SOLVED - Need help with Focal Solo6 Be repair?

[nOw]

The noise is definitely coming through the tweeter and not the woofer

 

[chilidawg]

The noise is definitely coming through the tweeter and not the woofer

Focal used a single power amplifier chip solution (LM4780) to drive the tweeter, and the circuit is a lot less complicated than the woofer amplifier section.

 

[chilidawg]

Nuts were glued on an aluminum piece using UHU that came off after a gentle twist. Seriously, these guys never heard of self-clinching nuts, or they are just to cheap skate to invest in a manual 1 tonnage arbor press, lol.



Different size of screwdrivers, a plier, and a box cutter just to get the board out, and a bottle of isopropanol plus paper towels to clean both of my hands from the thermal paste galore! Grrr...

All so that I can swap the positive signal mosfets with the negative signal mosfets and see if DC will follow them. If not, then the mosfets are good and they don't need to be replaced.

 

[chilidawg]

Well, believe it or not. Same exact problem with 2 years ago!

Here's the one that I repaired back then.
R311 (120K) measured in circuit as 120K instead of 15K like R307 in the negative signal section because there's no continuity to C312.



And here's the one that was measured 15K in circuit 2 years ago, but now is measured as 120K. I have continuity to C312, so I guess something else must have died and gone open circuit which is why this resistor (partly connected to opamp input) doesn't go down in resistance.



Added a solid core wire jumper, connecting C312 to the resistor, and voila! resistance in circuit is back to 15K. Powered it up, no more DC, no more crackling noise, and no pop when turning the speaker on/off. Hooray!!!



EDIT: Fixed typos (128K to 120K, and C132 to C312)

 

[Wavelength]

Chilli,
I feel your pain. Rob Fetters brought both of his 6Be in a year ago. One the two main power supply caps were bulging at the top. Easy fix, NOT the damn thing is almost impossible to take apart. The second one had some real whacky problem cut the wires because they were soldered to the drivers and sent the amp in. I have designed for Focal in the past and know these guys.... yeah it cost like $600 and they just sent back a new amp and tweeter grill.
Some nonsense about the defraction of the tweeter and the new amp it was required. Been designing drivers since 1981 pretty sure I was talking to a marketing guy.
Now the one I fixed is doing the same thing so Rob said send it in. I am not going to remove the amp because they charged me extra for that. So far had an RMA# for 2 weeks and they keep telling me I am like 250 in line for repair so don't send it yet???
Oh well... I have the 650's and turn them off when not using them as there is an easy power switch on the front. I think the problem with these is people plug them in 24/7 and you get some power surge or brown out and the damn thing kills itself.

 

[chilidawg]

No power supply problem here, but I do believe what I encountered is a flaw with the circuit design. A certain component will die in that positive signal section of the woofer amplifier after a decade of usage, causing R311 to lose connection and its resistance in circuit will no longer be 15K, but back to 120K (yes, it's a 120K resistor not 128K that I previously wrote, my bad...)

I don't know which component that died because I didn't have enough time to poke around to find out. The monitors are back with the owner. Maybe someday if another problem arise, lol. I think the big 6800uF 80V supply cap will need to be replaced soon considering its age.

 

[AXP]

First of all, thanks a lot to @chilidawg for this thread. It was helpful in troubleshooting my own problem with those monitors.

Some background. There are multiple versions of the plate amps used in Solo6 BE. I don't know how many revisions are there, but there are at least two distinctive features that tell them apart. One is the already known difference in the switches - the sensitivity and voltage switches on the backplate. The older version has a metal toggle switch for sensitivity and a plastic rotary switch for the mains voltage. The newer version has slider switches for both and is known to be more mechanically reliable.
Old version (image from the internet):

New version:

There's also another, more recent version of the plate amp, supposedly used in the 40th anniversary edition of the monitor. It can be bought as a spare here, and the "V4" in the name suggests there's also a 4th revision of the amp apart of the three I mentioned.


What no one seems to mention is that there are at least two completely different designs of the circuit - one based on integrated amplifier chips and another one on discrete amplifiers with Indigo BASH SIP module just for the PWM control. This thread has a lot of photos of the latter, e.g. this one:

As far as I can speculate, they initially built the woofer amp around the ST's STA5150 and STABP01 ICs (the first is the amplifier itself, and the second is the power voltage controller), but later ran out of parts (as ST discontinued the BASH line) and had to come up with an alternative design.

The monitor that I have has the old backplate (with the rotary voltage switch and no BASH logo), and the woofer amp is based on the aforementioned ICs:


Despite the differences, the failure symptoms are very similar to the ones described in the thread: the woofer either produced only hiss (no sound), or periodic crackling. Another one I haven't seen being mentioned is the dimming and blinking of the green power LED on the front panel.
After the initial troubleshooting a few years ago (which included replacing the STA5150 IC), the temporary workaround was to disable the standby/protection circuit by lifting the collector of the Q4 transistor (it pulls the standby/mute pin 11 of the amplifier IC):

That helped for about a year, but recently it started sporadically entering the LF crackling state upon power on again until eventually failing completely.

Interestingly, despite my two monitors having serial numbers a few digits apart, they had different revisions of the amplifier PCBs.
The failing one was labelled "FOCAL SM6 Issue 1.3" while the other, working one, had the "FOCAL SM6 Issue 1.31" label on it.
Nevertheless, the layout and component values both seemed to be exactly the same, so I've proceeded with comparing everything between the two PCB's. At this point I already had a particular suspicion that needed confirmation, so I've made a spreadsheet of all resistors - their marking and the measured values on both boards. I'm attaching a table with all values if anybody needs a reference.
Surprisingly, I've found a bunch of broken resistors! They all measure as open circuit. They are all in different parts of the circuit - some are in the preamp, some are in the poweramps, some are in the control circuit, so I doubt it's because of exceeded power ratings or something similar. More importantly, there's a suspiciously limited number of values that have been affected. For example, almost all 3.32k resistors are broken, so it seems like there was a bad batch of resistors.
After replacing all resistors of the affected values, and re-enabling the standby circuit, the monitor now works perfectly!
Hopefully this information will help others who happen to have boards from the same batch and are experiencing any issues.