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Bad output device?

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AJordan View Drop Down
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    Posted: 30 March 2018 at 2:27pm
I am attempting to repair the amp from a Laney TM200P active monitor for a friend. Said friend had been playing a bass guitar through it, he said it farted a bit then died. When I got it it was not powering on so opened her up and first things I saw were two F3.15A fuses on the transformer secondary were blown and the two 4700uf smothing caps were obviously shot.
My electronics skills are pretty basic so starting with the obvious I replaced the caps and fuses. Powered her up and I could hear a couple of seconds of gentle fizzing (from the speaker) another couple of seconds later both fuses went again.
Next I tested the bridge rectifier using DMM in diode mode, this tested OK (I think). Did a bit of googling and began to suspect the output devices (Motorola MJ15003 x 2). At this point I definately don't know what I'm doing, just attempting to follow what I found on google. Apparently these are an NPN transistor and I could do with some help interpreting the readings I am getting.
Again using DMM in diode mode and unsure which leg is base or emitter, devices still in circuit.
Device #1. +probe to case, -probe to either leg gives a low reading which slowly climbs to infinity(open circuit). -probe to case, +probe to either leg slowly climbs to around 600?milivolts?  Probes on each leg reads 100 in each direction.
Device #2. +probe to case, -probe to one leg reads 750, to the other reads 850. -probe to case, +probe to one leg reads 585, to the other reads 605. Probes on each leg reads 100 in each direction.
The fact that the two devices measure differently tells me there is a problem, other than that I am lost so, any advice would be gratefully recieved.

In addition to the above there are two green ceramic ?resistors? (no markings on them) which apear to be in parallel with the + and - legs of the rectifier and the smoothing caps. The cases of these apear to have suffered some heat marking, could they be part of the problem?

Cheers to anyone who replies! Even if it's to say scrap it cos it's not worth the hassle.
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jbl_man View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 30 March 2018 at 2:58pm
Whip the MJ15003 out,and test to see if they are shorted emitter to collector (from underneath is left hand pin, to metal case). These are quite robust transistors,but should be easily available. Also check the emitter resistor, as maybe dead too if the transistor is shorted.




Edited by jbl_man - 30 March 2018 at 3:40pm
Be seeing you.
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AJordan View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AJordan Quote  Post ReplyReply Direct Link To This Post Posted: 30 March 2018 at 4:10pm
Cheers jbl_man! Will try that. I'm guessing this can't be done with it still in circuit?

Are my other readings meaningless while still in circuit?


Edited by AJordan - 30 March 2018 at 4:12pm
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jbl_man View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 30 March 2018 at 4:12pm
It can be,but they are so easy to take out and test...if it keep blowing fuses,is usually a shorted output device.
Be seeing you.
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AJordan View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AJordan Quote  Post ReplyReply Direct Link To This Post Posted: 30 March 2018 at 4:28pm
OK will give it a go. Cheers!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Pinyorouk Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2018 at 2:19am
Usually when the output devices fail the driver transistors fail too. Don't just replace the output transistors as a bad driver can blow a new output transistor.
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AJordan View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AJordan Quote  Post ReplyReply Direct Link To This Post Posted: 04 April 2018 at 5:29pm
OK, so I took out the two output devices and tested them out of circuit. Both devices tested good, very different readings to when they were in circuit.

Out of interest I put a couple of new fuses in and powered it up while they were out. Powered up fine without blowing fuses and produced a constant hum from the speaker. Does that point to anything?

Wish I could find a schematic for this.


Edited by AJordan - 04 April 2018 at 5:55pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 04 April 2018 at 6:31pm
Take a good crisp photo of the PCB (Both top and bottom) and post it up? Might be useful to see what's going on.
Be seeing you.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote djeddie Quote  Post ReplyReply Direct Link To This Post Posted: 04 April 2018 at 7:05pm
If the OP devices are OK the next thing to look for is some dodgy soldering. The vibrations can shake them loose... had in on a couple of Wharfedale floor monitors.

Edit: As said above, post up a high quality pic of the PCB.


Edited by djeddie - 04 April 2018 at 7:05pm
Chas n Dave : it's like Drum and Bass but with beards.             E=mc² ±3dB
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AJordan View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AJordan Quote  Post ReplyReply Direct Link To This Post Posted: 04 April 2018 at 10:05pm
I might have missed something very basic here. Just had my hand in the cabinet for a feel of the speaker wiring and found that the red cable to the HF driver was detached from said driver and hanging loose in the cab. It's just wired in parrallel with the LF so piezzo I guess. Anyway, if his has been in contact with the - terminal on the LF would the short be causing the fuses to blow?

Edit: photos to follow, just charging camera.


Edited by AJordan - 04 April 2018 at 10:06pm
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jbl_man View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 05 April 2018 at 3:31pm
"would the short be causing the fuses to blow?"

Depending on the (if any,and i'm guessing there isn't one) protection circuit,yes,it would....a dead short would mean the amp overload,drawing to much current on the rails,and pop the fuses. Smile
Be seeing you.
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AJordan View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AJordan Quote  Post ReplyReply Direct Link To This Post Posted: 05 April 2018 at 4:00pm
Cheers jbl_man! Thought it might. Think I'll solder the devices back in and sort out the speaker wiring and see what happens.
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