| Modified SB-220 |
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| Fig 4. Less modified amp. Underside of chassis. Right side up. |
| Page 2----posted 31 Aug 05 |
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| Fig 1. (Repeated for convenience.) Extensively modified amp. Underside of chassis. Right side up. |
| Fig 4 shows a relatively unmodified SB-220. (The only mod in this view is the Harbach Soft Key positioned just above the fan opening.) Figure 1 is repeated alongside for comparison. Note that the shielding enclosure around the tube sockets has had the lower cover removed to show the wiring of the sockets as well as the bifilar choke. The shield virtually eliminates ventilation of the tube sockets. The plan will be to remove the shield to improve socket cooling. |
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| Fig 5. Closeup of shielding enclosure |
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| Fig 6. Shielding enclosure removed. |
| Comparing Figs 5 and 6 demonstrates that even after removal of the shield, the socket is a mess compared to Fig 4. My plan is to move the bifilar choke back to the original position and duplicate the original circuitry around the tube bases. The fan of the original kit has been replaced by a more powerful one, and when the xmtr is keyed, the fan increases rpm. Unfortunately the housing of the replacement obstructs flow to the underside of the chassis. I am planning to leave the antenna relay circuitry in place until I know whether the unit works satisfactorily with this modification left in place. |
| Another point of interest is seen when one compares Figs 1 and 4 above. The original position of the ALC circuitry was taken by the antenna relay mod. Fig 2 shows that this wiring was moved to the upper chassis compartment onto the load capacitor. |