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Adding tones to an older radio

Discussion in 'General Technical Questions and Answers' started by KC7JDS, Mar 21, 2005.

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  1. KC7JDS

    KC7JDS Ham Member QRZ Page

    I have a 1980's IC-251A 2m multi-mode. I have tried to add sub-tones so I can use it all an all around base station & on repeaters. I have attempted to add an external tone box, but... when I adjust the tone pk-to-pk voltage /modulation to where it accesses the repeater, all anyone can hear is the distorted tone. If I adjust the tone down to where it's not a bother, I can't access the repeater. What's wrong?
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Where did you attach the tone output from the encoder into your transmitter?

    Normally, the tone should be injected not along with the microphone input (audio from mike) but later in the transmitter chain, after the mike preamp and such. A few reasons for this, but one is that the mike preamp generally won't pass such low frequencies effectively, so you'd need to crank up the encoder output until it distorts in order to achieve any modulation at all -- not a good thing.

  3. W9GB

    W9GB Ham Member QRZ Page

    Bryan -

    As Steve mentioned, you do NOT inject a CTCSS tone at the microphone connector. This is also mentioned in the ComSpec hook-up literature.

    Icom IC-251A manual.

    You have a spare jack on the rear panel that could be used for connection/disconnection of the external CTCSS unit (smart idea).

    Are you using the ComSpec TE-32 external CTCSS encoder ??

    TE-32 instruction sheet

    ComSpec will have the specific hookup sheet (for the point of CTCSS tone injection) for your Icom IC-251A radio.  Here is the Icom IC-251A (Adobe Acrobat) hook-up sheet #4245

    The TE-32 is just the SS-64 module in an external box with a selection switch.

  4. KC7JDS

    KC7JDS Ham Member QRZ Page

    Thanks for your detailed input. I did inject the CTCSS into an aux jack pin at the back of the radio. I'm at work right now, but when I get home this evening, I'll look at the details & get back with you.

    Again, 73's.
    kc7jds, B Woodman
  5. W9GB

    W9GB Ham Member QRZ Page

    It looks like you attach the CTCSS output to the "wiper" of R59

    Request document # 4245 for IC-251A

    Adobe Acrobat version of #4245 (you can save)

    OR if you have a FAX machine handy, ComSpec has
    "Fax on Demand" for specific install sheets


    Good luck with it tonight !
  6. K9STH

    K9STH Ham Member QRZ Page

    An "aftermarket" CTCSS encoder is normally attached at the "wiper" of the IDC (instantaneous deviation control) potentiometer. This is the control that sets the maximum deviation ("modulation") on an FM transmitter.

    Normally you need to use a deviation meter of some type to set the deviation of the CTCSS encoder. These days setting it to about +/- 0.3 KHz (300 Hz) should be plenty. Back in the "goode olde dayes" you had to set the tone deviation between +/- 0.5 KHz to even as much as 0.7 KHz. This was when mechanical "reeds" were used in the decoders. Today virtually every repeater uses a solid-state decoder and those will often work when the tone is well down into the noise level.

    You need to set the tone level without any modulation from the microphone. After getting the tone correctly set, you need to activate the microphone and then use the IDC to set your deviation so that you do NOT exceed +/- 5 KHz deviation no matter what your microphone gain is set at and no matter how loud you talk into the microphone. The purpose of this control is to set the maximum. Then you need to set your microphone gain so that when you talk across the microphone when it is held up to the corner of your mouth that you reach the maximum deviation most of the time.

    Motorola used ot put in a small pamphlet with every microphone that came with their commercial two-way FM units. By talking across the microphone you do not get "breath noises" (caused by the air coming from your mouth striking the microphone element). You put the corner of the microphone to just where it touches the side of your mouth and talk across it. You will sound "much" better than when you talk directly into the microphone.

    Glen, K9STH
  7. KC7JDS

    KC7JDS Ham Member QRZ Page

    Ok, gents,
    Here it is. I pulled out some of my info to bring to work today to be more specific.
    I'm using a Communications Specialist TE-12P programmable CTCSS tone generator.
    I'm plugging it into the 24 pin accessory Molex jack at the rear of the radio. I'm getting +12V from pin 2; ground at pin 8; (and this is probably where I'm going wrong) and I'm putting the tone into pin 5, "Output from TX mic amp".
    I can't see any other place to handily plug in the CTCSS tones thru this rear jack.
    Pin 1 is "output from squelch control stage."
    Pin 3 is PTT
    Pin 4 is "output from the receiver detector stage".
    Pin 6 is 8Vdc available when TX.
    Pin 7 is external ALC voltage.
    Pins 9-15, N/C
    pins 16-24 are a mix of N/C, control or data signals.
  8. KC7JDS

    KC7JDS Ham Member QRZ Page

    After reviewing all the info (yours & mine)... I think what I'll need to do is break into the guts of the radio, and bring out a wire from R59 to a N/C pin at the Aux jack. Maybe a mini-coax, with one end grounded. That or an UTP (unshielded twisted pair).

    73's, kc7jds, B Woodman
  9. W9GB

    W9GB Ham Member QRZ Page

    Bryan -

    CORRECT. The INPUT for a CTCSS encoder is NOT present on the multi-pin Molex ACC connector. This radio was not originally CTCSS capable.

    YES, you are going to have to install a wire from the "wiper" of R59 to either the spare RCA jack on the back panel OR a spare pin (N/C) on the Molex ACC connector.

    ComSpec Install document #4245 (for the IC-251)

  10. K9STH

    K9STH Ham Member QRZ Page

    Make sure and use a shielded wire for this connection and not just a "plain" piece of hookup wire. This is to avoid problems with hum as well as r.f. getting into the modulator.

    Haven't ever used a plain piece of wire myself but have repaired a "number" of units that the owner did so and then had problems.

    Glen, K9STH
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