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-   -   Help! Repairing Arcam Alpha II (http://www.diyaudio.com/forums/solid-state/185566-help-repairing-arcam-alpha-ii.html)

underwurlde 22nd March 2011 01:33 PM

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

geraldfryjr 22nd March 2011 01:53 PM

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?

underwurlde 22nd March 2011 02:14 PM

Fuses are fine. PSU rails are fine, devices (I believe) are fine. Will post up a schematic when I get into photobucket... ;)

jaycee 22nd March 2011 02:58 PM

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.

underwurlde 22nd March 2011 03:41 PM

2 Attachment(s)
Arcam Alpha II schematics:

Attachment 215289

Attachment 215290

OK, will look at those transistors! Thanks,

Andy

underwurlde 22nd March 2011 03:44 PM

@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

jaycee 22nd March 2011 04:12 PM

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.

ostripper 22nd March 2011 04:15 PM

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. :eek:
PS... It would work ,a 2Q ccs has NTC.
OS

jaycee 22nd March 2011 04:48 PM

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"

underwurlde 23rd March 2011 02:45 PM

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).


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