Icom IC-575A/575H Transceiver Click here to go back to the home page
modification to allow 'scanning' with the 'squelch' open

by Chris Tran, GM3WOJ - chris.tran@zetnet.co.uk

(Note : After this modification, the 'scan' no longer stops when a signal is detected - you have to press the SCAN button or turn the main tuning dial)

The Icom IC-575 replaced the venerable IC-551, but has one or two design quirks - one being that the 'scanning' only works when the 'squelch' is closed - an unsatisfactory idea for 6m DX-ing where many of the signals are close to the noise level.

The squelch circuitry of the IC-575 is separate for FM and for SSB - the design of the SSB squelch is poor - it uses a signal from the 'S-meter' amplifier to 'open' the squelch. As we all know, all the real DX on 6m never reads on the 'S-meter' !

From the schematic diagram, it seems that cutting one wire makes the 'scanning' action independent to the 'squelch' action. Both the scanning and the squelch still work. When scanning, the VFO uses 100Hz steps, not entirely sequentially, which gives interesting effects on strong carriers !

  1. Remove bottom cover only (9 screws) - lay IC-575 upside down with main tuning dial towards you. (From now on 'left' means left as you are looking at the upside-down radio)
  2. Look at the 'Main' PCB - identify J16 (which has P5 plugged into it) - it is a 5-pin white connector (parallel to the rear edge of radio) about 4cm from rear edge and about 9cm from left edge of radio. (See manual Page 43 - upper photo - J16 is just below S3) J16 has 5 wires, in order from left to right :- Brown, Green, Yellow, Orange and Red.
  3. It is the Orange wire that has to be cut. (J16 orange wire = 'CSTP' line = 'Centre Scan Tuning Point?' +4.6V when squ. closed, +1.6V when squ. opens - from 'Main' unit to 'Front' unit) This orange wire runs to the back of the radio, then to the left - it is convenient to cut it near the rear left-hand corner of the radio (where it can be rejoined easily if necessary).
  4. Insulate both cut ends of the orange wire with small pieces of PVC tape or heatshrink tubing.
  5. Test radio - replace bottom cover.
  6. If necessary, remove top cover and set 'Scan speed' switch S1 to 'slow' - see manual P.43 We all have our own preferences - I scan a fairly small frequency range e.g. 50.098 to 50.122 fairly slowly - even the 'slow' scan speed is fairly fast (esp. on 'Memory scan') - the good old IC-551 has a 'scan speed' pot. instead of a switch. Slowing the IC-575 scan speed further should be an easy mod.

73 Chris Tran GM3WOJ Jan.1999

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