Vintage Synth Repair

Thanks John,

I don't have your regular address so I'm posting back
to the list...

I tried to replace both D4 and Q2 (with substitutes)
and got 0 voltage on the 5v rail, if I remember
correctly (I might be wrong but either the 5v or 7v
rails were 0). Not to cause any further damage I put
the original back... Should I replace only D4 and try
again ? Are there any parts which may cause it ? 

I just fixed the display problem I had on that unit
and it became that the bad component was a coil which
acts as FIP (fluorescent display) driver. I wonder if
that was caused by the bad unregulated voltage... if
so, I will have to fix that otherwise the new coil
will go bad as well...




--- John Brewer <member@...> wrote:

> Hi, I am replying off list from a host computer via
> web mail.  Please continue to reply to my usual
> address.
> If you have 11v Ac. on D4 then I would expect what
> is marked as 7 Volts to be nearer 18 volts or more
> because of the C11 charging to peak. Something very
> strange indeed is happening!!  Remove D4 completely
> then measure the 7 volt line again. It should be
> zero.  Have you checked the individual diodes in D4
> and is it correctly inserted in the board.
> If the 5 Volts is OK, then Q2 is ok and not feeding
> back positive voltage to the unregulated line.  The
> mistery deepens. 
> Best regards, John.
> 
> 
> 
> 
> 
> 
> 
> ========================================
> Message Received: Aug 22 2006, 06:22 PM
> From: "gil_we" 
> To: vintagesynthrepair@yahoogroups.com
> Cc: 
> Subject: [vintagesynthrepair] Re: What causes bad
> unregulated voltage on the power supply ?
> 
> Thank you ;)
> 
> Well here aew the results -
> 
> 1. 5v, 15v anb -15v are all ok.
> 
> 2. D4 does not show an "open" connectivity between
> its legs.
> 
> 3. ~ to ~ on D4 shows 11v AC...
> 
> So you think that Q2 went bad, cusing the +7v
> unregulated voltage to 
> become 11v (DC) ?
> 
> I just replaced a bad FIP display coil driver on
> that unit,
> and wondering if the bad unregulated voltage is what
> made it fail in 
> the first place....
> 
> 
> --- In vintagesynthrepair@yahoogroups.com, "John
> Brewer" 
> wrote:
> >
> > First of all, the circuit is a bit unusual in as
> much as the feed 
> for the 5v
> > regulator circuit is taken
> > from the +15v rail. This is ok to a point but if
> Q2 fails, 15 
> volts could
> > and will feed back through Q1. This would only be
> limited by R2 
> giving about
> > 11 volts on the 7 volt rail. You could try
> removing Q2 as a start 
> to see
> > what happens.
> > OK, Can you do a few things to track the problem
> down.
> > 1) Is the 5 Volts correct? Is the +15 and -15
> correct?
> > 2) have you checked D4.
> > 3) What is the AC voltage between ~ and ~?
> > 
> > Ignore for the moment the 'AC' reading you are
> getting. This could 
> be a red
> > herring depending on the model test meter you are
> using.
> > Best regards.
> > John. (U.K., where are you?)
> > 
> > ----- Original Message -----
> > From: "gil_we" 
> > To: 
> > Sent: Sunday, August 20, 2006 12:22 AM
> > Subject: [vintagesynthrepair] What causes bad
> unregulated voltage 
> on the
> > power supply ?
> > 
> > 
> > > I got a Roland MKS70 of which the Power Supply
> unregulated 
> voltage rail
> > > is too high. The schematics says there should be
> +7v but I read 11
> V
> > > when the DMM is set to DC and 28v when set to AC
> !
> > >
> > > This fits with the fact that the display of that
> unit is 
> overbright,
> > > showing all characters at once....
> > >
> > > But what can cause the unregulated voltage to be
> incorrect ?
> > > could this be bad regulators in the PS unit ??
> > >
> > > I already replaced all capacitors in the PS.
> > >
> > > Thanks all !!
> > >
> > >
> > >
> > >
> > >
> > >
> > > Yahoo! Groups Links
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > > --
> > > Internal Virus Database is out-of-date.
> > > Checked by AVG Free Edition.
> > > Version: 7.1.394 / Virus Database: 268.10.7/409
> - Release Date: 
> 04/08/06
> > >
> > >
> >
> 
> 
> 
> 
> 
> 
> 
> 
> Yahoo! Groups Links
> 
> 
> vintagesynthrepair-unsubscribe@yahoogroups.com
> 


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