Yet another Palomar 300a ? with a problem !?!

Discussion in 'Amateur Radio Amplifiers' started by WV9K, Apr 28, 2013.

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  1. WV9K

    WV9K Ham Member QRZ Page

    Hi and thanks in advance!

    Got one of these at a price I could afford, complete with the separate box and cable for the transformer.

    Plugged it in and the transformer buzzes to beat the band. Took it apart and tightened up the bolts / nuts that hold the laminations together. Helped some. Stuffed a couple wood shims between the transformer case base and the top which also helped, but not much.

    Hooked up a ~2W 10M rig (because it was handy :)) and attached my MFJ-949D to the output set on the dummy load and tried it on both low and high power switch positions. Outputs are close to dead on to the charts in the manual (~40W on low and ~175W on high). Preamp seems to work too.

    Other than the buzzing it seems to be working. I thought I'd ask about the buzz here before sticking my hand (hand, one in pocket!) in it.

    Yes, I did take the amp part apart too and brushed, vacuumed and blew the dust out of it. Nothing obviously hacked that I could see.

    Will have to say that seeing those filaments glowing was somehow very satisfying.

    Not the ideal amp. but it covers anything I am likely to be doing (40-10m). Also don't plan on running the pants off it, the values from the 2W test would be just fine.

    Any thoughts appreciated.

    73, Vikki.

    P.S. Forgot to mention that the buzz is constant whether it is just on or switched in and amplifying...
     
    Last edited: Apr 28, 2013
  2. KO6WB

    KO6WB Premium Subscriber QRZ Page

    The buzzing may be annoying but if you try tightening the bolts that hold the transformer together then you may reduce
    or eliminate the buzz.
    As for the suitability of the Palomar 300A for applications on the amateur bands I would be much more cautious. If used on
    SSB service the linear may not be that linear. It could make you very unpopular in that mode. For RTTY, CW, PSK31 or any
    100% duty cycle mode it might do okay. That's provided the output tuned circuits are adequate to suppress the harmonic
    and spurious outputs below the -43db level. You may need to add additional filtering to obtain that goal.
    The distortion products developed during linear service are not easily measured by just chatting with another station and
    asking how you sound. That won't tell you much except if it's way overboard or not. You can still present a high level of
    IMD, splatter, shotgunning, etc and still sound okay on the transmit frequency. To truly measure the IMD products requires
    the use of a spectrum analyzer or a really, really good receiver that can plot the exact transmit signal faithfully.
    So, you may have a lot more work to do before you can call the 300A good to go.
    This is just the nature of the product you obtained. It can be made better with some work.
    Good luck and have fun
    73
    Gary
     
  3. WV9K

    WV9K Ham Member QRZ Page

    Thanks for the reply Gary.

    A brief bout of sanity suggested I might take the transformer off the hollow door worktable and sit it on the floor. Wow, vast majority of the noise gone! Replaced the missing back feet on the RF deck and I think what I have now is mostly fan noise getting amplified through the hollow door.

    Going to move the whole mess to a solid table tomorrow and see what happens.

    Then we can address the other issues :).

    73, V.
     
  4. KO6WB

    KO6WB Premium Subscriber QRZ Page

    Yup Vikki, mentioning the hollow door worktable would have been great:rolleyes:!
    Had a guy at work trying as hard as he could to figure out why all the connections
    in a repaired contactor bucket were shorted. Took awhile to get him to understand
    the stabs on the back of the bucket were exposed and the bucket was sitting on a
    metal top workbench.....It happens:rolleyes:!!!
    Good luck
    73
    Gary
     
  5. WV9K

    WV9K Ham Member QRZ Page

    Problem is that I have never had an HF tube amp. A few 100W or so VHF/UHF ones so I really wasn't expecting all that goes with ~300W.

    It looks like the rest of the noise comes from the fan in the unit. Lot louder than just the exciter itself ;-).

    I'm pretty much a rather dedicated QRPer, but boots are nice to have if one needs them. My problem isn't being heard so much as not being able to hear. Fourth floor apartment (~25' EFLW inside) with crappy cable wiring (1960s RG-59) and an industrial park across the street.

    Not the greatest amp but not terrible based on much reading. Just glad it is one of the later ones with the bandswitch.

    Will have to do until I can come up with something better than just happens to coincide with sufficient $$$ :).
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Yeah, it seems to have been just acoustic coupling and placing the PS on the floor, if it's solid, should help a lot.

    My power supplies are all on the floor, and with my first-floor ham shack, there's "cement" almost right under them.

    As I'm sure you know, the Palomar is really a "CB" amp, but if you don't overdrive it, it should work okay. I'd recommend not ever using the "RF keyed" T-R circuit which places the amp in line when it detects a signal from the exciter...that's a bit of an accident waiting to happen, and also the detector in that circuit can create more harmonics than you need to have.
     
  7. KJ6SVX

    KJ6SVX Ham Member QRZ Page

    Can you elaborate on the accident waiting to happen risk?
     
  8. KO6WB

    KO6WB Premium Subscriber QRZ Page

    When you have full power from the exciter applied to the amplifier under RF sense keying, then
    the amplifier will trigger the keying circuit and for a moment there is no load on the exciter or the
    amplifier. This causes problems after a short time. Initially it'll work just fine.
    The other problem concerns the installation of a non-linear device (diode) into the area where it
    can be fed back into the input of the amplifier. A non-linear device generates harmonic energy
    and signal mixing.
    None of these products are a good thing to have applied to an amplifier circuit.
    Hope this helps
    73
    Gary
     
  9. KJ6SVX

    KJ6SVX Ham Member QRZ Page

    Thanks for the info.
     
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