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Heathkit SB221 Inspection

Discussion in 'General CB Services Discussion' started by 357, Nov 6, 2019.

  1. 357

    357 Walkin' the dog

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    Hmm thanks.
    I was hoping the amp sat many years and then when hamhifi got it and turned it on or someome previous, a tube arced and that did it. Dang I hope I didn't get a lemon


     

  2. nomadradio

    nomadradio Analog Retentive

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    The Ebay vendor is a legitimate guy. I don't believe he sells junky stuff knowingly. You might email him pics of the exploded R3. If he didn't power it up, there's no way he could know that this sort of gremlin could be hiding in it.

    Don't know what he would do in response, but you won't waste a lot of effort to find out.

    73
     
    357 likes this.
  3. 357

    357 Walkin' the dog

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  4. 357

    357 Walkin' the dog

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    ok I did the "short" test.
    I didn't leave it long.
    I hope thats enough...

     
    Woody-202 and Shadetree Mechanic like this.
  5. Dmans

    Dmans Sr. Member

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    Not a Lemon, just a 40 year old amplifier!

    Good Luck.

    73
    David
     
    SIX-SHOOTER, Woody-202 and 357 like this.
  6. nomadradio

    nomadradio Analog Retentive

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    The Harbach board with the snap-in caps is what we have been using for a while now. No failures in the field yet. I do one thing to reduce twisting stress on the caps' solder connections to the pc board. We solder one pin only of each cap. Having the caps attached to the board makes it less clumsy to insert the board into the egg crate and then tighten down the mounting nuts. Any slight sideways skew will now be relieved when that one solder connection on each cap is first remelted with the iron. You might see one or more of the pins shift just slightly in the hole when you do this. Once that's done, it's time to solder the other pin of each capacitor. This way the solder has solidified with the pins in their "relaxed" position. No sideways pressure on the solder. Solder is not a good structural material. Any kind of 'spring' pressure on it can cause fatigue cracks to form in the solder fillet after a few hundred cycles of operating-temperature rise and fall.

    A 5-Watt wirewound resistor is fine for R3 if you just want to replace that alone.

    I like the W7RY board mainly because he uses stouter diodes for the zener replacement. Jeff's Harbach meter/HV board does a good job on sideband, but I don't recommend it for AM use.

    The rectifier diodes on your original board are the 'small' type, and should probably be upgraded to the larger 1N5408 type used on both the Harbach and W7RY boards. The leads are thicker on that part. You would have to drill out the holes to simply upgrade your factory board to the larger rectifiers.

    A new pc board would be my recommendation in light of that.

    Your HV transformer passed the 'ground one side' test. Just say prayer for it every night before bedtime.

    Could be that the transformer's actual breakdown voltage is just a little higher than what your line voltage puts onto it. If it deteriorates, and that threshold comes down after a while, it will pop R3 again later on down the line. This is where putting 3500 Volts DC onto the red and red/yellow wires with a tester can improve your confidence in the transformer's expected life span.

    But for now it looks as if you're okay to rehab this one and put it on the air.

    Sure is a clean specimen. That's usually a good sign.

    73
     
    #21 nomadradio, Nov 9, 2019
    Last edited: Nov 9, 2019
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  7. 357

    357 Walkin' the dog

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    This is confusing, the schematic shows 1 and 4 after the breakers.
    Pictorials and my amp show it wired before and with the line in.


    2019127.jpg 2019128.jpg
     
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  8. nomadradio

    nomadradio Analog Retentive

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    Don't know where you got that diagram. Whoever did the redraw missed the mark.

    The one posted here is the original. And it matches your amplifier's wiring, with the line cord connected directly to 1 and 4.

    http://www.cbtricks.com/Amp/heathkit/sb220/index.htm

    73
     
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  9. 357

    357 Walkin' the dog

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    B. Bolin

    I'll put a red x so nobody uses it

    heathkit_sb221_sch.jpg
     
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  10. nomadradio

    nomadradio Analog Retentive

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    Well dern!

    Guess I never used Bennie's diagram enough to notice that detail.

    73
     
  11. 357

    357 Walkin' the dog

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    I was gonna ask about this board design, it seems like those resistor holes are right where the rim of the aluminum can are and with a clipped lead and solder, it looks like it would interfere and possibly arc over.
    I kinda thought that soon as I looked at it but maybe I'm paranoid?
    I think I'm just gonna go all harbach, this is gonna be SSB only so unless the ebay boards are really a better design.

    Thank you for your advice, Nomad

    s-l1600.jpg


    Also do you know what board Harbach uses now?
    It shows the older design in the picture at the site but I see a better version since 2016.

    P1090554.jpg

    Then I read the new design had fitment issues, I hope its fixed:

    The board as supplied WOULD NOT FIT so I had to build an offset mount so the board would clear everything. The new board was over .100 inches wider from the screw hole to the edge of the board and it ran into the cage around the high voltage section. Disappointing that the design was so far off. I moved it "north" about .250 inches.
    http://www.dasplace.net/SB221 Amp/SB-221Amplifier.htm
     
    #26 357, Nov 19, 2019
    Last edited: Nov 19, 2019
  12. 357

    357 Walkin' the dog

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    Compared to the board above, the Harbach version puts the solder points outside of the circumference of the capacitor by quite a bit
    Only 375 volts, should this be a concern?

    ES4SB220-1.jpg

    poster_156_z.jpg
     
    #27 357, Nov 22, 2019
    Last edited: Nov 22, 2019
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  13. nomadradio

    nomadradio Analog Retentive

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    So long as you clip the bleeder resistors' lead wires properly flush to the pc board there should not be a problem. One way to improve this detail is to solder the bleeder resistors from the component side, the side the resistors are mounted to. This will tend to make the solder fillet coming out the other side smaller and flatter. And easier to clip the lead flush.

    We have placed bleeder resistors on the solder side of pc boards just below each snap-in type filter cap for years without trouble.

    Just for grins, I sacrificed one board by placing high voltage from a breakdown tester between the pc board and the metal cans of a few filter caps. Took more than 2000 Volts to punch an arc through the plastic sleeve of the capacitor with this arrangement.

    The actual voltage difference between the bleeder resistor's solder fillet and the metal can of the cap mounted above it is the actual voltage drop across that resistor. The plastic sleeve covering the snap-in cap will never have more than the cap's voltage rating to insulate.

    Max.

    Not a risk in my experience.

    73
     
    357 likes this.
  14. 357

    357 Walkin' the dog

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    I grounded the grids, they are soldered very solid.
    Screws are tight on the lugs, hopefully this is sufficient.

    IMG_4815.JPG IMG_4816.JPG IMG_4818.JPG IMG_4819.JPG IMG_4820.JPG IMG_4821.JPG
     
    #29 357, Nov 24, 2019
    Last edited: Nov 24, 2019
    Shadetree Mechanic likes this.
  15. 357

    357 Walkin' the dog

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    Getting closer....

    IMG_4830.JPG IMG_4831.JPG IMG_4912.JPG IMG_4909.JPG IMG_4910.JPG IMG_4911.JPG
     
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