and charging is done through a MAX1873, which I guess needs a couple of volts headroom to charge to ~12.5V)

Scrounging around in my parts bin I found a NTP18N06, which is fairly similiar in terms of drive requirements, but a little higher Rdson (90mOhm vs 21.5), since I had lost part of the PCB I figured the exposed TO220 package would probably be best for heat dissipation and there was ample room on the PCB to mount it on.

Yeah, the size/packaging difference is huge.

A quick solder job and viewing the drive on the oscilloscope + output voltage measurement, verified nothing was wrong, no smoke and the boost circuitry output voltage was present (~14.5V or so for a couple of secs, I tried hooking up the original battery pack to see if it’ll start charging and let me test this section for longer but the pack was too far gone).

I scraped off a bit of PCB on the ground plane, flipped the TO220 package around and soldered the source pin directly to the FET, ran some insulated magnet wire to the inductor and gate pin on the PCB, held everything down with Dow Corning 3145 (electronics safe RTV).

Next I turned my attention to the battery pack, which had a bloated LiPo cell, I couldn’t source an exact replacement 3S battery pack (it uses a built in BMS to balance the charge, there are only 2 wires coming out of the pack, so non of the usual RC type LiPo packs with external balance connectors will work safely, at least not without wiring up a BMS on it). I removed the bad cell from the pack and clipped a random partially charged 18650 to it, attempting to charge it the battery came back to life (the BMS had stopped any voltage from coming out the pack with the 1 bad cell)

Got my hands on 3x new Molicel P26A 18650s and I decided to keep the original BMS.

A bunch of spot welding and soldering later, I’ve got a working battery pack with the original BMS.

Here it is in it’s original spot. I’ve elected to tape the original labels within the machine in case somebody/someone else needs to know what was in it originally. With these new cells, I’ve verified it’s able to charge and stop properly, (cut off at 12.5V) and it should last a bit longer given the capacity has now increased (original 2000mAH, now 2600mAH).

Finally, I re-soldered the input TVS diode and closed it all up. 😁

Mackie CR4 Repair

Over the years I’ve replaced all their internal capacitors, changed out the 12V zeners in the regulator area for beefier units (the stock ones are down on the PCB and keep dumping heat into what appears to be cheap PCB material, it’s definitely not FR4, I tried mounting them off the board to give it room to dissipate the heat), also had replaced the L-R switch and scrapped off all the nasty glue that they used during manufacture that was turning conductive and corroding components on the PCB (I saw a resistor lead almost disintegrate to nothing).

Right now they will come on, but will randomly get muted, I suspect there is even more damage to the 12V regulator area on the PCB and the mute circuitry was kicking in on the TDA6275 (all kinds of guides exist on the internet that just tell you to snip the Mute pin off, it’s not a proper fix as the Mute circuitry ensures +/- 12V exists before unmuting the amp section).

That’s the 12V section, after removing the associated components.

The PCB after I’ve cleared off the tracks that were heat damaged, it’s too far gone to try to patch up. During the process I found a cracked trace on a leg of a zener diode, that was probably why it was randomly muting.

I decided to remove all the circuitry around the +/-12V area, including the components that generate the mute signal and move it all to an external PCB.

The original +/-12V regulation was achieved thru value engineering with the below

The mute was done as below (I’d have to replicate this on my board)

Drew up a schematic and layed out a board. I’ve used TO220 7812 and 7912 for the regulators and all through hole parts.

Here it is populated and installed.

A picture of the clearances and mounting involved (I used a single mount point in between the 2 TO220 regulators, I wasn’t sure the PCB would be ok flapping in the breeze inside the speaker with just the TO220s for mechanical support).

And the rear of the entire PCB when I was done, I’ve marked out where the +/- voltages get tapped for this external regulator+mute board as well as where the +/- 12V regulated voltages go back in. I’ve left the burnt area as is (didn’t attempt to grind them out) but with a sharp scalpel cut off the tracks back to a good area, so nothing is shorting.

Putting it back together, I can confirm they work, time will tell if they keep working.