Results 1 to 6 of 6

Thread: PX-300-S Project recap and puzzle

  1. #1
    Join Date
    Mar 08, 2014
    Location
    Florida
    Posts
    519
    Thanks
    228
    Thanked 372 Times in 210 Posts

    Default PX-300-S Project recap and puzzle

    OK it has been several months since I (nearly) completed my PX300-S conversion to UHF. It all went well except I have a few post its (nags) attached to it.

    1) I have to touch up the RX tuning to span the entire 462-467 FRS/GMRS block. As it is it favors 462.6 and I can probably do over 5 MHz with little degradation.

    2) The power output is still at 5 watts. The actual UHF PX300-S claims 10 watts. I thought that by raising VCC to 12.5V per the schematic would raise the gain. I also changed the power control resistor per the schematic. As I don't have a complete manual, I don't have the parts list for the PX300-S. If anyone has a PX300 or PX300-S manual including the UHF parts, I can figure out if a module change is required. Yes I do know the PA requires a heatsink and a thermal control circuit added. I am guessing that I might need a higher gain driver stage, perhaps from a 2 watt MX that had no final stage.

    3) I found a D Cell battery box! Still looking for a NICAD box. So I filled this thing with 22 fresh D cells. It tops off at about 17.5 V. This seems to be a problem because the radio chokes up at that voltage and the TX won't key and the TX light lights dimly. I can unsquelch the radio and the sag will let me key the TX and all will be well when the battery loses a few hundred millivolts. But as soon as it rests, those cells rise and I am back to square one.

    It is a problem on the control board. The radio came from WHCA and already I found it strapped coded only with a jumper on the mode switch. I also found a deliberate foil cut at the point TX 8.0V feeds pin14 of U301 via a diode. My first thought was that WHCA wanted the TX and flashing low battery light disabled for security reasons. But the TX light works normally. I do know that the battery indicate pot needs to be adjusted because likely this radio ran from NICAD in its previous life and there is a lower low battery set point. But I cant see why having a high battery voltage would have an effect. Were there any bulletins on these radios as to the reason it behaves this way or why that foil would be cut?

    Any federal old timers out there that recall these?

    It is a neat radio and will be great for a temporary base, especially if I can find the high power UHF modules.


  2. The Following User Says Thank You to RFI-EMI-GUY For This Useful Post:

    uplinker (Dec 27, 2018)

  3. #2
    Join Date
    Feb 12, 2012
    Location
    Directly above the center of the earth.
    Posts
    2,618
    Thanks
    677
    Thanked 1,217 Times in 623 Posts
    Country: Christmas Island

    Default

    I have a couple of these but they are on VHF, so a lot of the PA info won't apply. I always wanted a UHF one, and tried to convert one to UHF without too much success. As for voltage it sounds like they were running it with D cell NiCd batteries, that would give a 13.2 V DC supply instead of the 16.5 V nominally you get with Dry Cell D batteries. They used to make dummy batteries for some radios that eliminated 2 of the cells in the 11-cell strings that make up the pack, for times when using the non-rechargeable batteries.

    As I recall, I thought the same thing about the PA - at least on the VHF version, it looked like they were using the standard PA; but as you said, running from a 12V B+ and on an external heatsink. I can't speak to the other strap mods, etc. Sorry, but those models, especially WHCA stuff, were rife with "SP" mods...

  4. The Following User Says Thank You to Alpha For This Useful Post:

    RFI-EMI-GUY (Dec 25, 2018)

  5. #3
    Join Date
    Mar 08, 2014
    Location
    Florida
    Posts
    519
    Thanks
    228
    Thanked 372 Times in 210 Posts

    Default

    They definitely have an optional battery box for alkaline and the instructions are for installing 2 banks of 11 cells.

    I think the problem I am having is a shorted transistor in the 12.5 V regulator. I should get the part in a couple weeks from HK. Looking forward to tweaking the RX and working on the TX power. I already found another 700 milliwatts by improving the grounding of the coax to the antenna terminal.

    I have some supercapacitors on order to give the DES memory retention an extension. The encryption key gets lost pulling the batter pack off.

    So this is a retro-modernization project as well.

    I can share my UHF mod notes if you wish to finish yours. It is like brain surgery.

    Sent from my SM-T350 using Tapatalk

  6. The Following User Says Thank You to RFI-EMI-GUY For This Useful Post:

    Alpha (Dec 28, 2018)

  7. #4
    Astro Spectra's Avatar
    Astro Spectra is offline T S - Moderator
    CS Forums $upporter
    Join Date
    Nov 22, 2012
    Posts
    803
    Thanks
    239
    Thanked 456 Times in 235 Posts
    Country: Great Britain

    Default

    I have a WHCA VHF unit too waiting on time to re prog the PROM (need to find some space or do the bank switch mod). I was super fortunate to inherit probably the only suitcase programmer in this part of the world.

    What was the original antenna for these things? Also was the black telephone handset standard?
    It is a fine thing to be honest, but it is also very important to be right

  8. #5
    Join Date
    Mar 08, 2014
    Location
    Florida
    Posts
    519
    Thanks
    228
    Thanked 372 Times in 210 Posts

    Default

    Quote Originally Posted by Astro Spectra View Post
    I have a WHCA VHF unit too waiting on time to re prog the PROM (need to find some space or do the bank switch mod). I was super fortunate to inherit probably the only suitcase programmer in this part of the world.

    What was the original antenna for these things? Also was the black telephone handset standard?
    For the VHF a 1/4 wave whip with a red ball tip was typical. I am trying a 5/8 wave for UHF but have some tuning to do.

    The handset seems to be a common feature on the WHCA units. The PT series used by Rail and Forestry seem to have the microphone as a common setup.

    I bought a NOS handset from a west coast seller. However the rubber cord and wire insulation was brittle . I bought a new handset cord from a telephone supplier in Canada and was able to change it and retain the molded Amphenol plug. It was a trick getting the molded rubber off. After much struggling I found an open end wrench slipped between the nut and the rubber put equal pressure on the skirt and easily released it. If you buy a handset on eBay, haggle the price because the cord will be bad.


    Sent from my SM-T350 using Tapatalk

  9. The Following 3 Users Say Thank You to RFI-EMI-GUY For This Useful Post:

    Alpha (Dec 28, 2018),Astro Spectra (Dec 26, 2018),Viper1-6 (Dec 27, 2018)

  10. #6
    Join Date
    Oct 09, 2012
    Posts
    214
    Thanks
    128
    Thanked 57 Times in 27 Posts

    Default

    I love the old lunchbox radios... have enough of them sitting around the house to prove it too!

    As far as handset vs mic... I have two railroad PT500's (N&W and Southern) with handsets, PX300 with a handset, Saber-X with a handset, and a low band PT300 I just picked up that has a mic. Doesn't seem to be any rhyme or reason to it, just what the customer ordered. The PX300 and Saber-X both have the capability to use the handset speaker only for privacy though.

    I would love to get the PT300 moved to my low band frequencies but it's unfortunately beyond my technical ability right now.