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Old 7th January 2006, 12:08 PM   #1
Clipped is offline Clipped  Thailand
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Default power supply fets keep blowing

need help guys, been trying to trouble shoot this for a while now
working on a 225 hcca (digital reference) taken it to 2 repair shops also and no one knows whats up.

whenever i try to replace the fets in the power supply they blow immediately as soon as i connect power....

the transformer windings arent shorted, i know this because i had purchased another amp of the exact same model...a working one with no problems.

if anyone has worked on this model amp, they should know of the double sided tape orion used to hold the fets to the retaining rail that screws down onto the heatsink....this is where my headaches started....

like an idiot i tried to remove the securing rail off the fets again, the tape stuck just like the first time and some of the power supply fets got destroyed again...the fets are smp60n06-18 ( i cant find datasheets on these fellas either)


so now i have two 225 hcca's that are scrap, just to see what happens i replaced the damaged fets on the second amp and the same thing happened -power supply fets fried immediately after connecting power.....i tried ssp60n06 and rfp70n06 but both types blew, i then tried taking the remaining good fets(the original ones) from one amp and transferred them to the other, same thing happened...


so my question is what is causing this? do i need to disconnect the drive transistor in the output section- connect power-disconnect power- then reconnect the drive transistor?

does anyone know what needs to be done to correctly fix this, because there is obviously a series of steps i need to follow to get these amps running again.

any help greatly appreciated
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Old 7th January 2006, 04:29 PM   #2
Eva is offline Eva  Spain
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The first thing that you should have checked is the gate drive section. You will blow as much MOSFETs as you can afford if the gate drive circuit is malfunctioning and turning both banks at the same time or for a too long time. Also, you may be unadvertedly damaging the MOSFETs when you mount them (too much ESD), there may be some insulation problem between the tabs of the devices and the heatsink, or some lost solder ball may be unadvertedly stuck in the wrong place...
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Old 7th January 2006, 06:04 PM   #3
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The first thing you need to do is to get some sort of current limiter for the power supply. I use a 2 ohm 50 watt resistor. Others have suggested using automobile head lamps. Even a low value fuse (5A) will save the FETs.

As Eva mentioned, the drive section could be at fault. There could also be something in the audio section that's drawing too much current from the supply. Either can kill the supply. If the FETs fail instantly, the problem is likely in the power supply itself.

These are some common problems with Orion power supplies that can cause the FETs to overheat/fail:

Defective driver transistors. The PNPs fail almost every time the FETs fail. The NPN transistors sometimes fail also. Both are inexpensive. Change both the NPN and the PNP transistors when replacing the FETs. They use MPSA06s and MPSA56s if they are too burned to read.

The 10 ohm resistor between the emitter of the PNP transistor and ground opens. This prevents the PNP transistor from turning the FETs off and will cause them to fail.

The zener diode connected to the gate of the FETs fails. When it does, it will prevent the FETS from being driven high. If only one is shorted (for 1/2 of the FETs), the FETs on the other side will run hot and eventually fail if the amp is driven into a load.

Sometimes, some of the gate resistors open. This will cause one or more FET to remain on which will cause it to overheat and fail. When it does, others will follow.

Check the output transistors for shorts.

Check the large capacitors on the 12 volts side of the power supply (~3300uf). Sometimes the legs burn when the supply fails. If they are glued down it may not be obvious.
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Old 8th January 2006, 03:23 AM   #4
Clipped is offline Clipped  Thailand
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ok, but what really trips me out is that these were perfect working amps before i got curious and pulled the securing rails off and damaged the fets because of the tape...

a month before that i had changed the capacitors in the rectification portion to Elna 1200uf from 1000uf....also the two smaller caps 1000uf to 820uf that are laying next to the horizontal caps...sound improved ALOT....

then i got itchy and wanted to see what else could be modified, pulled the rails and have been stuck in hell since then...

i dont hink that changing these caps should effect the power supply, but if im wrong someone tell me...everything worked fine and the amp even ran a bit cooler...

Quote:
The first thing that you should have checked is the gate drive section. You will blow as much MOSFETs as you can afford if the gate drive circuit is malfunctioning and turning both banks at the same time or for a too long time. Also, you may be unadvertedly damaging the MOSFETs when you mount them (too much ESD), there may be some insulation problem between the tabs of the devices and the heatsink, or some lost solder ball may be unadvertedly stuck in the wrong place...
what is ESD???



Quote:
The zener diode connected to the gate of the FETs fails. When it does, it will prevent the FETS from being driven high. If only one is shorted (for 1/2 of the FETs), the FETs on the other side will run hot and eventually fail if the amp is driven into a load.
i cant see any zener diodes in the powersupply section, but there is a light grey with black rim diode that is connected to the gate traces.....doesnt look the same as a traditional black diode.

there is one part of the amp that i think is suspect, i dont know what kind of circuit it is:

the layout of the fets and transistors are like this:

Power supply>f-f-f-f-f-d-n-p-p-p-n-n-n>>>>>> rca side

f=fet
d=diode
n or p = bipolar transistor

the 'n' between the d and the p....what is this guy? there are diodes on the base and emitter, the middle collector runs straight into a mylar cap, and the trace goes into the ground plane for the speaker outputs(i think)....this trace plane also has the rectification caps on it...on the other side of the pcb in the same position is the 'p' version of this trans...that side hasnt blown any fets yet.

the reason why im so confused is because these were perfect working amps...before i took the transistor securing rails off, there were no problems with them before, when i installed the new fets i checked all my soldering spots and made sure nothing would arc.

i replaced 'all' of the transistors and fets on the first amp and the same thing happened, although i'll admit i didnt check the drive fets when i put them in...but these were perfect working amps.

why is this happening?

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Old 8th January 2006, 04:42 AM   #5
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The zeners are gray/silver.

The N device is the current-boost transistor for the regulator. If you look closely, there is a diode on each leg. One diode is on the 'input' pin (the center pin). One diode is on the third pin (the output of the regulator). Both of those are standard diodes (1N400x types). The first pin of the transistor has a zener diode on it. There is a P counterpart on the other side of the heatsink. The NPN is the positive reg. The PNP is the negative reg.

When you initially pulled the transistors off of the clamp, you likely wedged a screwdriver between the clamp and the tab of the transistor. This can rarely be done without destroying the transistors. What you need to do next time is this...

While the board is still screwed down, pull the screws out of the clamps and insert a T20 driver in the hole in the clamp. Be sure that the driver does not extend down into the hole in the heatsink. While applying at least 50 pounds of downward pressure, pry back away from the center of the board. Start in the holes near the ends and work your way down the clamp. You will rarely break transistors when you do it this way. If you are applying enough downward force, the transistors will remain flat on the heatsink while the clamp pulls away. See atached image.
Attached Images
File Type: gif orionampclamps01.gif (16.0 KB, 379 views)
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Old 10th January 2006, 08:21 AM   #6
Clipped is offline Clipped  Thailand
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ok , gotcha those grey/silver are zeners
i got the A06...A56...zeners...10ohm R's today...but do you think i should change that 'n' transistor for current boost too?

if so, think it would be ok to use a 'ST' BD9112/911 instead of the 2n6488/91 instead because thats all i can find (crossreferenced)....as a precaution i would probably change the diodes on them also...

just to double check....the current boost transistor has a zener on the left pin...no diode that i can see on the middle pin...and a regular diode on the right pin...just want to make sure were on the same level...

the original zener diode in the power supply is a
1N47-42A-202....all i could get was...1N47-42A-806...that ok?

ok one more question the zener on the left pin of the current boost transistor is a 1N47-44A-208...would it be ok to use the 1N47-42A-806...since i got 20 pcs in one package when i bought them...

oh, and whats that vertical standing 2N6488 transistor for ???

im really close to doing this >>> to the amp

double thanx
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Old 10th January 2006, 08:41 AM   #7
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ok , gotcha those grey/silver are zeners
i got the A06...A56...zeners...10ohm R's today...but do you think i should change that 'n' transistor for current boost too?

**** If it ain't broke, don't fix it. Unless you physically damaged it, it's likely OK. I've never seen a bad one in all the ones I've repaired.


just to double check....the current boost transistor has a zener on the left pin...no diode that i can see on the middle pin...

**** It's likely about 1/2" away from the transistor but it's connected to the middle pin.


the original zener diode in the power supply is a
1N47-42A-202....all i could get was...1N47-42A-806...that ok?

**** Unless it's shorted, leave the originals. The numbers beyond the 42a are likely a date or manufacturing code.


ok one more question the zener on the left pin of the current boost transistor is a 1N47-44A-208...would it be ok to use the 1N47-42A-806...since i got 20 pcs in one package when i bought them...

**** If it's not defective leave it. If it is defective, you need to replace it with the same value. The 42 is a 12 volt zener and the 44 is a 15 volt zener. Using the lower voltage device will result in lower regulated voltage for your preamp section.


oh, and whats that vertical standing 2N6488 transistor for ???

**** It's likely the regulator for the gate drive. They use a crazy pulse VOLTAGE modulation instead of the normal pulse 'width' modulation. If you look at the gate pulses (with an oscilloscope) when the amp is driving a load, you can see the amplitude of the pulses change with demand (higher amplitude for higher demand).
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Old 10th January 2006, 09:38 AM   #8
Clipped is offline Clipped  Thailand
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ok found the middle pin diode, the top was on the same trace as the transistor, the bottom of the diode went to another trace on one of the PS rails... interesting design...

but the current boost transistor is going to have to be changed, the back of it is discolored like its burnt, already pulled it and the tape thing happened again (before you told me the right way to do it)

think its ok to use the ST microdevices BD911 and 912 for these current boosters? im not going to be able to find 2N6488/911's unless i buy 1000's...

do you have any recommendations for the power supply fets?
smp60n06-18's cant be found around here, but i have alternatives
like irfz44n, buz12, rfp70no6, ssp60n06, more...

i cant find a data sheet for the smp60n06-18, all i can find is a toshiba crossreference...the smp60n06-18 is listed as 60v 60a 125 watt device with .018rds

gonna be a couple days before i actually start replacing components, have to go pick up a new dmm...not reading 0 ohms when leads are touched.

really appreciate your help
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Old 10th January 2006, 03:43 PM   #9
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The BDs look like OK replacements.

Any of the transistors you listed would work but I'd suggest the 60n06s because they are likely the closest to the originals.

Did you try replacing the battery in the meter?

Did you check the leads?
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Old 11th January 2006, 01:35 AM   #10
Clipped is offline Clipped  Thailand
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yep tried changing the battery, how do i check the leads?

btw i checked the zeners in the first amp and they were both bad
used the ohmeter. the zeners in the second amp were ok.
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