Crown Amp DC300A - One channel distirted, other good.

[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.

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

[Agent24]

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

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

[petehall347]

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

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

[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)

[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.

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.

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.

[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.

[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.

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.

all from same batch should be pretty close

[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.

[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.

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.

[petehall347]

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

just a thought regarding outputs sharing current you could measure voltage across each output transistor emitter resistor .this will maybe be a bit out though because of resistor resistance tolerance .

[R_J]

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

Check and monitor the DC offset for that channel, it needs to be near 0 volts and if it is not or starts to go in the positive or negative, you still have a problem.

[redwire]

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

When you replace a transistor in a power amp, you also replace the one upstream or ahead of it as well because although it tests OK, it is likely damaged. This is from Sony service and I've followed it for many years.
You can see this (damaged transistor) with the diode-test E-B, C-B voltages being slightly off and leakage current present (on ohms).
This would be for the driver stage, pre-driver etc. as well as the VBE multiplier transistor.

Also, when a power amp transistor fails, the electrolytic capacitors (especially feedback and input ones) can see reverse-polarity and then they get damaged too. So check those capacitors as well, they make all kinds of noise and thumps as well.

You can also put the amp on light bulb power and heat or cool (freeze spray) transistors to find the dud.

[budwich]

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

Thanks for the "upstream transistor" hint. On the failure this last time, the second one in the "line" failed (was previously replaced along with its upstream one which had also failed) but the upstream "appeared" to be good ... although some form of leakage seemed present as the resistance checks across the various "modes" (ie. be,ec,bc) was "less than infinite" compared to a "new one". Currently, it is running with this one still in place but I will go back and replace it. Having said that, on some pre-checks of the "new ones", I have seen them present a similar "less than infinite" resistance on some of the modes of the testing so they may have an issue to start with.

I do use the "bulb limiter" as a "pre check" to help and prevent "significant happenings" as I can't guaranty my work is stellar... :-)

[budwich]

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

A new "symptom" showed up when I went to try the system this morning. I powered up the amp and there was a large hum coming from channel one (I think... speakers are pretty close together and I was wasn't paying close attention to sides). Somewhat scary. Power off, unplugged all inputs. The hum was still there. Connected in the bulb limiter and did some checks, the "expected" voltages seemed to be present on both sides (albeit reduced because of the limit). With a bit of time... a couple of turn on and off sequence just from doing multiple checks, the hum went away. Reconnected things. At this point, the hum was basically reduced / gone. Audio played normal from both channels.
Not sure if this was there from the start of this latest revival effort as I don't recall necessarily having speakers connected at initial start up points along the way (fear of blowing either the speaker or amp). So it might have always been there since this repair phase. It would appear to be some form of cold joint / ground issue maybe or possibly a failing capacitor somewhere (as suggested).