James Cameron
2017-04-06 11:24:57 UTC
I'm looking at a Yamaha Electone FS-20 organ from 1983.
Symptom is a loud crackling noise, like lightning on an AM receiver,
which comes and goes over a time period of ten minutes. It is
thermally related; if you turn the organ off for a few minutes, then
on, it takes a few minutes before it starts to happen. Once the
crackling starts, moving the tuning dial may stop it, or may make it
worse.
I've bought a copy of the service manual with schematics.
The design is a custom processor, ROMs, and a pair of four channel
DACs; huge custom packages, then the usual analog chain driving big
speakers.
Oscilloscope shows the noise is emitted by the DAC, and is a
square-ish wave; curved slightly by low pass filter after DAC.
Oscilloscope shows +15V, -15V, and +5V supplies to the main board are
reasonably clean, with only 10mV ripple, and they don't vary between
the perfect and symptomatic operating modes.
The front panel tuning dial (85k) emits a control voltage for a pitch
control clock oscillator (589.2 kHz), which drives a phase comparator,
to vary a master clock oscillator from which 4.7 MHz, 2.4 MHz, and 1.2
MHz clocks are derived. The CPU, front panel shift registers, bus
logic, and DAC are clocked from these sources; so the tuning affects
everything. Oscillators are made from discrete components, no
crystals present, using variable inductor cans.
Oscilloscope shows the 2.4 MHz and 1.2 MHz clocks are clean, and at the
correct frequencies, but I've not been able to access the 4.7 MHz
clock easily yet; it's on a card edge protected by steel yet to be
disassembled.
Someone before me has applied a spray oil over everything inside. The
oil has mostly evaporated from flat surfaces, but is discovered
between metal plates, under screws, and likely under components on the
PCBs. The whole organ stinks of it, especially the wood.
What excitement can this type of oil do to 1983 vintage electronics?
--
James Cameron
http://quozl.netrek.org/
--
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
Symptom is a loud crackling noise, like lightning on an AM receiver,
which comes and goes over a time period of ten minutes. It is
thermally related; if you turn the organ off for a few minutes, then
on, it takes a few minutes before it starts to happen. Once the
crackling starts, moving the tuning dial may stop it, or may make it
worse.
I've bought a copy of the service manual with schematics.
The design is a custom processor, ROMs, and a pair of four channel
DACs; huge custom packages, then the usual analog chain driving big
speakers.
Oscilloscope shows the noise is emitted by the DAC, and is a
square-ish wave; curved slightly by low pass filter after DAC.
Oscilloscope shows +15V, -15V, and +5V supplies to the main board are
reasonably clean, with only 10mV ripple, and they don't vary between
the perfect and symptomatic operating modes.
The front panel tuning dial (85k) emits a control voltage for a pitch
control clock oscillator (589.2 kHz), which drives a phase comparator,
to vary a master clock oscillator from which 4.7 MHz, 2.4 MHz, and 1.2
MHz clocks are derived. The CPU, front panel shift registers, bus
logic, and DAC are clocked from these sources; so the tuning affects
everything. Oscillators are made from discrete components, no
crystals present, using variable inductor cans.
Oscilloscope shows the 2.4 MHz and 1.2 MHz clocks are clean, and at the
correct frequencies, but I've not been able to access the 4.7 MHz
clock easily yet; it's on a card edge protected by steel yet to be
disassembled.
Someone before me has applied a spray oil over everything inside. The
oil has mostly evaporated from flat surfaces, but is discovered
between metal plates, under screws, and likely under components on the
PCBs. The whole organ stinks of it, especially the wood.
What excitement can this type of oil do to 1983 vintage electronics?
--
James Cameron
http://quozl.netrek.org/
--
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist