Help to repair - Focal Alpha 50 Studio Monitors - Page 1
Forum section: electronics / repair

author: jacksons
author_rank: Contributor
timestamp: August 08, 2025, 11:37:30 am
content: Hi everyone, hoping someone might be able to help with my speaker repair. The unit powers on with no sound. I'm a novice at this, I replaced capacitors and resistors on my previous KRK's successfully. I'm using my working unit to cross reference any measurements.Hi have found this similar post on here that is repairing a bigger version of mine -https://www.eevblog.com/forum/repair/focal-alpha-80-repair-(no-sound)/I replaced the same resistor and watched it glow red hot and burn out when I tested it. I replaced it with the same 10ohm and same size (1210), and just to double check that the power rating on them will be the same if they're the same size?I've measured the current AC coming into the board, and it's exactly the same as the working unit. The Bridge rectifier is the same DC reading too.I've measured the capacitors using resistance on my multimeter (mine doesn't have capacitance). The resistance slowly increases/ decreases the same as the working unit.I've attached images of both the broken and working unitsAny help would be appreciated, thank you

author: sean0118
author_rank: Frequent Contributor
timestamp: August 08, 2025, 12:53:19 pm
content: It looks like the resistor is connecting to pin 3 (right side) of U8? If so, I suspect U8 is faulty.U8 is a 12V linear regulator with pin 3 its input. With a short on it's output the current should limit to 150mA, which is 0.225W in the 1210 resistor (probably not enough to burn it out). So I suspect there might be a fault internally inside U8.With it off can you please measure resistance between pin 3 to GND and pin 3 to pin 1?Actually another possibility is that the input voltage into pin 3 is higher than 35V, in which case the ESD protection inside U8 will clamp (causing excessive current to flow).

author: jacksons
author_rank: Contributor
timestamp: August 08, 2025, 01:18:22 pm
content: Thank you sean!The resistance on pin 3 to GND was 57ohms3 to 1 - 1.65kohmon the working unitpin 3 to GND - 3k3 to 1 - climbs from 3k slows down at around 10.6kI turned the unit on and measured V on pin 3 to GNDBroken - 0.3VWorking- 20vI was thinking that with the broken resistor the U8 wouldn't be getting power

author: sean0118
author_rank: Frequent Contributor
timestamp: August 08, 2025, 02:22:07 pm
content: I'm pretty sure U8 has an internal fault given the low readings, is the resistor completely open circuit now? Worth checking that the voltage being supplied to the resistor is < 35V, otherwise a replacement for U8 will be damaged again.

author: jacksons
author_rank: Contributor
timestamp: August 08, 2025, 02:33:49 pm
content: The resistor is open circuit, no continuity.I measured from the negative on the bridge DC rectifier to the resistor and its 45V, but the working unit is also measuring 45VIs this the correct way of measuring? thank you

author: sean0118
author_rank: Frequent Contributor
timestamp: August 08, 2025, 02:45:08 pm
content: Quote from: jacksons on August 08, 2025, 02:33:49 pmSorry I should have said, you want the voltage between the resistor input and GND. It should be 20V like the working one.

author: jacksons
author_rank: Contributor
timestamp: August 08, 2025, 02:59:11 pm
content: Both the broken and working one are 20.5V.So it is probably the U8 that needs replacing? thank you

author: sean0118
author_rank: Frequent Contributor
timestamp: August 08, 2025, 03:02:38 pm
content: Yep, good work, to me it seems there's an internal fault within U8.

author: jacksons
author_rank: Contributor
timestamp: August 08, 2025, 03:08:45 pm
content: Thank you Sean! Appreciate it! I will order one and update with the results, probably be here monday

author: Kean
author_rank: Unknown
timestamp: August 08, 2025, 03:13:35 pm
content: Quote from: jacksons on August 08, 2025, 03:08:45 pmI'd suggest ordering more than one replacement... they shouldn't be expensive and there might be something faulty after that regulator that caused the original problem.

author: sean0118
author_rank: Frequent Contributor
timestamp: August 09, 2025, 03:57:47 am
content: Quote from: jacksons on August 08, 2025, 03:08:45 pmNo worries, Kean had a good point about ordering spares. Also when you remove U8 check that the output to GND, input to GND and input to output are high resistance / similar to the working board. If the resistances look good you should be fine to solder on the replacement U8 and resistor.

author: jacksons
author_rank: Contributor
timestamp: August 11, 2025, 03:17:01 pm
content: Hi again, unfortunately didn't fix it. Same thing happened. I read your new response after testing with the new parts, so un-soldered and took the measurements.I don't know if I've measured this right, I took these from the board where the U8 goes, measuring where their pins would bePin 1 (OUT) to 2 (GND) - This resistance climbed up to about 166kPin 3 (IN) to 2 (GND) - 10 ohmsPin 3 (IN) to 1 (OUT) - This resistance climbed up to about 166kOn the working board with u8 still solderedPin 1 (OUT) to 2 (GND) - 7.5kPin 3 (IN) to 2 (GND) - 3kPin 3 (IN) to 1 (OUT) - 10.6kI had to order 25 min so I've got some spares haha

author: Kean
author_rank: Unknown
timestamp: August 11, 2025, 04:04:45 pm
content: So you measure 10R from VIN to GND after replacing the 10R resistor between VIN and the 20V power rail.It seems like the 20V power rail is shorted to ground now. It can't have been like that before if it burnt the old resistor.Either there is a problem with your soldering of the R105 replacement, or the failure has induced another fault.

author: jacksons
author_rank: Contributor
timestamp: August 11, 2025, 06:08:43 pm
content: Sorry my typing error - it was 100ohmI've just checked all the readings again, the other two readings are at 4m (if this is significant that it has increased?)There's still 20.5v from the Resistor IN to GNDHow do I check for shorts if the 20V power rail is shorted to ground?Thank you for your help

author: jacksons
author_rank: Contributor
timestamp: August 12, 2025, 08:43:54 am
content: I managed to take these through the microscope.If I were to re-solder the new U8, would I be able to measure the output voltage to check if its working?With the broken or no resistor? The broken resistor does has continuity, unlike the previous

author: Kean
author_rank: Unknown
timestamp: August 12, 2025, 05:02:50 pm
content: Well 100 ohms is still very different to the 3k you have on the working board, but not as bad as 10 ohms.The soldering looks OK, but the replaced resistor does seem to have suffered again. Is it still measuring 10 ohms, or has it gone to 100 ohms? You mentioned it has continuity, but not its value.As you bought many replacement regulators, I don't think you'll do any more damage by soldering another one and taking more measurements.You just need to be gentle on the PCB pads as they don't like repeated soldering and unsoldering.What I would probably do is remove both 12V regulators (positive on U8 78L12 on negative on U9 79L12) and then check continuity while unpowered between the regulator pads.Then if no obvious shorts you can power up and check the unregulated +/-20V input rails.Note that the functional order of the pins is different between 78 and 79 series regulators.

author: jacksons
author_rank: Contributor
timestamp: August 12, 2025, 08:03:36 pm
content: Its measuring 38 ohms across the broken resistor, consistent with the previous broken ones.There's no continuity across the broken resistor, sorry I did double check my earlier measurements before sending, but apparently I was wrong.There's no continuity between U8 (Measured the Board) pin 2 (GND) and U9 (Measured the Board) Pin 2 (Vin)There's 20.5v in to the broken resistor, measured between Resistor In & U8 Pin 2 (GND) - bypassing the resistor?0.7v on the Resistor out to U8 pin 2There's 0.7v on the Vin on U8 Board - Between Pin 2 (GND) & Pin 3 (Vin)Is replacing the resistor with the U8 off and measuring the Voltage from Resistor Out to U8 Pin 2 a bad idea?I measured the Resistor In and turned the speaker on and it didn't surge about 20.5v, what could cause the V to increase? (I'm presuming that's what's blowing the resistor?)Is it the U8 drawing too much power? Currently nothing above the 35v rated for the U8 78L12 / U9 79L1220.5V on U9 Vin12v on U9 VoutSorry for my lack of knowledge. If you think this is out of my current ability I'll search out for a local professional. Thank you

author: jacksons
author_rank: Contributor
timestamp: August 18, 2025, 01:50:38 pm
content: Hi everyone, just want to follow up for anyone who might come across this with a similar issue.I took it to someone, apparently was a short on c102, one of the tiny capacitors after the U8.Thank you for your help, I nearly got it

author: Kean
author_rank: Unknown
timestamp: August 18, 2025, 02:56:50 pm
content: Thanks for the update.A slightly difficult one to troubleshoot remotely, but hopefully you'll be better prepared with what to look for when tackling the next one...