Background of the Forum-Vibe
The root of this work is based on JC Maillet's work back in 1996. JC had developed a 'vibe layout which loosely followed the original Uni-Vibe where the circuit wraps in a circle around the light sheild. I liked it and it occurred to me I might save some time if I was able to start with some pre-existing work, so I contacted JC about using his layout as a starting point for further development of my own and he gave me a huge thumb's-up. He liked the idea of using his work as a spring-board and even offered assistance. The ForumVibe board described here is a dedicated version of my R4 board layout R4-F.
We'll start by taking a look at JC's original work, the single-sided construction was a clever solution to an enclosure issue I believe. Here is what his original layout looked like as I had drawn it:
and here is JC Maillet's original PCB artwork:
Unfortunately I lost the pictures of my first vibe boards in a Hard Drive crash in 2002, but fortunately this picture of Moosapotamous's (CharlieB) board which was very nearly identical to mine, we both had flipped JC'c artwork and made the boards normal through-hole style.
I have long lost my pic but I do still have the layout artwork (see below) and though my board was nearly identical to Moosapotamus's I had already started moving the PCB traces around a little bit, mine used 1uF-25v BP Electro's instead of Tant's and I moved the two 4k7 resistors back onto the circuit board that had been relocated to the speed pot. I added standard trim-pot pads too:
With the R2 board I really got after-it, I moved the traces around a bit more and adding a couple of jumpers allowed me to open up some space. By getting rid of those two long traces going around the bottom left part of the board which allowed me to shift all the LFO parts over a bit.
The 1uF Electrolytics seemed a good place to add some potential and it seemed to me those Pansonic ECQ-V series stacked film capacitors would do nicely here. Their size (10mm x 10mm) being larger than the round 5mm-6mm Electro's made it clear that I needed to move components and traces around again to make room.
Also about this time I decided to look for another way of making a light sheild. The Film-Can thing had been a cheap and cool solution but as the world has switched over to digital cameras those plastic film containers were getting harder and harder to find. I don't like the idea of buying a roll of film just to get a plasic container.
I looked at a lot of things but most were not cheap or easily aquired. I settled on the little 3/4" Copper Pipe Caps found in the plumbing department of any local hardware store really fit the spec nicely at only 75¢/ea. and they were also smaller in diameter than the film canisters which allowed me to move components a little closer to the LDR/Bulb area and free-up more space.
I wanted to make space for other capacitor types besides Greenies and Ceramics by making the phase-shift caps have 5mm pad space for industry standard WIMA type capacitors.
The R2-b board quickly followed because it was time to fix a major anouyance. I decided it was time to make space for horizontal mount single-turn trimmers which are (in this case) much easier to deal with.
In my first few vibe builds I had used vertical-mount multi-turn trimmers but it was a hastle making adjustments, and impossible to know where your adjustment was. It was also extremely diffcult to know when you were at the end-of-travel of the multi-turn trimmer because one was usually playing the guitar with one hand and adjusting the trimmer with the other.
(ie; I couldn't hear the faint click-click sound they make when you are exceeding the travel)
Making room for the horizontal mount trimmer pots was quite a time consuming challenge as many parts and traces needed to move to make room, but I'm there now so it was worth it.
R2-b added the output mix mod I believe originally contributed to the forums by RG. This mod, while at first it seems like a really good idea but in actual practice turns out that a couple good 100k resistors work best IMHO and I only add this mod if it's requested anymore.
R2-b was also the time I started using an MPS-A13 transistor in the oscillator circuit. Every once in a while you can get a non-starting oscillator when using 2N3904'. My thinking was perhaps a gain issue and since it was simply a darlington pair I tried sub'ing a MPS-A13 which had much higher gain, it worked well and I haven't had a non-starting oscillator since. I decided to drop the option for a discrete darlington pair at this stage.
(Note: it was added again later to allow for "vintage" builds)
An accidental mod developed at this time from an error in the layout. If you look closely at the Offset-Adj trimmer (TR-3) you can see one end of it is connected directly to B+ supply instead of through the lamp as it should be. The vibe works fine but has a slightly different "throb" to it, a subtle difference for sure but I decided it should be an option later and you'll see it in the R4 board as jumper J5.
With the R3 board again more board space was needed for alternative film-n-foil .22uF capacitors. The small space where the .22 capacitor lived just didn't allow for much replacement potential so this area was up next for improvement:
I got busy shoving parts and traces around again, I could have just changed the size/shape of the board but for my self-inflicted parameter of retaining the original board dimensions, and this is what I came up with:
I wanted to use metal-film resistors and noticed 1.2M isn't a standard value in metal-film as it is in Carbon Composition, and since this is a stand-up resistor style layout like the 70's Japaneese PCB's I certainly did solder together two resistors (1M & 200K) in series like this:
but I thought it would be better if the board supported this properly, so I re-reworked that area of the preamp section to make space for two more resistors.
This is the R3 board layout, note the Offset-Adj (TR3) error was still there.
As my new layouts were working better with each rev, firing up without issues, sounding great, easy to adjust, easy to mod, a time came when I decided it would be cool to contribute a version of my new PCB layouts back to the DIY community.
Realizing most DIY folks don't prefer PIHER trim pots (as I do), I re-worked my R4 board to make pad space for a universal Bourns type footprint.
Personally I don't understand why folks shy away from the PIHER trimmers, unlike the Bourns trimmers they allow routing of a trace (or two) between their legs if needed, the Bourns style pads don't.
So to review, the options to have discrete transistors forming darlington pairs in the Osc and LFO was back, the jumper J5 "Throb" mod is back, the physical space for the .22uF cap now has room for a nice Poly, there are extra pads for the 1.2M resistors (1m + 220K) in the preamp section, space for the 10mm/sq ECQ-V caps, WIMA spaced pads for the phase shift caps, and so with that it became the R4-F Forum-Vibe layout.
ForumVibe.pdf (a 4.7MB Download)
(PIHER trimpots shown on the R4 board above)
© 2002 - Brad Burt