"Eeyore" wrote in message
...


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


Well, if that's the case, it ain't gonna get done, as he only sold it a
couple of weeks ago as a cheapy second hand job. As a matter of interest,
does the circuit employ feedback to the front end then ? I wasn't expecting
this to be an output issue, as the waveform seems to distort quite early in
the chain, but now you suggest this, I guess it makes some sense of the way
that the amplitude of the waveform 'bounces' a couple of times before
settling, when the level is adjusted, externally or front panel.

Arfa