Saturday, February 12, 2022

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).

Regulator

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

Burnt PCB

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

Reg Ori

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

Mute Sch

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

Sch

PCB

Here it is populated and installed.

PopulatedInstalled

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).

Mount and clearance

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.

Back

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

10 comments:

  1. Great work! i recently got a pair from the trash and was looking for a fix to that burned pcb, i'm a newbie at electronics so i'll try to aply the same fix as you but slowly because i don't know much about the topic, i have already removed all de 12v section jeje.
    Keep doing amazing things !

    ReplyDelete
    Replies
    1. I've still got spare populated boards available (only missing the regulators). I was kinda wondering if there'd be anybody interested in these boards. Contact me on my email if you need one.

      Delete
    2. Really!?... That would be great, i'm very interested could you share your email to me, i wasn't able to find it.

      Delete
    3. fox3.aim120SPAMANDDIE@gmail.com
      Remove "SPAMANDDIE". :)

      Delete
  2. Great work, I am glad you found my topic on Badcaps forum and implemented a similar solution. :-)

    The LM7X12 regulation removes the overheating of that area on the PCB.
    But, the "no sound" occurs due to a faulty C39 50V1uF capacitor which is the closest one to the big heatsink (hence on the direct path of the heat as it travels upward). Over time, the exposure to severe heat from the zeners and the heatsink causes this little capacitor to dry and leak current. It is used to make the transition between Play and Mute mode of Pin5 more smooth. But, when it starts to leak current, it slowly leaks voltage from the 20V rail onto Pin5 and causes the amplifier to mute (that is why cutting Pin5 as some people do removes the "no sound problem", but then the intended purpose of Pin5 is broken).

    Puting a high quality capacitor on C39 spot will likely remove this issue. Removing this capacitor is also a workaround and the TDA will then always work as intended - but, you will hear a faint "pop" sound every time you plug or unplug the headphones (because there would be no capacitor to make the transition between 20 and 12-13V on Pin5 smooth, but the Play/Mute function would still work).

    I did some research, the Cap-Top SK series capacitors used in these speakers are likely to be CapxCon knockoffs for this board, as it is the only SK general purpose 105° cap brand I could find. There is virtually no information about the Cap-Top brand per se.

    If Mackie had included the LM7X12 regulators in place of the zener/resistor combo, even these lower quality capacitor might have lasted longer or even be quite OK as the would be much less heat in the enclosed space.

    ReplyDelete
    Replies
    1. I'd previously replaced all the caps and replaced the Zener+resistor combo with higher wattage units and spaced them off the board, the heat is still insane though, ater about a year+ the surrounding capacitors have a noticeable increase in ESR (was using quality Nichicon or Nippon Chemicon 105deg capacitors), so yeah I cut out all the traces that were delaminated and stuck in the 7x12 combo. Had to implement the mute on the replacement board as those traces were delaminated as well, since they are next to the Zener+resistors.

      I believe mine was muting because one side of the supply (I can't remember +12 or -12V), had a crack in the board/trace. Looking at the board with a strong light I can see multiple cracks around this silly power supply section.

      Delete
    2. Yes, cracks can happen because the PCB burns and starts to shrink, therefore developing cracks. If a crack separates a trace, simply scrape the green cover at the cracked area and solder them back together.

      The only thing I havent done is replacing the L/R switch. It used to sometimes mute one of the speakers. I added some contact spray through the back plate where the switch is and hopefully it reached the sliding surfaces inside. So far it's not making any trouble. But the overheating and muting problem has been fixed since I intriduced the LM7X12 board. Like you said, the speakers will now probably last for a long time.

      Delete
  3. fwiw - for my Mackie CR3: The big 4700uF caps in the power supply section were obviously bad - buldging. I replaced those, but that didn't fix it - supply voltages were still off. So I then replaced the two 12V zeners (Z3 & Z4). That did the trick for me. Of course, keeping the original design means it still produces crazy heat, so it can't last forever. I had already replaced these speakers for the use with my PC tho. So I plan to use these as 'portable' speakers around my house, porch, patio, etc. And with that I'm going to leave the electronics package *outside* of the box so that the heat can dissapate into free-space. Time will tell how long that will allow these speakers to live on.

    As others have said - thanks for posting this blog entry to help us all repair these and keep them out of the landfill.

    --Jeff R--

    ReplyDelete
  4. Hi, i'd like to try your method.
    Could you share the list of any component you remove from original board?
    Thanks :)

    ReplyDelete
    Replies
    1. Fairly sure I removed for power supply "C66, C67, C78, C79, R1, R2, Z3, Z4, C68, C69, C80 & C81" and for mute "R86, R77, C74 & D1".
      R1, R2 and Z3, Z4 need to go for sure, the rest for the power supply section is non-critical.
      I only removed them + the mute portion and implemented them on the external board because the board itself was a charred mess.

      Delete