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Old 05-19-2019, 05:04 PM   #1
budwich
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Default Crown Amp DC300A - One channel distirted, other good.

I am the original owner of this DC300A. It drives some JBL cabinet speakers but doesn't get used much in the last 25 years... maybe playing an hour or two every year. :-(

Anyways, went to give it a run and one channel is distorted while the other is clean / clear. Tested the cabling to the speaker and did a swap test and the problem stayed with the channel. Check the cabling to the crown IC150 pre amp, along with a swap and the problem stayed with the same side.... so the amp channel channel appears to be the problem.

I don't know much / anything about audio amps other than basic electronics type level measurements, etc along with general setup.

Started taking it apart. Attached is the layout of the boards with the front panel removed and main control board "loosely mounted".

Because the one channel works and other is distorted, I started checking all the resistor / capacitors (resistance reading) and comparing across sides of the output boards. The only significant difference that I could find was a resistance difference on a capacitor across the output terminals. On the good side, it reads resistively as 2.96k while on the bad side, it reads 3.97k. Its a .1 MF 200v Filmatic ITW. I don't have an ESR meter, just a regular DVM.

I know that a resistance measurement of capacitor doesn't have much meaning / use but is that difference enough to indicate a bad cap. In "amp land", I don't even know what the purpose of the cap is going across the output terminals.... getting rid of high frequency noise?

Any guidance on this thing would be greatly appreciated as I would love to get it working again as it deserves a better fate.
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Last edited by budwich; 05-19-2019 at 05:19 PM..
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Old 05-19-2019, 06:22 PM   #2
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Default Re: Crown Amp DC300A - One channel distirted, other good.

clean the switches and contacts first .
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Old 05-19-2019, 07:44 PM   #3
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Default Re: Crown Amp DC300A - One channel distirted, other good.

can try.... but there is only a power switch (can't cause a problem to only one channel) and two volume / gain pots which don't cause any "crackle" when turned. Will look closer at the two output terminals.
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Old 05-20-2019, 01:17 AM   #4
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Default Re: Crown Amp DC300A - One channel distirted, other good.

Could be a transistor problem, leaky, shorted, missing power rail. Might be better to do symmetrical voltage measurements across channels.

Check for DC offset at outputs first I'm thinking. Shouldn't be more than a few milivolts.
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Old 05-20-2019, 06:25 AM   #5
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Default Re: Crown Amp DC300A - One channel distirted, other good.

thanks... will give it a try. Probably can't see much "offset" with just a meter, at least I can check if DC rails are there / "close". I have to figure out how to safely power up and still get access to various points (boards have no labeling).
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Old 05-20-2019, 01:35 PM   #6
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Default Re: Crown Amp DC300A - One channel distirted, other good.

You could do diode/resistance tests on all the semiconductors without power on. If something's shorted it will show up.
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Old 05-20-2019, 07:44 PM   #7
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Default Re: Crown Amp DC300A - One channel distirted, other good.

yes indeed, that was going to be my next "non-power test". I need to look at the board to see exactly what is connected to where... with no marking / labels its a bit harder to figure out what an inexperience eye sees. Once I figure that out, those checks should be good, assuming an in board test of the component won't impact the result... again given that one side is working, the comparison results hopefully will point at a "yeah" or "neah".
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Old 05-21-2019, 08:02 AM   #8
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Default Re: Crown Amp DC300A - One channel distirted, other good.

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Originally Posted by Agent24 View Post
Could be a transistor problem, leaky, shorted, missing power rail. Might be better to do symmetrical voltage measurements across channels.

Check for DC offset at outputs first I'm thinking. Shouldn't be more than a few milivolts.
Well... I didn't electrocute myself yet... :-)

Anyway, was able to power the unit up with the main board demounted so the access to the other boards is possible. Measured the two "rails" for each side. Basically, they were each about 61.75 / -61.75v (with some "drift"... up to .8) "unloaded" (ie. nothing connected to the input or output terminals). The nominal voltage for the rails is 60/-60v.

It would appear that the rails are good.

Doing some diode mode testing on the output boards didn't find any differences between the sides although without demounting the output transistors, it would not be possible to find an open circuit since they are connected in parallel. There were no shorts per se.

I haven't done any testing on the main board yet. Looks like maybe the problem lies in the "pre section" of the main board for that side possibly although still wondering why such a big difference in the resistance measurement of the capacitor across the output for each side.

Last edited by budwich; 05-21-2019 at 08:05 AM..
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Old 05-21-2019, 11:19 AM   #9
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Default Re: Crown Amp DC300A - One channel distirted, other good.

Attached is a schematic that I was able to dig up in my files.

One question that I have with respects to the capacitor across the output (c218 by the pen pointer), can I just remove / disconnect it without an operational issue for a brief test?

Crown assemblers must use some big soldering irons as everything has large solder points to attach to / heat up (ie. screw ends, terminal lugs, etc).
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Old 05-21-2019, 03:43 PM   #10
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Default 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.

Last edited by dom0; 05-21-2019 at 03:55 PM..
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Old 05-21-2019, 03:59 PM   #11
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Default Re: Crown Amp DC300A - One channel distirted, other good.

I don't have access to a scope at this time. :-( can feed a signal in from a test cd but without the scope, I don't think it will help at this point. Maybe have to call in some "favors". I can certainly see that looking at the output of the main board as it goes to the amp stage would certainly narrow things alot.
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Old 05-21-2019, 04:15 PM   #12
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Default 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.
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Old 05-21-2019, 04:18 PM   #13
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Default Re: Crown Amp DC300A - One channel distirted, other good.

I've never tried it, but there is "soundcard scope" software which uses the line in of a laptop / PC. Definitely want a voltage divider + AC coupling capacitor for that one, though!
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Old 05-21-2019, 04:23 PM   #14
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Default Re: Crown Amp DC300A - One channel distirted, other good.

Yeah, that would probably work great with an appropriate probe. Actually I think EA or SC magazine made a kit to do that with an interface box for various attenuation ranges etc.
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Old 05-22-2019, 05:02 AM   #15
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Default Re: Crown Amp DC300A - One channel distirted, other good.

interesting ideas /thoughts... thanks. It got me thinking. These suggestions use your ears (hearing the probing results) while a scope uses your eyes to see the results. So then, how about using my DMM meter to "hear" the "probing".... I can just put it on the "freq" range and hopefully it will "hear" the 1khz "tone" and put out a good frequency rate at points on the good side. On the bad side, if the signal is "distorted" (I need to get the title updated... it was a typo... distirted...:-) ) along the circuit path, then hopefully the meter will either put out some other rate or have problems "locking" on a frequency and "roll" or otherwise. May be it will work almost as good as hearing / seeing the test with my eyes / ears. Fingers crossed.


One question though... where in the circuit do I need to "probe".... the input to the bases of the output transistors or at the emitters?

Last edited by budwich; 05-22-2019 at 06:13 AM..
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Old 05-22-2019, 06:22 AM   #16
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Default Re: Crown Amp DC300A - One channel distirted, other good.

Quote:
Originally Posted by dom0 View Post
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.
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Old 05-22-2019, 08:28 AM   #17
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Default Re: Crown Amp DC300A - One channel distirted, other good.

Quote:
Originally Posted by Agent24 View Post
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.
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Old 05-22-2019, 12:58 PM   #18
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Default 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.
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Old 05-22-2019, 02:56 PM   #19
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Default 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. :-)
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Old 05-22-2019, 03:56 PM   #20
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Default 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
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