FX13 Gonkulator repair has me Gonked

[Cruton]

Greetings!

I just came into a non-functioning DOD FX13 Gonkulator (the 90s version with the silly control labels). It does not pass signal currently, but also does not appear to switch between bypass and active modes. The LED just stays on as long as there is power. I managed to find a schematic (hopefully this comes through in high enough resolution):



I first wanted to see if I could get it to pass the effected signal while stuck in the effected mode. Poking around, I found that the voltages on pins 1,2 of U2 (RC4558) and pins 1,2,6,7 of U4 (RC4558) were low. I replaced both op amps, and the voltages at those pins look good now, but still no signal passes. Using an audio probe, I found that the signal gets through to the output of U2A and is present at the appropriate legs of Q4 and Q7 (J201s), but doesn't make it through either one. I've confirmed that the bypass switch does make a contact to ground when depressed, so I think the switch itself is okay. I struggle to understand these FET switching systems (I think I've learned the Boss one like, 4 different times now and still forget the mechanics), so I thought I'd reach out for some help. *Edit*: I've found this copy of an old article that Mark Hammer has provided on his page that describes this switching setup: https://www.ampage.org/hammer/files/dodswitch.gif , but I'm still not sure where to start in terms of troubleshooting it.

Here are the voltages at each pin of each IC and transistor. They're labelled according to the schematic I've posted.

U1 (4007UBCP):

1:    0.01      14: 9.53
2:    9.52      13: 0.41
3:    0.41      12: 0.00
4:    0.00      11: 0.41
5:    0.00      10: 0.56
6:    0.00        9: 0.00
7:    0.00        8: 0.41


U2 (4558):
1:    4.74        8: 9.53
2:    4.76        7: 4.76
3:    4.76        6: 4.76
4:    0.00        5: 4.76


U3 (4558):
1:   4.60         8: 9.35
2:   4.76         7: 4.76
3:   4.76         6: 4.72
4:   0.00         5: 4.65


U4 (4558):
1:    4.77      8: 9.53
2:    4.76      7: 4.77
3:    4.76      6: 4.75
4:    0.00      5: 4.77


U5 (1496P):

1: 4.60     14: 0.00
2: 3.86.    13: 0.00
3: 3.86.    12: 8.66
4: 4.59.    11: 0.00
5: 1.18.    10: 7.00
6: 8.66.      9: 0.00
7: 0.00.      8: 7.01


Q1 (5088):
C: 9.53
B: 4.66
E: 4.09

Q2 (201):
D:  4.76
G:  0.04
S:  4.33

Q3 (5088):
C: 1.65
B: 0.00
E: 0.00

Q4 (201):
D: 4.65
G: 0.03
S: 4.72

Q5 (5088):
C: 9.53
B: 2.88
E: 2.43

Q6 (5088):
C: 9.53
B: 4.67
E: 4.14

Q7 (201)
D: 4.77
G: 0.00
S: 4.29

[PRR]

> start in terms of troubleshooting

Put jumpers on the JFET. Does signal pass?


However I am wondering about the CD4007.

[Cruton]

Thanks PRR, I'll try that here in just a few.

I think you may be right about the 4007 though. I got duped a few minutes ago-- I read a low resistance across the tactile switch with it open, and I convinced myself that this meant the switch wasn't fully open and was responsible for the low voltages on the 4007. So I desoldered the switch only to find that the 4007 voltages stayed the same. I destroyed the switch in the process so I'll be waiting for a new one in the mail now :/.

I'll report back regarding the jumpers.

[Cruton]

All right, I can confirm that it passes signal both in "must pass" and "should pass" jumper configurations as indicated in the diagram you provided. Jumpering Q2 and Q4 simultaneously gave me delightfully awful sounding gonkulation from the output jack, so it seems the effect circuit works fine now. I guess it's down to the 4007 then? I looked up the datasheet earlier and saw that all pins should be within a few hundred mV of the supply, which is definitely not the case here. I compared that to another DOD pedal I have on hand, and on that other pedal all the pins were either 0.00V or pretty close to the supply voltage.

[PRR]

> I read a low resistance across the tactile switch

In-circuit tests *require* you understand what is across the part. In this case the 470k is fairly obvious. However the chip is full of static protection diodes. One clue is (was) that the "resistance" seems different one way from the other.

> I'll be waiting for a new one in the mail now :/ .

Meanwhile you set two wire stubs and tap with a screwdriver. No good for gig but fine for the bebugging bench.

> 4007 then? I looked up the datasheet earlier and saw that all pins should be within a few hundred mV of the supply

Where do you see that?? If they are always very-near a supply, why not use a jumper? (CMOS chips used to cost money. Today they may be cheaper than jumpers?) No, logic chip pins typically go "low" or "high" depending what you put in it. (The '4007 is a little odd but will generally follow the rule here.)

[Cruton]

Yeah, after the fact I realized the "fairly obvious" in-circuit conditions were responsible for what I saw. I should slow down and think things through better.

As far as the two stubs and a screwdriver, that's a great suggestion until my switch arrives.  A quick tap with a jumper still doesn't give me any switching, presumably because of the 4007. Though I'm still not sure about the 4007 being the culprit, because I don't know how to tell aside from just replacing it.

Thanks for catching my mistake about the voltages. The data sheet said "For proper operation the voltages at all pins must be constrained to be between VSS -0.3V and VDD +0􏰁.3V at all times􏰁". I saw "within" instead of "between", which obviously means something very different.

I guess I'm going to order a 4007 and see if that solves it.

[Cruton]

I finally got the replacement CD4007 and it solved the problem. Thanks for all the help PRR, and for catching me on several lines of less-than-watertight thinking.