"Failure of a property changed by an external agent to return to its original value when the cause of the change is removed."It's like having play in a car's steering wheel, or backlash in a gear set. Too much hysteresis in a squelch circuit forces one to keep the squelch at a tighter setting, missing weaker signals when scanning or searching. The squelch hysteresis problem was first addressed in a 1984 article about the Radio Shack PRO-24 crystal controlled scanner.
Why do engineers design these circuits with any hysteresis? Basically, hysteresis prevents weak signals from "popping" the squelch open and closed rapidly, which would be very annoying to the listener. A squelch circuit with hysteresis requires a stronger signal to open the squelch than it does to keep it open.
Models cursed with too much hysteresis include the Uniden/Bearcat 800XLT and 760XLT, Radio Shack PRO-2003, PRO-2004, and others. Don't despair, the fix is simple for most radios. I've successfully reduced the squelch hysteresis by replacing a single resistor.
Hysteresis is usually implemented by providing feedback from the squelch gate input to output, through a resistor. The higher the resistor value, the less feedback there is, and less hysteresis. My modification involves replacing the "stock" resistor with a higher value resistor, as identified in the table below.
This technique does not apply to some of the newer Uniden models, because the squelch hysteresis is partially determined by firmware inside the processor IC.
This article cannot provide step-by-step instructions for each radio. I always encourage radio hobbyists to purchase the service manuals for their equipment. The most useful sections of the manual are the schematics and printed circuit diagrams. Good manuals will also describe the theory of operation and alignment procedures, and contain a complete parts list.
If your radio has too much hysteresis, and is not listed in the table, study the service manual. Motorola MC3357, MC3359, and MC3361 integrated circuits, which contain IF and squelch circuitry, are popular in scanners. In these receivers, look for the hysteresis resistor to be connected to these ICs.
________________________________________________________________ | Squelch Hysteresis Resistor In Selected Receivers | | | | Res- | |Receiver istor Stock | |Model No. Value Comments | |_____________________|________________________________________| |Radio Shack PRO-24 | R35 33K replace with 220K. | | | Located between | | | pins 12 and 14 | | | of MC3357 (IC1) | |Radio Shack PRO-2002 | R197 47K try 220K. | |Radio Shack PRO-2004 | R148 47K remove R148 | | | or replace with 220K. | | | On linear board, | | | between pins 12 & 14 | | | of TK10420 (IC2) | |Radio Shack PRO-2006 | R152 33K try 100K or higher. | |Radio Shack PRO-2022 | R91 33K replace 33K | | | with 220K. | | | Between pins 12 & 14 | | | on IC-1 MC3361 | |Radio Shack PRO-2035 | 100K replace 100K | | | with 220K. | | | Between pins 12 & 14 | | | of TK10420 (IC-2) | |Electra/Bearcat 220 | R75 2.7M may be ok as is | |Electra/Bearcat 20/20| R75 2.7M may be ok as is | |Electra/Bearcat 250 | R66 2.2M may be ok as is | |Uniden/Bearcat 760XLT| R58 22K replace with 6.8K | | | or other value. | | | (select value for | | | the particular radio.)| | | Located on pin 14 | | | of MC3359P IC | |Uniden/Bearcat 800XLT| R91 860K replace with 2.2M. | | | Located on pin 14 | | | of MC3359P IC | |Plectron P1 | R81 180K may be ok as is | |Plectron 700 | R96 180K replace with 560K | |_____________________|________________________________________|
Copyright 1998, Bob Parnass, AJ9S (firstname.lastname@example.org)