[SiriusHardware]

A little bit more detail about the operation of the Delta Tune.

In a 'Normal' 134 chassis without Delta Tune, the frequency of the 10.240MHz crystal is set by CT2 and C3. Once set, the frequency of the crystal does not move.

In a 134 set with 'Delta Tune' C3 is missing and there are two new capacitors which come into play, these are C123, 47pF (switched in or out by Q20) and C124, 18pF (switched in or out by Q21).

With the DT switch at '0' Q20 is turned off and Q21 is turned on so the 'normal' frequency of the 10.240MHz crystal is set by CT2 and C124 in parallel.

With the DT switch at '-', Q20 is turned on and Q21 is turned off so the frequency of the 10.240MHz crystal is set by CT2 and C123 in parallel and is lower than normal.

With the DT switch at '+', Q20 and Q21 are both switched off so the frequency of the 10.240MHz crystal is set by CT2 only, and is higher than normal.

If you go into TX mode, the voltage supplied from the TX-only supply rail through D17 effectively overrides whatever the setting of the DT switch is, turning Q20 off and Q21 on so that it is as though the DT switch is set to '0'.

It may be worth checking that D17 is present, someone may have removed it to make the DT shift active in both RX and TX modes.

I should say that C1 also influences the frequency of the crystal but since this is fixed and always in circuit it can be ignored for the purposes of the above description.