Crown Amp DC300A - One channel distirted, other good.

[budwich]

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

With the 100w bulb limiter, the two voltage rails on each channel are at +/- 56v as opposed to 60 which seems relatively OK... not full because of the incoming voltage drop from the bulb. Not sure what else to check at this point before removing the limiter. Maybe just praying... :-)

[budwich]

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

baby steps forward. Removed the bulb limiter and plugged in directly to power. Unit powers up and the rails are at +/- 61 which is closer to "normal".

Plugged in a preamp signal which feeds an audio test disc thru to the amp. I have sound.... YES! I need to do more checks to see if there is a channel difference but at "first listen", things seem OK although I haven't really pushed much sound for fear that the thing will go south.

[petehall347]

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

check idle current . measure voltage right across an output emitter resistor
also check dc offsets at speaker outs

[budwich]

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

thanks for this.... not sure about the "dc offsets" measurement but certainly can measure and calculate the resulting current.

I will add some new stuff. While I was waiting for parts and such, I gathered my "pennies" and picked up a small handheld one channel scope. First use. :-)

Attached is a picture of the display from the bad channel. The good channel (not posted) shows a "normal sine wave" while the bad channel appears to have lost most of the bottom half. :-( This was at the speaker output. I have not gone further to see where things went bad as the last time I did further "checks" with a meter on the good side, I wasn't careful and took out that channel completely so I am "trigger shy" at this point.

[budwich]

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

I measured dc voltage across a few emitter resistors on both channels with no connected input and did not get any voltage reading... ??? The output pots were at some low value.

read up on the dc offset. measured across the open output speaker connection. Good side was 15mv while the "bad side" was 31mv.

[petehall347]

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

offsets are acceptable .
does the amp have trimmers for idle current ?
the scope trace looks like there is a problem with the negative current . check output transistors collector voltage first .then base voltages that should be about 600mv

[budwich]

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

not sure about "trimmers for idle current", the only "adjustable board components" are on the main control board (none on either of the output boards) and they are an "input offset pot" and "output offset pot" for each side of the control board.

The "rails", plus and minus, on both channels are both showing 61.x volts (ie. + and -) accordingly.

Based on your "hint", I will see if I can carefully track the inputs toward each side to see where the signal get in trouble unless the output stage causes issues all the way back... maybe???

I am thinking almost that the trace almost looks like a "diode breakdown"... sort of if my recall is correct. There are two "opposing diodes" on the outputs "legs" (going up to each rail). I haven't checked those yet.

[petehall347]

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

would be worth looking at the drivers voltages . about 1.2v on the bases

[budwich]

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

As you suggested, I checked the voltages associated with the output transistors. On the positive "half" of the output "set", the base emitter voltages appear to be correct (one very low, one high... base versus emitter I think). I then looked at the negative "half set", the base emitter voltages were the same value at basically the -61 volts. :-(

correction... on the "positive side" both base and emitter are very low ... on the negative side, both base and emitter are at -61v.

disregard this post as I need to pay closer attention to details. :-(

[petehall347]

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

each base emitter junction should drop about 600mv . example 1.2v in 600mv out .

[budwich]

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

not sure what happened along the way, but somewhere the signal on the bad channel has gone to a "sawtooth" like wave form while the good channel has remained a nice sine 1khz sine wave.

In terms of your recent statement, how are those measured... to what reference point. IF I measure voltage across the base emitter, on the bad channel, I see no voltage (drop) as the voltage is the same on points (taken to chassis ground) where as on the good side, I see a "difference" of about .24 volts.

Not sure why the signal has gone to a saw tooth unless something worse happened.

[petehall347]

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

i would give that driver board a good looking at with power off . first solder joints then transistors and diodes and resistors . just compare readings from good channel to bad . remove and test again anything that looks bad to your meter .

[budwich]

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

I thought I would give a "signal sniff / scope track" to see if I could find the point where things are going south when compared on both sides. I thought I would look at the signal coming out of the "diff. op amp / ic" with the 1k input. I wasn't able to find the "expected sine wave" on either side.... even though I confirm the expected output at the speaker output jacks. Might be a triggering / level issue. Perhaps also the scope may be causing an issue for the IC although I wouldn't expect it to be so. My ground lead on the probe may be "flaky". I guess the question would be. Should I expect to see the "nice" 1khz sine signal coming out from the IC on each side / channel?

[budwich]

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

I took a closer look at the board layout versus the schematic as the main board has no labeling so transposing where / what component is where is somewhat difficult outside of obvious main components... plus my eyes ain't so good... :-)
Anyway, I think I located the points... just after the output of the op amps towards a first transistor (base pin). The "outs" are on pin 13 and 1 of the "dual op ic".
Attached is scope shots from each channel.... 1khz feed into the amp.

The first is the "good" channel. This shows a reasonably good wave form. Although not shown, the wave form is "superimposed" on top of a 60hz wave form... not sure why or what... maybe some form of hum / grounding issue. The "60 cycle" does not appear in a scope look at the speaker output jacks.

The second picture is the waveform come out for the bad channel. Not sure what to make of it other than the frequency is correct but the wave form is "unrecognizeable". The voltage value seems rather high almost like some form of both dc offset plus just plain large signal. I don't believe that the channel gain pot was turned up that high but I need to check.

It would appear that perhaps my channel distortion may originate in and around the op amp ic (and supporting circuits) as opposed to the output transistor boards... maybe?

[petehall347]

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

dc voltage marked on schematic for pin 1 and 13 .best check them

[petehall347]

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

waveform looks like positive dc on bad one .it should be ac .

[budwich]

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

dc voltage marked on schematic for pin 1 and 13 .best check them

The +/- 10 volts is there at pins 7 and 14 but I didn't check for any dc value at 1/13 (ie. output) of the op amp. I did a "dry check" (ie. no power) resistance measurement (to ground) on all pins comparing against equivalent at each channel and didn't find any "outstanding" differences.

[budwich]

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

waveform looks like positive dc on bad one .it should be ac .

perhaps a shorted capacitor somewhere near / the first "leg" off the ic that may not show up with "gross" dc resistance testing... maybe.

Thanks again for the "hints" as my understanding of general electronics is OK but how audio amps actually work / are designed is not really there.

I will do some more voltage checks... my fear is with "live" boards and small distances with shaky hands... :-)

[budwich]

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

With a 1khz input but with volume turn down / off, pin 13 reads 9v dc while pin 1 reads 2mv dc. Does this mean the IC is toast or that some things associated with the output line somewhere is "dead" / shorted (ie. capacitor)? I hope it isn't the op amp ic... not sure if replacements can be had.

[budwich]

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

looking at the schematic and transposing to the board, there is no "R207" ("r107") which are "option resistors" depending on the ic installed. These resistors "tie" the outputs to -10v "legs"... not sure for what reason beyond the viewing the schematic.

To me, this means that there is some for of "leakage" coming thru from the "other 10v rail" linked to the output via Q219 (2n4125)... maybe this is shorted or "turned on"?