Amcron DC300a overhaul. |
Post Reply | Page 123 4> |
Author | |||
jbl_man
Moderator Group Joined: 12 January 2005 Location: London. Status: Offline Points: 11155 |
Post Options
Thanks(0)
Posted: 24 April 2020 at 4:47pm |
||
Good evening guys,hope everyone is keeping well,and safe.
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 |
|||
Be seeing you.
|
|||
jbl_man
Moderator Group Joined: 12 January 2005 Location: London. Status: Offline Points: 11155 |
Post Options
Thanks(0)
|
||
|
|||
Be seeing you.
|
|||
Pinyorouk
Young Croc Joined: 31 January 2011 Status: Offline Points: 551 |
Post Options
Thanks(0)
|
||
Maybe you need to use an oscilloscope and signal generator to check signals at different points.
|
|||
kedwardsleisure
Old Croc Joined: 20 January 2009 Location: Staffordshire Status: Offline Points: 4949 |
Post Options
Thanks(0)
|
||
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
North Staffordshire |
|||
jbl_man
Moderator Group Joined: 12 January 2005 Location: London. Status: Offline Points: 11155 |
Post Options
Thanks(0)
|
||
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.
|
|||
Be seeing you.
|
|||
Earplug
Old Croc Joined: 03 January 2012 Location: Europe Status: Offline Points: 7216 |
Post Options
Thanks(0)
|
||
+1 The only way to really pin down the problem. |
|||
Earplugs Are For Wimps!
|
|||
APW
Young Croc Joined: 13 November 2012 Location: Kent, UK Status: Offline Points: 1174 |
Post Options
Thanks(0)
|
||
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 |
|||
jbl_man
Moderator Group Joined: 12 January 2005 Location: London. Status: Offline Points: 11155 |
Post Options
Thanks(0)
|
||
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 |
|||
Be seeing you.
|
|||
madboffin
Old Croc Joined: 03 July 2009 Location: Milton Keynes Status: Offline Points: 1539 |
Post Options
Thanks(0)
|
||
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... |
|||
jbl_man
Moderator Group Joined: 12 January 2005 Location: London. Status: Offline Points: 11155 |
Post Options
Thanks(0)
|
||
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.
|
|||
Be seeing you.
|
|||
GAZ.
Old Croc Joined: 16 January 2007 Location: Hertfordshire Status: Offline Points: 2433 |
Post Options
Thanks(0)
|
||
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? |
|||
100% Earth Moving Bass
|
|||
jbl_man
Moderator Group Joined: 12 January 2005 Location: London. Status: Offline Points: 11155 |
Post Options
Thanks(0)
|
||
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! |
|||
Be seeing you.
|
|||
Post Reply | Page 123 4> |
Tweet |
Forum Jump | Forum Permissions You cannot post new topics in this forum You cannot reply to topics in this forum You cannot delete your posts in this forum You cannot edit your posts in this forum You cannot create polls in this forum You cannot vote in polls in this forum |