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sb200 LOW POWER

Discussion in 'Amateur Radio Amplifiers' started by WA5WOH, May 4, 2021.

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  1. WA5WOH

    WA5WOH Ham Member QRZ Page

    I recently bought a SB200 from estate of silent key. It had been sitting for several years. It had had new capacitors and resistors installed and a soft key. High voltage is over 2000. Output maxes at 250 watts with 60 watts drive. Grid voltage is -1.9 when keyed. Plate current is 210 ma. Grid currrent is only to fist line, 10 ma I think. SWR to IC7300 is high at 2.5 - 3.0. I can use tuner to lower swr but output does not go up.. Any suggestions would be appreciated. Wa5woh, John Wolking.
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Which band?

    The SB-200 was always a bit squirrelly on 10m and maybe 15m, but on 20m and lower bands usually has a good input match (in my experience, having built one in the late sixties when it was new).

    There are "fixes." Lou W1QJ probably knows some, I forgot what they were and would have to go searching.

    Of course you could have a weak tube (or tubes). If the previous owner beat it to death, their emission might be low.
     
  3. W1QJ

    W1QJ Ham Member QRZ Page

    Funny thing about SB200 amps is that performance of them are all over the place. I have owned/repaired /refurbished hundreds of them over the years. I have a set of new tubes that I use in them all when I tweak them up. I use them specifically so that all the amps I work on can be compared in performance against each other. Some of them do remarkably well and make great power output on all bands, some no matter what you do only work so-so. Being a kit I suppose it is a progressive thing when someone does an outstanding job vs one that was poorly built. The outstanding ones work very well and the good ones work good and so on. I try to go through and get everything good so that it will work well. The most important thing is the input circuits. It is absolutely imperative that the input circuits are tuned in properly. You are striving to get a 1 to 1 swr on every band in the portion that the amp will be used. Over time those mica caps change value and the slugs need adjustments. If the mica caps have drifted to far out they will need replacing or else you will never get the 1 to 1 you are looking for. This requires that you have a decent meter to measure these caps so you can identify which ones are giving trouble and replace them. Some of the slugs are glued in place and they will need to be freed up so you can adjust them. 15 and 10 meters are especially hard to get adjusted properly and they will need special attention. The accuracy of the values of the caps is critical as small changes will throw off the match. Also a major problem exists if the amp has a soft key (H*rbach). On some units, (more often than not) the soft key can be susceptible to RFI on 15 and 10 meters , mostly 10. What happens is RF gets into the soft key circuitry and it causes the MOSFET to oscillate. The MOSFET does not remain fully low. The voltage obviously fluctuates up and down and the bias is all over the place. This causes the amp to exhibit very poor SWR on 10 meters (or 15 sometimes too) and you can not seem to get the swr down no matter what. Also the power output will fluctuate as the bias changes. Unfortunately some hams just give up and 10 meters never works that well. I only noticed this problem when I purchased a digital wattmeter (LP-100a) and used a radio driver with a digital type swr meter on it. You can easily see the oscillation of the swr and the power output, and neither are stable. The wattmeter endlessly displays numbers and never locks in on a solid carrier value of output. The swr meter which has dots that increase and decrease constantly and never settles in on a fixed value. When I noticed this I was sent on a wild goose chase to track down the problem. It took me a while to suspect the soft key as that did not seem like a likely suspect. When I rationalized the circuit and considered that the relay coil is part of the bias circuit I then suspected it. I simply bypassed it temporarily to see what would happen. When I did the problem went away and I was able to get 10 meters to work well. More often than not this is an issue. I also use the same soft key in the Collins 30L1 and it also does the same thing in that amp. The soft key needs to be modified to keep RF out of it. When it is modified everything is stable and it is just like it was not there. I am sure many have this problem but have not identified it because with analog meters you will not see it. You will see low power output and high swr and no matter what you do nothing works until you address the soft key.
     
    WQ4G and KD2ACO like this.

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