SB-220 C4 Overheating

Discussion in 'Amateur Radio Amplifiers' started by NE6I, Oct 18, 2014.

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  1. NE6I

    NE6I Ham Member QRZ Page

    I have an SB-220 and C4 is overheating. The original one had started smoking and was bulging on one end. I replaced it with a new Sprague of the same value. I plugged the amp in and kept an eye on the new cap. I noticed that within a couple of minutes, the cap was getting quite warm, so I turned the amp back off. A visual inspection isn't revealing anything obvious. Can someone point me in the right direction? Thanks.
  2. K9STH

    K9STH Platinum Subscriber Platinum Subscriber QRZ Page

    Where is C-4 in the circuit?

    I have a couple of different scanned copies of the SB-220 manual and, with both, the schematic is too blurred to read the component nomenclature.

    Glen, K9STH
  3. NE6I

    NE6I Ham Member QRZ Page

  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    I would take a look at the Diode D18 feeding it.

    Good Luck on your project.
  5. AF6LJ

    AF6LJ Ham Member QRZ Page

    That is most likely the problem shorted diode.
  6. NE6I

    NE6I Ham Member QRZ Page

    The diode appears to be good. My Fluke 73 shows .518 in one direction and open in the other. That's without disconnecting the diode from the circuitry.
  7. KA9JLM

    KA9JLM Ham Member QRZ Page

    Is the Voltage correct, on the cap ?

    120V DC.

    Can you measure AC ripple on it ?

    Is it wired for 120 or 240V ?
  8. W1BR

    W1BR Premium Subscriber QRZ Page

    Cap has the same working voltage rating, and capacitance? The capacitor polarity is correct -- the Negative lead goes to the chassis?

    Be careful when testing. You can easily damage the bias transformer, which is a very expensive item.

  9. NE6I

    NE6I Ham Member QRZ Page

    Yes, new cap is 20 uF at 150VDC. Cap is in the correct polarity with negative lead to chassis.

    Interesting finding when measuring the voltage across the cap. Powering up the amp resulted in the T/R relay clicking open and closed for about a minute and then stopping. The voltage was 118 VDC and would jump to 220 or something like that. Ripple is about .2 VDC.

    After the T/R relay settled down, voltage remained at 118 VDC and the cap did not seem to heat up.

    I'm also noticing that there is no indication on the plate meter. No grid or plate voltage shown. Not sure if that's related or a separate problem.

    The amp is wired for 120 VAC by the way.
  10. W9GB

    W9GB Ham Member QRZ Page

    I would never run this SB-220 HF Amplifier on 120 VAC.

    C4 is the capacitor ACROSS the BIAS Leads (RED) on the Bias/Filament transformer.

    1. MEASURE the AC voltage from that transformer winding (RED wires).
    Jumping to 220 Volts, indicates a problem. Intermittent, Short, Loose Hardware, etc.

    2. IF the Voltage is INCORRECT (Too High), THEN your issue may be with AC primary wiring.
    This could be improper 120 VAC hookup, broken wiring, an intermittent, loose chassis hardware, or
    a mistake in original wiring of the kit. DOUBLE CHECK WORK.
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