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Amcron MT1201 stuck.

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jbl_man View Drop Down
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    Posted: 29 October 2018 at 3:23pm
Afternoon chaps....have one of these here, powers-up..fan comes on ,both ODEP lights on..which indicate all normal.  Measure 0.2v DC across each channel output. However there is no audio output on either channel.

Had a quick look,checked all the output devices for shorts,(emitter to collector),all measure ok.

Has power to both boards. (88v)

Any ideas as to where to look? Is in good clean condition this one,so would like to get it running again.




Edited by jbl_man - 29 October 2018 at 4:30pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 29 October 2018 at 3:25pm



I did spot one resistor slightly scorched, but it still measures ok,so dont think this is the problem.

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M4trix View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote M4trix Quote  Post ReplyReply Direct Link To This Post Posted: 29 October 2018 at 4:06pm
Well, one thing is for sure, carbon resistors don't like scorching ! That resistor is a part of Zobel network and if the resistor across the output inductor is scorched then the amplifier was oscillating. And here comes the Captain Obvious... something else is scorched as well. Ouch 

Edit: strange, except the power supply (+/- 15V), channels aren't sharing any other part in the amplifier but still you have no sound on both channels. I would look around the signal inputs.


Edited by M4trix - 29 October 2018 at 6:38pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote djeddie Quote  Post ReplyReply Direct Link To This Post Posted: 29 October 2018 at 7:06pm
Have you done a signal trace on it Ian?

Chas n Dave : it's like Drum and Bass but with beards.             E=mc² ±3dB
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 29 October 2018 at 7:17pm
Not yet Eddie, am still trying to find a schematic for it...the only one i can find online wont open.Unhappy

The actual Crown service manual has no schematic in it!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote M4trix Quote  Post ReplyReply Direct Link To This Post Posted: 29 October 2018 at 8:06pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 29 October 2018 at 8:07pm
Thanks Mario.will have a look. Thumbs Up
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 29 October 2018 at 8:50pm
check the bias, (2 settings per channel), make sure it can be set. It's possible for the protection circuit to disable the low side drive if there's a fault with the high side, and vice-versa, so you wont get any current coming out of the amp. If one or other half of the output circuit is disabled, you wont be able to get it biased. It's one of the ways the amp can provide DC protection without having any speaker relays.

If both sides are the same then there arent too many things in common, (it would be bad luck to have 2 independent channels suffer the same obscure component failures at the same time). namely the LV power supply (4 rails). Check the ODEP voltages as they have the ability to control the VGS well before it sends the ODEP indicators off. THe odep voltage is generated by a balancing act of stable reference voltages, temperatures, currents and voltages. I regard it as a sort of simple analogue computer.

If it's an elderly amp then check for corrosion on the top board, dirt and muck accumulates around and under the switches and those flimsy connectors.

I think 0.2V dc offset is too much for a Crown. If the base drives are inhibited the output will float a bit, but when the output stages are fully active the error amp will pull the output to a few millivolts.

I would inspect the small ceramic axial caps around the LV power supply on the 'main' board for overheating as well. If one fails it can make the amp twitchy and prone to oscillate as that zobel resistor shows. Something's caused it!

I assume you've checked the back-emf diodes, driver transistors etc, though these should normally put the ODEP off.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote M4trix Quote  Post ReplyReply Direct Link To This Post Posted: 29 October 2018 at 8:57pm
Originally posted by kedwardsleisure kedwardsleisure wrote:

(it would be bad luck to have 2 independent channels suffer the same obscure component failures at the same time)


You mean, odds are greater to win on lottery than that scenario. LOL
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 29 October 2018 at 10:18pm
Thanks Kevin, will check those. Yes,does seem to be a lot of dust around the tiny slide switches,so will clean those well.

Only thing i can see common to both channels is a single 14-pin MC33079 quad op-amp IC,so will check that too.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 29 October 2018 at 10:24pm
never had that go, but there's always a first time
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Post Options Post Options   Thanks (0) Thanks(0)   Quote M4trix Quote  Post ReplyReply Direct Link To This Post Posted: 29 October 2018 at 11:20pm
Speaking of independent channels, not sure what Crown engineers had in their mind when they decided to supply both channels with +/- 15V from one fan/transformer ?
If this transformer kicks the bucket, both channels are out of business.

@Kevin, did you have any problems with this fan/transformer in Crown amps in your practice ?


Edited by M4trix - 29 October 2018 at 11:22pm
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