Table of Contents
- 功放板维修 / Amp Unit Repair
- 箱体维修 / Cabinet repair
- 喇叭单元维修 / Speaker Driver Repair
- 组装 / Reassembling
- 使用体验 / Experience Using the Speakers
最近花400元收了一对有问题的Alesis M1 Mk2有源监听音箱，这款音箱在2010年前后是很受欢迎的型号。不幸的是，因为电路板设计问题比较多，没有多少留存到现在了。
I recently acquired a pair of Alesis M1 Mk2 monitor speakers for about US$70. This was a very popular model around the 2010s. Unfortunately due to bad PCB designs there aren’t many left now.
The pair I got have one speaker working normally while the other one won’t power on. After tearing down I found the following faults:
|Part||One||The other one|
|Fried, with previous repair|
|Normal, with bulging caps|
|dried up ferrofluid|
|dried up ferrofluid|
|Pushed in dust cap|
Repair can be split into 3 parts: PCBs, drivers and cabinets. It was a lot more work than I imagined, and due to I only work on this project during weekend it took more than a month to finish all of the work.
功放板维修 / Amp Unit Repair
问题诊断 / Diagnosing
There is a 20-page long sticky thread on the badcaps forum (https://www.badcaps.net/forum/showthread.php?t=10910) about repairing the electronics of this speaker, which provided a lot of useful information. Based on these knowledge, service manual, and inspection and measurements, I discovered following problems in the non-working PSU board:
最明显的是R11已经烧糊了，说明Q1 S极有从主边滤波电容来的高电压（Q1 GDS全部短路，测量证实）。这个高压烧毁了采样电阻R15之后，烧毁R11。因此R10可能也有损坏。高压通过Q1 G极，烧毁Q3（Q3炸了）、Q4，因此C15、D9、R13、R12、C13可能也有损坏。去往光耦的线路有过流痕迹，但是没烧断，说明高压也去到了VCC网络，因此U1必然已经挂了，光耦也大概率损坏。
The most obvious fault was that R11 was obliterated, which means high voltage from mains filtering caps was present on source pin of Q1 (Q1’s GDS were all shorted together, confirmed by off-board measurements). This high voltage fried current sense resistor R15 before frying R11. Thus, R10 may also be damaged. The high voltage went through gate of Q1, fried Q4 and exploded Q3. C15 D9 R13 R12 may have also took damage. The silkscreen on the trace to the optocoupler were black suggesting over-current, which means high voltage was present on VCC network, so U1 must have been broken, alone with the optocoupler.
电路板维修 / PCB Repair
MOS管用ST的塑料封装600V 10A替换，D14 D16用SF18G 600V 35ns超快恢复二极管代替，光耦用CNY17-4替换。所有电解电容都换成了日本化工、红宝石或者尼吉康。
Other problems include fried 1N4007, exploded inrush limiting NTC, D14 D16 were replaced with FR107s which have poor performance, and bulging caps. So next step was to remove all offending components and test them. I also choose to replace any component that may have been damaged, especially semiconductors. I bought replacement parts from LCSC.
As UC3842 has been discontinued, I bought UC3842B by ST to replace the IC. UC3842B has smaller start-up current with other parameter being mostly identical or better, so there should be no problem using it.
MOSFETs were replaced with two 600V 10A ST MOSFETs in plastic TO-220 package. SF18G 600V 35ns ultrafast diodes were used for D14 D16. Optocoupler was replace with a CNY17-4. All electrolytic caps were replaced with either Chemi-Con, Rubycon or Nichicon ones.
电源板容易烧毁的原因是R3 R4阻值较小，发热大而且离VCC滤波电容C8太近，开关管的散热片也在C8旁边，将C8烤坏。R3 R4是主滤波电容的泄放电阻、均压电阻以及启动时VCC电源的来源。启动时高压直流通过R3 R4向C8充电，当电压达到15V左右时，U1启动，一旦开始开关，VCC电源由辅助绕组T1B经过D5整流提供。
The reason the PSU board is prone to self-destroy is that R3 and R4 have relatively small value thus generate a lot of heat. They were placed close to VCC filtering cap C8, and switching transistor’s heatsink was close to C8 as well so they constantly bake C8 until it dies, taking out everything. R3 R4 serves as discharge resistor, voltage balance network and startup source. On powering up, high voltage DC charges C8 through R3 R4, when voltage on C8 reaches about 15V, U1 starts and once switching is established VCC is supplied by auxiliary winding T1B and rectifier D5.
Based on the Badcaps thread and some analysis, I decided to move C8 to C14 to keep it away from the hot spot, move C14 to bottom side of PCB and increase R3 R4 to 68K. This should avoid or at least delay the same problem from happening in the future.
After the parts arrived, I soldered everything on as usual. C8 can be seen at the location of C14.
测试电源板 Testing the PSU
When testing the PSU, to avoid further damages and injuries to myself, I used the series-bulb method. A 100W incandescent light bulb was put in series between mains power and PSU. That way, if the PSU is fine, the bulb will light up briefly on plugging in the power, and remain off later. If there is a short circuit in the PSU, bulb will light up. As the power of the bulb is 100W it will limit the short-circuit current to below 0.5A.
测试没有问题之后我将正常工作的那块电源板进行了更换电容，将R3 R4 U1 C8 C14也进行了相应的更改。
I didn’t know this PSU cannot run under no load, which created a lot of confusion for me. Luckily everything worked out after I hooked up the amplifier board.
After some testing I decided it is good enough so I performed the same change of R3 R4 U1 C8 C14 on the still working board, and replaced all electrolytic caps.
打理功放板 / Work on the Amp Board
All electrolytic caps on both of the amp boards were also replaced.
I watched the EEVBlog video and didn’t want no component to be “flapping in the breeze”. So to prevent damage caused by vibration, I applied RTV silicone rubber to all large components.
功放组装 / Reassembling Amp Unit
When disassembling the amp I found that the screws holding the power amp IC were extremely tight, as if there were no threads on the aluminum board and they just screwed it in by force. All 4 screw heads were badly damaged, and it even managed to bite out one of my screwdriver bit. When installing it back in, I drilled the hole to 3mm and used M3 screw with nut to hold the amp IC.
修理XLR连接器 / Repair XLR Connector
One of the amp is missing a plate on the XLR connector, I designed one, printed with my Ender-3 3D-printer and superglued it in place.
箱体维修 / Cabinet repair
One cabinet suffered water damage, luckily the seam on the inside is still intact. I first remove all loose MDF that has been soaked.
Stabilize the surface with epoxy
I 3D printed a mock back panel to help with reconstruction
Reconstruct the cabinet using epoxy with wood flour
Fix everything in place with some makeshift mold
First try yielded some cavities, so another round was required
I sanded it down after epoxy has cured. To fix some mistakes I used superglue and wood flour to avoid having to wait another 48 hours for epoxy to cure.
喇叭单元维修 / Speaker Driver Repair
高音更换磁液 / Replace Ferrofluid in Tweeters
I ordered a small amount of APG L11 low viscousity ferrofluid. To replace FF in a tweeter, you take it apart, remove all old residues and put in new FF. I ended up putting about 100 micro-liter of FF into each tweeter. Reference: https://speakerrepairshop.nl/en/instructions/replace-ferrofluid-in-tweeter/c-36
When adding FF, it will be attracted into the air gaps automatically. This is what it looks like without any FF in the syringe.
组装 / Reassembling
The polyfill in the speaker which has bad PSU was ignited when the PSU exploded. The other one has its polyfill removed during repairing of the cabinet. So both speakers were installed with new acoustic material.
Next is to wrap the wires inside the speakers, connect the amp, reinstall the back plate, put on EVA shock absorbing pads on the bottom side and that concluded all of the repair. New screws were used to mount the amp units as some were missing.
And yes, socks in the ports.
使用体验 / Experience Using the Speakers
It has been 4 months after the repair, and there isn’t any problem. The sound of these speakers are well balanced, and bass content is very adequate for a woofer of this size.