SB-220 high swr on 10 meter

Discussion in 'General Technical Questions and Answers' started by KC9JYW, Jun 15, 2019.

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  1. KC9JYW

    KC9JYW Ham Member QRZ Page

    80-15 has great swr with amp on but 10m is high above 2.0 Running an ft-991 with dummy load and auto tune before turning amp on. Could the problem be the 10m coil needing adjustment? Thoughts???
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Sure, you can adjust it but with an SB-220 that's not the easiest thing; the designers did not make provision for tuning the input coils from outside the amp.

    Be careful.
     
    N8ZL and KA9JLM like this.
  3. KA9JLM

    KA9JLM Ham Member QRZ Page

    Seems the radio would tune 2:1 on the input of the amp no problem.

    Shooting for perfect on all frequencies can be hard.
     
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  4. KC9JYW

    KC9JYW Ham Member QRZ Page

    80-15 is giving me about 1000w with 1:3 being the highest swr to where 10 is giving me 200w and 2.5 or so. I would think a higher swr and lower wattage is normal on 10m but this seems a bit too much.
     
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    You can re-tune the input, just be careful since you have to do this "inside" the amp.

    The SB-220 always provided a bit less power on 10m (compared to lower bands), but should be a lot more than 200W.
     
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  6. N4ZAW

    N4ZAW Subscriber QRZ Page

    I hate resurrecting old threads, but I just learned of a 'fix' for this from Luke Miller (W7BBI)!

    Place a 47PF 500V @ 1%, silver dipped mica capacitor between the pass-though jumper bar on top of the relay, to the closest ground. In most cases, this 'closest ground' point is the braid connection of the input coax at the base of relay.
    I've tried this previously on my own, by placing a small air-variable at the inner connections of the input SO-239, (center conductor-to-ground), but it had little effect.. Apparently, it matters WHERE this cap is placed in the pass-through coax routing! It worked like a champ where Luke said to place it! From 2:1 down to 1:1.3! I could've probably gotten it dead-flat using an air variable, tuning for dead-flat on the MFJ259, then taking the air-variable out without moving the setting, and measuring capacitance, then replacing it with that fixed-value --- But I couldn't find one in that value range anywhere in the shop for some reason.
    [​IMG]
    Your input network coils inside only adjust the inputs to the TUBES' input, not the pass-through (amp off) input. Do not mess with those unless you experience a high SWR with the amp keyed up. And be very careful then. Use an insulated tickle stick, because it's one slip, and you're into 3KVDC!
     
    Last edited: Jun 8, 2021
    KA9JLM likes this.
  7. ON7YI

    ON7YI Ham Member QRZ Page

    This is the exact problem I am having with my SB-220; SWR of 2.5 on 10M and 200W only whereas on the other bands I can output 1000W ... Checked the input PI and Output PI and everything seems fine. Have you found a solution to output more than 200 W on 10 M?
     
  8. N4ZAW

    N4ZAW Subscriber QRZ Page

    Assuming the exciter you're using delivers the same output on 10M as it does on the other bands, and the amp is obviously keyed-up for the values you're reporting, I would first check the values of the mica caps on both sides of the 10M coil first, if you cannot adjust for flat SWR at the 10M input coil. If you need to replace them, I suggest using 500V at 1% tolerance, rather than the 500V at 5% OEM caps. Note where the tuning core is when it provides LOWEST SWR:
    If the 10M tuning core is bottomed-out, more capacitance is needed from the left side cap (as viewed from the front of the amp).... If the core is turned all the way out of the body of the coil, more capacitance is required on the right side of the coil to ground. Of course, either of these two caps on the 10M coil could cause the out-of-resonance condition you're seeing if they're out of tolerance. "If you can't get resonance through inductance, get resonance with capacitance".

    Also I would inspect the relay contacts, as well as the contacts on both front and rear band switch wafers, in particular, the ones in the 10M position (the rear fails more often). ANY black deposits, or soot can create an 'RF open' condition. That said, your looking for an 'RF-open', or 'RF-shorted" condition anywhere between the tube and the relay (relay contacts included of course).

    And while you're looking "in that neighborhood" anyway, locate and check the two 2.2K Ohm resistors for open, and the diode, on the agc terminal strip that is to the right of the front band switch wafer on the deck. The components in this little terminal strip "circuit" are the first to smoke if the amp is ever over-driven beyond 200W input, or if the amp was driven >50W while it's out-of-resonance for a long length of time --- since it already is out-of-resonance per your present issue with the input SWR, these should be inspected at least.
     
  9. ON7YI

    ON7YI Ham Member QRZ Page

    Thanks for the advises. In the mean time I solved the problem.

    The only reason the sb-220 was delivering much less on 10 M than on the other bands is simply because the transceiver would limit power to 20W due to the high input SWR on 10 (2.65).

    I powered off the amp and opened it up to make a lot of measurements on all bands using a nanovna2. Everything is working absolutely fine on the output and input (no bad contacts on the band-switch). Only on 10M, the pi input circuit is completely de-tuned. My measurements indicated the impedance of the two cathodes is 65 ohm on 10 M which is dam close to 50 ohm.

    What I have done is just to bypass completely the pi input circuit for 10 M (on the wafer) and after that I indeed obtained an SWR of 1.3 on 10M at full power (100W in). So, this solved the problem for my TS480SAT that is now happy to deliver 100W at the input of the SB-220 on 10M. The end result is 800-900W CW key down on 10M (note I didn't had to use the automatic tuner of the TS480SAT at any point in time) . This result is ok as my sb-220 uses 3-400Z tubes instead of 3-500Z tubes.

    Sadly, for my older SWAN 100 MX, of another age, it didn't help much for some reason. Looks like an SWR 1.3 is still too much for that old radio, the SWR protection mechanism continued to kick in even with an SWR of 1.3 or so. If I really wanted to use the sb-220 with my old Swan 100 MX , I would then have no choice but to re-tune the circuit to get to a swr 1.1 or so (but I didn't do it as I prefer my TS480SAT anyway).


    73
    Peter
     
  10. KM1H

    KM1H Ham Member QRZ Page


    Ive used Vishay 33pf 1000V NPO disc caps for many decades on literally hundreds of amps for several decades. This works especially well for 6M conversions of several popular amps.

    Carl
     
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