Print Page | Close Window

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: 29 March 2024 at 6:58am
Software Version: Web Wiz Forums 12.06 - 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...Confused




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!Thumbs Up


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. :)



Print Page | Close Window

Forum Software by Web Wiz Forums® version 12.06 - https://www.webwizforums.com
Copyright ©2001-2023 Web Wiz Ltd. - https://www.webwiz.net