Vox Wah V847 vintage modded...won't work...yes, I checked the transistors :)

[McQ7]

Hello all,

First post here, and first of all "Thank you."  I've learned a lot here in figuring out how to modify my Vox Wah V847, made in USA. 

I'm hoping to get any help or advice that I can about why my pedal won't work.  I've read the debugging thread and I hope I've included everything.  Here goes...

Summary:
I replaced the resistors, caps, and transistors, and now the Wah doesn't work when engaged.  It allows for bypass, so the signal comes through just fine when it isn't engaged.  Note:  the Wah had been modded for true bypass when I purchased it used, and it worked just fine.

I checked the voltage on the board using a multimeter and the values are all as they should be--consistent across the circuit, just like this example from a thread on this forum: http://img268.imageshack.us/img268/2203/img022w.jpg 
Connected black/negative to the ground lug on the pot, and took readings using the red/positive, with switch engaged and a jumper cable plugged into input and output.

Resistors:
I followed the advice of Castledine Electronics, http://www.wah-wah.co.uk/



"Vocal Mod"
Replace the 33K resistor in parallel with the inductor with=68K.

Gain and Bass Response
Replace the 470 ohm resistor (some have 390 or 510) at the emitter of Q1 with a lower value=330 ohm

Midrange
Replace the 1K5 resistor between the base of Q1 and the inductor with a larger value=2.2K

Volume
If you've converted to true bypass, replace the 68K series input resistor with a lower value=47K.

Capacitors:
I followed the advice of http://www003.upp.so-net.ne.jp/kazuhee/ewahmod1R.htm
and made sure that matched well with fuzzcentral's list of Clyde McCoy parts: http://fuzzcentral.ssguitar.com/mccoy/mccoypartslist.txt

Replace .01uF with Xicon polystyrene 50V .01uF caps, Mouser #23PW310.  I understand that these do not have polarity.

Replace .022uF with Xicon metallized polyster film 250V .22uF caps, Mouser #1430-2224.  I understand that these also do not have polarity.

Transistors:
Now, I know that one of the easiest ways to get a modded Wah to not work correctly is if you replace these without regard for identifying the Base, Emitter, and Collector.

I followed the advice of http://www003.upp.so-net.ne.jp/kazuhee/ewahmod1R.htm

Replace HMPSA18 with Central Semiconductor 2N5172, Mouser #610-2N5172

I tested and made sure the Base, Emitter, and Collector were correct using a multimeter: found the base, tested voltage, lower voltage pin is collector.  The HMPSA18 stock transistor is Emitter, Base, Collector from left to right with the flat side of the transistor facing you.  The Central Semiconductor 2N5172 is Emitter, Collector, Base with the flat side of the transistor facing you.  Therefore, I bent the legs so that the base would go in the middle position on the board.  I verified that none of the legs are touching.  I tested the transistor once installed, and the readings still verify positions.

My tests are supported by the data sheets:
HMPSA18, page 4:  http://www.datasheetcatalog.org/datasheet/hsmc/HMPSA18.pdf
Central Semiconductor 2N5172, top right of page:  http://www.datasheetcatalog.org/datasheet2/c/0gyxlyq5hqj0t5ocfdkrc6e7w9yy.pdf

The Fairchild brand 2N5172 is the same with the flat side facing you: ECB:  http://www.datasheetcatalog.org/datasheet/fairchild/2N5172.pdf

Here are some pictures:







Any ideas?

Thanks in advance for all the help!

[Paul Marossy]

Build an audio probe and feed the circuit with a continuous test signal and follow the signal until it dies. Where that is will most likely be the source of your trouble(s).

My guesses are either a bad solder joint or the PCB has a broken track on it that happened during the process of removing the existing components.

[Gus]

Polystyrene capacitors are heat sensitive.

[Fender3D]

lol orange-orange-yellow means 330K not 330 ohm...
I can't check wiring, but that true bypass looks weird to me...

check those pins in excess too (they shouldn't touch the case...)

[Gus]

Fender3D noted the 330K not a 330 ohm emitter resistor that could be all that is wrong. 

McQ7 what were the voltages at the first transistor?

I would check the bypass switch with a meter sometime switches go bad.

[McQ7]

Oh, wow--thanks! That's what I get for being a noob . I thought it was supposed to be 330K. I'll see if the radio shack down the street from work has a 330R ohm. If not, then I'll replace that with the original stock resistor to see if the wah comes back to life.  I'll check the voltages at that first transistor tonight.

The bypass appears to be based on this:


I know I need to get a better wire cutter to clip those ends. In the meantime, I've bent them down and out of the way--they definitely aren't touching the chassis.

[McQ7]

Replaced the resistor with 330R and it works!  Thanks to all!

Now I gotta get a pot with a more gradual sweep...

[Joe Hart]

Congrats!
-Joe Hart

[Paul Marossy]

Replaced the resistor with 330R and it works!  Thanks to all!

Now I gotta get a pot with a more gradual sweep...

Cool. Sharp eye Fender3D!

[Fender3D]

 

[McQ7]

Yeah--credit to Fender3D for spotting that one.  Thanks!

So...I've read good things about the Whipple halo inductor from the seller on eBay, http://myworld.ebay.com/indoblitydo/.  Is his Icar pot recommended as well?