Vox V847 Mods Questions

[Stephanar]

Hello guys! New member here.
Yesterday I bought a Vox V847 that needed some fixing. The guy is in real bad shape (at least it was very cheap). I'll need to disassemble the top, clean all the enclosure, wire a new pot (never heard a pot as bad as this one!), and while I'm at it I thought about doing a few mods. I started a research on this subject and looks like this is the place to talk about pedal mods. So, here I am.
I have some questions since this is my  first pedal mod and would appreciate any help.

- to disassemble the top (chromed part, don't know what to call it. English is not my mother language) I'll need to hammer the smaller side of the pin with something. Any recommendations on this? Risk of breaking anything?

- the board has some extra holes and tracks. What are those for?

- how fragile are the tracks?

- the inductor is strange. In the past I've had a v847 that came with that round plain black inductor. Is one better than the other? I don't plan on changing it yet since I'm on a tight budget.

- I plan to do some resistor changes, but instead of resistors I thought about trimpots + resistors. With just trimpots I would have a lot of dead spots, this looks like a good compromise. Example: instead of going for 68k or 100k for vocal mod, I thought about 50k (fixed) + trimpot 50k. That way with the pot at 0 I would still have an usable value. What about this? Good idea? Will I have trouble fitting this on the board?

- looks like there is a little debate about changing transistors but I plan to do that. It came with that transistor that everyone says it's too high gain for wahs. I plan on going for 2n5088 or bc108b, that's what I have access to. What do you think?

- when I measured the resistors in the board two values were strange. The resistor on the top left corner of the inductor should measure 33k, but it reads 19R. The one on the top right reads 0.05R. Is there anything wrong with them? Triple checked. The wah works as it should, but has a very bad scratchy pot. The 33k (vocal) I don't mind because I'll change it anyway, but the other one intrigues me.

- I don't plan on changing caps since I don't have access to tropical fish or other mojo components, but I used to have an awesome wah that used tantalum caps (those yellow ones). The tantalum I was able to find. Do you think I should go for it? Btw, I dont believe the tone of that wah came from the caps. It was a very fine wah with good parts, but since everything counts I decided to ask.

- the only pot I was able to find was a hot potz II. Some people say it doesn't work very well in vox enclosures due to the bigger travel. Any word on this? Actually, I was able to find a blacktop ICAR but it came in my last wah and it's life cycle is very short.

I guess that's it. Sorry for the long post, but this is my first project and I'm trying to feel safer. I did a lot a guitar electronics work, but my only experience with a pedal was changing a footswitch. Hope you guys can help me!



Link for full img
http://s27.postimg.org/4uzk5nvqb/image.jpg

[duck_arse]

Stephanar - hello and welcome to the forum. if you are lucky, Joe Gagan might come along soon and answer all your questions expertly, but I'll have a go at a few of them.

the "resistor" top right shows a single black band. this usually means a zero-Ohm link. it is just used as a bridge, and should read very close to 0R. as for the top-left resistor, I'd bet it was directly across the inductor, which will have a resistance of, at a guess, about 20R. check your circuit diagram.

as for the extra holes, it looks like you can add a resistor in parallel with the 33k without hacking up the board. also extra holes to fit different shape or replacement inductors.

chromed part is the treadle. the rest is for the experts.

[Stephanar]

Tranks dude! No bad parts, that's good.
To keep the thread updated, today I disassembled it for cleaning. I haven't planned to remove that small part that locks the pin, but the pedal was so dirty that I had to do it. Now it's much better. That small steel piece is oxydized. Should I bother?

[tubelectron]

Hello Stephanar,

I have exactly the same VOX V847 wah as yours, and it also needed restoration...

1 - I changed the original Clarostat 100K-EJ pot that was dead for a Pro-Pot Deluxe 100K ICAR pot from banzaïeffects that I had in stock, and it works well. Nonetheless, I think that a Hot-Potz II Dunlop wah pot should also work satisfactorily, with an extra-long life.

2 - I modified the switching operation to True By Pass with a DPDT FSW to avoid tone sucking when unengaged.

3 - I increased the wah feedback capacitor from 10nF to 15nF to have a more talky / less peaky wah (you can go to 22nF for a more vocal effect).

No other modification was necessary. You can hear the result here :

https://soundcloud.com/tubelectron/r05-0020-v847usreversed-tele

The guitar is a Fender Tele Mex, the amp is a Roland Microcube, the recorder is a Roland R-05.

A+!

[Stephanar]

Tubelectron,
Very nice tone! You havent changed the 68k resistor for more voume? Heard you gotta do that after the true bypass mod.
I placed the order for parts before reading your post, but I dont think I'll have trouble finding a 15nf around here. Ordered some trimpots, resistors, transistors (2n5088 and bc108b. 6 each, hope I'll be able to find a good matching pair around 300-350hfe), dpdt switch and a dunlop hot potz II. They'll be delivered until friday I think, plan to work on the mods on saturday.

[tubelectron]

Tubelectron,
Very nice tone! You haven't changed the 68k resistor for more volume? Heard you gotta do that after the true bypass mod.

Thanks !

It is nonetheless the tone (or very close) that you can have with your V847 if you modify it like me since it is the same model as mine. As you read it, I did not modified it so much : with the exception of the parts that I mentioned, all the rest of the pedal is factory stock, including the 68K input resistor. The True By Pass modification has no influence on the volume on the 2-transistors V847.

In fact, that wah did not needed an increase in volume : the balance was correct, conversely to my V846HW which needed an increase in volume. I noticed that It comes from the Hfe of the transistors : the MPSA18 on our V847 are specified circa 800-1000, and the 2N5172 on the V846HW is specified 100-500. Less Hfe means decreasing a bit the input resistor to increase volume, usually.

plan to work on the mods on saturday.

Unless you really want to tweak the circuit, I suggest you to limit first your work to the modifications that I have done, and have a sound check to see if it worth or in need to go further... But it is your wah, so you do what you want, indeed !

A+!

[Stephanar]

Keeping you guys updated!
Today I've had a little break and decided to remove the components I plan on swapping. Resistors were easy, transistors were a little trickier but it went fine.
Trannies reads 1150 and 900hfe. Pretty high.
Looks like the inductor was somewhat hacked, the solder joints looks pretty bad. Is that normal?

[duck_arse]

those solders are shiny, bright and pulling, the indicators of good joints. the brown gunk is the solder flux, it is not really a problem. it can be picked off with a spiked something, or washed off with alcohol and a cotton bud. the remaining flux indicates that the rest of the components were soldered and the board washed before the inductor was fitted.