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Old 24th May 2008, 09:15 PM   #1
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Default Blow Output Transistors Identified...What Next?

I've identified two blown outputs on the same channel of my Marantz 2285B. The first, Q001 (PNP 2SD557), is conducting in both directions between b-e and b-c. The second, Q003 (NPN 2SD427), is shorted from e-c. First question, do I replace all four transistors on this channel or just the bad ones? I've been suggested to use On-Semi MJ21193G and MJ21194G, Digikey has them. Second, is it apparent from the two damaged ones what might have caused them to fail? Do I need to pull all the other transistors on that channel or are some ruled out by inspection? Third, when I do replace the outputs, can I just solder them onto the replacements or do I have to wire wrap? The original installation was wrapped, but the brittle wire broke off while unwrapping. I do know I will need thermal grease and mica insulators, I'll get these with the transistors.

The schematic is here, but you have to be registered to gain access:

http://www.hifiengine.com/manuals/marantz/2285b.shtml

I do have a 400K version of the schematic I can email.
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Old 24th May 2008, 10:13 PM   #2
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Default Re: Blow Output Transistors Identified...What Next?

Quote:
Originally posted by DreadPirate
I've identified two blown outputs on the same channel of my Marantz 2285B. The first, Q001 (PNP 2SD557), is conducting in both directions between b-e and b-c. The second, Q003 (NPN 2SD427), is shorted from e-c. First question, do I replace all four transistors on this channel or just the bad ones? I've been suggested to use On-Semi MJ21193G and MJ21194G, Digikey has them. Second, is it apparent from the two damaged ones what might have caused them to fail? Do I need to pull all the other transistors on that channel or are some ruled out by inspection? Third, when I do replace the outputs, can I just solder them onto the replacements or do I have to wire wrap? The original installation was wrapped, but the brittle wire broke off while unwrapping. I do know I will need thermal grease and mica insulators, I'll get these with the transistors.

The schematic is here, but you have to be registered to gain access:

http://www.hifiengine.com/manuals/marantz/2285b.shtml

I do have a 400K version of the schematic I can email.

I would buy a new transistor and compare the resistance of the faulty transistors with a new one.
I have had MOSFETS fail fully but some failed with lower than normal resistances so I just binned them and bought new ones.

You need to investigate why the amp blew otherwise you could power it up just to blow a new set of transistors.
1/ Did you drive too low an impedance from your amp ?
2/ Did the amp run very high power for a long time ?
3/ Has a speaker fried, shorted and blown your amp ?
4/ Has your speaker cable failed and shorted out ?
5/ It could be some otehr component has gone in the amp and has applied volts to both output sets and caused them to blow.

My first step when fixing an amp is to remove the output transistors completely and feedback the driver output into the LTP. Only when I am convinced the driver is OK do I even consider adding the output transistors.

I enjoy fault finding, it can very challenging but very rewarding too.

This is why I tend to keep my own amp designs simple so they are easier to fault find.
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Old 25th May 2008, 06:17 AM   #3
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i would replace all 4, and check the other driver components for failure, soldering the wires is better
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Old 25th May 2008, 08:32 AM   #4
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I don't know what you're talking about with the wire wrap...I've worked on a few 2285B's, and the only wirewrap on the amplifier is on the male/female connectors that plug into the amp board. Hopefully you did not unwrap those. The transistors themselves are socketed.
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Old 25th May 2008, 01:13 PM   #5
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Quote:
Originally posted by EchoWars
I don't know what you're talking about with the wire wrap...I've worked on a few 2285B's, and the only wirewrap on the amplifier is on the male/female connectors that plug into the amp board. Hopefully you did not unwrap those. The transistors themselves are socketed.
Unfortunately, I did unwrap the connections to the socket rather than remove the sockets from the heatsink. I've made a bit of extra work for myself.

I believe the transistors to be checked are the heatsinked ones? I'll get these checked and all replaced and startup with a dim bulb setup.

How does one torque the transistors properly? Are there beam wrenches for this, they'd have to be awfully small...
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Old 25th May 2008, 03:52 PM   #6
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the rest of the circuit driving the transistors on the h\s needs checking, tighten to reasonable tightness the threads are easily stripped though
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Old 25th May 2008, 04:53 PM   #7
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one useful trick is to temporarily attach 1.5k/2w resistors in place of the B-E junctions of the output devices and power the amp up with no load. this will provide an indication of whether something else is wrong with the amp. it will also provide negative feedback to the diff amp and you can actually feed a signal to it to see if everything up to the output devices is functioning properly. you can also measure the quiescent bias voltage between the base connections of the output devices. this only works for emitter follower outputs, not CFP or Quasi-Comp.

if you let the magic blue smoke out of one of the resistors you will instantly know you still have a problem, but you only lost a resistor not an expensive output debice.
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Old 26th May 2008, 12:23 AM   #8
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"what might have caused them to fail? "

Nothing lives forever, it's 30 years old.

"conducting in both directions between b-e and b-c."

The driver transistors need to be replaced as well as all the outputs. Check the b-e and b-c of the drivers, if bad you must replace the bias transistor, Vas, and CCS for the Vas.

Check the emitter and base resistors for the outputs, the resistor betweem the emitters of the drivers, the bias transistor and diode string, and the bias pot.

Check the current limiter transistors.

Due to the age of this piece I would do the outputs and drivers on both channels, cheap insurance (and easier to do before it blows up).

Use a 75W lightbulb in series with one side of the line cord when you fire it up again.
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Old 26th May 2008, 12:27 AM   #9
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Quote:
Originally posted by djk
"what might have caused them to fail? "

Nothing lives forever, it's 30 years old.

"conducting in both directions between b-e and b-c."

The driver transistors need to be replaced as well as all the outputs. Check the b-e and b-c of the drivers, if bad you must replace the bias transistor, Vas, and CCS for the Vas.

Check the emitter and base resistors for the outputs, the resistor betweem the emitters of the drivers, the bias transistor and diode string, and the bias pot.

Check the current limiter transistors.

Due to the age of this piece I would do the outputs and drivers on both channels, cheap insurance (and easier to do before it blows up).

Use a 75W lightbulb in series with one side of the line cord when you fire it up again.

Dont forget to set the bias pot for min volts bias before you fire up again !

I forgot once and wiped out 6 MOSFETS !
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Old 26th May 2008, 09:15 PM   #10
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Heat-sinked devices on-board:

Pre-Drivers
Q706 2SA914 $1.35
Q707 2SC1953 $1.24

Power Amp
Q708 2SC1568 $.91

Drivers
Q709 2SA913 No Longer Available, Expensive and Rare
sub available: MJE15030G $1.30
Q710 2SC1913 No Longer Available, Expensive and Rare
sub available: MJE15031G $1.30

Varister
Q711 STV3HY $10.95
no sub found

I'll go ahead and purchase all of the above and substitute as I test for failure and also replace them on the good channel (but keep original outputs on that side). Does this sound like a reasonable plan or should I also spring for outputs on that side? Also, once subbed in I'll do a dimbulb startup will any adjustments prior to the startup be necessary or should I just go with it and take measurements after everything is working (well, hopefully working...)?

Resistors check out ok.
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