dod switching problems

[sanquilo]

Hi all, first off, i apologize if i'm posting this in the wrong section of the forum.

some time ago i decided to change the power jack of a dod fx72 bass stereo flanger to a normal barrel type jack. Pedal is from '89

I connected the +9 and ground to their respective pads on the board and it did work just fine. Since the pedal was already open, i changed the led and the CLR (10k) to 1k for brightness reasons, but then i had problems with the switching side of things since this resistor is also connected to a 5088 and the 14007 ic, so i instaled back a 10k and a red led (not the original, but same in size/ brightness).

Now here's the problem: when plugging in power it does power up and flanges as it should; also when it's off, clean signal comes through fine, but when i try to switch it back on the led turns on dimly and some weird sound comes out, kind of the same as when you unplug the power from it (this last sound always existed, even in stock form). 1 out of a thousand times it turns on.

so, i desoldered the wires going to the tact switch and jumpered them, with the same results, so thats not the problem. I'm guessing it's the 14007 or the j113 (q7)

For reference I used a fx 75b schematic which is the guitar version (the pcb even has "fx 75b" written on it) since i was unable to source a fx72 schem.

On to the readings, i measured voltage on the 14007, q7 (j113), and the 5088 connected to the status led and i did also measure while pressing on the footswitch

9.68 V at the power jack

W/O PRESSING ON THE FSW

14007

9.2 <-pin 1      9.22
9.2                   0
0                      4.62
0                      0
9.15                 9.2
9.15                 4.16
0                      0

J113

G    0
S    4.92
D    4.6

5088

E    0
B    0.64
C    0.06

Holding the fsw down...

14007

3.08 <-pin 1       3.13
3.12                     0.002
0.18                     1.3
0                          0.18
3.09                    0.02
3.1                       0
0                          0.002

J113

G   0
S    1.92
D   1.92


5088

E    0
B    0.28
C    1.87


pictures were taken with the fsw desoldered and before I installed a red led back in.





Any and all help is appreciated,

thanks and happy holidays

[Slowpoke101]

Hi, and welcome to the forum. You have posted your problem to the correct forum area (well, I think you have..Been wrong before).

Your voltages for the 4007 when the footswitch is not depressed seem OK - But when you have the switch depressed the voltages are very wrong. You appear to be pulling the pedal's main 9 volt supply line down but not fully shorted to 0 volts.

From the switch pad on the edge of the circuit board, trace the track all the way from that pad to pin 10 on the 4007. Make sure that there are no shorts (solder bridges, dags, etc) connecting the track to something else. One end of a 1M resistor connects to this track. Immediately under the red power wire there seems to be a lot of flux (or something). Clean this up as the switch track goes through this area. Give the board a good inspection and clean up any questionable areas.

See what you find and let us know. The LED transistor's voltages are a bit odd too but we will worry about that after you find what is causing the 9 volt line to be pulled down.

[reddesert]

It helps to include a link to the schematic.  Here it is: http://www.americaspedal.net/fx75/fx75schem.jpg

The section in the lower right is the switching.  A CMOS 4007 chip (three paired enhancement mode MOSFETs) is wired as a flip flop. This flip flop is triggered by the momentary switch and drives the indicator LED + transistor, and the gate of the J113 JFET, which is a switch - when the J113 conducts the effect is on, and when it blocks signal, the effect is bypassed.

When the momentary switch is open, the gate of a MOSFET, pin 10 of the 4007, is pulled up to +9V through a 1M resistor. This means that the MOSFET will conduct. When the the switch is depressed, it shorts pin 10 to ground, 0V, and interrupts the conduction. This should trigger a state change in the flip-flop, which is the part of the 4007 wired as two inverters - the triangles marked U6 in the schematic.  Because these are inverters, the output is the opposite of the input. In the first set of voltages, pin 11 is connected to pin 3, which is the first input, and is 0V. The output is pins 1,5 and is +9V: correct. That's wired to pin 6, also +9V, and then pins 8 and 13 are inverted from that, so 0V.

I'm assuming here that your IC voltages are listed corresponding to the physical layout of the chip, so the first column is pins 1-7 top to bottom, and the second column is pins 14 to 8 top to bottom; please confirm.

When the switch is depressed and released, the flip flop should change states and stay that way, so the 0 and 9V should change places. Pins 11 and 3 are +0.2 V, which isn't right, and the rest are also bollixed up. Whether this is a problem with the 4007 or with the connections that are supposed to make it work needs further testing.