Yamaha YST-SW150
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Although I've retired
I'm fixing (or attempting to fix) things for friends. When I
began my repair business it was chiefly domestic electronic equipments
such as video recorders and TV sets but as the price of these
sort of things fell to the point where repairs were not really
a sensible option I switched over to commercial stuff, in particular
lift and escalator bits and pieces. Now I've retired though I
don't mind moving back to cheaper items to keep my brain active.
The other day this sub-woofer arrived with the report that it
no longer responded properly to its input.
Reading the user manual and
looking at the block diagram the sub-woofer turns on when it
detects music so you'd imagine it uses a circuit which integrates
whatever its being presented with to decide whether or not to
turn on. Once turned on the manual tells me it will remain turned
on for a period which makes sense. |
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The Repair
Manual gives this block diagram reproduced below which
clearly shows how the sub-woofer derives its audio input and
how it turns on.
High power (but low voltage)
signals fed from the system loudspeakers are presented to IC11A,
the input amplifier, in parallel with IC4, the music sensing
detector circuit. Optionally a pair of phono inputs (at a higher
voltage) can be supplied for the same purpose. |
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I'm informed that the
thing isn't sensitive enough detecting music so I need to carry
out tests to determine exactly what's happening.
I noticed the Auto Power switch
is set to ON.
Below...You can see various
input resistors in the two sets of inputs whose values are designed
to provide equal response for music detection and the correct
level for proper operation in conjunction with VR3, the volume
control. You'll also note the Timer IC5. This is for maintaining
sub-woofer operation for a set period (described as "several
minutes" in the User
Manual) rather than turning it on and off during quieter
passages of music. To ensure balanced operation from a stereo
system both left and right audio inputs should be plugged in. |
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I turned on the sub-woofer
and applied a 35Hz audio input to one of the phono inputs. Initially
set to the minimum audio output I increased this to 100mV. The
audio amplifier turned on at roughly 96mV and remained on. Unplugging
the input the amplifier turned off after several minutes which
is what I'd expected. I applied the input to one of the loudspeaker
terminals but found the amplifier remained off despite an audio
input of 15 volts.
The rear panel detached after
removing 12 large philips screws and I was able to manoever the
panel so I could detach the input board and remove it by unplugging
its 8-way cable. By threading this cable back through the panel
I was able to power it up outside the box to make further tests.
I found that 66mV at one of the phono sockets turned the subwoofer
on then I reduced the input to draw 14W of mains power (the input
was then 11mV). The plan was to swap the input to the speaker
terminals and compare the results. During this process I noticed
intermittent operation when using the speaker terminals so I
resoldered every joint on the input board. Then, using the speaker
terminals I found the amplifier turned on when the input was
660mV. When the audio was turned off the mains power dropped
from around 16.5W to 13W and down to 1W after several minutes
delay. |
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I found that it was infuriating
and impossible to carry out meaningful tests because of a six
minute wait for the amplifier to respond to turning off coupled
with a two or three second delay from detecting the input and
closing the final relay to activate the output. The customer's
reported symptom was that once the sub-woofer was turned on it
would play for several minutes then turn off and it would only
then turn back on if the main amplifier was turned up to a fairly
loud level. Basically, to play at a comfortable lowish volume
wasn't possible for more than 6 minutes. If you look at the circuit
diagram above its clear that the Auto Power input is derived
from either left or right phono or either left or right speaker.
Ideally, because of balancing; both phonos or both left and right
speakers should be plugged in because of their probable different
mixture and amplitude of sounds. The sub-woofer adds together
right and left channels but only if both channels are wired to
the input panel.
The fraction of incoming audio
used for the Auto Power circuit is defined by the network of
various input resistors plus the value of C41 which also defines
the frequency response of the Auto Power circuit. Calculating
the actual resistances of circuits seen from the detector needs
some thought but the LS inputs each have a 1Mohm feed resistor.
Not all of the fraction of loudspeaker
voltage is supplied to the Auto Power circuit due to the method
of connection of phono circuit resistors but the phono level
isn't reduced as much by the loudspeaker resistors as vice versa.
This results in the requirement of a nominally higher voltage
from the speaker inputs than that from the phonos to turn on
the power.
In practice the turn-on voltage
requirement gets less as the input frequency rises due to the
impedance of C41 being less at higher frequencies.
But before proceeding I must
disable the long dwell time. |
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The timer chip is an M51848L
(IC5 Auto Power board... below) has a timed output governed by
a resistor/capacitor pair R C (R60/C46)
This has a formula T=1.1 x R
x C seconds.
This equates to 1.1 x 1Mohm
x 330uF = 363 seconds or 6 minutes.
I added a 12Kohm resistor across
R60 (1Mohm) and this resulted in a shorter delay of only 4 seconds
making it very much easier for testing. |
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With the long dwell time
reduced to 4 seconds I saw the voltage for the turn-on time at
the loudspeaker terminal was 500mV.
This still needed care to measure
because there was another built-in delay to cover switching noise
in the speakers. I used a mains power monitor to see when the
amplifier turned on which was about 2 to 3 seconds after audio
detection. Once turned on I was now able to measure the audio
level below which the Auto Power circuit dropped out. This was
half the turn-on value or 250mV. I carried out my tests using
50Hz audio.
As the customer wanted to play
bass at low audio power levels I figured out how to improve the
Audio Power circuit sensitivity. This was easy and was accomplished
by shunting each of the loudspeaker 1Mohm feed resistors R97
& R98 by a second 1Mohm.
The turn-on/off test was repeated,
again at 50Hz and this time the turn-on volage was 260mV and
once the amplifier had turned on it took below 130mV to allow
it to turn off.
I cut one leg of the 12Kohm
resistor to restore the 6 minute dwell time and reassembled everything.
Will it work as planned? |
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pending..... |
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