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An SB-220 needing lots of attention

Discussion in 'Amplifiers' started by Dmans, Jan 3, 2022.

  1. RedRiverII

    RedRiverII Supporting Member

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    Thank you for this post, it seems amazing that you know what the heck to do. Great job though, I know you know your business, muffling that fan rattle was a good idea. BTW I'm new and highly impressed.


     
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  2. Dmans

    Dmans Sr. Member

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    To continue..........
    The internal coax runs were replaced with more modern coax. Below the RG-303 from the relay to output RF connector is in place.
    IMG_2088.jpg
    Time to test and "tweak" pass thru SWR. I guess I didn't take a picture of the pass thru SWR measurement before modification but if memory serves it was in the neighborhood of 2:1 on most if not all bands. The VNA was connected to the input S0-239 and a dummy load was connected to the output S0-239. No A/C power, no tubes installed etc. In other words the amplifier was safe to put hands inside. A variable trimmer capacitor was temporarily soldered between relay terminal 9 and chassis ground and the VNA was set up to sweep from the low end of 80 meters to the upper end of 10 meters. when the "sweet spot" was found the trimmer was measured and a 47pF silver mica capacitor was added between relay contact #9 and ground.
    Below are a couple of measurements on various bands. I do have pictures of all band measurements using the VNA but you get the idea. In the end, all bands pass thru SWR were less than 1:1.1. I did remove the single 47pF 500V silver mica capacitor and used 2 paralleled 22pF 500V capacitors.
    IMG_2100.jpg IMG_2099.jpg IMG_2095.jpg

    The ALC circuit of the amplifier was removed. RF drive should be limited but most transceivers will not be capable of overdriving the 3-500Z's.
    This terminal strip contains the ALC circuit. Removal was easy enough and made it a bit easier to align the input tune circuit.
    IMG_2004.jpg

    More on the input tuning next time.

    73
    David
     
  3. nomadradio

    nomadradio Analog Retentive

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    A gen-yoo-wine unvarnished PITA, that. Hope it goes smoothly.

    If you have a HV breakdown tester, taking the red wires from the HV transformer loose, and checking for leakage current from the red wire to ground will reveal a transformer getting ready to arc over. Not fun when that happens.

    Or cheap. Seen it too many times.

    At least the two cathode pins on the tube sockets are the same color as the other three. If those two spring contacts are darkened at all it's a bad sign.

    73
     
    NZ8N and BJ radionut like this.
  4. BJ radionut

    BJ radionut Supporting Member and 6m addict

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    Yep, having a FLUKE solid state Meg'r has been a blessing!
    One of the fine tools had available as 40-year Journeyman Electrician.
    Same with fluke O-scope and couple different AC/DC DVOM clamp meters! (y):D
     
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  5. Dmans

    Dmans Sr. Member

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    Continuing on with the Tuned Input tuning.......
    Thankfully this amplifier is 50+ years old. There has been much written and documented to help me along the way with this amp. I could not have done it if not for all of those before me.
    Keeping in mind when this amplifier was developed most if not all transceivers were tube final radios and input matching was not near as critical as todays transistor final radios.

    This is the area of interest for this portion of the project. This picture only shows 3 of the 5 input circuits but the modification is very similar for each band.
    IMG_1795.jpg
    The goal is to increase the "Q" of each circuit. Too much "Q" and the bandwidth will be too small - SWR will be very flat on very narrow portion of the band.. Too little "Q" and the tuning match will be too wide and no amount of turning the inductor slug will bring the input SWR into an acceptable range.

    Rich Measure's states this better than I can.

    (Below copied from AG6K)
    Increasing both capacitances will lower the operating frequency of the tuned-input, so L must be decreased to bring the frequency back up to where it started. This can be accomplished by removing turns of wire from L, and by adjusting the inductor's tuning slug.


    It is important to keep in mind that the matching ratio of a tuned input circuit can not be changed by adjusting the slug-tuned inductor alone. In order to arrive at the optimum SWR, Two components must be adjusted in order to change the matching ratio of a tuned input circuit.

    The inductor for each band was removed one at a time and in most cases turns were removed from the coil and the input and output capacitors were (in most cases) increased in value.


    IMG_2111.jpg

    The assembly was installed and the circuit was tested using the VNA at the input connector and a dummy load at the output connector. Resistors were temporarily installed from the cathodes of the tube sockets to the grid pins of the socket to simulate the loaded resistance of the tube. The appropriate band was selected and swept for the frequency involved. The tuning slug was adjusted for the best match as reflected on the VNA. This was all done without powering up the amplifier but will need slight adjustment under full power when all other work is done.


    Below is the input section after the 80M, 20M and 15M coils and capacitors were modified. 40M on the top right and 10M on the bottom right.

    IMG_2328.jpg

    Now for a few "results" pictures.
    In all cases, each vertical graduation on the VNA is .2 SWR.
    i.e. The first horizontal line from the bottom of the scale represents 1.2:1. The second horizontal line is 1.4:1, the third is 1.6:1 etc

    80 meter result
    IMG_2329.jpg
    40 Meter result
    IMG_2160.jpg
    20 Meter result
    IMG_2135.jpg
    15 Meter result
    IMG_2119.jpg
    10 Meter result
    IMG_2151.jpg

    I agree with @nomadradio. A PITA!!

    On to the next episode.
    73
    David
     
    #20 Dmans, Jan 9, 2022
    Last edited: Jan 9, 2022
    Shadetree Mechanic, NZ8N and Robalo like this.
  6. Dmans

    Dmans Sr. Member

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    I was able to squeeze in some full power testing today. (This was done today 1/9/22)
    The amplifier is still wired for 110Vac input. The testing was done on 80 meters in the amp's SSB power position with the output terminated in a dummy load.
    I did not get a picture of my cross needle SWR meter between the Kenwood and the amplifier but it showed no reflected power. I'm thankful for that because I do not relish working on a powered up amplifier.

    Results below.
    Tested on 3.80010 LSB
    IMG_2358.jpg

    100 watts input power to SB-220
    IMG_2359.jpg

    Output from SB-220
    IMG_2360.jpg
    Grid Ma reading at 1800 watts output
    IMG_2361.jpg
    Plate Amp reading at 1800 watts output
    IMG_2362.jpg

    There was testing done on 80 meters at 30 watts input CW prior to this full power testing.I am impressed that the tubes appear to be good. As far as I know the amplifier has been in storage for around 20 years.
    It looks like this will be a survivor.

    73
    David
     
    #21 Dmans, Jan 9, 2022
    Last edited: Jan 9, 2022
  7. Robalo

    Robalo Well-Known Member

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    Congratulations! (y):) good job!
     
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  8. Ranch55

    Ranch55 Sr. Member

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    Looking very good. Lots of dedicated time and effort.
    Make him pay !!!!!
    LOL ........
     
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  9. Low_Boy

    Low_Boy Sr. Member

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    Nice Numbers.
     
  10. Dmans

    Dmans Sr. Member

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    Just a few more picture for informational purposes.
    IMG_1740.jpg IMG_1745.jpg
    IMG_1789.jpg IMG_1977.jpg
    IMG_2005.jpg 6/32 Rivnuts installed to eliminate sheet metal screws About 50 of them throughout.
    IMG_2008.jpg IMG_2052.jpg
    IMG_2084.jpg
    IMG_2085.jpg
     
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  11. Dmans

    Dmans Sr. Member

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    And a few more for those interested.
    IMG_2088.jpg
    Nylon 6/32 screws under filter bank to support the Harbach Soft Start module. Nylon stand off's were used under the chassis to hold the circuit board. One of the reasons the filter bank was installed at least 3 times (maybe 7 or 8!)
    IMG_2162.jpg
    IMG_2163.jpg
    New "Fish Paper" (insulator) being glued to the chassis after all fasteners behind it were secure.
    IMG_2165.jpg
    Connector added to all front panel wiring to facilitate front panel removal for service, adjustment etc without a soldering iron.
    IMG_2263.jpg IMG_2266.jpg
    IMG_2267.jpg
    This is what I called the "Beauty Shot". Full filament voltage to the 50 year old tubes.
    IMG_2219.jpg
    New Parasitic Suppressors installed
    IMG_2259.jpg
    Air conditioner coil cleaner was used on all the aluminum panels to bring them back to their original glory. Got to work fast with a scotch brite pad while wet or the panels get a white stain on them from the acid.
    IMG_2290.jpg
    73
    David
     
  12. nomadradio

    nomadradio Analog Retentive

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    Now THAT'S the way to do it! Rock on!

    And before bed, be sure to include the HV transformer in your prayers. It has already outlived a lot of its age bracket.

    73
     
  13. Ranch55

    Ranch55 Sr. Member

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    I told David to pay up.
    LOL...........
     
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  14. Dmans

    Dmans Sr. Member

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    It looks like this is a finished project.

    The last installment (yesterday) included changing the input to 220Vac, installing the Harbach Soft Start and applying Heathkit Service Bulletin SB-220-27-(Copied from a Word Document and pasted below). I was told that this amplifier had already suffered one low voltage transformer failure in its past. The drawing/schematic was not included in this bulletin that I found but it wasn't hard to figure out.

    April 26, 1985
    SB-220 Bulletin No:
    2 KW Linear Amplifier SB-220-27

    T2 Overheats And Fails When Operating On 220 Volt Line

    When the linear amplifier is wired for 220 volt AC line operation, and
    the contacts on SW2 [PN 61-45] or a high voltage transformer winding
    opens, excessive current will flow through the primary of low voltage
    transformer T2 [PN 54-238],
    causing it to fail. To prevent this failure, the black-green lead and
    the black-yellow lead of T2 are lifted and connected together with a wire
    nut.


    To do this, refer to the drawing at the right and remove the black-green
    lead at lug 3 of terminal strip AE [the lead from T2 at grommet AK] and
    the black-yellow lead at lug 2 of SW2. Connect these wires together with
    a wire nut [PN 432-199].

    Make this wiring change on units you receive for service which are wired
    for 220 volt AC operation. Let the customer know that this wiring change
    has been made. If he wants to operate on 120 volts AC, he'll have to
    reconnect these wires as shown in the assembly manual.

    I did not use a wire nut as suggested in the bulletin above but soldered the ends of the 120V primary leads together and covered with 2 layers of heat shrink.

    A couple of pictures below with the outer case on the amplifier.
    IMG_2395.jpg
    IMG_2396.jpg
    IMG_2397.jpg IMG_2391.jpg
    IMG_2392.jpg
    IMG_2393.jpg IMG_2394.jpg
    Red circled area is the two filament 120V primary wires connected together per the Heatkit Bulletin.
    IMG_2383.jpg
    I like the way this project turned out but am sure this look is not for everybody.
    I should build myself one!

    73
    David
     
    #29 Dmans, Jan 17, 2022 at 10:12 AM
    Last edited: Jan 17, 2022 at 11:03 AM
    Ranch55, Handy Andy, Robalo and 3 others like this.
  15. BJ radionut

    BJ radionut Supporting Member and 6m addict

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    David: Nice stuff Dude!
    I did the same color scheme to my last SB-220 when I converted it to 6 meters(y):)
    The band switch location now houses the "Tunable Input Circuit". I removed that junk "relative output/swr circuit" plugged that switch hole, now just have Grid/HV/Plate amp functions for meters.
    Anyway, nice work, hope the operator takes care of it.
    Oh, one last item, my last conversion I added a "Hot Rod" B+ transformer.
    The B+(HV) is close to 3350 DC (+/- due to local line voltage) made a nice change, reduces the sag down quite measurably.
    (1600 watts key down CW/100 w drive) never run it there!
    I like to see about 1100-1200w normal operation.
    All the Best
    Gary/W9FNB
     
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