MX 3000a early clip?
Printed From: Speakerplans.com
Category: General
Forum Name: Amp Forum
Forum Description: The 'Stopping Jake Fielder moaning constantly' forum description...
URL: https://forum.speakerplans.com/forum_posts.asp?TID=107516
Printed Date: 27 March 2026 at 8:53am
Software Version: Web Wiz Forums 12.08 - https://www.webwizforums.com
Topic: MX 3000a early clip?
Posted By: Greg32
Subject: MX 3000a early clip?
Date Posted: 26 November 2022 at 12:30am
| Hi Ive been trying to diagnose a fault in channel 1, when fed with a 100hz sine wave the positive side of the output waveform starts clipping at about 65vrms no load(seen on oscilloscope), the other channel is fine and clips evenly at about 95vrms. I have checked the rail step fets and diodes and they appear fine. i have another mx3000 that i have been cross checking voltages with and so far i cant find the fault. any thoughts on what else to check? Ta Greg |
Replies:
Posted By: kedwardsleisure
Date Posted: 26 November 2022 at 2:53pm
|
are the step driver boards showing the correct LED sequence? I would swap one over with the one on the other channel and see if the board itself is the problem. A tip with these is to arrange the boards so the faulty channel is the one on top. Saves time. ------------- Kevin North Staffordshire |
Posted By: Greg32
Date Posted: 26 November 2022 at 11:48pm
| Thanks Kevin, the step driver boards do appear to have the correct led sequence, I will try swapping the step driver boards from the working channel, are these repairable? or are they just replaced? |
Posted By: kedwardsleisure
Date Posted: 27 November 2022 at 11:26pm
|
the earlier ones are a thick film module and can be tricky but the later ones are conventional SMD and can be repaired. But let's see if they're faulty first! Obviously check for the silly things like the legs of the rail mosfets broken off or poor contact in the power supply connectors ------------- Kevin North Staffordshire |
Posted By: Greg32
Date Posted: 28 November 2022 at 8:27pm
| Thanks again Kevin, I've swapped out the step drivers and the problem still persists. I've checked for the obvious things ie power supply correct ,no visible damages I've metered all the semis for shorts etc. |
Posted By: kedwardsleisure
Date Posted: 28 November 2022 at 8:55pm
|
Is the clipping the original fault or has this happened after some other blow-up repair? ------------- Kevin North Staffordshire |
Posted By: Greg32
Date Posted: 28 November 2022 at 10:29pm
| The clipping was the original fault. |
Posted By: Randy Bohannon
Date Posted: 28 November 2022 at 10:43pm
|
I hesitate to suggest but have you checked for dry solder joints and bad connections? I had a strange clipping issue, only on the top half of the wave, in an amplifier of mine that turned out caused by a dry joint between a mosfet and it's source resistor, effectively making the amp run on two instead of three. The symptom was the issue got worse as the load impedance went down, indicating it was an issue with current delivery. https://forum.speakerplans.com/cloud-vtx-repair-and-restoration_topic107442.html" rel="nofollow - https://forum.speakerplans.com/cloud-vtx-repair-and-restoration_topic107442.html |
Posted By: Greg32
Date Posted: 28 November 2022 at 10:50pm
| Don't hesitate with suggestions please, there a a couple of suspect solder joints but I did meter them and they appeared ok , I will try a throught clean and resolder them. Ta Greg |
Posted By: kedwardsleisure
Date Posted: 29 November 2022 at 8:48am
|
there isn't alot of electronics in between the commutating fets and the drive boards, a couple of gate resistors (fusible, so they may fail without burning) but if the amp has had a hard life I would check the little bootstrap electrolytics and dropper resistors off each DC bus rail, these provide the decoupled power supply to the daughter boards (4 in total) ------------- Kevin North Staffordshire |
Posted By: kedwardsleisure
Date Posted: 29 November 2022 at 8:50am
|
PS in my notes there is a remark about checking any capacitors on the front board that houses the volume pots. Can't remember what symptom that causes. ------------- Kevin North Staffordshire |
Posted By: Greg32
Date Posted: 30 November 2022 at 11:31am
|
I was checking the bias as per the service manual, there is supposed to be 100-110mv across the driver transistors emitter resistors, mine were at 140mv, during adjustment the driver transistor failed and took out 1 output device before the main channel fuse blew. I read that onsemi have the mjl1302 and mjl3281 which are replacements for the toshiba devices. but I cant find the driver transistors the 2sc3298 and 2sa1306, are there any replacements for these? Ta Greg
|
Posted By: Randy Bohannon
Date Posted: 30 November 2022 at 12:39pm
|
Closest I have any experience of and is possible to get would be: https://www.mouser.com/datasheet/2/408/toshiba_2SC4793-587292.pdf" rel="nofollow - https://www.mouser.com/datasheet/2/408/toshiba_2SC4793-587292.pdf But I see that the original stands a collector current of 1.5A vs 1A and a base current of 150mA vs 100mA of the 2sc4793. Hmmm, perhaps not then...  |
Posted By: Greg32
Date Posted: 30 November 2022 at 8:01pm
| Thanks Randy, they might just work. collector current should be around 240ma. |
Posted By: Greg32
Date Posted: 16 December 2022 at 10:53am
| Update i have replaced the output devices with mjl1302 and mjl3281 and the pre drivers with MJE15032/33 the service manual suggests to set the bias at 100-110mv accross the driver transistors emitter resistors, this value only just turns on the output transistors with 1mv across the output transistors emitter resistors, that is about 4.5ma per device which seems a bit low. all other voltages seem to be good. more testing tomorrow... |
Posted By: Greg32
Date Posted: 19 December 2022 at 1:50am
|
I never tried changing out the step driver boards on the -ve side, the high rail step driver board has 2 leds that are illuminating as signal level is increased, I only saw 1 led on my other mx3000a. what is the correct sequence of led illumination? or what do they indicate? Thanks Greg
|
Posted By: Greg32
Date Posted: 26 December 2022 at 7:07am
|
Ok finally got time to try swapping out the negative step driver board, success it works. now i need to find a replacement board. qsc # ic-000090-00 
|
Posted By: Randy Bohannon
Date Posted: 26 December 2022 at 4:23pm
Well done on diagnosing it at last Greg!
|
Posted By: Greg32
Date Posted: 27 December 2022 at 7:14am
Thanks Randy, Me being impatient I had a go at repairing the step driver board, i couldn't get the sot-23 devices(quickly at this time of year) so i used the to-92 version 2n3904 and 2n3906. looks rough but functions as it should.  |
Posted By: Randy Bohannon
Date Posted: 27 December 2022 at 2:39pm
| Should be fine unless it's going gigging again. :) |
Posted By: xz_vice
Date Posted: 05 March 2026 at 7:15am
|
Hey!! I'm new to this forum. Greetings from Italy. I am 22yo and I do volunteering at my Church and I also contribute by offering my "technical knowledge" and my audio equipment for their outdoor/indoor events. I have recently acquired 4x JBL MPX1200 amps, and one of them happens to early clip on one channel. The channel happened to be the bottom board, so i flipped the main board to have the bad channel at the top. Apparently when the amp is cold it works fine, 5 minutes later the bad channel starts clipping on the negative rail, actually it says it's clipping but in reality it reduces the gain and it becomes unclipped but with max voltage==the smallest rail so i guess the 40V rail. I know the rails are driven by the SMD daughter boards. Is there any schematic of them? I could find the schematics for the whole amp, but these boards are defined as blocks in the schematic, not with actual components described.... It's like one of the two negative rail fails to activate when it heats up, and so the gain reduction circuitry triggers. The issue happens both when the amp output is loaded and also when it's not loaded, so it's not a manner of load i guess... Any help would be appreciated as we will have an event at the end of June!!! Tysm! :) Also, I am kind of scared of desoldering these SMD boards, they look kind of fragile... https://ibb.co/1Ys3Y34C https://ibb.co/nqNsVq8k Thank you!
|
Posted By: Greg32
Date Posted: 06 March 2026 at 12:37pm
| Hi, its late here atm but tomorrow I will have a look for the schematics for the drive boards. Greg |
Posted By: kedwardsleisure
Date Posted: 06 March 2026 at 1:12pm
are the step driver boards showing the correct LED sequence? I would swap one over with the one on the other channel and see if the board itself is the problem
The amp uses grounded bridge topology, which means faults on the positive half of the waveform can be caused by the negative drive. This is becuase the output section ground-references the power supply rails, and the power supply centre point becomes the output. ------------- Kevin North Staffordshire |
the positive side of the output waveform starts clippingPosted By: xz_vice
Date Posted: 07 March 2026 at 6:12am
|
Posted By: xz_vice
Date Posted: 07 March 2026 at 6:20am
How do i know the correct led sequence? Is there some kind of diagnostics procedure to follow regarding these daughter boards? Also my problem is related to the negative rail, as I tried to bypaas the gain reduction stage, and the positive rail was full, while the negative rail was clipping to the first rail (40V or so, so the second rail wasnt activating) Also, the daughter boards PCBs are burnt as hell (the back of the pcb became brown, probably because of a design issue? 😅)
I am not really experienced so I don't know how the amp works 😅, but yeah i figured out the output is the central point of the Power supply and that was really weird to me...
|
Posted By: Greg32
Date Posted: 08 March 2026 at 3:25am
|
uploads/417/QSC_SH-000009-00.pdf" rel="nofollow - uploads/417/QSC_SH-000009-00.pdf uploads/417/QSC_SH-000008-00.pdf" rel="nofollow - u uploads/417/QSC_SH-000008-00.pdf" rel="nofollow - ploads/417/QSC_SH-000008-00.pdf hope the attachments work, Greg
|
Posted By: xz_vice
Date Posted: 08 March 2026 at 6:23am
Heyyy!! Tysm! Do you happen to have also the schematic for the big smd pcb that controls clipping, protections and so on? It'd be great 😅 It's called IC-000010-00 iirc 🤔 Also, is there a check procedure that mentions also the led sequence of the SMD driver boards? |
Posted By: Greg32
Date Posted: 08 March 2026 at 7:29am
|
uploads/417/QSC_SH-000010-00.pdf" rel="nofollow - uploads/417/QSC_SH-000010-00.pdf
this one, Greg
|
Posted By: Greg32
Date Posted: 08 March 2026 at 7:31am
|
uploads/417/qsc_mx3000a_sm.pdf" rel="nofollow - uploads/417/qsc_mx3000a_sm.pdf
this is all i have, Greg
|
Posted By: xz_vice
Date Posted: 10 March 2026 at 10:12am
Heyyy!! Thank you so so much! Super useful! 🙏🏻 When i will have enough free time i will start investigating the issue 👍🏻💪🏻
|