Crown Amp DC300A - One channel distirted, other good.

[petehall347]

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

i am only guessing it will be ok because it has a miller cap in circuit to prevent high frequency oscillation . all you can do is try or get the right transistor .

[budwich]

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

OK... I haven't found where I picked up the bc639 as a plausible replacement... some where in my earlier travels thru forums a while back where some one was trying to revive / rework an old unit. I am hoping that I can remove 2n3859 without any damage (leads have been folded on the circuit board backside during soldering which gives me trouble some times for a clean "pull"). If I am lucky, I will measure the thing on a checker to see what it looks like.

One thing which I found while probing around with a meter (voltage measurement) with no audio input but gain turned up to hear the noise at bit, I could cause a audible change, slight increase in noise level at various points around the circuit which seems to indicate some sort of ground / common "issue".... or maybe just the noise floor fluctuating.

[budwich]

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

I pulled the 2n3859, hopefully without destroying its characteristics. I put in the tester and it shows an hfe 487 which seems rather high along with a 4.4 ma Ic current. I am not sure whether the high hfe was the intent of the "select" (ie. get one as high as possible) or that is the reason it is having a problem... not sure how hfe "ages" / fails.
I haven't dropped in the bc639 yet... mostly on fear that something bad will happen.

[budwich]

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

Soldered in the bc639. Nothing changed, perhaps even a bit louder buzz. Put back the 2n3859 for now.

[budwich]

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

with a 1khz tone input, looked at channel one. Maybe hard to see but it shows about a 1v DC level out. The other channel shows a similar output but less DC, ~.25 maybe. I don't know what the "block of pulses", that appear regularly across / over top of the 1khz, are. There are similar ones on the other channel. They are somewhat gain dependent. If I turn the gain to full on, they disappear. Adjusting the gain, intermediately, has somewhat little impact on their magnitude. At full gain and having them not showing, if I push up the input volume, they reappear. I guess some sort of input level dependency somewhere maybe? It appears that the frequency inside the "burst" is about 2.4khz. I would think that it would be audible although maybe my ears ain't that good. Is this the "buzz"? I haven't tried any heating of components to see what the resulting waveform is when the heat appears to improve / remove the buzz, nor have I tried tracing the signal back to see where it originates.

[petehall347]

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

dc is generally clipping . maybe something is being overdriven .is the sinewave clean all the way through the circuit ?
or it could be leakage i suppose .
whats the dc offset like at idle no signal ?

[budwich]

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

I haven't traced thru the signal to see what things are where yet. The idle dc value on channel one is high at 20mv (channel two is 2mv). When this repair saga started, once "operational", both channels were showing about 2mv. Its possible that some of the changes in components in channel one circuit may have caused a shift but I am thinking that its more likely some component is not functioning correctly still. I still need to check things after a "warm up period" to see if the characteristics change / are measureably improved. That may at least provide some clue (ie. temperature dependent).

[petehall347]

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

20mv is fine

[budwich]

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

my confusion continues even with simple signal checks along the circuit. I can see a "nice" 1khz at the gain output control going into the opamp. Check the outputs at pins 1 and 13, I don't see any 1khz but I do see the "block of pulses" as seen at the terminal output trace posted.... yet I hear the 1 khz tone plus the lower level "buzz". I am not sure why I can't see any 1khz in the trace at the output, perhaps a level issue or I don't know what I am looking at. The DC values at the opamp out is in line with the SM about 1.4v "nominal", depending on the volume / gain. The +/-10v for the opamp is there although I haven't verify its "condition" (ripple or otherwise).

[petehall347]

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

is the idle bias set correctly ?

[budwich]

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

sorry, I am not sure what you are referring to. The output bias at r132 (channel one output board) was around .4v, possibly not yet warmed up but higher than spec. This is similar on channel two (r232). The other resistor (r139... lower "leg") had higher reading of .51v. Again similar on both sides.

[petehall347]

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

i will have to check the manual but i think these are set at around 300mv

[petehall347]

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

if its same as 300a ii . its set at 340mv across r132 . r139 should be same 340mv
r232 r234 for other channel

[petehall347]

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

check out 7.3 in the manual and get the bias and offsets set to spec if you can then see how it is .

[budwich]

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

if its same as 300a ii . its set at 340mv across r132 . r139 should be same 340mv
r232 r234 for other channel

yes those are what I am referring to and yes they do appear to be high. Getting them down might help but it is still somewhat disconcerning about the buzz and the effect of heating a certain area improved that situation.
Having said that getting the appropriate resistance to adjust the bias might be a bit tough around here (lockdown, etc). I will see where that leads me.

Also still something fishy with the signal trace at the output of the opamp on both channels. So I have a few things to bang my head against. :-) thanks again for your hints.

[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 .

[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.

[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.

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.

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.