Help! Repairing Arcam Alpha II - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 22nd March 2011, 02:33 PM   #1
diyAudio Member
 
Join Date: Feb 2005
Location: S.England
Default Help! Repairing Arcam Alpha II

Hello!

I have an Arcam Alpha II Amp that has decided to give up the ghost.

BOTH channels are behaving strangely:

Observations:
1) If the preamp stage is connected to the power amp stage then the output transistors get instantly baking hot, (and if left for more than a few seconds will go *pop*).

2) Disconnecting the preamp stage then, no excessive current draw, but -32V appears across each speaker output.

Question is, how do I proceed fault-finding?

TIA,

Andy
  Reply With Quote
Old 22nd March 2011, 02:53 PM   #2
diyAudio Member
 
Join Date: Feb 2008
Location: Jackson,michigan
First Check fuses then the output devices for any shorted ones.
It is possible that there is no positve rail voltage.
Before you go any farther than that, do you have the schematic to post?
  Reply With Quote
Old 22nd March 2011, 03:14 PM   #3
diyAudio Member
 
Join Date: Feb 2005
Location: S.England
Fuses are fine. PSU rails are fine, devices (I believe) are fine. Will post up a schematic when I get into photobucket...
  Reply With Quote
Old 22nd March 2011, 03:58 PM   #4
jaycee is offline jaycee  United Kingdom
diyAudio Member
 
Join Date: Apr 2004
Location: Norwich, UK
If memory serves, Q6 or Q7 - I think Q7 - in the power amp circuit is next to the heatsink. This often fails. It is the current sink for the VAS transistor Q8 and it's failure to operate drives the outputs to one rail IIRC.

FST239/240 are obsolete, and Arcam replaced them with ZTX653/753.
  Reply With Quote
Old 22nd March 2011, 04:41 PM   #5
diyAudio Member
 
Join Date: Feb 2005
Location: S.England
Arcam Alpha II schematics:

Arcam Alpha II 1of2.jpg

Arcam Alpha II 2of2.jpg

OK, will look at those transistors! Thanks,

Andy
  Reply With Quote
Old 22nd March 2011, 04:44 PM   #6
diyAudio Member
 
Join Date: Feb 2005
Location: S.England
@jaycee, Looking at those tranny's next to the heatsink, they are labelled Q6 and Q106.

I guess they are part of a heat compensating circuit... I wonder why they are not therefore actually held onto the heatsinks instead of just being stood right next to the h/s, almost but not quite touching it.

Andy
  Reply With Quote
Old 22nd March 2011, 05:12 PM   #7
jaycee is offline jaycee  United Kingdom
diyAudio Member
 
Join Date: Apr 2004
Location: Norwich, UK
They are used for temperature compensation yes. Why Arcam didnt use the more conventional VBE multiplier arrangement I have no idea. The power amp circuit is the same as the A&R Cambridge A60 i believe.
  Reply With Quote
Old 22nd March 2011, 05:15 PM   #8
diyAudio Member
 
ostripper's Avatar
 
Join Date: May 2008
Location: Smoky Mountains , Tennessee
I can't believe arcam got away with this design. Using the VAS CCS as thermal comp. WOW ! The dual CCS's are cool , like my AX (3mA LTP / 12-14mA VAS current-arcam) but just one more device (a proper Vbe bolted to the heatsink) would go a long way, MTBF wise.
PS... It would work ,a 2Q ccs has NTC.
OS
__________________
Mongrel website , always current and updated :
http://www.fidelityforce.com/ostripper

Last edited by ostripper; 22nd March 2011 at 05:19 PM.
  Reply With Quote
Old 22nd March 2011, 05:48 PM   #9
jaycee is offline jaycee  United Kingdom
diyAudio Member
 
Join Date: Apr 2004
Location: Norwich, UK
ostripper, they got it from the A60 which was designed to a tight budget, in the very early days when everything was expensive Despite the crudeness they are usually quite reliable, and usually only fail when "cranked"
  Reply With Quote
Old 23rd March 2011, 03:45 PM   #10
diyAudio Member
 
Join Date: Feb 2005
Location: S.England
Weird, I've NEVER 'cranked' my amp. God know's why it popped and why BOTH channels exhibit the same failure mode.

Seen it all now!

OK, Q6 appears fine! (just buzzed it only though, lost my damn transistor tester).
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
arcam alpha II amplifier schematic underwurlde Solid State 3 30th January 2013 11:56 AM
ARCAM Alpha One weissi Digital Source 21 2nd October 2009 08:09 AM
Arcam Alpha Plus amc184 Digital Source 6 17th January 2008 07:01 PM
Arcam alpha 9 pumpkin Solid State 0 5th October 2005 07:31 PM
Arcam Alpha 5 + j baldam Digital Source 1 9th July 2003 11:20 PM


New To Site? Need Help?

All times are GMT. The time now is 02:56 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2