Euphonix AM713 psu repair

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ferrochrome

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I have a Euphonix AM713 AD converter unit that lost its regulated -18v rail. The regulator (Q4) has melted and delivers -9,8V. It's impossible to read anything on the regulator surface and I'd really like to find a replacement, hoping somebody here can help me out?

I removed the nut holding the regulator, screw not yet removed.

Measurements:
Pin 1: -27,4v
Pin 2: -9,8V
Pin 3: -18,8V
The PCB plane that it's screwed against: 0V
 

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ferrochrome

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I found a similar looking component searching the web, could this be a BD135, BD136, BD139 or a BD140 power transistor?
 

Jon S

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Always consult the schematic unless your are experienced enough to be able to reverse engineer the circuit.
Fit the wrong type of transistor with incorrect pin out and -27volts on the -18volt rail won't do it much good.
It is probably an MJE350.
 

ferrochrome

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Thanks Jon, thanks JR! Euphonix was taken over by Avid as far as I know and I think the schematics would be hard to come by. You know what, I found a magnifier and used better lighting, managed to read the inscription on the part. It's a MJE182. Obsolete part, but I believe a MJE182G will do the trick. Please shout out if anyone would advise me not to :)
 

JohnRoberts

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I would invest some time trying to trace out that circuit because that sounds like a simple bipolar transistor. It may indeed be a pass transistor for the minus supply but you know what they say about assume?

JR
 

ferrochrome

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I would invest some time trying to trace out that circuit because that sounds like a simple bipolar transistor. It may indeed be a pass transistor for the minus supply but you know what they say about assume?

JR
Do you mean that the problem may lie somewhere else than the power bipolar transistor? It looks melted to me but maybe another fault took it down, is that what you are suggesting?
 

ferrochrome

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If you are confident that is the correct part, replace it.

Good luck

JR
I'd say 100% sure. My attempt at reading it before my first post must have been very weak, and my camera is not functioning well, because its actually easily readable with the right tool :) I'll post back when I have the equivalent MJE182G installed. Thanks for helping me out all of you!
 

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ccaudle

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MJE182G is an NPN transistor. Do the connections make sense? Pin 3 (with the small trace) would be base, pin 2 (middle) would be collector, and pin 1 would be emitter. The collector would have to be more positive than the emitter, so it should no be connected to an unregulated more negative supply, i.e. pin 1 at -27V does not make sense for an NPN device.

I think you need to do some more circuit tracing and see where those connections come from. Something else is wrong, because those voltages don't make sense even for a blown transistor.
 

orphanaudio

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The burned transistor is an MJE-182. It is the pass transistor for the -18v rail and is driven by U28 (TL-071) which takes its reference from a V/2 resistor pair across the post regulated +/-18v rails. The LT1129 provides +18 regulation and monitors the 5v rail. As long as the pre-regulated +/-23v from AC transformer TX3 and its bridge/filter caps is good, the rest should be easy to repair.
Check U28, and Q3 (3904) as well.


Ken Hirsch / Director of Engineering
Orphan Audio www.orphanaudio.com
Quad-Eight Electronics www.quadeightelectronics.com
Electrodyne Audio www.electrodyneaudio.com


"Education is the cure for everything"
 

ferrochrome

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The burned transistor is an MJE-182. It is the pass transistor for the -18v rail and is driven by U28 (TL-071) which takes its reference from a V/2 resistor pair across the post regulated +/-18v rails. The LT1129 provides +18 regulation and monitors the 5v rail. As long as the pre-regulated +/-23v from AC transformer TX3 and its bridge/filter caps is good, the rest should be easy to repair.
Check U28, and Q3 (3904) as well.


Ken Hirsch / Director of Engineering
Orphan Audio www.orphanaudio.com
Quad-Eight Electronics www.quadeightelectronics.com
Electrodyne Audio www.electrodyneaudio.com


"Education is the cure for everything"
Thanks a whole lot for this information! I'll have it all checked. One thing maybe worth mentioning, C60, located beside u28, there's no capacitor there. Looks like there never was, looking at the solder on the pads, but could there have been, and can this missing cap be the cause for the blown mje182/off voltages?
 

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Bo Deadly

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Clearly with 8.6V between base and emitter, and because the transistor is badly discolored, it has died. When power SI parts get too hot they get burned up inside and stop turning on / off completely. Yes, MJE-182G is the right replacement. Replace that one part and see where you stand.
 

JohnRoberts

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Thanks for the schemo... that pass stage has an extra current limiter stage that shuts down the + rail above roughly 1/2A...

I used a similar topology (TL07x with bipolar pass transistors) for the high performance PS in my P-100 phono preamp back in the 1980s. One difference I ran the tl07x from the regulated rails that improved my PSRR a bunch. They are running the TL071 from the -23V but it should work just fine.

Typical failure mode for a pass devices is power dissipation so maybe look for something drawing excessive current from -18V rail. MJE182 is an 80V deice so should not experience enough voltage to punch through even with the - rail shorted to + rail.

The voltages you measure troubleshooting are clearly not linear operation, but not typical failure modes either. Bipolar devices often fail shorted collector-emitter, sometimes open base, but that circuit does not look like it has enough base drive to open a base.

Good luck. maybe measure the old transistor out of circuit.

JR
 

ferrochrome

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Thanks for the schemo... that pass stage has an extra current limiter stage that shuts down the + rail above roughly 1/2A...

I used a similar topology (TL07x with bipolar pass transistors) for the high performance PS in my P-100 phono preamp back in the 1980s. One difference I ran the tl07x from the regulated rails that improved my PSRR a bunch. They are running the TL071 from the -23V but it should work just fine.

Typical failure mode for a pass devices is power dissipation so maybe look for something drawing excessive current from -18V rail. MJE182 is an 80V deice so should not experience enough voltage to punch through even with the - rail shorted to + rail.

The voltages you measure troubleshooting are clearly not linear operation, but not typical failure modes either. Bipolar devices often fail shorted collector-emitter, sometimes open base, but that circuit does not look like it has enough base drive to open a base.

Good luck. maybe measure the old transistor out of circuit.

JR
Thanks JR, I'll see how it runs when I have the new MJE182G. Ordered a couple so I have a spare in case it fails again and I'll have to check for high current draw. Will report back with measurements once the new part(s) are in.
 

ferrochrome

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So, I replaced the melted mje182 and checked voltages before plugging in the connectors for the other boards. Measurments:
1 (emitter) : -27,5
2 (collector): -18,21
3 (base): -27,0

+18v rail measures 18,17v

So I plugged the card connectors back in and powered it back up. It took a second before capacitor C159 on AD card for inputs 1-8, caught fire. vert_fonce, do you have the schematic for the AD boards also?
 

ferrochrome

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