Lexicon RV-8 Service Manual page 79

Lexicon
Clip
The clip signal is derived from the output signal of the error amp. This signal is called V_ERR on the
schematic. V_ERR is fed into a window comparator set up by U4-A (LM339D) and U4-B. When the
signal coming out of the error amp exceeds ±12.5Vdc it triggers the window comparator. The output of
this stage is sent into another comparator stage set up by U4-C. The output of U4-C feeds a RC circuit
(R75, R76, and C31) that can be used to change the point at which a clip signal is indicated (in this case
the C is not populated so it is not being used). The output of the RC circuit feeds a fourth comparator
stage set up by U4-D. This comparator drives the clip signal. The clip signal is normally at 0Vdc and goes
to +5Vdc when a clip occurs.
Temperature Sensing
An NTC is thermally coupled to the negative heatsink for every amp channel. As the temperature of the
heatsink increases, so does the value of the temperature signal. At room temperature the temp signal
usually reads in the range of approximately 0.5 to 0.6Vdc.
DCLF
The card is also protected from DC signals and Low Frequency (LF) signals. The output is monitored and
will open the relay if either of these conditions exists. The relay will stay off for a few seconds and will try
to turn back on. If a DC or LF signal still appears at the output the relay will either turn off again or not turn
on at all. This cycle will repeat until a DC or LF signal is no longer present at the output.
The output of the amplifier is fed into a low-pass filter set up by R46 (200k), C23 (0.47uF), R47 (2.2k),
and C24 (0.1uF). The output of the filter is sent into a window comparator set up by U3-A (LM393D) and
U3-B. When the window comparator is triggered it turns on Q15 (MMBT3906), which then turns off Q16
(MMBT3904) and consequently turns of the output relay (K1).
RDY_CON
This is an input signal to each amp channel that is used to turn on and off the output relay. A 0Vdc input
or no input will keep the relay off. A 5Vdc input signal will turn the output relay on if a DC or LF signal is
not present at the output of the amplifier.
RDY_MON
This is an output signal of the amp card. The only function of the RDY_MON signal is to report the status
of the output relay. A 0Vdc signal means the relay is open and a 5Vdc signal means the relay is closed.
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