Crown Amp DC300A - One channel distirted, other good.

[petehall347]

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

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

[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?

[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?

[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.

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.

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

[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.

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

[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.

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.

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.

[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.

[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.

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.

[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.

Progress has been measured in "baby steps" over weeks / months.... some due to my slowness along with very slow "off shore sail boat times".... ultimately I hope I can remember how to put things back together if I ever get that far. :-)

I got some MJ15024 replacements for the originals output transistors. At this point, I only replaced two on each channel that I found which "appeared" to fail C-E resistance testing when compared to others (this is with the E isolated from the board... can't isolate B and C without totally desoldering / demounting).... either shorts or leakages. I did this at this point to limit the amount of "hands / soldering damage" that my "skills" will do to this thing.... :-)

Also replaced one .33 ohm resistor which was open.

Built my "bulb limiter" supply to hopefully reduce the "shock" of the first "plug in"... of course, I did a bunch of checks for shorts and comparing channel 1 to 2.
I plugged the amp into the bulb limiter and hit the switch. The bulb lit brightly for a second or so then basically dimmed to almost dark. The switch light remained lit so the limiter worked at limiting the current going to the amp and it would appear that I have no "immediate shorts". At this point, I am taking a deep breath, thinking on things and will go back to check for voltage at a few points to see if maybe there is enough juice going thru the limiter to get the amp "operational" at the basic electric level at this point. Hopefully, some baby steps will happen sooner as opposed to later.

[budwich]

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

continuing to do "on board checks", I can only further do E-B junctions on the transistors (no way to easily isolate the C-B due to multiple connection points). The diode voltage test of the junctions is good on transistors that had good resistance measurements across C-E. On the short transistors on the "blown side", the voltage was low / 0. On the two transistors that "failed" the resistance C-E test on the distorted side, they had good B-E voltages.... that would appear to confirm / indicated may be why the channel works but is distorted... maybe.... leakage across the C-E junction but functioning "control".

[budwich]

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

thanks... good idea with the "limiter bulb". The rest of the "hints" are a good guide also in my "learning curve". At this point, I am truly hoping to at least bring back the "blown channel" with some luck and carry that luck (and "learning") over to the distorted channel... maybe.

[circuitmonkey]

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

Just a little friendly advice. Maybe consider wiring up a dim bulb to share the current when an "oops" happens. While it won't completely eliminate damage to you or the components, it will greatly reduce the risk and in many cases mitigate damage. There are plenty of good diagrams on the Google machine. It also works pretty well as a fault tester. If you apply power and the bulb(s) stay lit, you have a significant short and no further distortion testing can really be performed until the shorts are gone. Also, without having an auto transformer, this will help limit inrush current when you first power it up. It's a cheap process if you're creative. Around $10 at Home Depot.

Channel distortion is usually (not always) caused by bad caps or leaky transistors in the audio signal chain. It's really difficult (by no means impossible, but a lot more guess work) to narrow down the noisy component(s) without using a scope or signal tracer. One mismatched transistor in the pre-amp section could cause audible distortion. While replacing one or two might fix the issue electrically, not matching the transistors could result in distortion. Might get lucky, though. Cold/cracked solder joints can also cause cracks and pops.

Also, film caps (which are not generally marked for polarity anymore) can cause distortion if they are not oriented correctly. While they are said to be bi-polar, they have a "foil end" and that needs to point to the path of least impedance. I don't think that is your issue as it developed over time, but something to consider if/when you need to change them. Also, hard to test polarity without a scope, although I'm sure someone out there has figured out a way.

[budwich]

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

If you say that you may have further damaged by blowing the working channel, do a no load check. By that I mean disconnect the speakers, replace the blown fuses with the correctly rated fuses, switch on the amp and see if you get any DC voltage on the speaker terminals of that channel. If there is DC present (I think it will be!) it means that you have blown output transistors. You could also have the driver transistors blown as well.
You could always check each transistor out of circuit. If you find even 1 of them blown, replace the whole lot. These direct coupled stages will damage everything if one gets blown.

While after a "deep sigh" and more reading and "thinking", I started back on this. I was able to "isolate" the output transistors (at least the collector to emitter section) without demounting them from the board.... demounting all transistors at this stage is probably not happening as this is significant effort and an unknowing experience (meaning I don't know how to go about the desoldering of such devices). Using a "gross resistance" test of the CE, I found two 2n5631's on the channel that I "took out with my probing"... these were shorted. I found two 2n5631's on the distorted channel... these had measureable resistance in both directions as opposed to "infinite" in one. I also found one .33 emitter resistor on the "blown channel" (all others on both channels are OK still).

Baby step forward... maybe... :-)