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AMP QUESTION ? SKIPPER BY PALOMAR AB1

Discussion in 'General CB Services Discussion' started by Switch Kit, Jul 12, 2008.

  1. Switch Kit

    Switch Kit Active Member

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    The amp in question is a 2 x 2879s Skipper by Palomar box AB-1 (mobile) everthing works on the box except the High and Max side of the box ? With 1 watt DK to 18 watts pep on AM , Low does about 10 to 40 and medium does 15 to 60 , and does the same on SSB . The numbers only change very slighty, if any, on High and Max but it still shows major amp draw on those two settings ?



    The amp looks very clean within (for what good that helps me,although the screws on one of the 239s look longer then the others so it's obvious to me that somebody was in there before ? ) Could 1 Transistor be bad on this or could it be something else ? Anything I could look at on this one ?
    [​IMG]
    #1
  2. Switch Kit

    Switch Kit Active Member

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    sorry about the second thread , no idea how that happened ?
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  3. Justme

    Justme Active Member

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    I've been looking at the picture for sometime, and only see the too long bolts on the one intput connector as you did, someone changed that along the line...


    The amp is reasonably classic in design, as most of these are buildt in the same configuration, with or without bias or class C or class AB1.
    This one has the bias, so clearly an classs AB1 for SSB.
    It has an rudimentary Pi filter, so it will be relative clean if it is not overdriven.

    Looking at the resistor network for selecting the drivelevel, i see no burnt resistors there, just check if they still have the value as read out from the coloured rings.

    2SC2879, 120 watt output with 7watt input gain 15 dB at 30 mHz

    You should see with adequate drive 200 watts output with an carrier of 12 to 20 watts bypassing the resistor network.

    If both transistors are good, but the amp doesn't have the output it should have, but does pull the amperage you have an faulty C or an fault in the output circuit.

    In there the HF partly goes to mass through an faulty leaky C or other faulty component.

    Without physically putting it on my testbench, you need to check the components.
    The amp looks clean, no ugly repairs or "mods" i can see in the picture.
    So mostly an component failed, relais has burned contacts, or bad solderjoint.

    Make sure the in and output connector make good ground.
    Rebuilding my KL500 to amateur use amp, the extra grounding straps there gave an almost 3 dB gain.

    Sometimes the in/output transformers are at fault.
    The copper tubing through the ferrites must be thoroughly soldered to the pcb wich is then soldered to the collectors and output or basis and input.

    The (teflon) wire through the transformer should not make contact with the tubing, check for nicks in the isolation there.

    Looks like an decent little amp, hope you get it sorted.




    The network from collector back to the basis , C, R and the L form an feedback from collector to basis depressing too much gain, and stabilising the amp.

    Checking the transistors is relative simpel, lift the basis flaps from each transistor, use an multimeter in diode measurement and seee if the transistors have about 600 Ohms in one direction, basis to Collector, basis to emittor, and thee other way round it should be almost infinite resistance.

    If you have that, the transistors are good, and the problem is elsewhere.

    73,

    Cor
    #3
  4. freecell

    freecell BANNED

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    "2SC2879, 120 watt output with 7watt input gain 15 dB at 30 mHz"

    db gain = 10LOG(POut/PIn)
    db gain = 10LOG(120/7)
    db gain = 10LOG(17.142)
    db gain = 10(1.234)
    db gain = 12.340

    the 2SC2879 is rated at 100WPEP output with 4.75W in @ just over 13db. gain at 28 Mhz.

    db gain = 10LOG(POut/PIn)
    db gain = 10LOG(100/4.75)
    db gain = 10LOG(21.052)
    db gain = 10(1.323)
    db gain = 13.233

    http://www.firecommunications.com/trdbase/2sc2879.pdf

    "use an multimeter in diode measurement and seee if the transistors have about 600 Ohms in one direction, basis to Collector, basis to emittor, and thee other way round it should be almost infinite resistance."

    the diode test function does not provide resistance measurements.
    Last edited: Jul 12, 2008
    #4
  5. Justme

    Justme Active Member

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    I know my dB stuff, but didn't want to complicate things.

    dB gain by an manufacturer is nice, in real life you have losses to compensate, in/output circuits, etc.

    If he uses an multimeter as i suggested he also has resistance ranges on that, and can use the diode test and/or resistance test.

    Finding an datasheet is for nobody an problem, using an Smithchart might be an challenge for some.

    Since i don't know Switch Kit's knowledge about repairing the amp, i tried to keep it to the point and simple, otherwise he wouldn't have asked help if he had thee knowledge to do the job himself.

    Trying to show off that you know how to calculate dB is stuff here for the novice license wich i passed in spring 1977

    Soon after that the full license.

    Main thing is here to help him to repair his P.A.

    My nick is not chosen like it is for nothing, Justme.

    I have no need to show off, i'm past that stage an long time.

    Have an nice day ;)
    #5
  6. freecell

    freecell BANNED

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    "I know my dB stuff, but didn't want to complicate things."

    no you don't because if you did then you would know that at 7 watts of input @ 15db. of gain the transistor output would be 224W.

    db gain = 10LOG(POut/PIn)
    db gain = 10LOG(224/7)
    db gain = 10LOG(32)
    db gain = 10(1.505)
    db gain = 15.051

    "dB gain by an manufacturer is nice, in real life you have losses to compensate, in/output circuits, etc."

    the manufacturers live in the real world, you live in lala land. if you bothered to look at the spec sheet you'll see that's a real live test circuit they used to obtain the specifications for the device. i think everything you were worried about was pretty much covered there.

    "If he uses an multimeter as i suggested he also has resistance ranges on that, and can use the diode test and/or resistance test."

    if he uses a multimeter in the diode test mode he's not measuring resistance and neither are you.

    "Finding an datasheet is for nobody an problem, using an Smithchart might be an challenge for some."

    not a problem here, although it's rather apparent that understanding a data sheet is a challenge for many. better save the smith chart until after you master the simple stuff.

    "Trying to show off that you know how to calculate dB is stuff here for the novice license wich i passed in spring 1977"

    then maybe you could get it right next time.


    Switch, how much current draw are you looking at?
    Last edited: Jul 12, 2008
    #6
  7. Switch Kit

    Switch Kit Active Member

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    Thank you Justme for your input on this. :) Amp draw is roughly 24 to 27 on SSB (ballpark) with 18 watts from radio off a Astron 50 amper on the max side of the box. (I honestly do not know how accurate the meter is on the Astron ?)

    You are right Justme , I honestly DO NOT know that much about amplifiers and how and why they work or don't work. I do know this is a fine well made AB1 amplifier and I know Freecell tends to feel the same way about these boxes.

    At times I look for an easy fix that I can do myself before I attempt to pay somebody else to do it. It also can be a learning experiance along the way. I surely appreciate all help given.
    #7
  8. Justme

    Justme Active Member

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    @ freecell

    Your social abileties lack severely ;)

    The TEST circuit used by an manufacturer on ONE frequency has nothing to do with the broadbanded circuit the transistors are in now.

    So, who is living in lala land now?
    Gain is lowered to stabilise the amp by the network from collector to basis, or didn't you read that conveniently?

    It was an answer not directed to you, nice to butt in, but it was directed to Switch Kit, who obviously didn't have the background to follow that.

    KISS
    Now, go back to your shop, and leave real life to real people.

    For Switch Kit, don't worry too much.
    The rest we dealt in private messages.
    Have fun, and experiment.

    Cor
    #8
  9. freecell

    freecell BANNED

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    "Your social abileties lack severely"

    this is a technical forum, not a dating service.

    "The TEST circuit used by an manufacturer on ONE frequency has nothing to do with the broadbanded circuit the transistors are in now."

    it has everything to do with the amp as the test frequency is in the operating range of the amplifier. the amplifier is designed to operate from 26 - 30 mhz. you do of course realize that 28 mhz. lies in the middle of that range, right?

    "So, who is living in lala land now?"

    that would still be you.


    Switch, there's no substitute for powered measurements under drive conditions but chances are good that one or both of the rf devices are toast. with your limited experience and resources the easiest thing to do would be to remove and re-install a new matched pair of 2879's at which time either one of two things will be obvious, either the original devices are defective or there's a problem on the board. the picture you provided is excellent but i see no visible signs of damage.

    transistors are simply diodes in front-to-front or back-to-back configurations.

    Electronics Tips: Measurements: Testing Diodes and Transistors

    if you can measure and determine whether or not a diode is functioning properly you can do the same with any transistor. in the case of a diode the multimeter in the diode test mode applies a voltage across the probes. with the positive lead attached to the anode and the negative probe lead at the cathode you should measure .5 - .7V and when reversed no current flows, voltage = .0000.

    the 2SC2879 is an NPN device.
    Last edited: Jul 13, 2008
    #9
  10. Justme

    Justme Active Member

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    I've read enough about your wisecrack help :D
    Again, you didn't read nor do you wish to understand.
    Single transistor, peaked input/output, compared with push pull in broadband configuration, what don't you understand?

    Looks like you do need an dating service.

    Replacing the transistors without at least measurement taking is nonsense, why not replace all parts?, then he can be sure it'll work again....

    Have an nice day ;)

    Hey, your problem, not mine.
    #10
  11. freecell

    freecell BANNED

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    "Replacing the transistors without at least measurement taking is nonsense,...."
    maybe you missed reading this part of my last post:

    transistors are simply diodes in front-to-front or back-to-back configurations.

    Electronics Tips: Measurements: Testing Diodes and Transistors

    if you can measure and determine whether or not a diode is functioning properly you can do the same with any transistor. in the case of a diode the multimeter in the diode test mode applies a voltage across the probes. with the positive lead attached to the anode and the negative probe lead at the cathode you should measure .5 - .7V and when reversed no current flows, voltage = .0000.

    the 2SC2879 is an NPN device.

    bye bye now.
    #11
  12. ExitThirteen

    ExitThirteen Grumpy, Cranky Repairman

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    Heya Switch Kit!

    Looks like someone replaced the pills in this one, are the 2879's standard on a Skipper? The one I had used a pair of MRF454's. Mine looked just like this one, board-wise too. I'm wondering if someone might have tried to upgrade the MRF 454's to the 2879's? I don't think they are "direct drop-in" compatible with MRF454's, that may be why you're not seeing anything real noticeable when you change power settings. I'm suspecting someone tried to just drop in a set of 2879's, expecting more power, and it didn't quite work... I'll see if I can find any notes on this amp in my file cabinet.

    -Exit
    #12
  13. freecell

    freecell BANNED

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    even if they were replaced as you say without the necessary changes they would still produce
    more than 60W. there's not enough difference in the series equivalent input and output
    impedances to justify that much of a difference in power output.
    Last edited: Jul 13, 2008
    #13
  14. Switch Kit

    Switch Kit Active Member

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    You know 13 , the thought came across my mind as well (maybe this amp isn't made for these 2879s ?) , but I could tell it hadn't been touched other then the 2 screws on the 239 (at least that's the way it looked to me), I lifted the braid myself to see that they were 2879s . Here's a picture (Thank you Zinger) of the same builder's blue face Palomar 350 , mine has 2 coils and this 1 only has one , but at the same time , I don't know for sure what these transistors are in this one ?(UPDATE : WAS TOLD THEY WERE 2879s) But all the 350s by this guy over the years I've seen had the 2879s in them.

    IM guessing ? this blue face looks older , I remember the pop rivets used on the 239s in the late 90s but then again he (builder) may still use them ? I just hate to put up the money for two new transistors and it be something else.
    [​IMG]
    [​IMG]
    Last edited: Jul 19, 2008
    #14
  15. freecell

    freecell BANNED

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    then test the transistors.
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