Marantz 1090 shorted output stage - diyAudio
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Old 11th April 2014, 11:48 PM   #1
jonsig is offline jonsig  Iceland
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Default Marantz 1090 shorted output stage

Hi my friends ,

i bought recently non working marantz 1090 , as far as i see one of the power transistor mounted on the heat sink is shorted . And one of the channel is reading 46VDC ! But the other one looks fine as far as i can see .

Any suggestions ? If the output transistor shorted out , what could it have damaged ?

Thank you
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Old 12th April 2014, 04:13 AM   #2
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Originally Posted by jonsig View Post
Hi my friends ,

i bought recently non working marantz 1090 , as far as i see one of the power transistor mounted on the heat sink is shorted . And one of the channel is reading 46VDC ! But the other one looks fine as far as i can see .

Any suggestions ? If the output transistor shorted out , what could it have damaged ?

Thank you
Output transistors nearly always fail in pairs. If one transistor fails and pulls to a rail the feedback is supposed to detect the error, turn off the bad transistor and turn on the good one to the other rail and go back to normal. BUT the shorted transistor won't cooperate and the good transistor attempts to correct it and IT dies trying. There is normally no way to figure out which one dies first. In your case something is way out of whack because it DIDN'T kill the other transistor. The input section or VAS may have died and it's _possible_ both outputs are still OK. This is one of those times that a curve tracer can be very useful.

G
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Old 12th April 2014, 12:43 PM   #3
jonsig is offline jonsig  Iceland
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Originally Posted by stratus46 View Post
Output transistors nearly always fail in pairs. If one transistor fails and pulls to a rail the feedback is supposed to detect the error, turn off the bad transistor and turn on the good one to the other rail and go back to normal. BUT the shorted transistor won't cooperate and the good transistor attempts to correct it and IT dies trying. There is normally no way to figure out which one dies first. In your case something is way out of whack because it DIDN'T kill the other transistor. The input section or VAS may have died and it's _possible_ both outputs are still OK. This is one of those times that a curve tracer can be very useful.

G

VAS ? Well i dont have oscilloscope or this expensive tracer so the duh way for me should be testing all he transistors in the input section ? could i get away with testing all the transistors on the main amp board?

Last edited by jonsig; 12th April 2014 at 12:55 PM.
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Old 12th April 2014, 02:17 PM   #4
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For an output stage repair, all you require is a descent DVM the right size screwdriver and other hand tools, a soldering iron and a methodical way. Remove the faulty side output transistors, read them to check them and replace as required along with any other faulty components on the faulty side. You have a good side to compare with.
Output transistors normally fail under stress of a shorted load or external damage. Always use the identical component or recommended equivalent! Check your service manual.
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Old 12th April 2014, 03:21 PM   #5
jonsig is offline jonsig  Iceland
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Im not the fist one to need new of these ..

2sd588 and 2sb618 replacements - AudioKarma.org Home Audio Stereo Discussion Forums

This guy has the same problem and he gets many suggestions , but it looks like those forum members dont agree about witch one is the "right" substitute for those broken transistors.

I did some quick test with the transistors "around" the problem area and they at least arent shorted .
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Old 12th April 2014, 04:36 PM   #6
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That should be an easy fix and I would replace the outputs and drivers and probably the VAS too (Q710) unless there is an obvious reason for the failure. Check the two 0.47 ohms carefully, check the whole PCB for dries, set R722 to its highest resistance and then power up the repaired amp with a bulb tester.

If all is well then check the quiescent current adjusts OK, then reset it back to minimum, then remove the bulb tester and power up on full mains. Set the quiescent current to the correct value with R722 and recheck and adjust when the amp is hot.
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Old 12th April 2014, 05:41 PM   #7
jonsig is offline jonsig  Iceland
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That should be an easy fix and I would replace the outputs and drivers and probably the VAS too (Q710) unless there is an obvious reason for the failure. Check the two 0.47 ohms carefully, check the whole PCB for dries, set R722 to its highest resistance and then power up the repaired amp with a bulb tester.

If all is well then check the quiescent current adjusts OK, then reset it back to minimum, then remove the bulb tester and power up on full mains. Set the quiescent current to the correct value with R722 and recheck and adjust when the amp is hot.


Thank you so much ! I tested Q724 and it read like two diodes while Q722 acted like transistor using my component analyzer .

could you please point out the drivers on the sch. Im not trusting my own judgement .And suggest replacement transistors? i dont care if the are not bat wing . I think the metal bar holds the transistors tight to the heat sink

Last edited by jonsig; 12th April 2014 at 05:46 PM.
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Old 12th April 2014, 06:18 PM   #8
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The drivers are Q718 and Q720.

Electrically something like the MJL21193 and MJL21194 should be good for the outputs and MJE340 and MJE350 for the drivers. An MJE340 should be good for the VAS too.

You would have to measure physically the outputs and see if these would fit.

You are saying Q722 and Q724 are not short circuit or reading faulty. If that is the case then it might be better to actually do some fault finding and see where the problem is. 95% of problems are down to failed outputs and drivers but there is always the one that isn't
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Old 12th April 2014, 06:38 PM   #9
jonsig is offline jonsig  Iceland
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Originally Posted by Mooly View Post
The drivers are Q718 and Q720.

Electrically something like the MJL21193 and MJL21194 should be good for the outputs and MJE340 and MJE350 for the drivers. An MJE340 should be good for the VAS too.

You would have to measure physically the outputs and see if these would fit.

You are saying Q722 and Q724 are not short circuit or reading faulty. If that is the case then it might be better to actually do some fault finding and see where the problem is. 95% of problems are down to failed outputs and drivers but there is always the one that isn't
Thank you so much! But 250V 16A 200W transistor sounds like overkill ! Then i need to replace the other channel also , having different output transistors on one channel will give me problems ??
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Old 12th April 2014, 06:55 PM   #10
Mooly is offline Mooly  United Kingdom
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There is no problem using higher rated devices because for the most part the performance of the circuit is determined by the passive components, particularly the frequency compensation and feedback networks. Audibly and there should be no difference between the two channels, measurement wise the modern devices may alter things in the 0.00xx region, and quite possibly for the better.

But... if you haven't found any problem so far with the devices fitted, then I think it makes sense to at least do a few basic checks and see where the problem really is. If the transistors really are OK then replacing them 'aint gonna fix it

Testing it is for another day...
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