My experience building a maestro filter / sample and hold

[PaulCurtis]

Selection:
This project was selected by my 8 year old son (the guitar player). He got a book on stomp boxes, it came with a CD. The CD contained samples from about 100 different pedals. He selected the Maestro fsh. I had never built a pedal before so I was a little nervous about all the warnings (do not try this with little or no experience). I asked my son if there was a different (easier) pedal that we could try and he said the the fsh was the pedal he wanted.  With that I said how much trouble can I get into. I have my scope, I have a EE degree from 25 years ago, and I hated analog stuff in college. With digital stuff I am fearless. Analog is magic.


I found a Bom, layout, schematics and pc board on general guitar gadgets.  I also found schematics and a pc board for a charge pump to convert 9 volts to 18 volts.


Parts:
I was faithful to the BOM.
I used a 3080E from small bear instead of the 3080T that is specified in the schematics.
I used 1/2 resistors as specified in the schematic.
I used 220 uF capacitors instead of 250 in the BOM.
I bought the parts (all except for the 1/2 watt resistors and the 9 volt wall wart) from small bear. I got the resistors and the wall wart from mouser.

Assembly:
2 1 hour sessions with my son  to solder in the resistors.
1 Hour session with my son to solder in the caps and IC sockets.
1 Hour session with my son to solder in the pots and other misc.

The 1/2 watt resistors did not fit all that well on the board. I suspect that the PC board was built for 1/4 watt. I had 1/2 watt, I used 1.2 watt.

The three diodes did not fit through the PC board. I drilled out the holes so that they fit.

I attach the pots to the back of the board. I spaced them off the board with a shim of cardboard. I soldered the connections using the parts I trimmed off the ends of the resistors.

The round transistor sockets that I bought from small bear did not fit in the circut board so I left them off. (If I did it again I would buy a strip of single pin sockets and use three of them for a transistor socket.

I drilled the holes in the box. (Only to discover that I drilled them in the wrong place.) I filled the incorrectly drilled holes with J.B Weld epoxy.

I found an article in this forum that said that when using the BF245A Jfets I needed to install them upside down. I unsoldered them, turned them around, and resoldered them.

Power up:
The first attempt at power up no smoke. Not much sound either. In filter mode the input made it through to the output but just barely. In sampler and hold mode all I got was a tick that's rate changed as I adjusted the speed pot.

I decided to look at the ticking first since it would be easy to see on the scope and didn't require any input.

I hooked my scope up to the output and started pocking around. I noticed the ticking showed up distinctly on the +9V and -9V. I moved wires around and noticed that if the +9V and the -9V input wires were close to each other the amplitude of the ticking was greatly reduced. I twisted the +9 and -9V wires together and things were greatly improved. I suspect that there should be some small capacitors in parallel with 220uF capacitors that are conditioning the input power.

The ticking was greatly reduced at this point but I still didn't get any music through the sample and hold circut, and nut much through the filter circut.

I hooked a signal (the guitar) up to the input and followed it through the schematic with my scope. When I got to Q6 the signal dissapeared. Found a bad solder joint at Q6 (one of the ones that I had removed to turn around). Fixed this and I started to get real music through the box in filter mode. Sample mode still didn't do much though there was no longer any ticking.

I looked at the noise generating circut and saw that there was not much noise being generated. About  +- .1 volt on the output of IC4 pin 6. I had been expecting difficulties here so that is where I looked first. I went out to the local Radio shack and bought a package of 15 2N3904s. 5 of them were no better than the one that I had in the circut. 5 of them got me about +- .5 volts. 4 of them got me about + - 2 volts, and one of them got me + - 4 volts.

I put everything back in the box and tried it out. It worked. There was a big grin on the face of my eight year old as the sample and hold circuitry made these funny popping sounds as it changed the notes. We adjusted the trim pots to maximize it. Closed up the box, tightened up the nuts on the pots, and the sample and hold stopped working well. I traced this to the back of the speed control pot shorting against something when it was tightened. Fixed this problem and we were done.

Results:
Great sounds. No ticking in any mode.

This being my first stomp pedal, or analog project for that matter, I don't think I could have gotten it to work without my scope. I used it to track down the source of the ticking, finding the bad solder joint, finding the pot short when putting thing in the box, and auditioning noise transistors. All these things could have been done in different ways by someone with a little experience but I am happy that I had my scope.

[Marcos - Munky]

Congratulations for your first effect. Now your son will want you to build more effects .

[Pushtone]

Hi,

Thanks for your post.
I'm just starting this exact effect/layout and your experience was helpful to read.
Particularly your debugging experience.
I'll look into that software scope that Fp has on the Tonepad links page.
Might need it for this build, eh?

[DDD]

Hi Paul,
I have to say that your first stompbox is not the simple one.
Hope you'll continue with this exciting hobby.

[PaulCurtis]

There isn't an excuse too small to get a scope

[dmk]

great stuff.
i feel inspired to try more difficult pedals now!

[moosapotamus]

Nice report! And, congratulations on your first successful build... very ambitious.
Now you should be able to tackle a nice flanger circuit with no problems.

~ Charlie

[captntasty]

Wow, your son sure picked a doozy for a first build!  I guess he has good taste.   Glad you had success... almost any effects you choose after this one will be downhill, tailwind cruising!  Enjoy and welcome.....

[PaulCurtis]

It turns out that I don't know what transistor I used for the noise transistor. I bought 15 of them in a package from radio shack. They said they were type 2N2222. I looked in the package and saw one that was a 2n3904 and thought that they were all 3904's. Silly me. Some of them are 3904's some are 2222 and there are a few other types. I have yet to crack open the box again to see what is really in there.

[Paul Perry (Frostwave)]

Paul, your son is much luckier than most kids.

[Meanderthal]

 Now that's an impressive 1st build! That took guts. Congrads!