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Old 05-22-2019, 03:58 PM   #21
petehall347
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Default Re: Crown Amp DC300A - One channel distirted, other good.

it wouldn't harm to check all those cement emitter resistors are good
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Old 05-22-2019, 05:32 PM   #22
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Default Re: Crown Amp DC300A - One channel distirted, other good.

I checked them all... they were within the value and "accuracy" of my meter... .3 ohms (value .33).
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Old 05-22-2019, 05:36 PM   #23
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Default Re: Crown Amp DC300A - One channel distirted, other good.

Quote:
Originally Posted by petehall347 View Post
you should use a blocking capacitor on the positive probe to measure AC .also if you use an audio tracer such as headphones . 0.1uf or thereabouts if i remember right
I was thinking of trying a fm wireless headphone... using the input to the base as the "probe"... which is a phone plug / rca plug and then wireless out to the headphones.

I am thinking that the issue might be the high DC levels around in places... so the capacitor is probably a good idea.

Last edited by budwich; 05-22-2019 at 06:52 PM..
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Old 05-22-2019, 05:57 PM   #24
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Default Re: Crown Amp DC300A - One channel distirted, other good.

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Originally Posted by budwich View Post
I was thinking of trying a fm wireless headphone... using the input to the base as the "probe"... which is a phone plug / rca plug and then wireless out to the headphones.

I am thinking that issue might be the high DC levels around in places... so the capacitor is probably a good idea.
always a good idea to use low value blocking capacitor .
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Old 05-22-2019, 06:03 PM   #25
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Default Re: Crown Amp DC300A - One channel distirted, other good.

in my experience any amp left in a working state then doesn't work properly is either corrosion or dampness got into the board or resistors . corroded connections being the main thing .
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Old 05-22-2019, 06:58 PM   #26
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Default Re: Crown Amp DC300A - One channel distirted, other good.

I agree. I have been measuring components at the component wires. I should probably do some more check at the board layer / junctions just in case it is a poor solder joint as opposed to a faulty component.

This unit is over 40 years old so capacitors of their time might have an issue (drying out or otherwise) which to me is a "distortion maker".
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Old 05-22-2019, 11:13 PM   #27
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Default Re: Crown Amp DC300A - One channel distirted, other good.

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I have to investigate the meter but I am sure that I have used it to check the incoming AC frequency on a few house circuits...
Some probably can, I know mine doesn't.
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Old 05-23-2019, 06:05 AM   #28
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Default Re: Crown Amp DC300A - One channel distirted, other good.

I think the problem with the meter use was what point I was probing. I don't really know where to look for signals.... at the base inputs to the transistors, at the emitters, other places? In the case of the "first attempt", I put the positive (red lead) on the base of the output transistor "group" that is associated with the positive rail. Nothing really got measured there but the meter did try to do some counting as but basically sat at 0. The next attempt was at the base lead of the transistor group of the negative rail. It is quite likely that this caused a current flow into the based from the chassis ground (black lead) towards the red lead. I suspect the meter doesn't expect to see "reverse flows" and has limited / no "blocks" to prevent it... maybe.

Anyway, I will do some more thinking / looking / checking before blindly going forward probing things.
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Old 05-23-2019, 06:16 AM   #29
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Default Re: Crown Amp DC300A - One channel distirted, other good.

First place I'd be checking is the output pin of that dual opamp, IC1.
Looks like pins 1 and 13 are the outputs. Not sure which channel is which. They seem labeled 1&2 not L&R.

Hard to read the text though. Can you take a better photo at all, or do a scan?
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Old 05-23-2019, 06:54 AM   #30
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Default Re: Crown Amp DC300A - One channel distirted, other good.

:-((

I guess I did some damage somewhere with my "unknowing probing". Replaced the speaker line fuse. This was on the good side of the amp. At the time, I was checking to see what I would see on the good side with the 1khz input signal.

Anyway, after replacing the fuse, turned on the amp at low volume, then tried to listen to the test cd again. The amp ran for about 30 seconds, noticed no sound coming from the good side. At this time, the amp shut down because it had blown the main line fuse (10 amp)... oh oh. So it appears I made my problem worse with some sort of short somewhere but not an immediate short as it did power up for a bit as opposed to an instant fuse blow.
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Old 05-23-2019, 07:20 AM   #31
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Default Re: Crown Amp DC300A - One channel distirted, other good.

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Originally Posted by Agent24 View Post
First place I'd be checking is the output pin of that dual opamp, IC1.
Looks like pins 1 and 13 are the outputs. Not sure which channel is which. They seem labeled 1&2 not L&R.

Hard to read the text though. Can you take a better photo at all, or do a scan?
not sure if this helps as my scanner is pretty old. The circuit diagram is a "fold out" so it takes two pages to get the whole schematic in. Note the page separation and even though it looks like it aligns, it is an overlapped view and not "thru flow" from one page to the next.
Attached Files
File Type: pdf crown amp.pdf (329.6 KB, 6 views)
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Old 05-24-2019, 09:46 PM   #32
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Default 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.
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Old 05-25-2019, 01:11 PM   #33
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Default Re: Crown Amp DC300A - One channel distirted, other good.

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Originally Posted by budwich View Post
not sure if this helps as my scanner is pretty old. The circuit diagram is a "fold out" so it takes two pages to get the whole schematic in. Note the page separation and even though it looks like it aligns, it is an overlapped view and not "thru flow" from one page to the next.
Thanks, that is a lot better than before, but can you try at a higher resolution? 300 dpi? Some of the component values are still hard to read, unfortunately.
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Old 05-25-2019, 02:19 PM   #34
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Default Re: Crown Amp DC300A - One channel distirted, other good.

see if this is any better
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File Type: pdf Crown DC-300A Service Manual.pdf (5.27 MB, 5 views)
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Old 05-25-2019, 07:46 PM   #35
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Default Re: Crown Amp DC300A - One channel distirted, other good.

thanks for the assist, RJ
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