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repairing a texas star 667

Discussion in 'General CB Services Discussion' started by bigred222, Jan 25, 2014.

  1. bigred222

    bigred222 Key Up City Radio

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    ok so i have decided to fix my 667. After getting on 38lsb with my new (for me) cobra 146gtl and trying to make contacts across the pond. i know the output combiner is fried and im assuming all the transistors are fried so i am going to order a matched quad set of DEI 2879s, i would appreciate any help on anyone who could point me in the right direction of getting this thing sorted out and back running again. im guessing its not as simple as just replacing the transistors and output combiner then hooking it back up again and flipping the switch. i figured it will cost me less to replace the parts then it would be to replace the whole amplifier or build one myself and itd be nice to have an easy 500 watts in the mobile and would probably talk pretty good also counting on the fact that im going to be rigging up something to mount a 3 element beam for when i post up at the local beach.
    i have attached a photo for those of you to look at it and tell me what you think. should i start by pulling the transistors?
    you can see the fried output combiner near the coaxial cable on the top of the board towards the faceplate of the amplifier


     

    Attached Files:


  2. nitroturbo4u

    nitroturbo4u Member

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    667 troubleshooting.

    Hi Dan here in Conway S.C. AKA TURBO.
    Here is a cheat sheet i made up...Use this first.

    No receive:

    when the power is on, does the light behind the meter turn on?

    if the meter light turns on without being keyed up, then the keying transistor is shorted. Remove the 2n2907a which is located behind the relay.
    now the lamp should not turn on.



    if the meter lamp does not turn on and there is not receive, I expect that the receive transistor is not working. Replace the transistor behind the receive switch.

    Use a 2907a



    No transmit output:


    let's back up and start from scratch.

    Turn the amp off. Key the radio. The radio should pass radio power straight through it and should have a low SWR. If the straight through condition is not A-OK, we need to fix that before we do anything else.

    Turn on amp and key up radio. Does the relay click? Does the meter lamp light up while radio is keyed up?

    If the relay doesn't click and meter lamp turn on, replace the keying transistor with a new 2n2907a.

    Now if the relay does click and if the meter lamp does turn on while keyed, is there any power output from the amp.

    No output can mean blown power transistors (commonly called pills).

    Checking the Pills:

    Now we need to determine if the transistors are ok or if they are blown.

    With the power turned on read the voltage on the top lead (collector) of the transistors. We want full B+ here, 13.8 volts d.c.

    More than that and you blew the transistors. Less than that and you have an open circuit. This is connected directly to the fuse connector.

    Now check the voltage at the bottom lead (base) of the transistors. This should read 00 volts d.c.

    If you read any voltage here now, you have leaky transistors. They will need to be replaced. Read the cautions in next section.

    You can check the transistors without removing them, but you do need to get them out of the circuit. This is called floating the transistors. Carefully lift the top tabs (collectors) so they do not touch the traces. Now carefully remove the bottom tabs (bases) so that they do not touch the traces. The transistor pair is now out of the circuit. Now you need to check the resistances.

    Connect the black lead of your ohmmeter to the top tab and the red lead to the bottom lead. This is your collector base junction and it should read about 600 ohms. Now connect the black lead to the ground trace (emmiter) and the red lead to the bottom tab (base). This should also read about 600 ohms. It will be common to find open or shorted junctions. This indicates a blown transistor. If these resistances are as I say, the transistors are good.

    Before changing transistors please read the cautions in the next section.

    Burnt bias resisistors always indicates blown transistors.

    When changing the two 10 ohm resistors you will be well advised to also change the 1.6 or 1.8 ohm resistor also.


    Changing the Pills:


    When replacing transistors on a Texas Star amplifier, it is imperative to ALWAYS check the bias circuit of the transistor pair.

    There will always be two 10ohm resistors from the bases AND a 1.6 (or 1.8) ohm resistor from the bead wires to ground.

    Look for about 1.3 ohms to ground from the input transformer (or the transistor bases).

    Note: If you read 5 ohms that’s the 2 tens, and the 1.6 is open.

    (exception is 667 and 667v- front board has 2 - 10s and a 2.7 ohm and driver has a 6.2 ohm to

    ground for equivalent resistance)

    Now, if you are sure that the effective resistance is correct, manually key-up the amp and check the voltage at the input transformer (transistor bases).

    It must be 0.6 volts d.c. **** *** OR DO NOT key rf into the amp.

    If this voltage is 0.0 volts there is a short in the bias circuit

    If this voltage is > 0.6 volts then there is a bias resistance problem or a leaky transistor


    Manual key-up procedure:

    Behind the relay there is a 470 ohm resistor. The side closest to the relay is the emitter of the

    keying transistor. Short this point to ground with a screwdriver and the relay will manually key up



    Burnt toroids:


    Amplifier boards each have a pair of transistors. Two boards can be combined for a more powerful amplifier. The input signal will go to a separator toroid and the outputs will combine at another toroid.

    Simple concept. But both amp boards must be exactly the same as each other. If one has less power or fails entirely, the combiner toroid will burn. Therefore, a burnt toroid indicates that the transistors on one board or both are blown and require replacement.


    Precautions:

    Make sure that you are using a properly installed ground plane antenna system. Run ground straps from your vehicle frame to the antenna bracket. Even if this is a mirror mount. Do not ever use a magnetic mount antenna system.


    Make sure that the coax connector grips the antenna center conductor. These connections commonly fail and you will have an effective open circuit.


    Make sure that supplied voltage is at 13.8 volts d.c. and not more or less. Insufficient power connections will cause low power output.


    Make sure that the radio output wattage is not more than the rated input for the amplifier.

    Troubleshooting:

    First we need to really determine exactly what is wrong.

    I will describe some common conditions and go from there.

    No receive:

    when the power is on, does the light behind the meter turn on?



    if the meter light turns on without being keyed up, then the keying transistor is shorted. Remove the 2n2907a which is located behind the relay.

    now the lamp should not turn on.



    if the meter lamp does not turn on and there is not receive, I expect that the receive transistor is not working. Replace the transistor behind the receive switch.

    Use a 2907a

    Hope this helps.:pop:
     
    Robb, Robalo, Eldorado828 and 3 others like this.
  3. TonyV225

    TonyV225 W9WDX Amateur Radio Club Member

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    (y)(y)Turbo your an outstanding guy for taking the time to simplify this trouble shooting method and typing it out for him and It will be helpful to many others as this will work with most other amplifiers out there. This is a great starting point put into laymens terms for all people to understand.

    We have the best forum on the net here and I will never change my mind on that or say different, We have a great family here at the WWRF. (y)
     
  4. Demetrios

    Demetrios New Member

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    Can someone please show a pic of the 2n2907a
     
  5. 357

    357 Walkin' the dog

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    Ranch55 likes this.
  6. Bama512

    Bama512 New Member

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    Is this the same info pretty much for a 3 pill Dave made box? Can't get relay to make and contact.. God bless
     
  7. TheRealPorkchop

    TheRealPorkchop Certified Sith Pimp

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    No clicky clicky means bad transistor for keying most likely. Or possibly bad relay. That’s probably a 2N2222 keying transistor.
     
  8. nomadradio

    nomadradio Analog Retentive

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    Nope. Texas Star uses a PNP transistor. Originally a PN2907A. There are literally hundreds if not more substitutes that will work.

    A burned combiner tells you only that one pair of RF transistors was (past tense) putting out power, and the the other pair was not. Or not as much. Only takes an imbalance of power between the one push-pull pair and the other. Naturally if one pair shuts down, that's a total imbalance.

    Doesn't tell you why. A capacitor that comes unsoldered can do this.

    Blown RF transistors is a much more statistically-common cause. Especially if you observe any of the four 10-ohm base resistors burned. That's a total giveaway.

    Never forget to replace the bias resistors, the quarter-watt one that goes to ground from each 25-ohm 5-Watt resistor, one on each circuit board. They're R16 and R23 on the schemo.

    They typically blow when a transistor shorts and fails. New transistors will run terribly hot if either or both of those resistors is blown.

    Until they blow out.

    73
     
    Shadetree Mechanic likes this.
  9. TheRealPorkchop

    TheRealPorkchop Certified Sith Pimp

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    I must be high. I was thinking it said Davemade. Damn, I got to go to bed, it catching up with me finally.
     
  10. loosecannon

    loosecannon Sr. Member

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    LOL TRPC, you weren't wrong, he did say he had a davemade, but nomad was pointing out that he cannot just use the info for the texas star in order to fix his DM box.

    the TS used a PNP keying transistor, and the DMs and all the other similar boxes use an NPN.

    I mean don't get me wrong, you might be high and confused but not in the way you were thinking LOL.
    LC
     
  11. OldTech03

    OldTech03 Well-Known Member

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    Who said that? I didn't see that? did you see that? who is on 2nd anyway? Now I know what he said. No wait he didn't say that.
     
  12. nomadradio

    nomadradio Analog Retentive

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    Whups!

    Gotta learn to look at the post date at the start of the thread.

    The 667 guy was from 2014.

    Doh!

    DaveMad or clone is probably a NPN transistor like the 2N2222.

    Reading a number printed on the old part would be like cheating.

    73
     
  13. TheRealPorkchop

    TheRealPorkchop Certified Sith Pimp

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    Y’all stop. You’re confusing me even more. I’m running off 3 hours of sleep here in 2 days.

    I got to bail on this and go to sleep.

    Oh, I’ve never been high either, I was sheltered as a child and seems as an adult too. Maybe I should get high, maybe then I could sleep.
     
    Shadetree Mechanic likes this.
  14. Bama512

    Bama512 New Member

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    Thanks for the help, just changed relay and keying circuit , still will not key the relay.... Still baffled by this box..
     
  15. nomadradio

    nomadradio Analog Retentive

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    Starts to make it sound as if maybe there is a "remote" switch hookup that's disconnected.

    Or maybe someone wired it wrong before you got it, and copying that setup just would not work in the first place.

    No wiring diagram for those.

    Have you ever seen it key properly the way it's set up?

    73
     

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