[sdiy] troubleshooting [what should be] a simple problem...

[Merlin Zener]

Hi there,

I have an Ensoniq MR-rack; it's a nice unit built in 1995,
so it's reasonably modern in design and build [lots of
surface mount]. Everything is pretty much on the one big
board aside from power supply and front panel [LCD, two
rotary encoders, Vol ctrl, H/phone skt and some buttons]
and it's built really well - quite drop proof, strong
metal case etc.
Only trouble is, Ensoniq is out of business now, and parts
and comics etc will be hard if not impossible to get...

I'm an ex-technician - I had a business back in the 90s,
but for the last few years I've just been playing. I used
to be on this list then, but I've only recently re-subscribed
and been lurking a while.

ANYWAY... <gr>

So one day the outputs of the MR-rack stopped working. No
reason, the unit hadn't been dropped or moved or anything,
just turned off overnight and turned on again, and no sound
from the MR. But get this: the headphone socket still has
sound!!!
I've been thinking this through - clearly this unit is not
using the old 
D/A >>> opamp >>> outputs >>> H/phone amp >>> H/phone skt
signal chain - otherwise there would be no sound at the
headphone.

I've had a bit of poke around but being a double sided board
and lots of SM and no comic, I'm not able to ascertain much.

One thing: the outputs on the back panel are balanced, so there
are a couple of other possibilities:
D/A >>> opamp >>> outputs 
    >>> H/phone amp >>> H/phone skt

or even:
D/A >>> VCA [from Vol ctrl] >>> opamp >>> outputs 
                            >>> H/phone amp >>> H/phone skt

The other thing I was thinking of is maybe there are some
FETs across the outputs for muting [I remember some Rane
equipment around the same vintage had that idea] and maybe
they've gone short or are being switched on by leaking
voltage somehow. Not that there's any signs of moisture or
cracks etc - the unit looks like new inside.

There are a bunch of 3 legged SM devices with "CR15", "CR16"
etc on the screenprint; they have "SD" or "5D" on them.
The only other transistors in the general area, well the
print is really small and hard to make out, but I think
it's something like P10, with 59 at right angle. I'm assuming
transistors - the screen says "Q12", "Q11" etc.

Anyone got any ideas?
A circuit or block diagram maybe?
Or a contact or website etc for someone doing Ensoniq repairs
and parts these days?

TIA for any help... :)


--
Merlin Zener
Piano, Synthesizer
Pattaya Thailand
http://www.merlinzener.com
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