Q2 Audio Compex F760X-RS User Manual - page 8
The ATTACK and RELEASE parameters offer a wide range of possible combinations to handle any situation most ef
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fectively. The ATTACK control determines the speed at which the Expander/Gate opens from the attenuated mode; the
RELEASE controls the rate at which the device attenuates along its particular slope towards the maximum attenuation
pre-set by the RANGE control.
When using fast release (as will be normal) any tendency to hunting can be reduced by slightly slowing the attack rate
ranter than slowing the release time.
When using ‘M’ or ‘S’ attack positions it will be found (particularly in the case of the ‘S’ position) that the release speed
possible becomes limited to slightly longer times since there will be the phenomena of release beating attack so that the
Expander/Gate never fully opens and the green indicator will remain on. In the ‘M’ position this will only apply to the fast
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est position; in the ‘S’ position a medium or longer release time will only be possible.
F760X Series Maintenance
Printed Circuit Boards: An F760 re-issue channel consists of two PCB’s, a main PCB with the original ‘A’, ‘B’ and ‘C’ sec
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tions and a front PCB containing the user controls and attack/release resistors,etc. The power supply PCB is separate and
mounted to the chassis wall. The power supply creates two 24V DC outputs, one for audio and one for powering relays
and LED’s.
On the Main PCB, the ‘A’ section comprises the Compressor Side-chain, Control Voltage Mixing (from all side-chains),
Inverting Amplifier and FET Bias control. The ‘B’ section consists of the variloss FET controlled amplifier (30dB gain), the
Line Amplifier (4dB gain), and the Peak Limit Side-chain and indicator switching circuit (RED). The ‘C’ section has the
Expander/Gate Side-chain with associated Indicator switching circuit (GREEN).
With the FET Bias pre-set correctly adjusted, and with the Input/Output attenuators fully open, the overall gain of the sys
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tem will be 34dB (transformerless). (30dB variloss amp; 4dB Line-drive amp – this 4dB is lost in a pad when in the direct
signal (bypass) mode to maintain unit gain).
Normal maintenance would consist of checking system gain and making and necessary adjustment to the FET Bias; mea
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suring frequency response; checking distortion and stereo matching.
System Gain: In checking the gain ensure that attenuators are fully open and that no gain reduction is taking place (i.e.
switch the Compressor to 1:1, Peak Limit and Expander switches to ‘Out’). Insert signal level of -34dBm and the output
should read 0dBm. Slight drift may be experienced due to temperature changes but this will not affect the dynamic charac
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teristics until the gain rises to 37dB. Therefore a normal tolerance of +/- 2dB would be acceptable. On stereo units if there
is a difference, it should be the same difference of course.
Measuring Frequency Response: Care should be taken to ensure no gain reduction is taking place for the effect will be to
flatten out any error and conceal it. 20HZ = -0.5dB, 20kHz = -1.0dB (+/-0.25dB) relative to midband/1kHz.
Distortion: This is normally checked at 1kHz; if measured at low frequencies care should be taken to have the release on
a long setting; or at least to note the effect of the release at low frequencies. This is covered in the set-up procedure which
follows. Generally, when measured under conditions as described, THD is approx. 0.2% or slightly less. In recent years
(2011 onward) it has been difficult to source FET’s which will adjust much below 0.25% (although some have). We are not
concerned about this as it is mostly pleasing second harmonic distortion and is really more of what you want a Compex
for!