Build Your Own Clone Message Board

This is my first post and first build. Unfortunately not all went to plan.

I just assembled a FET preamp - very meticulously - and it works, LED comes on, makes the sort of sound its supposed to make - however the volume pot makes scratchy static when turned and the pedal swells in and out of volume - very odd & unpredictable...

If anyone could help with this it would be much appreciated!

A crackling volume control in that circuit could be an indication that you have DC current flowing through that 1M pot. The clockwise end of that pot is supposed to be DC isolated from the rest of the circuit by C4 and C5 (both 10uF). Are you sure you have these two caps wired into the PCB the right way round? If so reversing them may work but replacing any electrolytic that has been reverse polarised is probably better. Or it could just be a faulty cap.

You could try turning the control to maximum clockwise and seeing if you can measure any DC voltage across it. If you see anything more than a few millivolts one or both of those caps is leaking current

The other possible cause of your problem is a faulty pot. Or maybe a bad solder joint.

Tark - thank you for your reply.

I have double checked all 10u caps in the circuit - they are correctly oriented as per building instructions.

I tried taking a multimeter reading but I am not sure which 2 points you wanted me to take a reading from?

Lastly - as for some more particular details - first I didn't previously mention that there is a faint high-pitched frequency when the unit is turned on. Also regarding the volume pot, when 0-1/4 volume there is NO sound - 1/4 to 3/4 is extremely cracking and erracticly modulating - at full volume the problem kind of goes away, the tone or sound doesn't feel right but there is less stuff going on with the volume maxed out. Also - this may be due to the fact that the pedal is not functioning properly but neither tones (bass, treble) nor 2 way switch have any discernible effect?

I have attached a picture of the build for reference.

Across the two outside tags of the volume pot with the volume maxed.

One other of the 2 10uF caps could still be leaking current or as I said you have a faulty pot.

The volume control in the FET pedal works by connecting the variable control, via a capacitor so it is AC coupled, in parallel with the feedback resistor around the op-amp. If you removed the volume control form the circuit altogether the op-amp would be running at the maximum gain determined by the fixed resistors in the feedback path. So as far as I can see there are only two possible component faults that result in it crackling - a bad pot or DC flowing through the pot. I suppose it is also possible that you have a short or bad connection somewhere. The PCB isn't shorting onto the metal treble pot case by any chance?

... Neither the switch or the treble and bass have any effect....

Now that is truly weird because the gain switch is before the crackling volume pot gain stage and the tone controls are in the feedback loop of the second op-amp which follows the gain stage.

I think that indicates you have something fundamentally wrong with your build. Are the op-amps both in the right way around??

Ok - so I have gone through your above notes to the best of my abilities (this is my first build ever - so needless to say its frustrating and disheartening) and this is what I can tell you:

1. Volume pot readings when maxed - On all my DCV multimeter settings (there are 5) I got a reading across the volume pot lugs of "0" except on the 200m setting which gave me a reading of 0.001 - 0.002. I would assume this would rule out the leaky caps?

2. Op Amp orientation - I have triple checked this as well as pushed down to make sure the connectors were firmly seated. All seems well.

3. I tried a battery hookup vs. the DC adapter - no change.

4. Solder joints - I did another visual inspection and poked prodded everything I could and all seems sound.

5. 2 way switch - unlike previously mentioned there does seem to be some effect coming from the 2-way a bit fuller sounding one way - a bit thinner the other.

6. Fresh ears and new description - so I plugged everything back in again and started with fresh ears and here is the best description of what is happening:

- Turn pedal on - a noticeable but not brutal pop from footswitch - makes the reverb tank splash a bit...
- 0-1/4 volume no sound is emitted from pedal at all
- 1/4 to 1/2 volume - faint static in pot but not "crackling" like you would get from a dusty guitar pot with your amp turned up - much more subdued. However the sound at this point is that of a blown 5 inch speaker - mixed with a crackly radio transmission - with random volume shifts from audible to faint and back.
- 1/2 to 3/4 - all of the above symptoms but louder with more distortion and less apparent volume spikes.
- 3/4 to FULL - the volume spikes and sputtery-ness are more smoothed out and seems like the most normal - obviously there is significant distortion/breakup coming from the amp because of the boost but the tome doesn't seem right. The blown speaker/intermittent volume spikes are reduced though.

While all this is going down I can't get any discernible effect from the 2 tones knobs - they go from full right to left with no change. That being said the sound is so static and crackly below 3/4 volume that you couldn't really tell anyway. But at full volume still no noticeable effect.

I hope this detailed description helps - as I am really out my league trying to diagnose this pedal. Of course there was some (or numerous) errors made but like I mentioned a first, I tried my best to be very clean, accurate and methodical when putting this together and everything seems to be correct with the build instructions.

if you cannot measure any significant voltage across the volume pot yes you are right that would seem to rule out capacitor leakage. In that case it would seem you have a faulty pot (or IC1 is damaged). Or the crackling is from a bad joint or other fault earlier in the circuit and that gain stage is actually operating correctly. You would probably need to use an audio probe to follow the audio signal through the pedal to see if you had poor signal / crackling before the first op-amp. Now I think about it the fact that you have radio pickup could be an indication that this is the case.

It is perhaps worth noting that all pots have a neutral 'safety' or landing zone for the wiper at either end of their rotation. It may seem hard to believe but on a lot of pots this extends for up to 30 degrees of their rotation, so within 30 degrees of minimum and 30 degrees of maximum nothing actually changes. I'm assuming all the pots are in the right place.

It sounds like the the low cut switch is rolling off the bass OK.

The fact that the tone control circuit has no effect is odd - the second op-amp is dedicated to an active Baxandall type tone control that should produce very obvious bass and treble boost and cut. This circuit is AC isolated by capacitors at input and output, and largely self contained, so it is not likely to be affected by faults in the previous op-amp stage. One possibility is that the second op-amp is dead. You could try swapping the op-amps around to see what happens. I seem to remember a mention that some people have found these chips to be faulty as shipped with their kits.

You should also check that both op-amps have +9V on pin 8 and about +4.5V on pin 6.

BTW - BYOC claim this circuit has a 3.3 Meg input impedance BUT they have slapped a 470K pop stopper resistor across the input, so really the input Z is around 470K. If you want the high input impedance either lose the pop stopper or fit a much larger value. Oh and the output impedance is around 470 ohms - not 100K.

Ok - that brings some clarity.

To be clear - 1. I am not picking up radio interference - but the SOUND coming from the pedal is that of a of tune radio station and a blown speaker combined. (distorted, intermittent, crackly) when strumming a single chord it seems like little fragments of the sound is coming through at random times (much like that of a blown speaker)

Regarding the volume pot - I could be wrong - but I have a feeling that the issue does not like there as it is doing its job of adjusting gain/volume just fine and the crackling is not directly tied to the adjustment of the pot.

My feeling is the signal is being corrupted somewhere in the board and the sound coming out is a result of that.

I am going to try and switch the Op Amps see what that brings. What is the IC1 that you were saying could be faulty?

Hi Scott - great looking build, especially for your 1st one!

We're likely going to need you to take the pcb out of the enclosure and re-do the solder joints on the bottom of the pcb, and take some additional photos of the pcb. But before that, try switching the two op amps with each other to see if your symptoms change. That might help us narrow down the issue.

Edit - oops never mind! You guys were talking while I was posting...

Thanks Morgan.

Ok so now this is blowing my mind. I switched the 2 Op Amps (made double sure they were oriented correctly) and this is what happened:

1. All distortion/crackling/volume modulation gone!

2. EQ's are active and very functional - crisp/silent etc...

3. The volume stage now is in reverse! There is also an overall volume drop of about 50% of amp natural vs. pedal engaged with pedal volume on MAX.
Max however is full counter-clockwise now. There is no crackling/distortion etc is the volume pot or gain - all of those symptoms disappeared - however like I mentioned - the volume now operates backward and there is approx. 50% volume drop.

I have changed nothing else at this point.

thoughts?

I don't want to jump to conclusions but it seems to me now a strong possibility that one of the op-amps is faulty. The working op-amp would now seem to be in the tone control circuit since this did not work and with the chips swapped now it does. IC1 and IC2 are how the two op-amps are labelled on the circuit diagram. I think you need to get at least one more HA1457W chip.

That makes sense.

Can you tell me which one of the 2 chips is the "functioning" tone circuit. Chip closest to the input jacks or chip closest to the footswitch? (looking at the circuit board as per my photo)

IC2

Contact BYOC about a replacement IC. There have been a few cases of folks getting a bad op-amp from time to time.

The tone control amp is IC2 so it is the chip furthest from the jack sockets. That means you need to replace the other chip - the chip closest to the sockets.

Thank you everyone for your help - it was indeed a faulty chip - the replacement came and now all works great.

Great sounding pedal BTW.

Scott

Brilliant! I love it when a plan comes together.

Amazing how good a pedal with high impedance MOSFET input and a real active tone control can sound, innit?

I found good help in this thread and would just like to share my experiences so that anybody with a similar problem might benefit from them.

I have built 4 BYOC kits without any problems. The fifth - the FET Preamp - is the first build to give me any trouble. I found this thread and it helped me locate the problem.

The symptoms were:
- Red LED comes on. Good, we have power.
- Volume control works "in reverse". From half the bypass volume to silence.
- The tone controls have very little effect on the sound. In fact only the bass and low cut controls made any discernible difference.

After reading this thread I of course figured that the OpAmp IC's were faulty - it couldn't possible be a problem with the build

Just to make sure, I went through all the suggestions in this thread.

Switching the IC's had no effect. In fact removing both of the IC's didn't change much. Maybe the tone controls had less effect but not by much.

IC2 had 9V from pin 4 to pin 8, and 4,5V from pin 4 to pin 6. IC1 was a different story. No voltage on pin 6 or pin 8.

Turns out I had skipped pin 4 when soldering the socket for IC1. After fixing this small problem I measured the expected voltages on pin 6 and 8.

Now everything works, and I agree, it is "Amazing how good a pedal with high impedance MOSFET input and a real active tone control can sound, innit?"

Thanks for the help.

Poul

PS: And for my next build I have the E.S.V. 2-Knob Bender (OC75) from my local distributor. Sorry, they are now out of stock too.

Glad to hear you solved your problem pchris.

Just noticed I said MOSFET. The 2SK246 is of course an n channel junction FET.