Recording - August 2013 - 61

With the fan on, the drum machine
worked perfectly. I fixed it! Sort of.
Not wanting to tie up a fan and a mic
stand forever, I took the unit in to my local
repair center. They know me so well they
have a picture of me on the wall over the
door. Unfortunately, they wanted $110 to
diagnose the problem, as it was “vintage” gear. I wanted them to fix it so I
said, “Okay.”.
A week later I got their diagnosis: a
long list of parts they wanted to replace
and things they wanted to check and
adjust. $800. Gulp. No way I’m paying
that much to fix it. It may not even be
worth that much. So I took it home and
went to Plan B. What else could I see?
Into the belly of the beast
I unscrewed 5 screws on the bottom and
3 screws on the back and went inside. It
opened up nicely like a clamshell. (Photo 3)
At this point let me warn you: If you’ve
never been inside your gear before, it can be
dangerous. Large capacitors can shock you
even when the unit is unplugged, and when
your unit is plugged in, the voltages on the
wall-current side of the transformer are lethal.
Even if you don’t hurt yourself, while poking
around in there you can do more damage
than you are trying to fix. Be careful!
On the other side of the hot spot (which
got too hot to touch) was a small voltage
regulator (VR) attached to a hexagonal
metal plate. The plate was a heat sink but it
had no fins. I wanted to help it out so I fashioned a heat sink out of a thin piece of metal
and gave it fins. I carefully removed the nut
holding the VR in place and attached my
heat sink. The result is shown in Photo 4.
I turned it on without the fan and it ran
for 15 minutes. I had made some progress
but apparently I still needed a fan. On further inspection I found three more VRs, two
on the motherboard that got very hot, and
one under a ribbon connector that only got
warm. When I aimed the fan at the VRs on
the motherboard, the drum machine
worked without failure.
That’s when it hit me. Computer CPU
fans are the perfect size to sit above the
VRs inside the unit and keep them cool. I
went to my local electronics store and
bought one for $6. You can also find
them online. It’s 60 mm square and only
10 mm high. It runs on 12 Volts DC.
Back in the shop I looked inside the
SP1200 and followed the 110 Volt AC
wires from the plug through the switch and
the fuse to one side of the power transformer. The wires coming out the other side
have various voltages so I followed them to

a junction block on the back panel. I powered up the unit and tested the DC voltages
from the terminals on the junction block to
ground. I found 12 Volts! (If I had not found
anything near 12 Volts inside, I would have
powered the fan from an external 12 Volt
supply such as a wall-wart. That’s not nearly as elegant, but it would’ve worked.) By
the way, that was the most dangerous part
of this job; you must stay away from the
wires on the wall-current side of the power
transformer while it’s powered on, if you
value your safety.
Next, I powered the unit back down again
and unplugged it, then I connected a wire
from the ground post on the back panel to the
black wire of the fan. Then I connected the
fan’s red wire to the 12 Volt terminal on the
block by pulling out the connector a little,
putting the wire through a hole in the terminal
and pushing the connector back on. Easy.
Testing, one, two...
I powered up again and the fan worked
like a charm. When the fan was supported
over the VRs the machine ran great. I made
four legs for the fan out of an old chopstick
and glued them into place, making sure the
bottom of the feet were level. Then I positioned the fan and put a tiny drop of glue
at the base of each foot. I was careful not
to put any glue on any exposed trace or
component, as the glue might affect the
resistance or capacitance of the circuit at
that point. Photo 5 shows the final layout.
After the glue dried I sealed up the
SP1200 and put it into service. Problem
solved. Six bucks or eight hundred?
Which would you rather spend?
Side effect goes away
There is one minor side-effect to this
mod. When the drum machine output volume is low (say, 9 o’clock) you can hear a
slight clicking of the fan on the output
audio. I usually run the output level around
3 o’clock, which for some reason eliminates the clicking, even when the drum
machine is not playing! Go figure.
If you’re like me and you don’t really
know what you are doing, it’s always
exciting—and a little scary—fixing your
own gear. (Remember what I said about
capacitors and wall current!) But it’s
worth it to experience the joy of winning.
It also feels good knowing that you own
your gear and it doesn’t own you.
Jon Bare (bare@recordingmag.com)
loves the smell of solder in the morning.
Check out his music and videos at
www.jonbare.net.

http://www.royerlabs.com http://www.jonbare.net http://www.royerlabs.com

Recording - August 2013

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