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Amcron DC300a overhaul.

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jbl_man View Drop Down
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    Posted: 24 April 2020 at 4:47pm
Good evening guys,hope everyone is keeping well,and safe. Smile

Whilst sitting here stuck at home, took the opportunity to overhaul this lovely old amp....

Crown DC300a from 1973/74.  Bear in mind this is 47 years old.

I'm getting 0.01 and 0.02v DC  both channels at output, so all ok there, but sound is VERY weak and muffled (max output is about half a watt,on both channels.) Output PCB rails read fine, both channels giving + - 51v. OK. All output transistors tested,all fine. All preamp board diodes and transistors test fine.....i'm getting the correct + - 9.5v power supply at the ic pins....must be something on that main centre preamp PCB, as is affecting both channels.....any ideas? As i'm a bit stuck now.

Ps,i have to say,this is beautifully made,lots of nice shiny copper on the PCB's, really nicely built back then.









Edited by jbl_man - 24 April 2020 at 7:13pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 24 April 2020 at 4:48pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Pinyorouk Quote  Post ReplyReply Direct Link To This Post Posted: 24 April 2020 at 10:34pm
Maybe you need to use an oscilloscope and signal generator to check signals at different points.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 24 April 2020 at 11:08pm
interesting it's both channels. Dodgy IC socket?
Old silver mica capacitors as used by crown suffer from what the yanks call 'silver mica disease'. They can go leaky and eventually short. Might be worth checking. Not something generally used in european amps.
Kevin

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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 25 April 2020 at 11:39am
Cleaned those four presets, now have 0.00v DC at output both channels, but output very weak and distorted. Fitted new ua739 ic, still the same.  Geek
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 25 April 2020 at 12:31pm
Originally posted by Pinyorouk Pinyorouk wrote:

Maybe you need to use an oscilloscope and signal generator to check signals at different points.


+1

The only way to really pin down the problem.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote APW Quote  Post ReplyReply Direct Link To This Post Posted: 25 April 2020 at 2:15pm

Check C4 and C5 (the two caps closest to the Red Switch) as they are part of a diode pump (in conjunction with D1,D2, R1,R2 and D3) that generates about 70volts that feeds both channels driver circuit. Check you have about 100volts with respect to ground on one end of the 2k2 resistor R2…. R2 should have about 30Volts across it.




Edited by APW - 25 April 2020 at 2:24pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 25 April 2020 at 2:33pm
Thanks APW,will have a look.....i did find a couple of resistors way-out of value, (r101 and r201) on the input offset on both channels, should be 2 meg, one was reading 780k,and other 540k,that cant be helping, have ordered some new ones. Will check what you suggest when i have fitted these. 


Edited by jbl_man - 25 April 2020 at 3:50pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote madboffin Quote  Post ReplyReply Direct Link To This Post Posted: 25 April 2020 at 5:30pm
On anything like this, I would change all the electrolytic capacitors on the circuit board. 50 years is well over their expected service life and they will all have gone leaky or acquired a high ESR (equivalent series resistance). Their service life depends (inversely) on temperature, so it's best to replace them with 105 degree rated types instead of the standard 80 degree version.

I'm surprised that you have found resistors gone low in value - carbon types normally drift high as they age.

I must get around to servicing my vintage Turners...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 25 April 2020 at 5:35pm
Yup...have just changed the 10uf 150v, that Alan suggested above as was physically bulging and leaking. Will do,have ordered a batch of of other ones too. These old things are so nice to work on, lots of room around the PCB tracks, simple to solder, not everything crammed in like the modern stuff,with a hairs-width between the tracks. One slip with the iron,and it's scrap. Yuk.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GAZ. Quote  Post ReplyReply Direct Link To This Post Posted: 25 April 2020 at 11:29pm
And I’m sure with a few new components this amp will live on for another 50 years.
I can’t see any of these new amps surviving that long.
Any evidence of previous repairs or does it all seem original?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 26 April 2020 at 1:30pm
All completely original Gaz, original Crown (RCA) output devices, only repairs i can see is new front volume pots, not suprising after nearly 50 years. There are 3 service stickers on the back,dating from the 80's,90's and 05's so i guess someone has cared for it regually.

 Looking at the mechanical construction and attention to detail, i can see why these were twice or three times the price of anything else back then.

Yes likewise, love to see a Powersoft or FFA or MC2 after fifty years. Wont be much that can be repaired i bet! LOL
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