Amcron MT1201 stuck. |
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jbl_man
Moderator Group Joined: 12 January 2005 Location: London. Status: Offline Points: 11154 |
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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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Be seeing you.
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jbl_man
Moderator Group Joined: 12 January 2005 Location: London. Status: Offline Points: 11154 |
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I did spot one resistor slightly scorched, but it still measures ok,so dont think this is the problem. |
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M4trix
Registered User Joined: 23 June 2009 Location: Croatia Status: Offline Points: 445 |
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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. 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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djeddie
Old Croc Joined: 26 April 2004 Location: Bristol Status: Offline Points: 4125 |
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Have you done a signal trace on it Ian? |
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Chas n Dave : it's like Drum and Bass but with beards. E=mc² ±3dB
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jbl_man
Moderator Group Joined: 12 January 2005 Location: London. Status: Offline Points: 11154 |
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Not yet Eddie, am still trying to find a schematic for it...the only one i can find online wont open.
The actual Crown service manual has no schematic in it! |
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M4trix
Registered User Joined: 23 June 2009 Location: Croatia Status: Offline Points: 445 |
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jbl_man
Moderator Group Joined: 12 January 2005 Location: London. Status: Offline Points: 11154 |
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Thanks Mario.will have a look.
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kedwardsleisure
Old Croc Joined: 20 January 2009 Location: Staffordshire Status: Offline Points: 4938 |
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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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Kevin
North Staffordshire |
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M4trix
Registered User Joined: 23 June 2009 Location: Croatia Status: Offline Points: 445 |
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You mean, odds are greater to win on lottery than that scenario.
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jbl_man
Moderator Group Joined: 12 January 2005 Location: London. Status: Offline Points: 11154 |
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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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Be seeing you.
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kedwardsleisure
Old Croc Joined: 20 January 2009 Location: Staffordshire Status: Offline Points: 4938 |
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never had that go, but there's always a first time
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Kevin
North Staffordshire |
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M4trix
Registered User Joined: 23 June 2009 Location: Croatia Status: Offline Points: 445 |
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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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