Sunday, November 16, 2008

Saving Private Fizmo

As I posted back in September, I acquired an Ensoniq Fizmo that needed the factory voltage regulator replaced.  I finally found time to do the replacement a couple of weeks ago and the Fiz is up and running how.  Here's how it went:  

Fizmo on the workbench:  

To get to the CPU board, which contains the voltage regulator, you have to flip the synth over and take the sheet-metal bottom off.  (Rest the edges of the top on a couple of lengths of 2x4 wood, so that the weight of the synth isn't bearing on the knobs.)  Upon doing so, you are greeted with this:

There are six screws on top holding the CPU board in.  (In the above photo, all but one have been removed.)  There are also three that go through the back panel.  All of the rear panel jacks are mounted to the CPU board, so once it is loose, you have to slide it forward, tilt it out, and then very carefully slide it out from under the two grey ribbon cables.  There is one connector, just visible at the upper right, that needs to be disconnected.  Like so:

With the CPU board removed, you can see the panel board underneath.  The ribbon cables are going to a third board, underneath the metal shield.  I didn't remove the shield, so I don't know what that looks like.

A closer view of the CPU board.  The voltage regulator is at lower right:

Just for the heck of it, while I had it open, I took this shot of the underside of the pitch/mod wheels:

Here, I have already drilled out the pop rivet holding the regulator down, and removed the black heat sink out from under it.  The regulator's leads are still soldered in:

Here is the board with the regulator completely removed.  The silkscreened lines show where the regulator and the heat sink go:

A close-up of the culprit:

Here is the new one, soldered in, but without the heat sink installed yet.  The synth came from the buyer with the replacement regulator and decoupling caps in baggies; apparently he had gotten them from Ensoniq, but never got them installed since he didn't know how and there isn't a tech in this area.  The new regulator is an RoHS part and getting solder to stick to the leads was an absolute pain.

You have to mechanically secure the regulator to the heat sink to ensure good contact.  Without the heat sink, the regulator will burn up.  I found some machine screws that I had left over from a cabinet hardware installation, and a matching nut.  The screw was too long, so here I am cutting it to length with the screw cutting mechanism of a 3-in-1 tool:

The heat sink, ready to install.  The white stuff is heat sink compound, which helps with heat transfer from the regulator to the heat sink.  There was also a plastic electrical insulator between the old regulator and the heat sink (not shown here) which I re-used.  

The complete installation.  The board is pictured here mounted in a Panavise circuit board holder.

And from the back.  There's a small metal washer under the nut.  I only finger-torqued it, to avoid cracking the board.  I put Permatex thread locker on the nut to keep it from backing off.

Soldering on the decoupling caps.  These may not have been strictly necessary, but one theory has it that lack of decoupling encourages the factory regulators to fail.  In any event, it can't hurt.  Only problem is, there's no place on the board for them, so you have to improvise.  Most people use tantalum caps for this, but the ones that came with the repair kit that I got with this Fizmo were polystyrenes.  That's OK with me; polystyrenes are non-polar, so you don't have to worry about getting them backwards.  However, they had huge thick leads which were pure heck to get soldered to the regulator's solder-averse leads.  Here is the result; the white and red stuff is heat shrink tubing, ready to be shrunk.

You can't just leave those things dangling out in space, so the only alternative is to tape them down to the board.  I throughly mummified them in tape, just to preclude any possibility of a short.  

When I bough the synth, the little red lens that covers the alphanumeric display had come off; the owner had it in a baggie.  I needed to find something that would stick it back down without damaging the display; there is very little contact area for it to adhere to.  The perfect thing was electrically insulative, clear RTV silicone.  I put a bit all around the edge with a toothpick, put the lens on, and left it to set up overnight.  Here it is before I put the lens back on:

And finally, it's playing again!  The Fizmo in action:

The wall wart power supply gets pretty warm.  It's rated 1A so, with the new regulator (which probably dissipates more current than the old one), it's running right on the edge.  Wall warts rated higher than 1A are kind of hard to find.  I might replace it with a 1.5A open-frame linear.  

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