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The following text I posted to on May 2007

Hi C500-Fans,

Last week end I started my surgery-project on my C500. I was asked
to share this experience with you, so I start a new thread on this
topic here.

opening the organ I was surprised, that the organ has almost no plug-
and-socket connections! All wires are wrapped on long pins which
makes it look a bit chaotic. But the trouble of bad plug contacts is
not a topic for this organ! I started with the

Power supply
The mains lead cleat was broken and the cable was rubbed nearly to
the copper. I was lucky that I did not get an electric shock until
now. Further the full tension of the cable was held by the 3 pins on
the power supply board and they were badly bent. Luckily, they did
not touch other low-voltage pins until now.
The major issue were 2 capacitors which drained the voltage down.
After their replacement, the symphonizer worked again, as well as
some sounds sounded better than before.
There is also a potentiometer to calibrate a proper 13V. The voltage
was 14V before, this was possibly the reason for the blown octave
divider in the oscillator board. The previous owner already replaced
some of them, the labels on some ICs are different. So I hope I've
found the real culprit.
I advise everyone to check this voltage in his C500, because these
divider ICs are hard to find; I got not a replacement until now :-(

I also glued small heat sinks on the TOS divider chips, because they
are getting really hot and they shall face the next 30 years also.
However, there is a replacement TOS set in the Genie board...

Aged Resistors and capacitors
Identifying all defect resistors and capacitors will get hard work.
I already found one resistor that has altered from 1M to 20k, it
prevents the diapason register from switching off. Possibly I need
some months to find all remaining bugs. I have no comparison, how
each single voice sounded ex-factory to identify which one is wrong.

I would say, each C500 today will sound individually due to the fact
that all thousands of parts might have changed their values
differently. This is the fascinating story with this old analogue


Hi friends of Celebrations,

this is my continued story to repair my C500:

Divider ICs
one divider chip was missing and I looked for replacement. I soon
noticed that the divider is no longer available on the market. offers a retro kit for 21.50$. By googling around I
also found someone who designed an own replacement Kit for his
Polymoog and he offers now some pieces at ebay. You can read his story
2 days later, the chips arrived and worked fine. Unfortunately their
pins are too thick, the chip slipping off the sockets and I damaged it
with my screwdriver experiments - with power on... Now I am ordering a
second set from Chris, 2,35 pounds per chip is worth having a second
spare part also. Chris was also so kind to make the chips with thinner

Symphonizer Chorus
I found one transistor mounted wrong into its place! This means my
organ never had a chorus with some symohonizer voices. Having this
fixed, I was astonished, how good Horn or Piano sounds.

Short Cut
One tab did not swith off. The reason was a grounded pin of the
switchin FET. I noticed that many long pins of the electronic
components were not cut off from the copper side of the PCB. When the
PCB get older, they bend down to the grounded chassis and made the
short cut.

one CMOS IC (MM74C74) was blown, that's why my orchester Wah doesn't
work. I hope the replacement Chip will arrive in the next days. Some
other tabs depend on this key-on signal as well.

After that, the most visible problems with the C500 are fixed. Next to
come are optimizations like checking the chorus, I think the modulator
waveform is not correct. Checking the photo-modulators, and finally
replacing the capacitors, as Ed recommends.