The following checks and tests for the 9210 Stack-in-a-Box are for the power areas, then sections related to the sound or audio signal through the unit or lack thereof.
If the power LED is not lighting double-check to be sure the wall mount power supply is indeed the specified 12v AC type (not 12v dc) and that the power switch is on and the outlet live.
The dc voltages derived from the ac supply may be less than noted. The positive and negative dc supplies can be measured on pins 8 and 4 of the 5532 op-amp ICs. Reference the ground/common circuit that originates at wiring point B, the neutral end of the power supply transformer. I have notes for +10.19 and -8.98 v dc readings on these pins.
IC3 connects to ground and the positive dc supply on pins 8 and 1. Pin 1 should have about 10.19v and pin 8, zero. It is set up as a multivibrator that toggles from zero to about 10.19v and three of its sections are parallel connected to drive the following diode and capacitor section. Pins 2, 4, and 6 should each be the post audio rate toggling and with a dc check should read at about 6v average dc. This voltage, or not, switching causes the dc voltages on the plus legs of capacitor C27, then C11, and finally C5 to step up along the way. Check for about 9.6, 18.6, and 27.7, v dc on these points.
This higher dc voltage from this section goes to two 270k resistors Rs 4 and 5 which connect it to the plates of the tube sections. The color coding of these resistors is the reverse of 4700ohms used elsewhere in the kit. Check to be sure there wasn't a mix-up in the installation of the red-violet-yellow-gold 270k and the yellow-violet-red-gold 4700ohm.
A mis-wire, bad connection, or malfunction of the c.c. (closed circuit) phone jacks can lead to trouble. These connectors have a switch (shunt) contact that should touch the tip contact when a plug is not inserted. Look to be sure these two contacts are touching and if not it might be possible to bend one or the other so they do contact.
The order of the readout on these connectors can vary according to our source. If the Ground (sleeve), Hot (tip), and Switch (shunt) terminals get connected in place of each other, it can cause the signal to be grounded or have an open circuit when plugging into this connector on the Guitar Input position. You can look at the side of the connector and follow the bends in the contact/terminal metal to confirm that the tip which carries the audio signal in from the guitar is getting into the circuit board input, wiring point C.
Inserting the tube into the socket requires enough force that it might lead you to believe the spring in the top of the socket cover holds it against the socket contacts, but this is not the case. The bottom of the tube should meet the surface of the socket.
For a while we had builders modify a solder eyelet socket for installation through holes in pc board and if it wasn't done just right, the legs could be too short to reach all the way through into the solder joint and this was an easy way to get an open circuit. We've since revised the board and the entire eyelet reaches on through without any trimming. If the socket installation is with the trimmed eyelets, inspect the joints for evidence of a pip in the solder that is the pin extending into the joint. If there is just a smooth dome of solder, it could be an indication the pin hasn't reached through and into the joint and is an open circuit. Desolder and use a piece of wire to extend the pin length by soldering it to the pin and the solder pad.
If you get a lot of boost in the guitar signal, but find that there just isn't good overdrive and distortion effects through the unit, it is probably OK. Normally, the sound through the unit leans towards a clean sound.
I have a mod to the unit that makes it work more as a tube distortion effect, but it is only on paper and I would have to put it in the mail to you or a fax would be possible. Let me know a postal address or a fax number and I can get it to you.
It sounds good to have some solid-state fuzz/overdrive/distortion and then run this through the SiaB for the tube effect. A fuzz/overdrive/distortion unit patched in series between the guitar and the SiaB can do this for you, or, as in the mod I have, the guitar input gain stage can be altered so as to allow for clipping through this section giving the tube a square-wave signal to 'treat'. The other part of the mod I have for the unit adds capacitive coupling and alters the biasing into the second, 'Drive', tube stage to remove some scratchiness in the sound associated with adjustment of the control and provide for symmetrical clip points at the upper and lower waveform extremes. Note this is as pertains to the kit as we supply it with, the Sovtek 12AX7WA.
The increased gain on the guitar input stage is obtained by substituting a greater value potentiometer for the 10k at the Crunch control. I put a 250k on the one I have here. This yielded such extreme gain that it helps to add a 0.001 at points O and P on the bottom of the board to help control oscillation. Just keeping the setting lower reduces the amount of gain though and a lower value pot could give more useable adjustment range (say, 50k or 100k).
The Drive part of the mod subs an 82k for the 33k at R8, inserts a 0.1uF between the wiper of the Drive control and the tube grid, and adds a 270k to ground at the grid end of the added 0.1uF.
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