Mackie Powered Speaker Repair - Page 1
Forum section: electronics / repair

author: RubiconJoe
author_rank: Newbie
timestamp: November 22, 2016, 03:47:38 pm
content: I am attempting to repair a Mackie (TH-15A) powered speaker's amplifier. The original fault was that the unit would not power on.After opening her up and referring to the schematic I found a few burnt components and replaced them. The final amplifier chip (TDA7294) as well as the resistor on its output were damaged. Good news was the amp now powered on (and didn't blow up the new parts). Bad news the LED that indicates the unit is powered on lights up but then dims almost to off, then back on periodically. I checked the +- 28V, +-15V and 3.3V with the all other boards connected, and supply is still fluctuates. Then disconnecting the amplifier board, supply still fluctuating. Finally disconnected the pre-amp board and the rails all became solid with correct voltage levels. Now I'm kind of stuck. I'm not sure what to suspect on this pre-amp board. There is nothing obviously damaged and many surface mount components. Unfortunatly, I do not have much in the way of test equipment, only a DMM. Any suggestions on how to proceed would be greatly appreciated!Thank you for anyone who can help!

author: CJay
author_rank: Super Contributor
timestamp: November 22, 2016, 03:57:36 pm
content: A picture of the boards is always useful to help debug things remotely

author: RubiconJoe
author_rank: Newbie
timestamp: November 22, 2016, 10:52:12 pm
content: See attached. First picture is a picture of the power supply and amplifier board. Next front and back of pre-amp board, then DSP board that plugs on top of the pre-amp board.Also pdf of schematic at the link below.https://bmamps.com/Schematics/Mackie/Mackie_Thump_TH-15A_PCB_Schematics.pdfHope these help.~Joe

author: RubiconJoe
author_rank: Newbie
timestamp: November 23, 2016, 03:08:41 pm
content: Correction: There are two connections to the pre-amp board. One power supply connection with +-15 & 3.3 V, and the 28V connection goes only to the amplifier board. Then the signal connections between power and pre-amp boards.After retesting, I have found that voltage fluctuates on all rails only when the power-amp board is connected, either by itself or with the pre-amp connected. The rails are solid when the pre-amp board is connected alone. Looks like there is another fault on the power-amp board, not the pre-amp. Sorry for the confusion.What would cause my supply to fluctuate? A bad cap? If it were a short somewhere, I assume more components would burn. How can I test for this type of fault?Thank you in advance!~Joe

author: PedroDaGr8
author_rank: Super Contributor
timestamp: November 23, 2016, 05:28:05 pm
content: Quote from: RubiconJoe on November 23, 2016, 03:08:41 pmPower supply entering protect mode, then coming out of it. Seeing the fault again and re-entering protect mode.Sent from my LGLS992 using Tapatalk

author: RubiconJoe
author_rank: Newbie
timestamp: November 26, 2016, 06:22:07 am
content: Thanks for the reply! That does makes sense. Any suggestions on how to further troubleshoot this issue?

author: richnormand
author_rank: Unknown
timestamp: November 26, 2016, 06:00:50 pm
content: Quote from: RubiconJoe on November 26, 2016, 06:22:07 amI assume the power amp is the board on the right in your first photo.Also you mentioned that you have a DMM for checking.Considering that the power supply chokes up when the power board is in circuit you can assume there a pretty high load from something that should be close to a short.As a first pass I would measure the resistance between ground and the voltage supply input rails to the power amp board (with PSU disconnected from it).Look for a shorted/leaky capacitor or diode on the board.Look at the amp chip itself too. It may have been damaged the moment you plugged the replacement unit in. Again that would show up as a near short.Using the diode test range on the DMM applies enough voltage to go in conduction so you should repeat the resistance measurements in both directions for diodes transistors and ICs.

author: geobee
author_rank: Contributor
timestamp: November 26, 2016, 08:40:29 pm
content: Hi there, I have worked on these Mackie units, and I would say the supply is going into protect mode, due to a short in the power amp section. They usually use a digital amplifier IC as the output, and these go down very easily. Not easy to replace unless you have a rework iron with a heat gun unit to extract the old one, even then be careful of board track damage. Best leave this job to a pro workshop, sorry, probably not what you want to hear. I am not a fan of these digital power amp modules, or the switchmode supplies used in musical equipment these days, nut they are lightweight, which is what the consumer wants, and cheaper to produce, less copper etc that a conventional transformer, but more difficult to repair when they turn up their legs so to speak.

author: geobee
author_rank: Contributor
timestamp: November 27, 2016, 12:53:39 am
content: Sorry, got the model wrong re it being a digital SM ic as in some of the Mackie models. But the part about a power amp short would still be my best bet. If the IC has been replaced, it could be a diode shorted or leaky or even, but less likely a cap gone down. Also, check any other transistors on the board, best to remove and check if any funny readings are coming up.

author: RubiconJoe
author_rank: Newbie
timestamp: January 28, 2017, 05:47:12 pm
content: After a very busy couple of months, I have finally returned to try and repair this speaker's amp. I am kind of going in circles on this trying to figure out what is wrong here.Here is what I have tested so far.Power supply rails (power supply disconnected from all other boards)+28 V measured +29.3 V-28 V measured -29.3 V+15 V measured 14.2 V-15 V measured -14.2 V+3.3 V measured +3.2VShould I be concerned with +-15 V rail being low at +-14V?Power supply rails (with conn. 3 plugged in, Power amp board only) All voltages remain the same.Power supply rails (with conn. 11 plugged in, pre-amp board) All voltages start fluctuating as power supply goes into failure mode.Interestingly enough, when I unscrewed the pre-amp board and pulled it away from the chassis to test, I tried to power it on and the power supply then became stable. I cannot seem to find any loose connections on this board. I wiggled everything that wiggles on this board and cannot replicate the problem when the board isn't mounted. After then screwing the board back onto the chassis, powering it on, the voltages fluctuate again. Maybe a grounding issue? I cannot seem to find this fault.I have also tested all transistors and diodes that I can access with a DMM, across both pre and power amp boards and none are shorted.Hope someone can point me in the right direction to fix this unit!Thank you for your help,Joe

author: Armadillo
author_rank: Super Contributor
timestamp: January 28, 2017, 07:21:51 pm
content: With only a DMM, I would segregate the trouble-shooting by eliminating assumption as much as possible. The steps involved qualify the power supply, then the power amp then the pre-amp.To qualify the power supply;Remove the power amp and pre-amp from the circuit. Measure the current [in series] available across the +28v and -ve28v line when say a incandescent bulb [ use anything > 100 Watts say 50 + 50 + 50 in parallel ] is connected  across them. Pass if STABLE current flow. Failed otherwise. [Proper way is to use programmable DC load bank but of course.]. [no need to bother with the 15v and 3.3v line].To qualify the power amp include measuring the quiescent current draw at zero input signal [short the input to ground]. [Quiescent Current = ? amp]Then Connect up the speakers to your amp [without preamp], connect the input to your PC audio out, play audio tone or something [AT LOW VOLUME], Pass if speakers happy, fail otherwise.To qualify the preamp..... to be continued.

author: zepster44
author_rank: Newbie
timestamp: March 01, 2017, 03:10:00 pm
content: I have 3 of the Mackie Thump 15a speakers and 2 of them will not put out any volume until you turn the gain up all the way on the back of the speaker. Once the volume is up, it sounds terrible. I know it's an issue with the amp because I took my 1 working speaker and connected the amp from that to the cabinet that wasn't working and it sounded just fine. Any idea what would cause this gain issue on 2 of the speakers? Is this something I could easily fix with some my shade tree electronics repair skill? Or can anyone recommend a good repair service? Cheapest I've found is $119 on Ebay plus $15 for shipping a piece.thanksChris

author: firstcolle
author_rank: Regular Contributor
timestamp: May 30, 2017, 08:44:49 am
content: Quote from: zepster44 on March 01, 2017, 03:10:00 pmhi,same issue here... any news?

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: May 30, 2017, 05:50:28 pm
content: That LEVEL knob looks HUGE and vulnerable to SMACKING during use, and transit, etc. If it does get smacked, it cracks the volume pot behind it. It then becomes a switch, instead of a pot. The switch makes contact at full clockwise rotation. You get full gain, and unless your input signal is drastically reduced, a lot of added distortion. Replace the pot., and then protect from further SMACKING.

author: zepster44
author_rank: Newbie
timestamp: July 10, 2017, 02:16:12 pm
content: Quote from: firstcolle on May 30, 2017, 08:44:49 amStill no luck. One person said it could be the ASP board that needs replacing but, I'm not technically inclined to verify if that is the issue. instead I contacted a local shop to get one fixed and once I find out their pricing, if its within an acceptable range, I'll get it done and post what they did to fix it on here.Modify message

author: Armadillo
author_rank: Super Contributor
timestamp: July 10, 2017, 03:35:19 pm
content: Same, Check your input signal first; It should be XLR input. Remember turn down the volume control.https://www.eevblog.com/forum/repair/odd-output-signal-jbl-gto-75-4-amplifier/