6720 troubleshooting tips...
The power supply section allows either dual-dc (bipolar, split, positive and negative dc supplies with respect to G/circuit common), or, 12v ac input from a wall mount transformer. Two dc adapters can be used to get a dual-dc supply by wiring them in series and using the midpoint (the plus of one to the minus of the other) as ground/common/0v and then there is a free plus wire that is the +V dc and a free minus wire that is the -V dc. Since a single wall-mount (or other) ac transformer can be used, two adapters aren't the best choice, but the description helps convey the idea of a dual/split/bipolar supply as used by the circuit. The ac input voltage is rectified to be positive and negative dc supplies before going to the on-board +5 and -5 volt regulators. Note too, there are two 'dc power connector/connection' positions at the corner of the board labels (+ G SG -). The (+) and (-) points are for the positive and negative dc supplies and there are separate ground points for power-related and audio-related circuits (with an external dual-dc supply, they'll run to the supply over separate wires and join at the supply but with the ac transformer voltage connected, they must have a jumper wire linking them together). The PAiA 9770-series supplies use this scheme and one connection area is for input to the board and the other is a tap for connection to other equipment downstream.
So, there are a few things to be sure of in the power supply area. When powering by external dc, there must be both a positive and negative dc supply and it is best to have the two grounds G and SG going to the single ground on the dual-dc supply over separate wires. When powering by a 12v ac transformer supply, the G and SG circuits must be linked by installing a jumper wire between the two at one of the DC connection points located in one corner of the board. It should be noted too that with a direct connection it does not matter which 12v ac wire goes to which connection point on the 6720 G or AC, but if an external connector is used on a metal chassis containing the unit and one terminal of the connector is in contact with the chassis, then this terminal must have the wire going to G connected with it (because the G is the ground/circuit common/0vdc circuit that runs to the panel mounted phone jack sleeve terminals). Also, having the transformer ground to the chassis at a connector like this can be a source of ground noise due to the panel mounted jacks grounding there too, and insulating the connector would prevent this (or using a connector that isolates the terminals from the chassis)).
Tests for power supply voltages can be made using a multitester (vom, dvm, etc.). Note with a dvm you can touch the black probe to ground (G/SG) and make both positive and negative dc tests and for the negative readings the display will show a minus along with the digits. For an analog unit with a needle that swings across a meter, set a dc polarity switch to the negative setting, or, touch the red probe to ground and use the black probe to measure an expected negative dc point. Then, the needle won't try to go less (negative) than its minimum zero setting. Another way around this is to put the black probe on the negative dc supply and consider all readings positive from here (ie, referencing the -5v supply would give a +5v reading on G/SG and a +10v reading on the +5v supply). A test lead with an alligator clip at each end can hold the black probe in place during tests to free up one of your hands.
Test for the +5 and -5 vdc supplies on the IC pins. Count up in a ccw direction around the part from the notched/pin1 end (as viewed from the top of the board). The 4136 op-amp ICs get the negative dc supply on pin 7 and the positive dc supply on pin 11. The 4016 switch ICs gets the negative dc supply on pin 7 and the positive dc supply on pin 14. Another dc test that can be revealing is to measure the output pins of the op-amp sections, pins 3, 4, 10 and 12. These should be near zero volts but if there is trouble in a section or the connections to it, the voltage can read at nearly the power supply voltage, or near +5 or -5 in this circuit.
There are also dc states that can be checked in the bypass circuit sections including the electronic switch IC and transistor Q1.
Transistor Q1 is in the footswitch circuit and operates the Footswitch LED and sends a control signal (dc level) to one electronic switch section. Check to be sure there isn't any contact between the tip part of the footswitch jack J3 and ground (the sleeve part). A push-on/push-off, alternate action footswitch makes the connection between the tip circuit and the sleeve circuit or not (I've made panel mount arrangement for mine using a phone plug with a spdt toggle switch soldered to the two terminals of the plug). The negative 5v dc on the emitter of transistor Q1 causes the base to be a more positive voltage when it is at the zero volt ground potential and this turns the transistor 'on'. Current flows through the LED and R1. The LED lights and voltage on the collector of transistor Q1 changes from 5v to -5v. This voltage level connects with the control pin of electronic switch section IC4D. The positive level 'closes' the switch, the negative level 'opens' it.
The IC4D 'control' signal from the transistor collector also operates a section connected with the output stage selecting the guitar input stage as its input. When the control is positive and the LED is off, the IC4A section is on connecting the guitar input stage with the output stage.
The IC4D section connects its end with R35, R49 and D12 with -5v or not. When not connected, it's at 5v. So it's an inversion of the Q1 voltage levels and it goes to the other switch section connected to the output stage and the Attack control circuit. When these sections get a positive control signal, the Attack control should be operational and the fuzzed signal should be selected for the output stage.
Note it is not unusual for there to be an oscillation or tone if there is not anything connected at the input and the Attack control is advanced more than about halfway. And, the tone control would work as a frequency change on this oscillation. A guitar or some other input source presents some resistance to ground/common that will lessen the tendency for the circuit to oscillate or howl. The Loop Send is an output from the fuzz sections before the tone control, so it would be useful to know if connecting the amp here sounds OK or not when checking for trouble with no audio output. Connecting a cable from the Send to the Receive jack would bypass the tone circuit. If the trouble is there, this would eliminate it. Trouble in this section might be related to there being two halves to this control and a possible mix-up in the connections to it. Double-check this area.
When there isn't a plug in the loop send connector, the switch or shunt contact should be resting against the tip contact and passing the signal on to the tone control via the wire from J10-x to the 1 and 2 terminals on R46 of the section nearest the panel. A 2200ohm (red-red-red) resistor ties terminal 3 on this section over to terminals 1 and 2 of the other section along with wire Y. Wire Z should be connecting to terminal 3 on this outermost section. Look to be sure the J10 normally closed contact is indeed contacting--sometimes the tip gets bent outward and doesn't spring back against the switch/shunt contact.
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