Crown Amp DC300A - One channel distirted, other good.

[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.

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

[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 .

[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.

[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.

:-( 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.

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.

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

[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.

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.

you could use 22uf 25v NP

[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?

[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.

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.

[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.

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).

[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... :-)

[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.

[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.

[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 .