Hi, my Comprehensive Drive build worked well at first, but after a couple days it became a little buzzy/cheap sounding at even low drive and now is a complete fuzz-fest even when drive is at its lowest setting. The signal is clean entering the IC, and coming out super distorted. All components between IC input and IC output are outputting fuzz signal, including potentiometer in and out, as tested with an audio probe. I have attached photos, showing the pedal with an audio probe temporarily installed. Does anyone have any suggestions, advice or insight? This is my third pedal build, after building two Propolis Fuzz. I'm curious why it would start fine and then become messed up. Maybe there was a short that ruined the chip itself? I have a multimeter but I wouldn't know what exactly to test to see if this is the case. Thanks ahead of time!
Comprehensive Drive stopped working, super buzzy/fuzzy.
- Thread starter alsiv23
- Start date
DailyDovetails
Well-known member
I might check the resistance between legs 1 and 2 on the IC. It should go up and down when you adjust the drive pot between maybe 10k and 500k as a guess.
Thanks for responding! The IC is a Texas Instruments RC4558P from Mouser.
My readings are:
1 - 7.4
2 - 7.4
3 - started at 7.4 then gradually kept going down 0.02 at a time. I let it get down to 2.8 before I stopped testing. Pin 1 and 2 voltage always matched pin 3 after I started bleeding (if that's what was going on).
4 - 0.0
5 - 3.56
6 - 4.56
7 - 4.56
8 - 8.9
Thanks for responding! My resistance is jumping around wildly, even when just testing a single resistor out of circuit. Something must be wrong with my multimeter. I'll see what I can do about getting my hands on a better one and will post my findings here. This one was free from Harbor Freight.
DailyDovetails
Well-known member
What I was wondering was if something was wrong with the drive pot or one of the adjacent resistors. That part of the circuit sets up how much gain the IC will have. The higher the resistance the more gain it will have. If there is a bad solder joint or pot in that area it can increase the resistance, setting the gain too high and cause clipping.
It’s possible you moved the component to where it was making good contact again. I would try giving things especially the drive pot a jiggle and see if you can recreate it.
Brett
Well-known member
Your voltages on pins 1-3 seem off, but it's early and it's possible I'm entirely wrong. I'll measure the voltages on my build to confirm.
Brett
Well-known member
I just confirmed, your voltages on pins 1-3 are wrong. You're looking for 1/2VCC on those pins. Something has gone awry with your 1/2 supply bias network (R15, R16, C12). I'd check closely around those parts to make certain you don't have a bridge somewhere.
I don’t see any bridges, do you know why the voltage of pins 1-3 goes down slowly and simultaneously when I test pin 3 with my multimeter? The voltage goes back up gradually as I play after bringing the voltage down. Do you have any hacks that you can suggest to get the desired reading without identifying the specific problem? The pedal sounds amazing after bleeding some of the 1-3 voltage. Maybe I can adjust some values in 1/2 supply bias network?
Brett
Well-known member
No, I wouldn't cobble together a makeshift solution personally. I'd find the problem.
In the case you've presented, if I was absolutely sure that I used the correct values in R15/R16, I'd start by trying to isolate VREF_A to ensure that the bias voltage is correct. As it currently stands, you should have roughly 7.4V at the junction circled in red below. If that's the case, you can isolate the VREF_A supply by disconnecting components that are fed from that supply. You'd need to lift the resistor leg on the side of R12 and R14 that connect to VREF_A (circled in pink below). Only lift one leg on one resistor at a time and use your DMM to measure voltage at the junction of R15/R16 (circled in red below) each time you disconnect that part of the circuit from VREF_A.
Let us know what you find.
Thanks, I have the correct values and measure 4.42V with my DMM on the righthand legs of both R15 and R16. The voltage of pins 1-3 increases by 10mV every seven seconds, and decreases rapidly by 30mV every second I measure pin 3 specifically with my DMM. When I began testing this evening the voltage of pins 1-3 was around 3.8 and after letting it sit in from of me powered on it reads 6.8 and climbing. The voltage of R15/R16 remains consistent regardless of pin 1-3 voltage. What do you make of this?
Brett
Well-known member
In lifting one end of those resistors and breaking the connection(s) to VREF_A, you never measured any voltage that exceeded 4.5V (1/2 VCC) at the junction circled in red or at IC1's pins 1-3?
As it should.
andrewsrea
Active member
A few things came to mind from your description:
1.) C9 isn't properly connected to ground via R11 (10K) and is storing up too much DC voltage by, instead of keeping Vref A steady. Ensure those solder joints and values (note that the cap orientation looks good).
2.) Vref A circuit (you already checked). If one of the component values is wrong or the connections are not solid, it can throw off the voltage being divided.
3.) RC4558 is shorted or internally compromised between pin #3 and #8. I ALWAYS socket my ICs, FETs, transistors, etc. because they are a PIA to unsolder if one fails.
1.) C9 isn't properly connected to ground via R11 (10K) and is storing up too much DC voltage by, instead of keeping Vref A steady. Ensure those solder joints and values (note that the cap orientation looks good).
2.) Vref A circuit (you already checked). If one of the component values is wrong or the connections are not solid, it can throw off the voltage being divided.
3.) RC4558 is shorted or internally compromised between pin #3 and #8. I ALWAYS socket my ICs, FETs, transistors, etc. because they are a PIA to unsolder if one fails.
Junction R15/R16 always measures 4.42V regardless of which resistor leg is lifted. On testing again, if 1K05 is lifted pin 1 reads 6.86, 2 reads 6.8, and pin 3 descends from 7.2. If 47K is lifted pin 1 6.2 and increasing, pin 2 5.8 and increasing, pin 3 6.2 and decreasing.
Similar threads
- Question
