OldSynths

I would be pretty sure that the regulator would be the standard 723 adjustable type. ARP used this on a lot of their synths and used the same circuit configuration. If you pull up the 723 pin-out you should be able to compare with the schematic. If only the negative rail is bad then it’s likely that the associated op-amp (probably a 301!) is what needs replacing.  

I would also take the opportunity to replace any tantalum polarized caps in the regulator circuit – use electrolytic types or heavily de-rated new tantalums

mw

-----Original Message-----
From: Bob Weigel [mailto:sounddoctorin@...]
Sent: Saturday, November 01, 2003 12:35 PM
To: oldsynths@yahoogroups.com
Subject: Re: [oldsynths] Re: ARP Pro Soloist problems, part 2

Hmm.  Not likely.  It's probably the regulator.  The trim pot could have
some effect in certain types of circuits even though the regulator is blown.
If you know what the voltage is supposed to be, I think you said -15V...you
could just replace with a 7915 while cutting  out all related parts of that
circuit except the input capacitance and an output capacitance of at least
a.1uF to kill oscillation.  Usually those circuits use a local .1cap and
then nearer the components being driven put a 10uF electrolytic or like
that.  Anyway, your other regulator MAY have associated components like a
resistance and maybe a diode in a leg that goes to ground.  You direct
ground the 7915.  Remember it's a negative regulator so put the negative of
any cap towards it's output rather than ground!
  Anyway working renditions of that number through Sam's software, I get no
cross ref.  It's some oem number it sounds like.  Nothing I've ever seen.
78xx and 79xx chips are rated for a little over an amp usually if they have
a great heat sink.  So usually one has plenty of current capacity to feed 50
chips or like that doing 10 to 20ma each. -Bob

----- Original Message -----
From: "dshoemaker93" <dgsaa@...>
To: <oldsynths@yahoogroups.com>
Sent: Saturday, November 01, 2003 10:54 AM
Subject: [oldsynths] Re: ARP Pro Soloist problems, part 2


> Hi,
>
> Thanks for the tips, Bob.   Concerning the regulator--which I'm assuming I
need to
> replace--I've been trying to find a modern replacement--is there a easy
way to do
> that?  It's listed in the schematic as a Fairchild U5R 7723393, and it's
an IC.  I can't
> find that part number at Digikey or NTE or even the Fairchild website.
Any tips?
>
> Also, the trim pot for the negative voltage only trims it down to
about -18.5.  Does
> this mean it could be the *trimpot* that's blown and not the regulator
itself?
>
> --David
>
>
> --- In oldsynths@yahoogroups.com, "Bob Weigel" <sounddoctorin@m...> wrote:
>  >     Yow, I'd double check that -19V!  BE CAREFUL THOUGH!  Some of those
> > chips may be rated for 15V...I can't recall...surely a 301 isn't rated
for
> > more than 18V I don't think.  Pretty old chip.  My farfisa organ got a
> > couple tos chips knocked out and I had to buy retro modules for it which
> > were expensive...due to a bad regulator.  But before you turn it on get
> > ready to turn the powersupply adjustment and slowly turn it to make sure
you
> > don't go the wrong way immediately on turn on and get that set right.
> >
>
>
>
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