>Hi Morrie,
>
>I bought an SB220 a couple of years ago. Here are my thoughts.
>
>You said the amp has the original tubes. If it has had heavy use, the
>tubes will probably be "soft". If not, the amp may have seen extended
>periods of non-use, in which case the HV filter caps could be in bad
>shape. My amp had been on the shelf for years, and the HV caps blew when I
>flipped the mode switch on the amp to SSB. I replaced the entire HV cap
>assembly with a new one from Harbach Electronics (see below).
>
>The SB220's T/R switching runs on 120V dc. Modern transceivers can't
>handle it. You need to rig an interposing relay or install the Soft Key
>module from Harbach.
Another solution is to remove the wimpy amp control relay from the radio
and replace it with a 300V, 0.5A NPN switch transistor.
See diagram in:
http://www.somis.org/440.html
Search for: "Transistor Installation Note ".
Advertisement: We sell a parts kit for doing this mod. Price = 50-cents.
>
>The SB220 uses an old-style, slow, open-frame relay for T/R switching.
>Modern transceivers switch much faster. This means that your transceiver
>will be pumping RF through the amp relay contacts before they have had
>time to close. At the very least, this results in a momentary spike of
>reflected power, which will cause your exciter output to momentarily
>"fold back" before coming back to full output. Maybe not serious if you
>are an SSB ragchewer, but an intolerable situation for a CW contester or
>DXer. QSK is out of the question, unless you install a good vacuum relay
>QSK system like the one sold by Rich Measures. Use of an interposing
>relay will exacerbate the problem.
>
>Because of the 220's smart power supply design, filament inrush current is
>not really as big a problem for the SB220 as it was for other amps of the
>same time period. The damaging effect is mitigated if you start up the
>amp in the CW position (lower plate voltage). Harbach sells a mod that
>cures the problem entirely.
However, after doing the aforementioned high-speed switching mod, the
original relay can be used for stepstart service. To do this, 2, 10 to
25 ohm power resistors are needed plus c. a 2k-ohm 2w resistor. Diagram
@:
http://www.somis.org/D.a.01.GIF.
>
>SB220 mods by Harbach can be found at: http://www.harbachelectronics.com/.
> It seems that he has suspended business temporarily, but will return
>soon. These are excellent mods, and well worth the money.
>
>I love my 220. It just keeps cranking along. A fair price, I think, would
>be no more than $400 USD.
>
>Good luck!
>
>Tony
>KM0O
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>
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