Crown Amp DC300A - One channel distirted, other good.

[budwich]

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

thanks for the assist, RJ

[R_J]

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

see if this is any better

[Agent24]

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

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.

[andy1]

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.

[budwich]

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?

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.

[budwich]

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.

[Agent24]

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?

[budwich]

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.

[Agent24]

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

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.

[budwich]

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".

[petehall347]

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 .

[petehall347]

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

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 .

[budwich]

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

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.

[budwich]

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).

[petehall347]

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

it wouldn't harm to check all those cement emitter resistors are good

[petehall347]

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

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

[budwich]

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

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... but the mind does play tricks. Anyway, will push towards the scope.... won't be the first time. :-)

[Agent24]

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

The frequency input in your DMM likely only works at TTL logic levels, e.g. 5v. Check the manual to be sure.

Probably wouldn't work anyway, even if the 1kHz tone gets distorted, it'll still be at 1kHz.

The best option in your case is likely an audio signal tracer using sound card or other amplifier. Unless you can borrow a scope.

[budwich]

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

Without a scope you can use another amplifier (or build one from something like an LM386) as a signal tracer, then follow the signal through until you find where it becomes distorted.

This would work great in the preamp/input section just as a plain input.
In the power section you will need a voltage divider/clamp on the input to avoid overloading because of the high voltages. A DC blocking capacitor would be good too, in both cases. Make sure it has a rating higher than your main voltage rail value.

oops.... I needed to pay more heed to this. :-(

I was "playing" with the "DDM frequency mode"... and bad things happened when I probed from red on one of the emitter incoming lines (from the main board to amp) with black on the chassis ground.... large noise immediately...scared me and blew the fuse on the lead to the speaker.

I was expecting some sort of DC block in the meter or otherwise. Need to go back to the drawing boards and think on this a bit. I certainly don't understand how the circuit works and what to expect where.

[budwich]

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

One of the quickest ways of troubleshooting just one bad channel is to feed the amp with the same, say, 1 kHz sine on both channels and then just working from either the inputs or the outputs, checking the signal shape on the good and comparing it to the bad.

I usually start from the outputs, because the higher the power, the more often it breaks.


Edit: Removing C218 might result in HF oscillation of the amplifier, but it probably won't.

thanks for the warning on the capacitor... I think I will try the "dmm in frequency mode" to see if I can at least isolate whether the problem is at the main board or at the output / amp board.