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dc300a guru needed


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#16 joust

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Posted 18 March 2013 - 02:55 PM

Hey cronie, I've come once on a 'similar' problem, where the output components isolators were made of mica, and with time, started to be conductive! Solution: replace all micas with silicon or plastic isolators. The mica isolator has a small tab at one of its ends, looks greyish and metallic.
Do you see the described noise at the output of the main IC?
How about the hi-volt biasing circuit? There's a HV cap that is usually very problematic...

Alain

#17 cronie

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Posted 19 March 2013 - 02:00 PM

I just had all ten transistors out of the ch2 board.Plus two transistors out of the good side to compare to. They are the mica ones with the tab. I wiil change the isolators.  Channel one would also have distortion on it on startup but would go away after about two minutes. A year later the same happened to ch 2 but would not go away. So far I have swapped the large beer can power cap and retightened the screws to see if it helped- same symptoms. Then I changed the 4 nonpolar timing caps and two other white vertical caps on the main board.  Its funny when i was doing the ohm tests on the transistors i was wondering if the dow compound was conducting. It was smeared on pretty heavy. There is one more large yellow cap on the main board is this the hv cap?  I will replace it also.

#18 karlos the dc300a

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Posted 20 March 2013 - 03:22 PM

Hows it going?

#19 cronie

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Posted 21 March 2013 - 06:30 AM

carlos or joust--is the2n3773 a drop in replacement for th rca 62085 which i have now or do I need to to replace all eight with the same number. Two of the rca 62085 transistor read a little different out of circuit and I would like to replace them. The 2n3773 is readily available. Thanks Cronie

#20 joust

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Posted 21 March 2013 - 09:39 AM

Crown has used the 3773 in the past. It seems like a good replacement for the outputs. Perhaps Carlos can concur.
When you say that two transistors read a little funny, what is the difference?

#21 karlos the dc300a

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Posted 21 March 2013 - 01:02 PM

To be honest im not sure about the 62085.

2n3773 are usually ok, the trouble these days are finding decent original genuine transistors, there are so many fakes about.

use a reputable supplier farnell rs comps etc etc

#22 cronie

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Posted 21 March 2013 - 04:58 PM

View Postjoust, on 21 March 2013 - 09:39 AM, said:

Crown has used the 3773 in the past. It seems like a good replacement for the outputs. Perhaps Carlos can concur.
When you say that two transistors read a little funny, what is the difference?
    First , I have a hewlett- Packard 412A analog Vacuum tube  volt ohmmeter which i love and is extremely accurate. I pulled an output transistor from the good channel and it read like the crown manual said it should read. I pulled all eight trans from the bad side and six read identical to the known good transistor.  One of the others read  .4 eb  and it should have read  "large" like the others.The last  transistor read 10ce and should have read "large".like the others. Im not sure what scale I had it on but It  was very sensitive to the touch. For comparative purposes the scale shouldn't matter. Each transistor was measured twice each leg-each polarity.  These 2 transistors may be ok but i would rather have eight that read the same.  Thanks for listening-Im still new to transistors and am trying to learn as i go.   I have new isolators ordered.  Cronie

#23 vinyvamos

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Posted 31 July 2013 - 11:02 AM

Hi Cronie,

I am currently going through a similar repair process and was interested when I found this thread. I am not sure what is blown in mine yet but I cannot seem to source "genuine" 2n5804 transistors. There is only one seller showing on fleabay and he has unbranded parts. I may not need them yet but I am trying to find a source/alternative for all transistors in the amp. Could you please tell me where you sourced your 2n5804 parts?

I am planning to go with 2N3773 parts for the outputs should I need any replacing. Still can't find an equivalent for the pos. pre-driver RCA61061 though. Any thoughts? For the neg. pre-driver I will use 2n3637, available from Farnell.

Thanks!

Vinny