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DC 300A


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#1 Wilder Bill

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Posted 07 August 2005 - 07:57 AM

My DC 300A started having a little distortion, which at first would clear up after a few minutes.
It sounds like voice coil drag, but isn't.
Now, my right channel is dead, as well.

I did see the D 150 queston that suggested it might be the UA 739 op amp, but I'd like more opinions, please.

Also, just how long are the filter caps good for on these babys? No hum or smoke yet, just curious.

#2 KWhitehead

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Posted 09 August 2005 - 04:58 PM

The power supply is made up of a plus and minus filter cap so if one of those where to go bad both channels would be affected by it. If the cap opened causing excessive ripple distortion would be heard on both channels so my guess is the filter caps are still good. I have seen D Series amplifiers in service 15 plus years old with the filters caps perfectly fine. Now there is a 10 volt supply to the op amps on these amplifiers and this is what I have seen the most to fail. These small electrolytic caps do dry out and open up causing a distortion on the output. This could be what is causing your problem, distrotion wise.

The right channel out could be something more specfic to that channel such as and output device or driver. The 10 volt supply is common to both channels. You can send it in for an estimate, our service dept. is $70 per/hr plus parts. If just the caps in the 10 volt supply, my guess is it would still be under $100 for repair. If it's that plus an output device then maybe around $100-$150.

Thanks,

Kip Whitehead
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#3 Wilder Bill

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Posted 09 August 2005 - 07:44 PM

QUOTE(KWhitehead @ Aug 9 2005, 04:58 PM)
The power supply is made up of a plus and minus filter cap so if one of those where to go bad both channels would be affected by it. If the cap opened causing excessive ripple distortion would be heard on both channels so my guess is the filter caps are still good. I have seen D Series amplifiers in service 15 plus years old with the filters caps perfectly fine. Now there is a 10 volt supply to the op amps on these amplifiers and this is what I have seen the most to fail. These small electrolytic caps do dry out and open up causing a distortion on the output. This could be what is causing your problem, distrotion wise.

The right channel out could be something more specfic to that channel such as and output device or driver. The 10 volt supply is common to both channels. You can send it in for an estimate, our service dept. is $70 per/hr plus parts. If just the caps in the 10 volt supply, my guess is it would still be under $100 for repair. If it's that plus an output device then maybe around $100-$150.

Thanks,

Kip Whitehead
Crown Technical Support

Thanks Kip,
Mine has been trouble free since '76, I guess it's time for some attention.
Not to say anything bad about your service dept, but I will probably be putting my AAS in Electronics to work, here.
My point in asking for educated guess, is that I don't feel any speicial need to re- invent the wheel if someone has already been there, done that and there are enough of these beasts around, even now, that I know some one must have faced this a few times.  tongue.gif
I can see where electrolytics in the 10 V supply could sure mess up op amp function.
That would fit with the gradual onset of it's troubles as well. Slow leakage, getting worse.
It's funny how you come to take these things for granted, year after year, after year, until one day it stops being perfect!  sad.gif
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#4 Wilder Bill

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Posted 16 October 2005 - 09:38 AM

OK, I found a shorted driver on the dead channel. While I was at it, I replaced the UA 739 and the three small electrolytics.
The dead channel works like new, now. Cost - less than $20.  biggrin.gif
The other one still distorts, kinda raspy and a little weak.  sad.gif

I think this amp is a 76 model, with original filters! I agree, they must be still working or I'd have at least a lot of hum.

I did talk to a guy that ran sound systems for concerts back in the late 70s and early 80s. He hates DC 300s. He said that at every show at least one of the dozens they used would die and put DC on the speakers, smoking the voice coils.
Maybe an open driver?
His 'cure' was to toss the offending amp in the river on the way home.
Hey, he coulda given 'em to me!  tongue.gif

I'm wondering if it would be practical to add large value, 100V (or more) caps at the outputs to block DC, in case that ever happens to me.
I know it would be a high pass filter, say 20Hz and up, but that's OK, as long as it isn't able to pass DC.
I know it can happen, because I had a Peavey amp fry a great Altec-Lansing 421, the same way.   blink.gif

Any ideas on the distortion?
I will check for bad solder joints (I can see that someone once worked on that channel).




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