SOLVED More Mach 1 issues

[fable_instruments]

I’m having some issues with a Mach 1 I built for a buddy of mine awhile back. This is the same pedal that had the screwy gain pot. I replaced the pot, tested the pedal, retested it, then tested it a couple more times for good measure, and everything worked fine. Take it to my friend, we play it, he’s happy. He called me up the other day saying it’s not working. It passes sound in bypass, and the led comes on when you hit the switch but it’s dead silent. I swapped op-amps and no change. The VDC on the op-amp (from leg 1-8) reads 2-2-1-0-2-1-2-8. My next step is gonna be re-flowing the input and output joints on both the board and the switch, but beyond that I’m running out of ideas. Do any of you have any suggestions on what else to try?

 

[Chas Grant]

The op amp isn't biased correctly. You should have about 4V on all pins, except pin4 = 0V, and pin8 = 9V. This voltage is set by R101, R102 and C101. It is applied to op amp at pins 3 and 5. Check voltages on R101 both sides of resistor.

 

[fable_instruments]

I checked the voltages. Viewing from the silkscreened side, the left pin of R101 reads 8V and the right reads 3V. Sorry if the numbers are imprecise, I’ve got a cheap DMM that only reads whole numbers for VDC.

 

[Chas Grant]

No problem about the DMM. Now we need to figure out why the voltage is low. R101 is 4.7k and R102 is 5.6k, that’s a total of 10.3k. R102 should drop a little more voltage thanR101, 5.6/10.3=.54, .54X8=4.3. So your voltage between 101 and 102 should be 4.3V. You have 3 volts here, and since pin 3 should be the same but you measured 1V on it, I suspect the problem is in that area. C1 could be leaking, this might do it, measure dc volts from in input Jack tip to ground, it should be 0, if you have any dc voltage there C1 is the culprit. You can also check resistance from R102 to ground, if it’s lower than 5K then something is grounded.

 

[fable_instruments]

No DC from tip to sleeve on the input jack, and unfortunately the aforementioned cheap DMM only reads resistance up to 2K and doesn’t really give a reading on any of the resistors I tried

 

[Chas Grant]

Darn, let me see what I can come up with. If you could post a picture of the board it might help

 

[fable_instruments]

The tone pot blocks a lot of the joints on the back of the board. Should I remove it for the pictures/troubleshooting?

 

[Chas Grant]

No, just what ever you got. The front has more info

 

[fable_instruments]

Here ya go

 

[Chas Grant]

OK, looked over the board. You have the right values, which is a good thing. I see a number of solder joints on the resistors that look cold. Reflow the resistors and add a touch of solder to them. If you look at C9, you can see there is a fillet on the top side of the board. That's a good solder joint. Its hard to get the fillet when reflowing, but it usually takes care of cold solder joints. As soon as you see the solder flow add a small amount of fresh solder and it should be good. When reflowing, I try to keep the iron on the joint to a 4 count after the solder melts. Since C1 isn't leaky, a cold solder joint on R101 and/or R2 would give you similar voltages.

 

[fable_instruments]

I reflowed and added solder to R101, R102, any other resistor providing bias voltage and any other joint that looked suspect. This got me proper VDC on the op-amp and sound when the effect was activated, but it was quiet and sounded shitty. Switched out the op-amps and it sounds perfect. Before I box this up, is there anything else I need to fix that might’ve caused the original op-amp to crap out?

 

[PJS]

One thing that I will say is that all your jacks have plastic barrels. That is not a problem, but it does mean that you won't be getting automatic grounding of the enclosure. You will need to deliberately ground the enclosure to reduce noise. My favourite scheme is to solder a lead to the little waffly washer that goes under the switch, and then solder that lead to a ground connection somewhere.

 

[fable_instruments]

Hard to see in the photo, but the black wire coming off the sleeve lug of the output jack is my enclosure ground. It gets sandwiched between the back cover and the screw when I close the enclosure up. Any thoughts on the op-amp? Could it be as simple as wear end tear on the pins going in and out of the sockets, or is it an issue with the power supply that made it crap out? I’m a little hesitant to call this finished if there’s a lingering issue that might burn out another perfectly good op amp.

 

[Chas Grant]

There’s a good chance that the cold solder caused the failure of the op amp. As the joints with cold age they can pull away from the lead of the component, this will alter the resistance and therefore voltage and current will change. A small vibration can cause the connect to get better or worse which will pulse the op amp. This over time can mess up the op amp. I had a similar situation, except I totally missed a solder joint. Tried all kinds of things to figure out what was going on. I finally got new glasses and could see again, that’s when I saw the missing solder. The lead was situated so it made contact with the pad, it caused the pedal to start sputtering and cutting out when I turned up the gain. So it is possible the cold solder joint caused it.

 

[fable_instruments]

Thanks for the reassurance and all the help @Chas Grant

 

[Chas Grant]

Thanks for the reassurance and all the help @Chas Grant

Glad I could help and glad you got it working, always a plus!

 

[ADAOCE]

Hmm reading this thread as I ponder my own Mach 1 issues.. I’m going to try some of the measurements mentioned but any thoughts would be appreciated. I just posted my thread in troubleshooting