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MX 3000a early clip?

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Greg32 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Greg32 Quote  Post ReplyReply Direct Link To This Post Topic: MX 3000a early clip?
    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
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post 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

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Greg32 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Greg32 Quote  Post ReplyReply Direct Link To This Post 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? 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post 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

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Greg32 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Greg32 Quote  Post ReplyReply Direct Link To This Post 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. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 28 November 2022 at 8:55pm
Is the clipping the original fault or has this happened after some other blow-up repair?
Kevin

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Greg32 Quote  Post ReplyReply Direct Link To This Post Posted: 28 November 2022 at 10:29pm
The clipping was the original fault. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Randy Bohannon Quote  Post ReplyReply Direct Link To This Post 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.

  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Greg32 Quote  Post ReplyReply Direct Link To This Post 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 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post 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

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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post 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

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Greg32 Quote  Post ReplyReply Direct Link To This Post 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
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