VOX (JEN) Tone Bender - circa 1966 - 197(?) [schematic]

[beedotman]

Awesome post Sinner!

[boblob]

Ace thread.

Just want to add some stuff I've seen on Jen version. I've seen a few come up on eBay and not seen any with a 3n3 output cap. They've all been 320nf tropical fish, like this one:

http://img.photobucket.com/albums/v315/bieke2/Italy/JEN_tonebender_in.jpg

And another on Acid Fuzz's Italian Fuzz (which has a cap switch on the output - can see a 4n7 tropical in there, which I guess is the Vox setting):
http://www.acidfuzz.com/italian-fuzz

I'd definitely recommend the cap switch - it can get punishingly bright esp. with single coils/Fender-y amps. I'd agree that low gain + low leakage is key if you want it to bias properly and sound as it ought to. I like the 2k2 instead of the 1k too, gives it more output and seems to push the mids a bit more.

[joeyshakespeare]

Hi guys, I'm having an issue with an old tone bender.
I just finished doing some re-wiring on it and it seems to be fine in that sense.

Bypass works perfectly but no output when engaged.
Measured voltages and theres nothing going to the board.
I get 9volts on the jack pin the battery is connected to but nothing after that.

As a side note, my transistors look different than anything I've seen before. They're like little silver canisters almost so I'm hoping they work when I get power going to the board.

[sinner]

Can you post some innard shots?

[Govmnt_Lacky]

Measured voltages and theres nothing going to the board.
I get 9volts on the jack pin the battery is connected to but nothing after that.

Is this a Positive Ground circuit?

Did you wire your battery lead to the wrong lug on the DC jack?

Pics would help A LOT!

[joeyshakespeare]

Here's a few photos.
I need to clean the wires up a little but they all seem to be connected right.
I got a few close up of my DC jack. I grabbed a stereo cable and plugged it in and the battery is connected to the Ring lug.

http://s123.photobucket.com/user/joeysh ... e%20Bender

[Govmnt_Lacky]

A few possible things to try...

Make sure that your -9V and 0V are connected properly. Here is a schematic to use:

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

You can also use this for further troubleshooting. (DISREGARD THE PARTS IN BLUE)

Also, it looks like some work has been done to the original PCB. As you can see in the linked pic I provided, original TBs did NOT use those smaller carbon resistors. This was probably a replacement/mod.

You should be able to solve your problems now as this is a fairly simple circuit. All you need now is your DMM

[Electric Warrior]

Also, it looks like some work has been done to the original PCB. As you can see in the linked pic I provided, original TBs did NOT use those smaller carbon resistors. This was probably a replacement/mod.

No. There are plenty of Vox TBs with that type of carbon film resistor around.

[joeyshakespeare]

The thing is I'm not sure how the voltage is supposed to get to the board. I've never put the power directly to a stereo jack in any pedals I've built or fixed and I'm fairly new to it anyway.
Everything on the board itself seems to be ok, I get a signal into it when engaged, but as soon as it hits those transistors everything goes quiet (which I'm assuming is because of the power issue)

[Govmnt_Lacky]

Do you have a Multi-meter?

Take some DC voltage measurements and post them according to which transistors you are measuring. If you need to know which leg is which according to the schematic, just Google it.

Cant really go much further unless you provide some measurements

[Govmnt_Lacky]

Also, it looks like some work has been done to the original PCB. As you can see in the linked pic I provided, original TBs did NOT use those smaller carbon resistors. This was probably a replacement/mod.

No. There are plenty of Vox TBs with that type of carbon film resistor around.

Thanks for the correction. Seeing as you have a TON more experience with these EW.... perhaps you could offer some repair advice as well?

[joeyshakespeare]

Do you have a Multi-meter?

Take some DC voltage measurements and post them according to which transistors you are measuring. If you need to know which leg is which according to the schematic, just Google it.

Cant really go much further unless you provide some measurements

Theres no voltages anywhere, that's the issue.
Everything has 0 volts except of the jack lug that I put the 9V battery to.

[Govmnt_Lacky]

Theres no voltages anywhere, that's the issue.
Everything has 0 volts except of the jack lug that I put the 9V battery to.

You have to install a guitar cable to the input jack for the power to flow.

Insert a guitar cable and then measure.

[Electric Warrior]

Also, it looks like some work has been done to the original PCB. As you can see in the linked pic I provided, original TBs did NOT use those smaller carbon resistors. This was probably a replacement/mod.

No. There are plenty of Vox TBs with that type of carbon film resistor around.

Thanks for the correction. Seeing as you have a TON more experience with these EW.... perhaps you could offer some repair advice as well?

I don't have any experience with Vox TBs. I just collected a couple of pics.

No advice here other than basic trouble shooting. Going through the circuit with a meter to find out what the issue is is the way to go. I can't see anything wrong in the pics.

[joeyshakespeare]

Theres no voltages anywhere, that's the issue.
Everything has 0 volts except of the jack lug that I put the 9V battery to.

You have to install a guitar cable to the input jack for the power to flow.

Insert a guitar cable and then measure.

OK perfect, so some progress.
I got the battery to make a connection. However, the battery immediately drains down to less than 1 volt.
I put a DMM across it and watch the voltage go down...
Would this be a bad cap?

[joeyshakespeare]

Ok cool, I got it working. The 8.2k resistor was broken internally and not passing the signal.
Thanks for all your help!

[Electric Warrior]

Ha! Glad you found the problem.

[sonicmojo]

I recently came across one of these and I'm trying to tune it. Compared to an identical one that I've borrowed to compare, mine is about half the volume output and not nearly the overall balls. Unity gain for mine is all the way up on level. Unity gain for the louder one is not nearly that far, more like halfway., with gobs more overall boost . I can't figure out the internal difference but I kinda want mine to sound more like the other one. What is normal on these? Is there is that much of a swing based on the transistors in each unit .Or is there something else that I should try to check - hopefully short of pulling all the transistors and measuring them. I'm kinda new to fuzz circuits and transistor tuning in general but I do have a DCA55 for measuring if it comes to that.

Further details to help:
Electrolytics have been replaced on both recently. I've checked bias of Q2 and am getting about 6V for both. I installed a 20K trimmer on mine in place of the 8.2K so I can adjust bias on Q2 better. Voltages look similar everywhere on both. There is variation in parts used in both. The other one has carbon comp, mine has carbon film resistors (but same values). Different cap manufacturers but similar values. Both have SFT337 in Q1. The other has SFT363 and mine has an 0C76 in Q2. I could post pics or more voltage readings if that would help.

Thanks for any advice!

[sonicmojo]

Well, I did a little more poking and found that the carbon comp 1K output resistor on the "louder" one has drifted to 1.75K. Mine was still reading about 1K. I popped in a 2.2K to mine and it significantly upped the volume. I think this is the majority of the volume difference between my two specimens. I'm pretty sure the transistors are mostly responsible for gain/fuzz in these units.

[ringworm]

When I view the image of the circuit board magnified, I can just make-out 2N5580, or something like that on one of the Si transistors, the other one is a bit harder to see.....

I think it's 2N5550 which is NPN with hfe 60-250, making it a good candidate.