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Palomar 300a key up and pre amp problems

Discussion in 'Amplifiers' started by dozerman, Nov 6, 2017.

  1. dozerman

    dozerman
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    hello, its me again

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    just got a palomar 300a from an older gentleman that was getting out of radio due to lack of interest. It seems to be a great amp. It's 2-6kd6 driving 2-6lf6's. on my pres Dwight it does 550 pep under normal modulation. Not as stout as my maco's but much cleaner. Anyway, I started having problems with the rx dropping out. Flipping the pre amp switch helped for a minute then it would drop out. About the same time, the keying relay went out, or rather the relay wouldn't kick in and I had full output, but my rx was gone until I flipped the standby switch off. I ordered new transistors for both, thinking it might be those. The keying transistor had been changed once upon a time. My obvious question is, what is causing this and what are the pin outs for the transistors that are in there? I'm having a helluva time trying to figure out the keying relay. Thanks in advance


     

  2. Tallman

    Tallman
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    W9WDX Amateur Radio Member, KW4YJ EXTRA class

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  3. dozerman

    dozerman
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    hello, its me again

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    I'm not real good with schematics, but the keying transistor pinout looks like emitter, collector and base by the schematic. I'm sure I've read that wrong tho. I've tried that transistor in so many variations and I can't get the relay to kick in. I can get key up in operate, but it kills my recieve.
     
  4. Tallman

    Tallman
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    When keyed the receive is disconnected, but I'm sure you know that. I listed the points for the keying transistor. The actual pin outs can change from different makers.

    palomar_300a_sch.jpg
     
    #4 Tallman, Nov 8, 2017
    Last edited: Nov 8, 2017
  5. nomadradio

    nomadradio
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    Much-more common cause is the 1000uf electrolytic filter cap for the 12-Volt DC relay power. It's located on the pc board under the chassis that has the two large HV filter caps. It's the small one at the front corner towards the meter.

    When it goes bad open-circuit you just don't get enough voltage to close the relay.

    If it goes bad and shorts, this will damage the rectifier diode just next to it. This can cause 12-Volts AC to be applied to the DC relay circuits. Bad juju when that happens.

    All the small 1000-uf caps should get changed if they are original. Just too old to trust.

    The published schematic is just one of several production versions of this model.

    Might or might not be an exact match for your amplifier.

    73
     
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  6. dozerman

    dozerman
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    hello, its me again

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    @Tallman ive tried the replacement transistor for that 2905 in that particular pattern, When I switch from stand by to operate, my recieve is gone. I get key up but have to flip the switch to get recieve again. @nomadradio i just ordered all of the caps and transistors for this unit. The original 2950 transistors. I'm tired of trying to get the replacemts to work and don't want to ruin the thin foil on these boards. They should be in within a week or so. Maybe then I can get this unit back inline.
     
  7. dozerman

    dozerman
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    hello, its me again

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    On a side note, @nomadradio, I tried the shielding trick you showed me on my Maco 200. It helped dramatically. I had crazy swr problems before and now it barely moves the needle. I have it inline now and everyone says it's much cleaner sounding. Thanks for the advice
     
  8. nomadradio

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    Rock on!

    So far, so good.

    73
     
  9. Tallman

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    Sounds like the amp might be oscillating or has a feedback problem.
     
  10. nomadradio

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    Does this one have two nearly-square plastic relays marked "R50-E2-Y2"?

    They are famous for going bad. Repeated failure of the keying transistor sounds like a spike diode gone bad. Each relay has a rectifier diode in parallel with the two coil pins. It serves to "catch" the pulse of reverse-polarity voltage that comes back out of the relay coil when it is de-energized. Reverse polarity is toxic to that kind of transistor, even just that brief pulse. That diode almost never goes bad, but it might explain keying transistors pooping out.

    And the R50-type relay is famous for causing preamp problems.

    One other famous way to make the preamp fail is to use the "SSB" switch. The preamp circuit will get a brief pulse of your AM carrier when you key the mike if the preamp is turned on. There is a small glass diode that serves to protect the tiny preamp transistor from the radio's transmitter output. But it's only sufficient for that brief pulse of AM carrier.

    The SSB switch adds a capacitor to the keying circuit that slows down the keying relay to stop it from chattering on sideband. It also slows down the relay's response when you key the mike. Slows it down enough that a much-longer pulse of your transmitter power now gets pushed up the anus of the low-power preamp circuit. Blows it out with surprising reliability.

    The preamp and the "SSB" switch are NOT compatible. You can use one or the other, but not both at once.

    73
     
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  11. Onelasttime

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    Recap the unit right off the bat. BCS Bad Capacitor Syndrome.....After you recap the electrolytics than test it to see if it is the relay or the keying transistor. You do not need to power up the amp to test the relay test it with out the amp powered up far safer! Keying transistors are usually $.20-$.60 cents so just replace it. TO make shipping worth while do order all the caps, diodes,and transistors you need at once so shipping makes sense becasue I normaly get hit with $9-$14 for shipping no matter if I order 3 parts or 300.
     
  12. dozerman

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    Yeah, you mentioned recapping, I've ordered all the caps in this amp. Along with the 2905 keying transistors, diodes and most of the resistors. The problem in the pre amp and keying I was having turned out to be a 4005 diode and a 47uf cap. All the other stuff should be in by Friday so I will post what an improvement they make, if any. Running it behind my Dwight D, I dk 180 and swing to 550-ish. It's not as simple as my Maco's but I'm liking it for sure
     
  13. dozerman

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    Sorry I skipped this post, but yes it does. The transistor I was having problems with is behind it. Well the emitter attaches to one of them. The pre amp and keying relay work now, replaced a 4005 diode, 2905 transistor and a 47uf cap. Without the preamp, it kills A LOT of ambient noise but I kinda like that. Doesn't hurt someone's signal, just the white noise
     
  14. Rok55

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    Don't mean to hijack the thread but ........ I have a Palomar 200, functional so far as I can tell but unproven. Is it susceptible to the same Preamp / SSB issue?

    And what is this "Shielding" trick?
     
  15. nomadradio

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    A lot of amplifiers with a preamp share this vulnerability. The issue is how fast the relay takes the preamp out of the antenna circuit when you key the mike. So long as your transmitter is feeding RF into the output side of the preamp circuit, this low-power circuit is being overloaded. The question is how long it can tolerate the overload. So long as the relay changes over rapidly, like when your AM carrier kicks on, the short pulse of carrier RF won't cause damage. But on sideband, your "hello" may take longer to activate the relay. If it takes too long the preamp transistor gets toasted by the radio's RF output feeding upstream into its output circuit for just a little too long.

    The shielding trick refers to amplifiers that have a driver stage and a final stage. If the tubes for each separate stage are too close together, an accidental feedback circuit results. Makes the amplifier oscillate, and produce its own RF power separate from what the radio is feeding into it.

    Bad juju when that happens. A shield of small-gauge chicken wire called "hardware cloth" placed between the stages can suppress this by shielding the driver tubes from the final tubes.

    Not typically necessary in a Palomar, but Maco amplifiers are famous for this bad habit.

    73
     
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