9407 Tube MicPreAmp trouble-shooting tips/suggestions...
General
For no-go situations power supply and missing input or output signals can be the trouble(s):
Inspect solders to be sure there isn't one that was missed or is 'cold' and hasn't flowed to the component
lead/wire and the pad.
Parts out of place or mis-installed can cause troubles.
With two kits, you can use the working one as a point of reference in making checks, comparisons, and voltage
tests, etc.
Check to be sure both mic input signals are getting through. If you hold a piece of wire in contact with your
thumb and forefinger and touch the two wires going over to the mic polarity switch one at a time, a hum should
be introduced in the audio output. If one or the other isn't 'live' it could be a short to ground. The switch is a
place this can occur, but it could be the drain melting through and contacting the wire on the red or black of the
twin-ax. With the mic removed, touching the xlr receptacles that wire over to the switch should introduce hum
too and if not, it could be pointing to the solder not making a good connection on the connector terminals or the
polarity switch. Listening for hum, turn the blend from ccw to cw to find out if trouble is particular to the Solid
State (ccw), Tube (cw), or both.
Tube path....
Check to be sure the tubes are completely seated in the socket.
A common trouble is the tube not being fully seated in the socket because it takes more force than you might
expect to get it in the first time. This coupled with there being a spring in the cover can make it seem like it's
held in place with the cover/spring, but this isn't the case. It is possible to peek through an opening near the
mounting ears in the base of the socket to confirm that the tube is in all the way, but also, if taking the cover off
allows the tube to drop out or if it comes out very easily, then it isn't in good enough and this can result in
intermittent/marginal operation.
Check too to see if both Symmetry trims have a 100k marking.
It is easy to get yellow-violet-red and red-violet-yellow (4700ohm and 270kohm) resistors in place of each other
and there are 270k ones associated with the tubes.
.
Drive LED not lighting...
In some instances an input signal might not be strong enough to cause enough signal to develop on the tube to
light the Drive LED. It lights about the time the signal is strong enough through the tube that it will begin to
overdrive and clip the signal. Listen to determine if you're able to hear overdrive distortion in the signal when the
Blend is set fully cw for 100% tube path. If so, the Drive LED should be lighting. Check the LED flat/cathode to
confirm it matches the circuit board marking.
Drive LED always lighted...
Check installation of the LED D11 flat/cathode to confirm it matches the circuit board marking. Check C16 to be
sure it's polarity markings agree with the circuit board marking.
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