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MT-2400 blowing O/P transistors


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#1 smokinleroy

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Posted 25 May 2006 - 01:06 PM

Hello,
I'm working on a Micro-Tech 2400 that had a channel out. I located a bad O/P PNP on the "LOW SIDE" output. It had a direct CE short, but base was not shorted. I replaced all three PNPs of the bank because the original "sort" was not available. After re-assembly, amp worked fine for a while. While pushing around 100W into 4ohms, the channel quit again. This time, a PNP in the "HIGH SIDE" went with CE short. Now, on power-up, after a few seconds, speaker hums and lights dim for a second or two. I replaced that PNP with one of the left-overs from the "LOW side" and it worked fine for a while. Again, however, while pushing just a few watts, one of the new "LOW SIDE" PNPs went, CE short. No hum, or dimming lights though. Fortunately, there was a spare PNP of the new "sort" so I replaced the bad one. I'm out of new PNPs so am nervous about pushing any power. I compare the voltages on the 12-pin ribbon connectors between the good and bad channels, and the voltages match. I read the voltage across the emitter power resistors during idle and there is considerable variation on both channels from zero to maybe  11mV on both channels. Anyone seen this problem before? I'm not sure how much more info to provide, so please ask.

I found the service manual here, but there is no schematic. Does anyone have a pdf?

Also, what is a generic sub for both NPN and PNPs?

SL

#2 KGring

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Posted 26 May 2006 - 09:35 AM

SL,
Something had to have originally caused the low side out put to fail. Check diodes d09-d12, and lift the emitter resistors from circuit to verify that one has not opened. You did the correct thing by replacing the 3 paralleled low side PNP outputs but i would also recommend the matching 3 NPN low side outputs as well. A factory repair would have automatically replaced those and probably the low side drivers as well. A low side repair should not affect the high side outputs, unless there was an inadvertent solder splash or bridge between the two.
I am attaching the schematics for you.


QUOTE(smokinleroy @ May 25 2006, 01:06 PM)
Hello,
I'm working on a Micro-Tech 2400 that had a channel out. I located a bad O/P PNP on the "LOW SIDE" output. It had a direct CE short, but base was not shorted. I replaced all three PNPs of the bank because the original "sort" was not available. After re-assembly, amp worked fine for a while. While pushing around 100W into 4ohms, the channel quit again. This time, a PNP in the "HIGH SIDE" went with CE short. Now, on power-up, after a few seconds, speaker hums and lights dim for a second or two. I replaced that PNP with one of the left-overs from the "LOW side" and it worked fine for a while. Again, however, while pushing just a few watts, one of the new "LOW SIDE" PNPs went, CE short. No hum, or dimming lights though. Fortunately, there was a spare PNP of the new "sort" so I replaced the bad one. I'm out of new PNPs so am nervous about pushing any power. I compare the voltages on the 12-pin ribbon connectors between the good and bad channels, and the voltages match. I read the voltage across the emitter power resistors during idle and there is considerable variation on both channels from zero to maybe  11mV on both channels. Anyone seen this problem before? I'm not sure how much more info to provide, so please ask.

I found the service manual here, but there is no schematic. Does anyone have a pdf?

Also, what is a generic sub for both NPN and PNPs?

SL
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Kevin B. Gring
Crown Audio
kgring@crownintl.com

#3 smokinleroy

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Posted 26 May 2006 - 10:38 AM

QUOTE(KGring @ May 26 2006, 09:35 AM)
SL,
Something had to have originally caused the low side out put to fail. Check diodes d09-d12, and lift the emitter resistors from circuit to verify that one has not opened. You did the correct thing by replacing the 3 paralleled low side PNP outputs but i would also recommend the matching 3 NPN low side outputs as well. A factory repair would have automatically replaced those and probably the low side drivers as well. A low side repair should not affect the high side outputs, unless there was an inadvertent solder splash or bridge between the two.
I am attaching the schematics for you.


QUOTE(smokinleroy @ May 25 2006, 01:06 PM)
Hello,
I'm working on a Micro-Tech 2400 that had a channel out. I located a bad O/P PNP on the "LOW SIDE" output. It had a direct CE short, but base was not shorted. I replaced all three PNPs of the bank because the original "sort" was not available. After re-assembly, amp worked fine for a while. While pushing around 100W into 4ohms, the channel quit again. This time, a PNP in the "HIGH SIDE" went with CE short. Now, on power-up, after a few seconds, speaker hums and lights dim for a second or two. I replaced that PNP with one of the left-overs from the "LOW side" and it worked fine for a while. Again, however, while pushing just a few watts, one of the new "LOW SIDE" PNPs went, CE short. No hum, or dimming lights though. Fortunately, there was a spare PNP of the new "sort" so I replaced the bad one. I'm out of new PNPs so am nervous about pushing any power. I compare the voltages on the 12-pin ribbon connectors between the good and bad channels, and the voltages match. I read the voltage across the emitter power resistors during idle and there is considerable variation on both channels from zero to maybe  11mV on both channels. Anyone seen this problem before? I'm not sure how much more info to provide, so please ask.

I found the service manual here, but there is no schematic. Does anyone have a pdf?

Also, what is a generic sub for both NPN and PNPs?

SL
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#4 smokinleroy

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Posted 26 May 2006 - 10:51 AM

Well, De De Dee (ala Carlos Mencia). I ran the thing on low power and felt the transistors individually. One of them seemed to get a lot warmer than the others. Eventually I came to realize that I hadn't completely tightened down the screws to the heat sink. Apparently I left them all a little loose to allow for aligning the transistor pins through the board holes. I did, however, replenish the heat sink compound. After tightening all, everything works great.

I'm guessing the original transistor failure is due to silicon fatigue after ten years of professional use. Them things don't last forever.

Thanks for the schems.

SL





QUOTE(KGring @ May 26 2006, 09:35 AM)
SL,
Something had to have originally caused the low side out put to fail. Check diodes d09-d12, and lift the emitter resistors from circuit to verify that one has not opened. You did the correct thing by replacing the 3 paralleled low side PNP outputs but i would also recommend the matching 3 NPN low side outputs as well. A factory repair would have automatically replaced those and probably the low side drivers as well. A low side repair should not affect the high side outputs, unless there was an inadvertent solder splash or bridge between the two.
I am attaching the schematics for you.


QUOTE(smokinleroy @ May 25 2006, 01:06 PM)
Hello, 
I'm working on a Micro-Tech 2400 that had a channel out. I located a bad O/P PNP on the "LOW SIDE" output. It had a direct CE short, but base was not shorted. I replaced all three PNPs of the bank because the original "sort" was not available. After re-assembly, amp worked fine for a while. While pushing around 100W into 4ohms, the channel quit again. This time, a PNP in the "HIGH SIDE" went with CE short. Now, on power-up, after a few seconds, speaker hums and lights dim for a second or two. I replaced that PNP with one of the left-overs from the "LOW side" and it worked fine for a while. Again, however, while pushing just a few watts, one of the new "LOW SIDE" PNPs went, CE short. No hum, or dimming lights though. Fortunately, there was a spare PNP of the new "sort" so I replaced the bad one. I'm out of new PNPs so am nervous about pushing any power. I compare the voltages on the 12-pin ribbon connectors between the good and bad channels, and the voltages match. I read the voltage across the emitter power resistors during idle and there is considerable variation on both channels from zero to maybe  11mV on both channels. Anyone seen this problem before? I'm not sure how much more info to provide, so please ask. 
 
I found the service manual here, but there is no schematic. Does anyone have a pdf? 
 
Also, what is a generic sub for both NPN and PNPs? 
 
SL
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#5 kevvywevvywoo

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Posted 25 June 2006 - 07:29 AM

QUOTE
Also, what is a generic sub for both NPN and PNPs? 


C8188 are MJ21193

C8187 are MJ21194

However they are 'selected' and should all be changed such that their gain-group is the same in the amp. Crown-supplied transistors have 'grades' marked on them and are supplied as such....a bit like 'matched pairs'.  

Substituting non-selected generic types is likely to make for a very unreliable repair since power-sharing will be lop-sided leading to hot and cold spots. Don't do it!!