Trouble could be with the closed-circuit phone jack connectors that are used for the Guitar input and the Fx Output? They can be the cause of an open circuit that could interrupt the signal from the input to the output, or, if there is a mis-wire due to parts being supplied with a different terminal read-out than what is shown in the manual, again, an open circuit can occur (on the Guitar input).
If you look at these connectors from the side, the contacts can be followed through bends, between the brown insulating layers, through more bends, and to the terminals.
The tallest part of the connector is the prong that contacts the tip
of the plug when it is inserted. Follow it down to the bend beneath
the insulator and to its terminal/solder-eyelet. This is the tip
terminal/contact labeled H for Hot on the SiaB diagrams. On the
Guitar Input this is wired to point C and on the Fx Out this is
wired to point D.
The next section of the connector is the shunt or switch. Its job
is to normally be in contact with the tip contact unless a plug is
inserted to move the tip contact away. Follow this shunt down,
through the layer and to the terminal. For the Guitar input, this
shunt terminal/contact with the S label should be going to the resistor and capacitor junction R35/C19. For the Fx output, this
shunt terminal/contact should be going to switch S2, lug 3.
The G terminal on each of these connectors is the sleeve part and is
the same metal as the threaded mounting bushing. It just wires with
If this is OK, be sure the tip and shunt contacts do indeed touch each other when there is not a plug in place. For the Fx Out, this is the way the signal couples from the first tube stage to the second. A cable can be inserted here to input this Fx output signal to an amp for a test of whether or not the signal is getting through the guitar input stage and the first tube stage.
The output signal to the amp can also be taken from either of the 1/4" output connectors (+), in-phase with the input signal, or (-), out-of-phase with the input signal. These will sound identical, but one is direct off the Output level control, J5(+), and the other, J6(-), is from inverting amp stage IC2:B. Try both just in case the trouble is particular to the IC2:B section of circuitry.
If it is still no-go, then the wire to the J6 output could be temporarily disconnected for use as an amp input test probe. With J6 patched to the input of an amp/speaker, the free wire will be a live sensing probe that can be touched to various circuit points to 'hear' any signal being input and passing through the SiaB circuit. An alligator-clip test-lead could be used to extend the wire. You should notice that touching the free end of the wire causes a buzz in the amp/speaker just as touching the end of the cable to the amp does.
It might be simplest if you can put a constant signal to the Lo-Z input such as a keyboard with a key taped or weighted down, the output from a cd player, etc...
Tracing the signal through from input to output, you should notice input signal at the following circuit nodes. Only touch the points specified so that dc levels are not sent to the amp input. Identify the points on the schematic and parts placement diagrams.
Wiring point C/R25.
Wiring point P/R15.
Wiring point D/J1 tip.
Wiring point R/R10/C6.
Wiring point J.
Wiring point L.
This will help to isolate the trouble to a particular circuit section and from there we can zero in on the problem.
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