Vintage Synth Repair

Please note that the wall-connected 120VAC power supply is dangerous to test.  Have you mapped a schematic of the power supply circuit (with the power off)?  Do you know which points on the circuit board have life-threatening voltages and which have benign voltages levels?  Do you know which capacitors will store large charges after the power has been turned off?

  The DX21 will run on relatively small currents, less than one amp for +5volts and about 50 millamps of +/- 15 volts.  These voltages and currents can be produced by a couple of wall-warts (or an old PC power supply).  

 Separate the DX21 power supply from the rest of the DX21.  Cut traces on the PCB with a Dremel tool if necessary (you can always reconnect them with a wire later).  Get a wall-wart power adapter that is +9-12 volts and about one amp.  Get a 7805 voltage regulator IC and add a piece of metal to it to be a heat sink.  Verify that the 7805 puts out +5V.  Use the +9-12V 'raw' power for the +15V source.   Connect these two to the DX21 and check that it works (even partially).  If so then the digital/analog circuitry is OK and only the power supply is affected.  Probably one of the big electrolytic caps is burned out.

  If the DX21 is working with the wall-wart then you can either use the wall-wart as a power supply for the unit or repair the original DX21 power supply (if possible).  I would recommend getting a bigger wall-wart adapter and just mounting it in the DX21 case.  Use a 7660 -type chip to invert the +12 - +15 to -12 volts.  The +/- 12_15 volts is only used for the output op-amps.  Everything else is done by +5V.  Be sure to use voltage regulators on the +12 voltage for op-amps.  This voltage doesn't have to be +12, it can be as low as +9 or so.  But it must not have any ripple AC voltage on it or this ripple will show up as hum on the audio outputs.

  Try not to be obsessive about repairing any power supply.  The DX21 is not a vintage collectable synth that has to kept 'all original parts' to retain its value.  Plus power supplies are, again, dangerous, complex, and under-documented.  They are also cheap and easy to exchange as a unit.

  Good luck.


--- On Sun, 8/1/10, Mark <casiotone1331@...> wrote:

> From: Mark <casiotone1331@...>
> Subject: [vintagesynthrepair] Re: Yamaha DX21 will not power up
> To: vintagesynthrepair@yahoogroups.com
> Date: Sunday, August 1, 2010, 8:46 PM
> Yeah, the fuse was the first thing I
> looked at. I'm now starting to think it may be a bad diode
> or resistor or something. Could the transformers be the
> culprit? If so I can't find them anywhere on the net. They
> are marked TM205 and 3021A. 
> 
> --- In vintagesynthrepair@yahoogroups.com,
> "Quazimodo" <noddyspuncture@...> wrote:
> >
> > You checked the main (mains) fuse - either on the
> machine or in the plug...!?
> > 
> > 
> > 
> > --- In vintagesynthrepair@yahoogroups.com,
> "Mark" <casiotone1331@> wrote:
> > >
> > > OK, I just plugged in the supply on my workbench
> and tested the pins. I also checked and tested all of the
> traces and found no cold joints or cracks in the PCB. Just
> to be safe I reflowed all of the joints and cleaned up the
> board to no avail. Anyway, these are the voltages I should
> be getting, and what I AM getting. There is literally NO
> power leaving the PSU.
> > > 
> > > Pin 1+2 should be 5V, I get 0V
> > > Pin 2+3 are ground
> > > Pin 5 is 0V
> > > Pin 6 should be +15V, I get 0V
> > > 
> > > What should I do next? I'm not too used to
> working on power supplies, so any direction is a good
> direction.
> > > 
> > > Thanks!
> > > Mark
> > > 
> > > --- In vintagesynthrepair@yahoogroups.com,
> Alan Probandt <alan_probandt@> wrote:
> > > >
> > > > I would assume that it is a problem in the
> power supply.  I suggest getting a cheap used PC power
> supply and bypassing the DX21 supply just for
> testing.   I got a recent Dell supply from
> one of their mini DeskTops for $5.  It's only 15"x5"x2"
> in size.  It puts out +5, + and - 12V, +3.3v
> regulated.  For a PC supply it's unusual in that it
> works without any minimum load.  Usually PC power
> supplies need about 0.5 amp load on their +5V line in order
> to operate correctly.
> > > > 
> > > >   There is a DX100/DX27
> schematic floating around and possibly here in the files
> section.  It shouldn't be that much different from the
> DX21.
> > > > 
> > > > --- On Sat, 7/31/10, Doug
> <dougslocum@> wrote:
> > > > 
> > > > > From: Doug <dougslocum@>
> > > > > Subject: [vintagesynthrepair] Re:
> Yamaha DX21 will not power up
> > > > > To: vintagesynthrepair@yahoogroups.com
> > > > > Date: Saturday, July 31, 2010, 6:42 AM
> > > > > Mark,
> > > > > 
> > > > > Rhetoric: Troubleshooting anything
> without seeing it is
> > > > > always difficult, at best.  :-)
> > > > > 
> > > > > That said, I'm not familiar with the
> DX21 specifically, but
> > > > > I would be looking for voltage
> measurements and values from
> > > > > a oScope to determine if it is acually
> the power supply that
> > > > > is at fault here.
> > > > > 
> > > > > I wouldn't be too quick to judge the
> power supply, just
> > > > > because it's the easier thing to
> troubleshoot.
> > > > > 
> > > > > Doug
> > > > > 
> > > > > --- In vintagesynthrepair@yahoogroups.com,
> > > > > "Mark" <casiotone1331@> wrote:
> > > > > >
> > > > > > I recently scored a non-working
> DX21 on the cheap, and
> > > > > it will not power up. No lights,
> response, or life
> > > > > whatsoever is in this machine. I don't
> even hear the audible
> > > > > "click" when I turn the machine on. I
> tested the power cord,
> > > > > the switch, the fuse, and replaced most
> of the capacitors on
> > > > > the main PSU board, and it still is
> doing the same thing.
> > > > > > 
> > > > > > What worries me is that a previous
> owner replaced the
> > > > > big black cap in C5 with a 100uf 50V
> capacitor, and I know
> > > > > that it is MUCH too low of a voltage to
> be there. I'm not
> > > > > even sure if the capacitor is even the
> right value... I'm
> > > > > used to seeing things like 2200uf and
> such in power supplies
> > > > > like this one. If anyone could point me
> to the proper
> > > > > replacement, that would be great.
> > > > > > 
> > > > > > Anyway, before I start to go nuts
> tearing this thing
> > > > > apart, I was wondering if anyone here
> could possibly help me
> > > > > out, I had great success bringing a
> dead TX81Z back to life
> > > > > thanks to the collective minds on this
> group. Where should I
> > > > > start? Many thanks!
> > > > > >
> > > > > 
> > > > > 
> > > > > 
> > > > > 
> > > > > ------------------------------------
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> > > > > Yahoo! Groups Links
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> > > > >     vintagesynthrepair-fullfeatured@yahoogroups.com
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> > > > >
> > > >
> > >
> >
> 
> 
> 
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