SB-401 Changes for Full Break-in


Summary of Changes

To achieve full break-in the original transmit/receive control relays were disconnected from the CW control path but retained on SSB.

The aerial changeover relay was removed and replaced with a cathode follower T-R Switch.

The bandswitch was rebuilt to add an extra position for top band and an extra position marked on the front panel using white Letraset.

A grid block control system was installed using the -150V supply.

A multiway socket was added to the rear panel to enable PTT control and CW monitor lines to connect to the a matching but completely rebuilt SB301 receiver.

The following block diagram shows the revised control system:

The changes mean that I can listen through the key-up periods on CW up to about 25wpm without any clicks, bangs in the receiver headphones or mechanical relay noise.

There is no intention of doing any further modifications as an SB-101 is in the process of being completely re-engineered to go with the already rebuilt SB-310.

See Valve Circuits 3 for more information on grid block keying and T/R switches.