Crown Amp DC300A - One channel distirted, other good.

[petehall347]

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

bias is set at idle .hence why its called idle bias ..it will change in normal use ..its sort of like a starting point . tickover if you like .
lets say its set too low it will never get high enough in use and distortion will happen ..if set too high it will go over the top in use and maybe hot things up a bit .
it can be set more accurately with a distortion analyzer . i have never had one and never been a problem .i do have various scopes i could use to look at crossover distortion and set so its gone away but never bothered . i just go off the manual settings if available . and see how it sounds and check it isnt getting too hot .

[budwich]

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

Its been running, medium volume, for a while, 1 hour here and there at a time. One thing that I am wondering about, when measuring the "bias voltage' on r132 / r232, it is within the spec of .314 or so (no gain / input). However, when playing some input, the voltage goes up into the .44v or so area on the r132/r232 which are associated with the positive side of the outputs while on the negative side, the voltage go down to about .2v on the r139/r239. Of course, I don't know what to expect but maybe was expecting it to remain somewhat "balanced" / similar for both "legs".

[budwich]

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

:-))) operational again. Checked the bias (r132/r232). Channel two was within spec at around .314v while channel one was low at about .28v. Changed out the "selectable bias resistor"(r129) for a lower value (82ohm-> 47ohm) brought the voltage up to .32v.
Will let it run again for a while with my fingers crossed. :-)

[budwich]

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

checked out the output from the ic150 pre-amp. Didn't see anything unusual thru its volume range. At "full volume" on a 0db 1khz wave, its output was 1.8vP-P. So that's seem reasonably OK. Probably found the last of the blown components on channel two, q220 (2n4125) which is the equivalent of the channel one component that was taken out during its "not working" previously in this exercise. Still doing some resistance checks / comparison from side to side before powering back up, more for general "scope of design numbers / what's being seen" as even with the blown components, the amp powers up (no input / output). Hopefully, I will get back to some form of normal shortly.

[budwich]

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

OOPS... I screwed up the transistor numbers earlier... they are mj15024 not mj21194. sorry.

[petehall347]

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

yes the 2 emitter resistors 0.33 ohms .
you can leave the others there as won't be connecting to anything with outputs removed .

[budwich]

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

I assume the "r234" was a mistype and you mean r239 (the negative leg). To do the suggested test, of only one output pair, I would pull the .33 resistor on all the others? Of course, this would be with no speaker attached.

As for having all the same output tr's, that would be tough at this point.

I am also going to check the pre-amp (ic150) a bit to look at the signal output along its volume range (0-10).

[petehall347]

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

i would be testing with one pair of outputs . q210 q215 . and making sure voltage across r233 r234 is about 10 or 20mv ish .
you should really have all the same outputs installed so they match .
or you could remove all outputs and sub in 1k resistors between where base emitters connect at q210 q215 . this is to keep the feedback path intact . then check output dc offset 0V and also base voltages that should be about 600mv at idle . do not have any loads attached to speaker terminals for now .

[budwich]

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

There are currently three replacement transistors (mj21194) in the channel two output stages. One on the negative leg and two on the positive leg. All three appear to be blown; one open, and two shorted. None of the original transistors appear to be impact although I haven't demounted them for complete checking. I will replace the replacements to at least see if I can get back to some semblance of good. Go from the result. I guess "amp rocket science" IS rocket science. :-)

[budwich]

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

I have only been running with all output transistors in place. I am not sure that I understand how the outputs work to provide signal / current. For example, as a "quick calculation", the 5w .33 blew, so that it indicating more than 5w. Then, that's indicating just under 4A flowing thru that one. IF so, that doesn't seem like enough current to blow the transistor. Further, IF the totally output of the amp was possibly fused on the speaker for 100W (2A) then I am not really understanding why the higher current assuming that it stages was "reasonably balanced" which would be 16A (4 "stages" X 4). Obviously, I don't understand some "subtleties" especially around "matched outputs"... :-) Having said that, I am not sure how a "minor mismatch" could cause such a wide current range difference on each stage taking out one stage and not all the others.... especially if one stage fails, then would not the others try to make up the current demand causing them to blow also?

[petehall347]

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

too much current flowing through the outputs will burn those resistors . and the outputs . need to do a regroup on this one as cant remember where we are up to as in like are all outputs installed or just a pair and stuff like that .

[budwich]

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

Unwisely, I tried a "full volume test" ( set on the pre amp). It causes the speaker line 2amp fuse to blow on channel two. :-( The channel no longer operates, channel one continued to function (is still functioning). First check, blew the .33ohm emitter resistor on the Q212 output transistor (second from the "end of series"). I have to go thru more checks to see what else is gone as I don't believe a blown emitter resistor would stop the sound out of the channel.
It was sounding good prior to the poor judgement call.

[budwich]

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

I got my numbering mixed up. The original part is an mpsa93 which I have replaced with mpsa92's.

edit... while replacing the replaced mspa92 didn't help. I, then, "adjusted" my heating technique which might not be as good as it needs to be. Anyway, cap C5 which is beside the mpsa pair seemed to be "helped" with the heating. I replaced it and things appear to be good / clean across the spectrum of test tones. I will run it a bit then let it cool off and see what the result of some "cold starts" will be. It could be a good day today. :-)

[budwich]

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

shielding the heating away from c6 and more on to the two transistors (mpsa92 - q101 / q201) cleans the signal up. Back at the start, I had found a problem with q201 whereby its was not conducting which cause channel two to have high DC. I replaced it with a mpsa93 which has a greater voltage spec and it was functional. Not sure I ever looked close at the 1khz tone other than ear listening. Later, while chasing channel one issues, changed q101 with the same mpsa93. Sometime there after, the warmup buzz started (not sure how immediate is was).
I am pretty sure that the MPSA93 might be the issue... more with channel one than channel two so perhaps some characteristic in their makeup is important and different. I have a few others that I will check and compare parameters with the checker to see if any pops out. I guess I will drop in another. One thing, the two are in a "clip on" heat shield that isn't grounded, just plugged with legs into the board. Wondering if maybe some form of "EMI" is "coupling" into the bodies so some strange form. ???

[petehall347]

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

c6 would affect both channels . it looks like its job is smoothing that 5v supply to the input offsets .
it could maybe affect the biasing as well .

[budwich]

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

went "heat hunting" again. I think I found that heating C6 (100uf 16v electrolytic) associated with the 5v "rail" cleaned up some noise in the top half of the 1khz test tone for both sides. This might not be related to the warm up buzz but maybe some form of ground buzz coming thru that cap into the input section and then flowing thru thereafter.

[budwich]

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

I changed the resistor down to 47ohms (its in series with R128 - 780ohm). It does not make any impact on the bias voltage measured on r132 / r139) which is basically at .14 mv. I am not sure how changing such a limited value in series with 780 ohms is really doing much, but this is way past my understanding of operation.

The warm up "buzz" seems to be less but that might be just today as opposed to anything that I have done. It is quiet with no input connected at any gain setting which seems different. I have to do some scope checks again to see if I see anything different.

[budwich]

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

Slow progress... really slow. I mis-ordered resistors (low values 1/2 w) for making changes to the bias as per the manual. :-( However, I tried cobbling up a bunch of 15 ohm resistors to give me 60 ohms for channel one (with the warm up buzz). The existing resistor is 82 ohm. The "60 ohm DIY" didn't make any change in the bias voltage that I read but seemed to help a bit with reducing the pre-warm up buzz. I am somewhat worried about how low that I need to go to raise the voltage OR perhaps the manual is "incorrect" with its direct.... " bias voltage low go with a lower value replacement resistor".

[budwich]

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

Crown DC300A Series II schematic, service manual. It looks like yours?

Overall- there's no electrolytics in the signal path, only two in the protection circuit 25uF 15V NP (per ch) and one for 5V DC offset power for both channels C6 100uF, which I would check.

For a quick check if things are reasonable, I would look at +/-10VDC power for the op-amps, check the idling bias current 0.34V across 5.6R driver stage and the op-amp should have around 1.35V at its output.

It's strange to have no bias current trimpot but input offset and output offset trimpots. I think transistors were hand selected and it's a tight design.
This means changing transistors could give bias current (heat) too high or an offset problem. For now, I would just take some voltage measurements to see what is off center.

I am sorry that I never properly replied to your post. I was not aware of the potential significant differences in the various crown dc300a. It appears that my unit is a series I (maybe or series "nothing" as there is no designation anywhere in its user manual or on the unit) and not the series II shown in the service manual that you attached.

Looking further thru this service manual and the one posted in by RJ (post 34), shows a difference in the bias setting checks / adjustment.

I should have paid more attention to the differences and the posts.

Anyway, measuring the voltage across the 5.6 (R132,R139 - channel one, R232, R239 - channel two), they are showing less than .2 v (.14-.18 range). According to the SM (RJ post, section 5.8), the voltage should be in the range of .31-.34, so there is significant difference in the bias on both sides.

[petehall347]

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

i would go through all the settings then look for any other problems .
offsets and bias first . if input offsets are wrong it might cause a bit of a nuisance .