Discussion in 'Amateur Radio Amplifiers' started by W2PPJ, Nov 3, 2017.

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  1. W2PPJ

    W2PPJ Ham Member QRZ Page

    operate switch 47.1 up . zero down . there is a small pot in those pics i downloaded . where is that tiny board in the pic located on the drawings???thanks peter
  2. G3YRO

    G3YRO Ham Member QRZ Page

    When you say Zero with the switch down, I presume you really mean infinity? (ie open circuit)

    OK . . . well in that case you have a short AFTER the big bias resistor . . . . in fact it must be either C206 (a .01uF disc ceramic) or C209 (a 2uF electrolytic) that has gone short circuit

    (I think if either of the grid de-coupling capacitors C201 and C202 were short, you would see a higher resistance with the switch up)

    So disconnect one end of C206 and see if it's short circuit . . . if not, check C209.

    Can't see any pot in your photos . . . and that small board contains the components that derive the ALC feed: C204, C206, C207, R204, D201 & D202.

    Roger G3YRO
    Last edited: Nov 9, 2017
  3. WA7PRC

    WA7PRC Ham Member QRZ Page

    You didn't specify where you were measuring. According to the schematic I posted:
    (click for bigly image)
    C209 is 2μF/25V (electrolytic). Unless there's a wiring fault, I'd almost bet the farm on C209 being shorted.
    WQ4G likes this.
  4. G3YRO

    G3YRO Ham Member QRZ Page

    He was measuring where I asked him to - on the RY terminal at the back !

    Well I already said it muct be C206 or C209.

    Roger G3YRO
  5. WA7PRC

    WA7PRC Ham Member QRZ Page

    So did I... first:
    Except, C206 in the ALC circuit has zero to do with RL1. OTOH, C208 is a ceramic capacitor; C209 is an electrolytic capacitor... and MOST likely to be defective.
  6. G3YRO

    G3YRO Ham Member QRZ Page

    No, I meant C208 ! (typing error)

    I suggested to check C208 first, as in my experience, electrolytics are more likely to go open circuit than short . . . whereas disc ceramics can easily develop a short if there's a big spike.

    But we shall see !

    Roger G3YRO
  7. WA7PRC

    WA7PRC Ham Member QRZ Page

    Electrolytics often become leaky. Ceramics, not so often.

    ! <-- exclamation mark added for equal emotional effect.

    Peter: Follow the instructions in my PM in order to quickly determine the fault location, and for a link to the FL2100B documentation I accumulated and created.
  8. W2PPJ

    W2PPJ Ham Member QRZ Page

    is c208 where my screwdriver is and c 209 is on the backside.

    Attached Files:

  9. G3YRO

    G3YRO Ham Member QRZ Page

    Well I think the coil under your screwdriver is the Grid RF Choke L201 . . .

    Can't see them in your photo, but if I remember correctly, C208 is there, but I think the electrolytic capacitor (C209) is under the chassis, connected directly to one end of the big green wirewound resistor (R4)

    So unsolder one end of each, to test whether either have gone short-circuit.

    Roger G3YRO
  10. WA7PRC

    WA7PRC Ham Member QRZ Page

    ... one at a time. If after lifting one lead of the first capacitor, it "plays" correctly, there's no reason to lift one lead of the other capacitor.

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