> Merrill <kg4...@...> wrote: > I have a MR4 receiver that the squelch will not close > on any ideas . > Merrill > KG4IDD
Typical squelch operation for the MR4 type of receiver is to use no-signal present audio white noise to drive a switch circuit. Many receivers of the MR4, Hamtronics, VHF Engineering, Uniden, Radio Shack & GRE Scanners... yadda, yadda use one of the popular receiver chips like the MC-3359, MC-3357, MCxxxx & Mxxxx (xxxx meaning you add your chips number in place of...) I often find these same chips with house numbering placed into many amateur and commercial radios (GE Phoenix is one such commercial radio using one of the MCxxxx chips). There is an MR4 information sheet in/at the repeater builder web site... but I just had a look and it seems to be missing the receiver diagram. Inside a portion of the "integrated FM receiver subsection" chip, the white noise normally present/heard when you open a receiver squelch is sampled & separated in an adjacent section/stage (inside the chip). The audio white noise is then amplified to limiting and routed to a simple rectifier stage where the out is a changing DC voltage (based on how much white (audio) noise was driven into the rectifier circuit). Naturally I just read where the MR4 does not use the internal chip squelch circuit... but the mentioned RB web page information does indicate receiver board diodes CR5 & CR6 are the squelch rectifiers. Regardless of the circuit location (internal or external to the chip).... the squelch operation should be pretty much the same for many receiver sections. Separated, amplified, limited, (sometimes frequency range selected or optimized) no-signal audio is rectified (changed to DC) and generates a DC voltage value set up to toggle an internal (to the chip) or external electronic switch, which is set up to mute/un-mute the audio path to your speaker amplifier section. This location is also where many receivers toggle the COR/COS output logic to your repeater controller (external devices). Normally... using the chip internal squelch circuit. The following does not directly apply to your circuit. Some basic testing... with a scope is easy (because you can quickly see the audio noise displayed) or a regular Multi (volt meter) set up to read low value AC voltage. This type of testing is a place where the previously mentioned Radio Shack Mini Test Amplifier would be very helpful (actually in some cases would work in place of the scope and multi meter. http://www.radioshack.com/product/index.jsp?productId=2062620 (one of the best spent $14.95 test equipment items you'll ever buy) The amount of audio noise fed back into the chip is often routed and controlled by the squelch pot. So a quick test of the signal present on the squelch pot... one terminal might be ground, the opposite end the inbound audio noise signal and the center wiper the return back to the chip. Do you see an AC voltage on the squelch pot "high side"? And of course is that AC voltage present and vary with control rotation on/from the center wiper? If so... you move onto the section of the receiver chip after the AC audio noise is rectified (changed to DC) to find the section where the resultant DC voltage changes with rotation of the squelch pot and on frequency received signal input. At some circuit selected value the logic circuits controlled by the DC switch voltage should change. One portion of the control circuit should be set up to mute the audio amplifier. **** I had typed all the above in before viewing the circuit operation description text, which states your circuit uses the rectified DC Voltage (level) on the squelch pot to set a level driving transistors Q15 and Q16. So... using the circuit description and hopefully an available diagram... it would not be a huge pain to sort out your squelch problem... The first step would be measure the DC voltage on the squelch pot and see if it varies with signal input. www.repeater-builder.com/kendecom/kendecom.pdf If we can find an easy web copy of the MR4 Diagram... the above described can be better detailed for you. cheers, s.
