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Crown DC300A Problem


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#1 mtarr

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Posted 26 March 2006 - 12:23 PM

I have a Crown DC300A that I bought new in 1975.  This amp has lived a comfortable life in my home for the past 30 years.  It has performed well for a long time.  Over the past year, it has developed buzz and hum in both channels.  The output is also distorted.  

Since sound does get through, I'm thinking that the output stages are still good.  Also, when I turn the unit off, it takes about a minute for the amplifier to stop driving the speakers, so I think the 13,000 uF main filter caps are also still good.

I would like to repair the amp; I have some experience repairing audio equipment.  I know that it is easier to cause more damage than to get it right.   I am concerned about damaging devices that are still good and are difficult to find today.  I have not started to debug the problem.

I have two questions:
1.  What is the most likely cause of buzz and distortion on both channels?  The buzz seems a little louder on one channel.
2.  Is there a way to current limit the power stages to minimize the chance of causing damage to them while I'm working on the amp?  With some amplifiers I have been able to add current limiting resistors in series with the V+/V- supplies to the power PCB.  If that is possible, what values/wattage do you recommend?

Thank you,
Morty

#2 Lou

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Posted 27 June 2006 - 06:22 PM

The caps could be bad on a 30 yr old amp.
Does it do it with the volume turned down?
Did you feed a "hot" signal into it and blow up the input stage?

#3 mtarr

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Posted 27 June 2006 - 07:32 PM

QUOTE(Lou @ Jun 27 2006, 07:22 PM)
The caps could be bad on a 30 yr old amp.
Does it do it with the volume turned down?
Did you feed a "hot" signal into it and blow up the input stage?
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Hi Lou,

I changed C5 and it worked for about a week (C5 was open, both times).  I then changed C4 and C5 and it seems good.  Occasionally, I hear what might be a low level oscillation in one channel.  Each time I was ready to debug, it was not there.  

To answer your questions, the buzz was there all the time, even when the inputs were disconnected.  It did not get appreciably louder with the input pots turned up.  The input stage is not blown, but it is not ruled out as the source of the possible oscillation.

Thank you for your help,
Morty

#4 Lou

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Posted 29 June 2006 - 07:22 AM

The source of the low level oscillation is probably caused by poor decoupling of one of your supply rails. This is caused by dried up electrolytic caps.
What I would do is replace all the electrolytics in it. Its certainly past the age at which this should be done.
If you're going to keep it and its in good cosmetic shape there is no need to replace it. (You could buy a new "digital" amp and you would be opening a can of worms.)
Get the parts list out and circle all the electrolytic caps. The call Digikey and place a cap order.
When the order arrives, just plan on sitting down with a soldering iron for a couple of hours.

#5 joust

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Posted 08 July 2006 - 01:25 PM

If you have a low level buzz that cannot be removed with the level controls, replace all the output devices' mica isolators: they probably have started some type of leakage. Replace the isolators with silicone type tissu.