Crown Amp DC300A - One channel distirted, other good.

[budwich]

Re: Crown Amp DC300A - One channel distirted, other good.

Its still running at medium volume. Attached is a thermal shot of the control board. There was not much happening at the output boards so I didn't include those. The control board seems to get pretty "toasty" although I don't know what it should be and whether this is contributing to any subsequent failure there and elsewhere.

[budwich]

Re: Crown Amp DC300A - One channel distirted, other good.

Passed the "bulb test" this morning. Then powered up with nothing plugged in and left idling (1/2 hour) to see if anything tipped over. Nothing happened, rails check out. Plugged in preamp and cd player, and now have "clean" sound from both channels. Will let it run at low to medium level for a while and check some areas. Also as a test, I will run at a bit higher level and check things with a thermal imager to see if possibly there is an issue therein (ie. the replacements are running hotter than expected or whatever). I am reluctant to do much more... ie switching input, pushing volume, etc at this point. Hopefully, joy will return and stay a while. :-)

[budwich]

Re: Crown Amp DC300A - One channel distirted, other good.

that would mean 16 need to be replaced. Not sure that is going to be cost effective at this point.

[petehall347]

Re: Crown Amp DC300A - One channel distirted, other good.

all from same batch should be pretty close

[budwich]

Re: Crown Amp DC300A - One channel distirted, other good.

I made an error in the previous post, the failed "3990" was actually a mj15024 that I had replaced during the earlier effort. Summary, 2 of 3 on channel two failed while none (there are two there) failed on channel one. I am still doing checks.

matching vbe might be difficult and wold require pulling all of them... or at least one to determine the existing "vbe". I would think matching them would require significant stock and / or supply from someone who would indicate the value. As you say, if and when I get it going again, I can perhaps cross the "vbe bridge".

[petehall347]

Re: Crown Amp DC300A - One channel distirted, other good.

they should all be same and matched for vbe . you can maybe find 4 good ones out of whats there and get the amp running before deciding what to do .

[budwich]

Re: Crown Amp DC300A - One channel distirted, other good.

the "slope appears slippery"... I found two output transistors bad on channel 2. One was previously replaced by me (mj15024) while the other is an original 3990...:-( note: one of the other previously replaced mj15024 does not appear to be bad (meter testing only).... there are two mj15024's on the other channel.

It would appear that there is some "science" to this as mentioned which might be an issue or possibly the replacements aren't very good (substandard or "fake"). Probably continue to slice thru this to see if I can get it going again and then wrestle with what to do there after with the loss of 3990 which has been indicated need to be matched (set?)... not sure I can or want to go there at this point.

[budwich]

Re: Crown Amp DC300A - One channel distirted, other good.

Quick checks of the transistor pins to "case mount bolt" show low resistance on channel 2 which was the original failed channel. It will take a bit of desoldering of resistors to isolate which transistor has failed (as all have some form of "parallelism" involved). Channel 1 which was the most recent one to get attention (distorted) appears to be relatively ok resistance check wise. I will hopefully find the bad ones and go from there. I will use the "bulb limiter" to hopefully prevent any initial destruction should I not find everything on this attempt. Thanks again for the hints / help.

[petehall347]

Re: Crown Amp DC300A - One channel distirted, other good.

transistors generally fail shorted b to e and sometimes b to e but not as often .

[budwich]

Re: Crown Amp DC300A - One channel distirted, other good.

this is likely where the problem lies. I used some replacements (alternatives) for some of the output ones that I found were initially bad at the start of this "exercise" not necessarily understanding the possible impact. The bad ones were all 3990. The replacements that I used (based on reads on the net) was mj15024. Anyway, I am tracking thru the output boards to see where things are there. Initial gross resistance checks of the rails do not show any direct shorts and both channels look the similar.

[petehall347]

Re: Crown Amp DC300A - One channel distirted, other good.

from audiokarma .
Output Transistors 3990 = 2N5631 ~ MJ21194G ~ (or 2N3773 with mods noted in the service manual)
Driver Transistors 5402 = 2N5804 ~ ( I've also seen 2N6306 used)

[petehall347]

Re: Crown Amp DC300A - One channel distirted, other good.

output transistors should be matched for vbe so they all share same current .

[Agent24]

Re: Crown Amp DC300A - One channel distirted, other good.

Outputs: Output transistors (usually the most expensive ones)
Subs: Substitutes

[budwich]

Re: Crown Amp DC300A - One channel distirted, other good.

sorry but I don't understand what this means... what are you referring to as "subs"... the main capacitors???

[petehall347]

Re: Crown Amp DC300A - One channel distirted, other good.

hope the outputs are still good ..if not you can sub in a couple of low cost subs for testing . only needs 2 for getting it going .

[budwich]

Re: Crown Amp DC300A - One channel distirted, other good.

I guess at my level, anything is possible... :-) I did check voltages as per schematic at various point shown once the channel was repaired. Things looked OK. The pots / switch seem ok as when it was running, audibly, I didn't hear anything like crackle , etc prior to the "thud" of the fuse going. I don't know anything about its "internal operation" or design... beyond my knowledge limits. I will see if I can isolate the power inputs from the boards and see if the thing stays on and go from the result.

[Agent24]

Re: Crown Amp DC300A - One channel distirted, other good.

I find it interesting it broke when you 1) changed inputs 2) changed volume

I can imagine each of these actions has possibility to create a noise burst from dirty switch and potentiometer contacts.

I wonder if some feedback etc network or such in the amp designed to handle transients is not working properly (e.g. Bad capacitor) and the amp becomes unstable due to sudden change in signal, be that due to high level, fast transition time, etc.

(Again, wild speculation with no real evidence or possibly even sanity)

[petehall347]

Re: Crown Amp DC300A - One channel distirted, other good.

maybe its oscillating or bias tracking isnt right .

[budwich]

Re: Crown Amp DC300A - One channel distirted, other good.

Another short lived "joy" moment... :-( It was playing at medium volume for about 2 hours or so. I did some input switching and everything remained fine. I then decided to turn up the volume on the pre-amp and then "thud" as the external fuse (10 amp) on the amp blew. New fuse blows on power up so something has gotten significantly worse this time. I guess its back to the "bulb limiting power up" to see what's up.

[budwich]

Re: Crown Amp DC300A - One channel distirted, other good.

I tracked back poor / high positive dc voltage on the output of the op amp, compared to less than .6v on the good side. This is similar to the previous trouble of the other side. Then traced back (resistance... power off) and found low resistance at the output from the mpsa92 (itself tested good) on the side that is distorted (compared to the good side). It traced down to q120 (2n4125) which appears to be in the feedback path from the output stage. It was blown. I replaced it and now things seem to be up and running. From my reads on the net, people seem to do an R&R on all 2n4125. I have not at this point but if another failure occurs, I may follow suit. I haven't done a "full operational test" yet in terms of frequency along with input switching. I will let the thing run for a bit to see if there is some temp issue possibly causing a problem with time.