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. 😁