Little v847 mod project with big problems. Need help!

[Sokar]

Hello.  First post here.  This forum appears to be the Oracle of pedal knowledge.  Glad to be here.

I'm working on a project to mod my Vox v847 pedal to Clyde specs.  I'm pretty much replacing everything on the existing board.  Carbon comp resisitors, polystyrene and polyester film caps, transistors and inductor.  I'm also making slight adjustments to resistor values to get the 'vocal mod'.

I will base my comments using this layout since it is pretty much exactly like the stock PCB:

http://www.generalguitargadgets.com/diagrams/wah_847_lo_tb.gif

Step 1 – I replaced all 10 resistors on the board with the carbon composition replacements.  Between each swap, I plugged the board back up and tested and it worked every time.

Step 2 – After the resistors, I replaced C4 and C5 with .22uf poly caps.  Again, I tested after switching each and everything worked like a charm.

Step 3 – I then removed the stock transistor at Q2 and replaced it with BC109B transistor I bought as a pair of eBay.  Suddenly, my wah turned into more of a crappy volume pedal.  I thought it might be because I have to replace both the transistors, so I removed Q1 and replaced it with another BC109B.  On testing this step, I moved Q1 around which was only slightly soldered because I didn't want to commit until I knew this was the problem.  At first, it still sounded like a volume pedal, and then it went silent.

I tried to clean up the soldiering on the transistors thinking I may have bridged something.  Instead of my normal sucker that I use to remove solder, I used a braid.  I removed both transistors and cleaned up the pads when I noticed that part of the middle pad on Q1 was removed with the braid. 

At this point, I decided I needed a full stop.  I had concerns about the BC109B's and ordered some 2N5172 transistors from Mouser.  I needed a break anyway.

The parts arrived yesterday and I installed the new transistors at Q1 and Q2.  Pedal is still dead.  I went ahead and replaced the caps at C1 and C2.  Still dead.

I could really use some help from someone on this.  I have no troubleshooting skills at all, but I am armed with a multimeter.  The only test I've done with that is check continuity between the center leg of Q1 (where the pad partially came off) and the right leg of C1 and there is continuity.

I've done some reading here and see that one of the troubleshooting steps that has been asked for is voltage readings on all the pins.  I think I can handle that.  I did some readings already with the circuit engaged and have got some wildly different readings across the board and a few spots that are 0.  I did this by ground the black probe to the pot ground and touching with the red probe.  The dial was set to DCV 20v on my meter.  Was that the correct setting?  If so, I can go ahead and collect all the numbers.

Another thing I read about is creating an audio probe.  I'm working on creating one of those right now.

Any help would be greatly appreciated.  I'll provide you with whatever information I can to ease pinpointing the failure.  I can stuff boards all day, but if I can't troubleshoot, I'll never finish a project (this is my second, the first scratch project was a big failure).

Thank you.



Shot of the partially missing pad on C1 center leg.  Continuity between this point and the right leg (left looking at the back) tests ok.



Ready for DC testing?  Input and output have a patch cable run between them.  Battery is fresh.  Black probe stuck under third lug of pot.  Multimeter at DCA 20v.  The 3PDT switch is not wired at the moment, only mounted.  The original switch (hanging off to the side is still wired up).

[Gus]

Compare the pin outs of the transistors to the original ones.  Google etc the transistor numbers.

Does the wha work with the original transistors reinstalled?

[Sokar]

Compare the pin outs of the transistors to the original ones.  Google etc the transistor numbers.

Does the wha work with the original transistors reinstalled?

No.  I tried putting the originals back in and nothing worked.

The originals are marked MPS A18 EBC.

From everything I've researched, 2N5172 is the proper old school replacement, then again, I thought BC109B was right too.  I'll try some searches to find what the differences are.

Also, I found a 4.7uf cap laying around so I'm about to finish my audio probe.

[frokost]

Welcome to the forum! With that attitude and will to research, you will accomplish great things!

Double and triple check the pinouts of the original trannies and the replacements and crosscheck against the schematic. Make sure they're in the right way. The pedal stopped working when you replaced those, so it's most likely that the fault is somewhere around that region.

Also, go through the procedure in the DEBUGGING thread and post back. Someone is sure to know what's wrong when that information has been collected. It may even be you.

[Sokar]

Welcome to the forum! With that attitude and will to research, you will accomplish great things!

Double and triple check the pinouts of the original trannies and the replacements and crosscheck against the schematic. Make sure they're in the right way. The pedal stopped working when you replaced those, so it's most likely that the fault is somewhere around that region.

Also, go through the procedure in the DEBUGGING thread and post back. Someone is sure to know what's wrong when that information has been collected. It may even be you.

Thanks for the encouragement, frokost.  This is extremely frustrating.

I just now found the link in the forum here:

http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

...and I'm going through the list.  Looks like I was headed in the right direction with some of the things I needed, but I need a few more others.  I'll post my results here when I collect them all.

I'm still trying to get a feel for what 'C, B, and E' is on the transistors and I'm not sure how I should differentiate the left pin and right pin on the resistors and caps.  Should I just say left= and right= if you were looking at the board from the component side?

[frokost]

Hm, no. What you should do is to know where the emitter, base and collector is on the board, and make sure that it corresponds with the pins on the actual transistor that you have in hand. Trannies have different pinouts, and, as Mark Hammer states from time to time, the possible variations that three pins give seems to be endless  . The only way to know for sure is to google for the datasheet of the specific transistor and be absolutely certain that the pins go where they're supposed to.

I'll quote part of R.G. Keen's law (found on http://www.geofex.com/effxfaq/keenslaws.htm) number 6: "On all transistors watch the pinout like crazy . Assume that the device you actually have in your hand is probably different from the device specified in the effect and go look up the pinout for what you actually have. Never assume that a replacement has the same pinout - check it! This one step would probably eliminate half the non-functional first-timer's pedals. The data sheets are available on the internet in almost all cases. Go do the search. Find them. "

[ACS]

To answer one of your first questions - yes, that's the right way to take your voltage reading

Here's the schematic for your original circuit, which should help a bit. 

Use this to identify which of the three holes is the collector/base/emitter for each transistor, then (as above) reference the datasheet for the specific transistor your intend to replace it with.  You may find that just because the 'flat side was on the left' on the original, it's not necessarily the same for the replacement.  For example, that schematic tells me that for the first transistor, the collector should be connected to a 22K and a 470K resistor, the base is connected to a 0.01µF cap and a 1K5 resistor, and the emitter is connected through a 510 ohm resistor to ground.  Map this to your PCB and you're there

And don't stress about that damaged solder pad - all thet means is that you applied too much heat to the trace and it lifted.  I'd recommend putting transistor socktes in once you've identified the orientation as above.  This gives you two benefits - 1. you can use the two 'intact' pads to anchor the socket, then connect the pin for the damaged pad directly to the remnant of the trace with a short off-cut of resistor wire; and 2. you run zero risk of overheating your transistors and damaging them!

Good luck!

[Sokar]

Ok, here's the layout with the voltages I picked up.

I see there have been a couple of replies.  I'll take a look at those shortly and have more info.  I need to take a short break and get my head together.

Thanks for the continued help.

[Sokar]

Ahaa!

I just found that the emitter is pin 1, base is pin 2 and collector is pin 3 on the MPSA18.

http://www.datasheetcatalog.org/datasheet/motorola/MPSA18.pdf

On the 2N5172, the emitter is pin 1, collector is pin 2 and base is on pin3!

http://www.datasheetcatalog.org/datasheet/fairchild/2N5172.pdf

Although they don't number them the same (Fairchild uses letters), that's the three of them top to bottom of you were holding the flat side of the transistor to the left.

I'm going to do some twisting and re-soldering.  Be right back.

[Sokar]

Woooohoooooooo!

IT WORKED!

Twisting the transistor leads around to make them fit the to the board pinout did the trick.  That is, it worked after I realized that I had broke one of the leads to the switch.

I can't believe it!  I learned more about transistors here in one thread than I've ever known before.  You guys are great.  This pedal sounds FANTASTIC.

But brace yourselves, I've still got the inductor, true bypass switch and FoxRox buffer left to finish this project.  My first successful pedal project.  Sweet.

One last question - Should I consider wiring this FoxRox buffer to be always on instead of only on when the circuit is engaged?

Thanks again everybody.

[frokost]

Great! Congratulations!

About the buffer - do a search. Buffers have been debated over and over before. Reading through some of these discussions will reveal a great deal of knowledge. Most people would true bypass it, though.

[Paul Marossy]

Cool, glad you got it working! 

Yeah, you gotta watch the pinouts on these transistors. Sometimes they can bite you in the rear. 

[Sokar]

Yeah.  I took a break last night and did some checking on that.  Running it through the buffer all the time would be a dumb idea.  It's when I start over-engineering things like this I get into trouble.

I'll just install it the way the manufacturer says it should be installed.

Here's a quick update.  I did get the inductor in (used one called a Whipple).  Easy install.  Fit was perfect.

After that I put it on my main amp (instead of the little Roland Cube in the repair area).  Unfortunately, I've got some nasty buzz, both switched on and switched off.  Plus the sweep is too wide.  So I've got more voicing issues to look into as well.

But I'm going to put the project on hold for right now.  It's going to be a busy weekend anyway.

I've decided to stop screwing around with the values and go with the confirmed values from a genuine v846 found here:

http://www003.upp.so-net.ne.jp/kazuhee/ewahmod1R.htm

So I placed an order for more (ridiculously expensive) carbon comp resistors and they won't be here until the first of next week.  Maybe replacing the 6 resistors I'm going to replace will clear up the buzz.  If not, I'll bump this thread and start looking into what will probably turn out to be more advanced troubleshooting.

Looks like I may get to use that audio probe I built after all.

Thanks again.

[Sokar]

LOL!  From the link I posted above...

Italian V846 has 2N5172 transistor. We can purchase Fairchild 2N5172 and Central Semiconductor 2N5172 at Mouser online shop. I bought both of them and replace V847's unknown transistor with them. The pin assignment of Fairchild 2N5172 and Central Semiconductor 2N5172 are different from V847's transistor. V847's transistor's pin assignment Emitter, Collector and Base(from left to right). And Fairchild 2N5172 and Central Semiconductor 2N5172's pin assignment are Emitter, Base and Collector(from left to right). So I bend the legs of Collector and Base of the transistor to cross each other without touching. First, I replaced the two transistors with Fairchild's 2N5172. The sound was almost the same as V847's transistor. Next, I used Central Semiconductor's 2N5172. The sound became fat for mid frequency and has natural saturation. They has Vintage vibes ! The sound started to sweep like "Coowahoo"

I tried BC109 transistor as well. They are good also. However, 2N5172 has more vintage vibes.

I guess I didn't read that page as nearly well as I thought I did.

[zombiwoof]

When you replace the MPSA18's (which are very high gain), with something like a BC109B (those should be in the 300-400 hfe range for vintage sound, according to Fuzz Central), you will probably have to change a couple of other resistors also.  The Vox and Dunlop wahs that use those MPSA18's had some of the other resistors changed to balance with those trannies.  What I did, and encourage you to do, is to go to Fuzz Central's site, check out the Clyde McCoy wah stuff he has there, and replace the trannies that Dunlop put in that were different with the original values, to get it closer to Clyde specs.  It's a good starting point, and you can go further with mods from there if you want.  There's only about 3 or four that are different in the Vox pedal (which was also made by Dunlop, unless you have the newer 847A which is made in China), so it's not a lot of changes.  It worked for me.  I also changed the two .01uf's to the original type, which was polystyrene, but I don't know how much that made a difference.  Good luck!

Al

[Sokar]

Zombiwoof.  That is exactly the plan.  When I started the project, I used the page you mentioned as a guide.

http://fuzzcentral.ssguitar.com/mccoy.php

That's where my direction for the BC109B's, different values at some resistors and component types (polyester film, polystyrene caps, carbon comp resistors) came from.

For some reason, I didn't get the 'stock' Clyde values when I ordered resistors, and went straight to playing with values.  Great components but I screwed up.  With the transistor problem fixed, I did have some major buzz.

Since then, I've based the bulid completely on this site:

http://www003.upp.so-net.ne.jp/kazuhee/ewahmod1R.htm

The values in at least 3 locations I changed slightly were changed to strictly follow his guide.  Those values weren't that far of from what was installed, by the way.  To make a long story short, my buzz is gone now.

So I've pretty much finished this one.  I even installed the 3PDT and FoxRox buffer.

I wish you guys could hear this pedal.  It's freaking unbelievable now.  It sounds exactly the way I think in my head a wah is suppose to sound (I should admit Voodoo Chlid Slight Return is my favorite song of all time, lol).

It's a whole new level and then some.  Thanks again for all the help guys.

[Joe Hart]

Nice! And welcome to the forum!

Any sound clips or pics?
-Joe Hart

[Sokar]

Nice! And welcome to the forum!

Any sound clips or pics?
-Joe Hart

Thanks, Joe.

The only option I have right now is to record it on my cell phone, and that would probably sound like ass and then everyone would wonder why I had such horrible taste in wahs.   

I actually have a SM57 laying around and not a single damn thing to plug it into.  I should really consider one of those cheap USB interfaces.

I'll get some pics up of the finished mods when I get back home.  Me and my kids are hanging out at the house of the mouse in Orlando right now.