Amcron DC300a overhaul. |
Post Reply | Page 123 4> |
Author | |
boominbob
Registered User Joined: 19 July 2011 Location: uk Status: Offline Points: 61 |
Post Options
Thanks(0)
Posted: 26 April 2022 at 9:38pm |
Hi guys.
There are 7 different main pcbs on dc300a / dc300a mk2 With overlap between mk1 and mk2 5 different versions on mk1 3 different versions on mk2 Early mk2 had mk1 pcb. Essentially early mk2 was a mk1 turned upside down with different mains switch and relocated level pots. Apparently mainboard was redesigned due to the main 739 ic going out of production. Mk2 had drain resistor across one smoothing cap. Drain resistors across pre drivers and a solid state switch across main ic supply. All these mods were to stop farting and whistling at switch off. Always follow number on underside of pcb Spent a lot of my life working on them. Good luck.
Edited by boominbob - 26 April 2022 at 9:46pm |
|
kedwardsleisure
Old Croc Joined: 20 January 2009 Location: Staffordshire Status: Offline Points: 4947 |
Post Options
Thanks(0)
|
Possibly the result of a long production run. Components become obsolete or expensive, better parts become available, feedback from the customer, new design staff, all conspires to change the design over the years. It's the same with the macrotechs. There are many circuit revisions and one bears little resemblance to another.
|
|
Kevin
North Staffordshire |
|
jbl_man
Moderator Group Joined: 12 January 2005 Location: London. Status: Offline Points: 11155 |
Post Options
Thanks(0)
|
Yes,this exactly. There appears to be three version out there,unfortuantly none relate precisely to my amp here,so was a lot of guesswork involved filling in the blanks. |
|
Be seeing you.
|
|
jbl_man
Moderator Group Joined: 12 January 2005 Location: London. Status: Offline Points: 11155 |
Post Options
Thanks(0)
|
Thank you gents. I will not be beaten by a 47 year old amplifier! Of course,it doesn't help that the official schematic is wrong....the
voltages they give,and some of the components dont match at
all,including at least half a dozen resistors,transistors and capacitors
that simple dont exist on my board. I guess they modified these on a
yearly basis,and no-one bothered to update the schematics.
I even emailed Crown direct,and asked if they had the correct schematic for my amp, the reply, "Thank you for your enquiry, we do not release schematics,these are only available to our official service centres" suggesting i take the amp there! As if anyone is going to do that with something this old. They even had the cheek to send me one of those customer satisfaction surveys afterwards.....so that will be Zero, zero,zero,zero,zero and zero then. In the slim chance of anyone from Crown ever reading this,dont be so bloody stupid,it's not as if anyone is going to steal the design of this thing after all this time. |
|
Be seeing you.
|
|
odc04r
Old Croc Joined: 12 July 2006 Location: Sarfampton Status: Offline Points: 5482 |
Post Options
Thanks(0)
|
More than likely! Or the schematic that made it into the net archives was just one of many in the original library given out at one point.
|
|
kedwardsleisure
Old Croc Joined: 20 January 2009 Location: Staffordshire Status: Offline Points: 4947 |
Post Options
Thanks(0)
|
great result. Always a relief when you fix that elusive fault and hopefully a satisfying sense of acheivement
|
|
Kevin
North Staffordshire |
|
APW
Young Croc Joined: 13 November 2012 Location: Kent, UK Status: Offline Points: 1174 |
Post Options
Thanks(0)
|
As Kevin pointed out on the first page of this thread old silver mica caps can suffer from "silver mica disease". I doubt it was the issue in this case but in something this old I'd change them as a precaution.
Edited by APW - 13 July 2020 at 5:15pm |
|
djeddie
Old Croc Joined: 26 April 2004 Location: Bristol Status: Offline Points: 4125 |
Post Options
Thanks(0)
|
Bravo Sir!
|
|
Chas n Dave : it's like Drum and Bass but with beards. E=mc² ±3dB
|
|
4D
Old Croc Joined: 13 November 2008 Location: Winchester Status: Offline Points: 4257 |
Post Options
Thanks(0)
|
Nice one Ian persistence pays off...
|
|
DMZ. "The bass was intense. Girls were literally running up to stand next to the subs"
|
|
Earplug
Old Croc Joined: 03 January 2012 Location: Europe Status: Offline Points: 7216 |
Post Options
Thanks(0)
|
Nice one. Glad to hear that you got it sorted.
|
|
Earplugs Are For Wimps!
|
|
madboffin
Old Croc Joined: 03 July 2009 Location: Milton Keynes Status: Offline Points: 1538 |
Post Options
Thanks(0)
|
Well done, good to know it's working again. As for the economics, a restoration like this is really a form of entertainment, or a hobby, and we can expect it to cost us something!
For future information, I'll be very surprised if all those brown capacitors really needed replacing. They are silver-micas and are usually very reliable. And they often have a "burnt" look about them, it's just the brown encapsulation. Carbon composition resistors are always suspect, especially high value types (100k upwards). They tend to drift high in resistance over the years (even in storage) and should all be checked and, if necessary, replaced with modern film types. |
|
jbl_man
Moderator Group Joined: 12 January 2005 Location: London. Status: Offline Points: 11155 |
Post Options
Thanks(0)
|
And the parts tally...
|
|
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 |