"Meat Plow" wrote in message
...
On Tue, 25 Nov 2008 06:00:02 +0000, Eeyore
wrote:



Arfa Daily wrote:

"Eeyore" wrote in message
Arfa Daily wrote:

Does anyone have a set of schematics for the C-Audio RA3001 that they
could let me have fairly urgently please ?

It's not a 'head', it's a 19" rack mounting pro power amp with line
level
inputs. What do you want to know btw ? They are notoriously
unreliable.
First job is to replace all the PSU caps that have been cooked by the
hot
exhaust air. Then check how many mosfets are open.

I know an awful lot about it, having rejected it as an ODM product for
Studiomaster. I think it was that model.

The links to the service manual on the supposed C-Audio 'website',
just
seem to take you round in ever decreasing circles via other third
party
vendors of everthing you can imagine.

Harman UK bought them. And I could tell you a story about that too !
Ask
there. Except it's not Harman any more it seems but google that for
starters.
They've moved to Letchworth. I think most of the C-Audio parts ended
up
shuffled
into a corner (I could tell another story) so they might be willing to
get
rid of
some unwanted and unloved bits. Unless they already put them in a skip
(dumpster for USAans)

The guy who owns the shop where all this stuff comes from, and who is a
musician himself with 'many years in', tends to call anything that's
rack
mount, line input and 'power', a "head", as opposed to an "amp" for an
instrument level standalone unit like a Marshall, or a "combo" for the
same
thing with speakers built in like an AC30 or Fender Twin. I just follow
his
lead, and call them the same. What do you describe as a "head"?

A 'head' has preamp level inputs. A guitar head has sensitive Hi-Z inputs
for
guitars, a 'PA' head has mic inputs. The old name for what an RA3001 type
amp is
a 'slave amp'.


Anyways, as to what's wrong with it. The "A" channel has an *extremely*
asymmetric output.

Typical C-Audio !


On a sine wave, looks like the output from a half wave
reccy with no filter cap on the end. It also 'bounces' in amplitude when
the
input level, or front panel level, is varied. The problem is right up
the
front end somewhere. There are pukka sine waves around the TL074 input
amp,
but soon after, the signal has taken on this asymmetry, which is then
faithfully amplified by the output stage. The "B" channel works
perfectly,
so I guess I can start comparing between the two. Just would have been a
lot
easier with a set of schematics to see where exactly the signal goes.
It's
got that nasty 'feel' about it, that says it's going to be something
like an
o/c resistor, which looks perfect to the naked eye ...

I see a couple of bunches of four TO92 transistors, so I think that may
be a
good place to start looking.

Unlikely.

Do you know how to test Hitachi lateral mosfet output devices ? Not much
different from any mosfet except they made true complements.

Remove them all. I expect they're TO-3 so easy to get them out and test.
You
will find a fair few are totally open. Replace these and you're good to go
but
DO replace the reservoir caps too. They will have been roasted by the
searingly
hot air passing over the heatsink. Don't even think of using 85C types.

One of the worst laid out designs I've ever seen in my entire life.

In tests I measured 150C *on the HEATSINK*. God knows what the junction
temp
was. Your trouble will be getting genuine Hitachi replacements as they
stopped
making them and the Semelab/Magnatec and Exicon replacements aren't
exactly
identical.. I may have a few but you WILL pay megabucks for them.

Graham

Ran into the Hitachi mosfet issue years ago after acquiring a Sound
Code Systems (SCS) rack mount power amp. Had the 2SJ49/2SK134 pairs
inside if I recall correctly. I replaced them with J50/K135 devices
which at the time were still available. Now adays there are
replacements but with a little modification since they are not in a
TO3 package. I've had that amp now for 15 years after repair and
used it for 5 years as a bass guitar amp strapped mono. Now adays it
is the PA amp for my home studio. I've only seen one SCS amp and this
is it.



IIRC, the C-Audio RA series commonly used BUZ Mosfets, both T03 and T0247,
which are still easily available.

http://uk.farnell.com/jsp/search/bro...estid=48642 0



Gareth.