Callsign
ad: w5yi
Page 2 of 2 FirstFirst 1 2
Results 11 to 15 of 15

Thread: Heathkit sb-200 repair tech

ad: L-HROutlet
ad: l-assoc
ad: l-sarc
ad: l-rl
ad: l-tentec
ad: l-innov
ad: l-WarrenG
ad: l-gcopper
ad: l-Waters
  1. #11

    Post

    It turned to be the 1.5 ohm resistor. It was open. and burnt.
    I replaced it with a 1.5 ohm half watt resistor. (half watt is ok???)
    Sounds like R15, which leads from 572B grids and to Antenna Relay.

    w9gb
    Nullius in verba

  2. #12
    Join Date
    Apr 2001
    Location
    north central Connecticut
    Posts
    1,051

    Default

    R15? If it was my amp I'd want a metal oxide in there, and at a higher wattage than what Heath used.
    Adding Back to back protection diodes across the multimeter coil might be a good idea.

    If a tube flashes and blows the resistor while you're measuring grid current, I suspect it could take
    out the meter too. Maybe someone else can comment, since i am more familiar with the SB-220 meter
    circuit and I could whistling Dixie, and the schematic I'm using is a very poor copy.

    But at a quick glance that looks like the grid current metering resistor. Nothing good takes out that resistor.

    Pete

  3. #13

    Default

    Ok, well I thought that the 1.5 ohm might be the problem that's why I asked you to check it. You may want to up the wattage on it and use a good 1% or 2% rated one. They are pretty cheap these days. You should be OK for a while if you don't hold key down too long. Glad you didn't have to ship it out. Now you have 90MA idle and that is right on the money.

  4. #14

    Default

    Thanks again for the help, I will upgrade the resistor!!

  5. #15

    Default

    Quote Originally Posted by K1ZJH View Post
    R15? If it was my amp I'd want a metal oxide in there, and at a higher wattage than what Heath used.
    Adding Back to back protection diodes across the multimeter coil might be a good idea.

    If a tube flashes and blows the resistor while you're measuring grid current, I suspect it could take
    out the meter too. Maybe someone else can comment, since i am more familiar with the SB-220 meter
    circuit and I could whistling Dixie, and the schematic I'm using is a very poor copy.

    But at a quick glance that looks like the grid current metering resistor. Nothing good takes out that resistor.

    Pete
     Indeed Pete. . . There are two possibilities when a grid resistor blows. To find out which one it was remove the tubes and measure the anode to grid leakage current at 2x the rated anode potential with a breakdown voltage tester (BVT). If the measured current is less than 10uA, the tube did not flash over. Instead, the burst in grid-I that blew the resistor was caused by an unloaded condition - for which there are also two possibilities. 1. the antenna became disconnected or 2. there was an intermittent burst of VHF energy that was blocked by the HF-tank -- which acts as a LPF. To discourage VHF parasitic oscillation, reduce the Q of the VHF suppressors It's also a good idea to incorporate extensive glitch protection in any amplifier.
    Rich, AG6K, [url]www.somis.org[/url] 805-386-3734

Page 2 of 2 FirstFirst 1 2

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •