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Broken Resistor On a Palomar 255 Amp

Discussion in 'General CB Services Discussion' started by Buckshot555, Jul 25, 2010.

  1. Buckshot555 Member

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    I have a Galaxy 225 Amp that is an EXACT copy of a Palomar 225, blue face. I picked it up used at a cb shop for 50 bucks, the owner said it didn't have full output power. It seems to be a pretty sturdy little amp, but the problem is that it only puts out about 50 watts.

    When I inspected it, I found a resistor that has broken one leg off of its solder joint, the rest of the amp seems fine. The pre amp works amazingly and the first radio check I requested got me a "you sound somewhat scratchy" but that was with a cheapo mag mount antenna on my truck and the reply was from another CBer 41 miles away in the woods!

    Its my first amp and though it works great already I just want to get all the power out of it.

    There is another resistor on the board that broke in the same spot that has been fixed by the previous owner. It seems he didnt even remove the board from the chassis and just put new solder on one leg of the resistor and connected it right to the board.

    I have tried to remove the board but it seems to be impossible without desoldering the front switches and drilling out the rivits that hold the SO 259 connectors to the chassis. I tried to remove the four screws that hold on the faceplate, but it still was connected to the frame somehow.

    My question is how can I remove the board from the chassis to solder the leg back on? Also, could I just patch it the way the previous owner did and not have to remove it?

    I know that boards have a solder side and a component side but it seems that the pathways on the board are on the component side as well and the components are just soldered to parts of exposed pathways on the component side. If this is the case, I should be able to get away with just soldering the resistor back on the component side right?

    Its my first amp and I'm really looking forward to a response so I can get this guy up and running and make contacts from 82 miles away!!!



    I have added a few pictures to help everyone get a better idea of what's going on. The second and third pictures are of the broken resistor and the fourth is of the broken resistor that was fixed by the previous owner.

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]
     
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  2. GLR Keeping It Good Natured

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    82 miles?

    awesome
     
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  3. Buckshot555 Member

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    Ha ha. Well I was saying that because getting the other pill to fire up would double my output power and maybe double my distance....

    In all fairness, the other guy was 41 miles away with a tower..... :laugh:
     
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  4. bob85 Supporting Member

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    buckshot, the 820pf compensation cap is missing from the output transformer
     
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  5. Buckshot555 Member

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    Is that the brown square looking capacitor I saw in some other pictures of this style amp? What does it do? Can I buy another one and solder it in place?

    Also would you happen to know what it is that the broken resistor controls?
     
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  6. mackmobile43 Jock Supporter

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    It's one of 2 feedback resistors..
     
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  7. bob85 Supporting Member

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    the cap compensates inductive reactance in the transformer primary, you can buy one and solder it in,
    the unsoldered resistor is as mack says one of the feedback resistors, along with other feedback components they do several things, lower gain, lower input/output impedance and often stabilise unstable amplifiers,
    the other resistors that have had some hammer look like part of the input attenuator.
     
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  8. mackmobile43 Jock Supporter

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    Input padding which Bob85 eloquently stated which without the use of the smokeless resistor would failed long ago, Galaxy don't like 10ohm smoke smell.

    A guy could up the wattage of that resistor and rid one's self of the ugliness but it's an easier solution to just keep the input power from the radio below 5 watts which that resistor is rated for.
     
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  9. Buckshot555 Member

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    Thanks for the replies guys! So its good to hear this amp is fixable. Now that you mention it, it does look like it fried off and the previous owner just threw it away.

    Since this amp is a Galaxy and not a Palomar, is the 820p capacitor the right one? If I get the wrong capacitor, could I run the risk of frying the amp?

    I'm not the best at electronics but I am a computer repair and network technician by trade, and I hold a basic Amature radio liscence. I know a few things about resistance, capacitance, waveform, propagation, etc, but I don't know much about how amps work. I also don't have the money for another amp, and I don't want to blow up what I have.

    Where exactly is the cap soldered? I see a circular type cap that is soldered on top of the transformer that is near the front panel with two ferrite beads. It seems to be soldered to both sides. Is this how the 820p cap is supposed to be? I've attached a picture where I think it should be connected. Please correct me if I'm wrong.

    Also, I cannot seems to find an 820p capacitor at radio shack online, or any other online shop for that matter. If someone could point me in the right direction I would greatly appreciate it.

    I thank you all for taking the time to answer my questions and I'm am enjoying the learning experience, because after all, thats what its about isnt it?

    This first picture is where I think the cap should be soldered. Am I right?
    [​IMG]

    This picture is of another palomar 225. The cap is the brown square one. If I'm not mistaken isn't an 820p cap a circular one? Does this matter?
    [​IMG]

    [​IMG]
     
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  10. mackmobile43 Jock Supporter

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    The cap required for that application should either a silver dipped mica(brown rectangular cap) or a metal mica for stability reasons when higher voltages and or exposed to high heat situations, don't really care for disc capacitors for that reason.

    Different transistors require different values at certain frequencies and your amp I believe has MRF455 transistors and I would believe Bob85 if he says your amp should have an 820pf cap in that location, he's a [​IMG]
     
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  11. Buckshot555 Member

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    Correct. My amp takes MRF455's. Ok so I'll get ad brown square 820pf cap. So am I correct in thinking the cap needs soldering on the top of the transformer in my picture? Also can I fry anything if I'm wrong? I'm trying to locate the correct cap at the moment with no luck so far so any help would be appreciated. :)
     
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  12. Buckshot555 Member

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    Well looking at the picture of the other palomar, the cap appears to be 820pf, 5% tolerance, rated for 500 volts DC.

    Is this correct for my amp? Would this cap work?

    DM19FD821JO3: 820pF 500V Dipped Mica Capacitor
     
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  13. mackmobile43 Jock Supporter

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    yes and Heres a much better deal on some.
     
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  14. loosecannon break on through

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    yes, the cap gets soldered to the points you laid out in your pic.
    LC
     
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  15. Buckshot555 Member

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    Once again, thanks everyone for your help. I'm going to pick up a few of these caps when I get back to the house. I'll solder them in the spot where the old used to be and fix that feedback resistor.

    Hopefully I don't fry anything :laugh:

    I'll let everyone know how it went.

    So is this cap prone to failure, or was it user negligence that caused the burn up, maybe from keying to long?

    One more question, I hope I havent been too anyoying. My radio is a Cobra 19 ultra, I tuned it and it deadkeys at 4 watts and peaks to about 12. Would this be too much drive for this amp?
     
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