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Argonaut 515/2SC1969 p.a.

Discussion in 'QRP Corner' started by G0DRK, Oct 17, 2020.

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  1. G0DRK

    G0DRK Ham Member QRZ Page

    Hi everyone,I,ve replaced the p.a. transistors in my 515 from MRF475,s to an easier to obtain 2sc1969,s I,m having difficulty setting the bias current.I believe the 2sc1969,s will need more than the 10ma of the originals. The 475,s that were fitted were replacements by a previous owner but had gone QRPp.
    The resistors on the p.a. had been changed from R1/100ohms R2/330ohms to R1/47ohms R2/220ohms to alter the bias current.
    Has anyone whose changed to 2sc1969,s got the values of the two resistors i need to fit to start the p.a.?
    Any help would be great
    73 Kev
     
  2. G4COE

    G4COE Ham Member QRZ Page

    I can't help you with the resistors.... hope you know the MRF475 use the emitter as tab and being the middle lead while the 1969 has the collector tab in the middle.

    We also need to be extremely careful where you got the 1969's from because there's a lot of fakes knocking about - I've got half a dozen of em' I got from a Ebay supplier, other folks on the forum has been 'conned' too, messing with such devices will send you going round in circles and getting nowhere.

    If you look at the schematic here you'll find they are MRF476, page 3-25, may be you better check the circuit board number as there *may* be variations.

    Dave

    Edit:

    Using 1969's means you will need to isolate them from chassis with proper insulating kits, mica and plastic bushing.... don't for get the heatsink compound to aid heat transfer.
     
  3. G0DRK

    G0DRK Ham Member QRZ Page

    Thanks for the reply.On closer inspection the p.a. transistors i,ve taken out are actually mrf475,s Obviously replaced by the previous owner after the 476,s became unobtanium.
    The 2sc1969,s i bought were "new,old stock" from nevada who had sold 268 so i think they should be genuine.Commonly used i believe as a cb p.a.
    I have them insulated with mica washers etc but the current is too low to start them as i see it. Ten tec states 10ma for 476,s and 2sc1969,s approx 40ma.I,m actually stuck at the moment......not sure what to do...maybe try and find some 476,s and put it back as it was??
     
  4. G4COE

    G4COE Ham Member QRZ Page

    2SC1969 are used in CB pa stages as you say and 'burners' as the CB brigade calls them and usually ran in class C rather than linear mode.

    I wonder, would its not be feasible building a PA using a modern device, possibly using a FET of Motorola RD series and placing it between driver an output filters?.

    You mention the 1969's not drawing current, I can only think of R1, D1 and R2....or the transistor well, I trusst you have checked thesew components, the transistor can soon be sorted in one or two ways-

    1) Use a transistor tester, don't be surprised if there's a diode between collector and emitter.

    2) Lob a 12V 60mA lamp (a meter lamp is ideal) in series with collector and +12V, emitter to negative.... then get a good 1.5V battery with 2.2K preset across it negative to emitter the wiper going to base of the transistor..... before switching ON ensure the preset wiper is turned down to the negative end, by slowly turning the preset up the bulb should start to glow..... if it doesnt measure the voltage on the base, should it get to about 0.7V that transistor should be turned fully ON and the bulb lit if not that transistor (or the bulb) is faulty.

    I can only stress beware where you get transistors from... fakes may work a day or two, may work a few seconds.... or not at all and to add to the fun some may produce only low power...! They will have you suspecting other faults when there isn't.

    Dave
     
  5. G0DRK

    G0DRK Ham Member QRZ Page

    Thanks for your input.I,ve put original mrf475,s back in but still cant get it working.I,m going to put it up for sale so someone can fix it and use it again.Shame for it to go to waste.
     
  6. KA5NLY

    KA5NLY Ham Member QRZ Page

    Dang! It's a shame that I'm broke and will be for a while. Love those Argonauts!

    Smiff
     
  7. N8AFT

    N8AFT Ham Member QRZ Page

    I ain't broke but badly bent...
     
    KA5NLY likes this.
  8. G4COE

    G4COE Ham Member QRZ Page

    Are we getting any drive to em?

    Dave
     

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