Crown Amp DC300A - One channel distirted, other good.

[redwire]

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.

[budwich]

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

Thanks. I haven't completed all the checks yet... doing baby steps. I confirmed that the startup noise is only on channel one.... two is quiet. The +/- 10v powering are both there and 10.05 +/-. I am trying not to let the thing run too long at any instance so that the noise remains in place to help with tracking things done as opposed to waiting for it to return after it has dissipated (I don't know what timeframe that occurs at... although left overnight, its there). It certainly appears to be some form of weaken capacitor.

I did a quick check of the dc value at the speaker output (no input / 0 gain setting). Channel one (start up noise) has .015v while the other is 0v. There appears (confirms) to be some small dc value showing up at the output.

I have yet to pull the capacitors to check their values.

Maybe as suggested earlier, I will do a heat check on the capacitors first to see if that helps first with start up and go from there.

[budwich]

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

ok... I heated up the c108/c109 (they are close together so they can't really be heated individually). The good news is that the channel noise is gone at the start up so it is likely that these have lost there value when cold. I will pull them to confirm and locate a replacement source. Related to that is there anything special about these other than be "NP" electrolytics 25/15's ? Thanks again for the hints. I can feel joy returning... :-)

first glance... it seems like a replacement for 25/15 NP might be hard to readily find... :-(

[budwich]

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

I manage to mangle one of the capacitors trying to get them off the board, one lead pulled away. :-( so now I need to track down some equivalent substitute for a 25uf15v NP electrolytic. Suggestions for alternate values. 33uf 25v polarized?

[petehall347]

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

you could use 22uf 25v NP

[budwich]

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

OK... will give those a try. They seem more available (at least off shore). I guess with normal tolerances, they should be with range of the originals. Thanks.

[petehall347]

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

where is it placed in the circuit ?

[budwich]

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

as redwire pointed out, the two are in the "associated" feedback circuit... not directly in its path but the supporting circuitry / drive.

[budwich]

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

So the "slow boat" finally came in and I got some 22uf 25v bipolar caps in... not sure of the difference between NP and BP. Hopefully, it won't impact the sound / design. I only replaced the two associated with the noisy channel 1. I now have "clean" sound again on both channels, at sound levels and at idle (quiet, no sound input). I will let it run for a while to see if anything tips over with run time.
Thanks again for the help, hints, and guidance on this. It is much appreciated and has helped improve my troubleshooting and outcomes along the way.

[budwich]

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

:-( not quite there yet. On a cold morning start up, just powering up the amp, channel one has a significant buzz (again). Was surprising since the initial startup from the other day did not exhibit such a sound. Of course, there was some soldering involved which might have changed something in terms of temperature. Need to do some checks again to see what's different. Previously, I did some prechecks of voltages for the 10v, +/-60v along with output voltage at the opamp (looking for any DC disparity).

checking the voltage on the opamp output shows a significant dc voltage (2+ volts) on both channels. I was wrong with my initial hearing of buzz as it is actually on both channels (speakers are too close together).

[budwich]

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

Playing with the gain controls, with no inputs connected, the buzz increases (on either channel) as the gain is increased but does drop off as you move towards max gain. The DC voltage on the output of the opamp increases but then drops off in step accordingly. At this point, the buzz does not seem to dissipate with time (ie. as run warmup happens).
It would appear to be some form of grounding issue maybe??? Looking at the schematic, there is supposed to be a 2.7 ohm resistor between the control board ground and the earth / chassis ground (although I am somewhat unclear with the ground icons in the schematic... /// versus = ). There resistor has always been hanging in "mid air". One end is connected to the common / ground areas of the board (where the signal shields all terminate) while the other end is just hanging there. Not really sure why it is like that. Is this related to the issue? Is it possible crown dropped the connection for some reason? The resistor is sitting on the backside of the printed control board, almost like an afterthought / temporary mod.

edit: It probably has no bearing as the amp is normally plugged into a "cheater plug" (3 to 2 prong adapter) probably to prevent any possible ground loops (and the back side of the preamp switched power only has 2 prong outputs for the same reason).

[budwich]

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

Went "hunting with heat" (ie. hot air station), and found the buzz on channel one (which is slightly greater than that on channel 2) was quieted by applying heat to the area of C119 (200 pf mica).
and Q102 (2n3859a)... not sure the hot air can be directed well enough but I suspect the cap rather than the transistor.

[petehall347]

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

i say transistor . or poor solder .
i will look at schematic to see why the caps there .
edit
appears to be a miller cap to stop oscillation . wont harm changing it .

[petehall347]

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

heat the transistor then cool it with a drop of isa on a cotton bud .

[budwich]

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

It appears that you are right as I focused more heat towards the transistor and it eventually quiets the channel.... :-( It would appear that the q102 (2n3859) as some form of issue. The bad news is it identified as "selected" and probably means the replacement of the other one on that channel and trying find those two in some "closeness of selection". I am not sure where I read or saw somewhere a suggestion to replace them with bc639's (which I have... not sure why but related to this repair at some time at the start). They have a slightly higher voltage rating with the same current rating. Do you think that would work or cause me more problems... oscillations, otherwise? This is way past my knowledge and experiences to date

[petehall347]

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

what part number is q203 ? am wondering if they should both be changed as complementary pairs .

[budwich]

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

did you mean q103 (for channel one)? its a 2n4125... for the negative "leg". I never looked close enough at the circuit and figure the "select" referred to a "match" between the "top" and "bottom" of the circuit, but I see from your comment / question, that they appear to "complementary pairs" (npn, pnp). So then the question would be to select a bc639 that is similar to a replacement 2n4125? or am I totally missing the point?

[budwich]

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

Not sure what they mean by "select" but measuring a bc639 and a 2n4125 with a transistor checker shows an hfe for bc639 at 220 ish while the hfe for 2n4125 was 120 ish. This seems like a sizeable difference. Possibly a bc640 instead of 2n4125? The data sheet for the bc639 seems to indicate a "normal range" of hfe similar to that of the 2n4125. Perhaps a "select" is harder to accomplish with a limited count of devices if indeed, the hfe is what is being considered.

[petehall347]

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

i dont know if they are complimentary .i did a quick search for the numbers and came up with different numbers for pairing them .

[budwich]

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

Is there a great risk at using the higher gain (hfe) bc639 substitute... ie will it cause other components to blow and out of control oscillations or otherwise?