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Thread: Racal PRC-6725 or PRC-139 schematic help

  1. #1
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    Default Racal PRC-6725 or PRC-139 schematic help

    Hi,

    I've got hold of a pair of Racal PRC-6725 transceivers, (30-88MHz), they are effectively the PRC-139 I believe when sold in the USA.
    Odd thing is both of them are low in frequency by about 3.7KHz on both transmit and receive leading me to think it's the reference frequency for the PLL that is off.
    Would anyone have a service manual or schematic of the synth / pll section, the radio has of 2 PCB's stacked ontop of each other, and on the opposite side, an 'RF deck' which is a separate module, used I believe several Racal transceivers of the same era, there were also UHF and High band modules.
    Googling has yielded zero so far, I know they are programmed from a dedicated hand held programmer, but don't know if the frequency can be adjusted from said programmer to get it spot on.

    Regards
    Tony


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    Hi Tony,
    The handheld programmer does not have the capability to adjust the reference frequency. It only sets squelch level, tone, frequency, and power level for each channel. I've had different versions of that handheld radio and found that the different versions have slightly different physical dimensions such that modules that determines the frequency band VHF low, or VHF hi, or UHF don't fit the different versions. So it's likely modules for the '139 will not work with the '6725 and vice versa. The programmer did not properly work with both versions either. Racal made some small changes in the interface messages. I've never seen a schematic or service manual. It would make sense that the radio body does the IF processing and provides a freq. reference to the band module which does the RF/IF conversion. I've got a repeater setup that uses those radios. Good luck with it!
    Trevor

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  4. #3
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    Hi TrevorB,

    Apologies for the late reply, i've only just spotted your reply.
    As I said, the ones I have are both low band (30-88MHz) and are both low in frequency by about the same amount, I know it's not the test set as that is GPS locked to a 10MHz reference, and it works fine with other radios too.
    I guess i'll need to take one to pieces and have a look inside to see if there is anything around the synth that I can tweak.
    At least you've made me feel a bit happier that I don't require the hand held programmer to adjust the frequency.

    regards
    Tony