Mesa V Twin problem.

[davefromtheattic]

Hello everyone,

I have a V Twin pedal that I plugged the wrong polarity power supply into. Anyone got any ideas to point me in the right direction of the fault. The lights still come on but no sound. Even just pointers for testing it would be brilliant.

Thanks in advance,

Dave "pedal destroyer" O'Neil.

[bulldogguitars]

Would you happen to have a schematic we could see.

[davefromtheattic]

Sorry, here is the link to the schematic.

http://www.tubefreak.com/V-Twinp.gif

[bulldogguitars]

I would start by verifying your voltages to the voltage regulators and also check the 4007 and 4148 diodes on the input of the regulators.

[davefromtheattic]

how do I check them? Sorry, I am an electronics novice, although I'm trying to learn. I could have put the v twin in to get fixed, but I thought it would be interesting to try myself. I am trying to gather information on building a valve amp, but just now I need to learn some of the basics.

[bulldogguitars]

Hook your voltmeter to ground. And with your positive lead, meter set to read dc volts, check to see if you have voltage to the Vi on the regulator. If not check to see if you have voltage out of the diodes feeding it. I bet one of the diodes is dead.

[davefromtheattic]

I'm tying to locate the diodes on the board just now. What will the regulator look like?

[bulldogguitars]

Those should be in a TO-220 package, marked 7812 and 7912. The diodes according to the schematic are D13 and D18.

[davefromtheattic]

I've got a voltage on one side of D13 diode but not the other. Is that looking like the likely cause? Thank you so much for the help btw. I've just ordered a new diode, I'll kep you posted how it works out.

Many thanks.

[bulldogguitars]

If the voltage you read is on the input side, make sure you check the output side. There should only be half volt difference or so.

[Tom Phillips]

...I have a V Twin pedal that I plugged the wrong polarity power supply into....

Dave,
Hopefully you are on your way to a fix with the help you already received. However, I'd like to point out that the schematic shows a 12 Volt AC supply. Polarity is not an issue in this case as it would be with a DC supply. Over voltage or hooking up a DC supply would be a problem since the V-Twin has no protection for those cases.
I think it would be a good idea to double check your power supply with your meter. It's also possible that your problem is just a coincidence.
Good Luck,
Tom

[davefromtheattic]

Thanks for posting Paul. The thing has been lying broken for a while (I lost the power supply and tried another without properly checking the rating), it was only when I started looking into building an amp that I decided to fix it. My thinking was that if I concentrate on building a preamp first it would break down the project. I then had the idea of trying to fix the v twin to start my learning curve. I am a complete novice at electronics, apart from what I remember from physics at school. I am very keen to learn, but I'm not sure of what material to read to learn. Any suggestions?

Anyway, were you saying that if I hooked up a DC supply that the pedal is broken beyond fixing?

The status just now is:

-Led's work,
-plugging in with bypass selected I get buzzing when I tap the tip of the cable, but when a guitar is connected it has a low drone with no guitar present,
-the other channels have nothing.

I could just put it in to a service place to get it fixed, but I want to learn about the problem.

Thank you so much for the help.

[Steve Conner]

Dave, if you lost the wall wart, are you sure the replacement you're using now is the correct one? It sounds to me like you still have a DC one, and Tom Phillips reckons it should be AC. If this were the problem, then changing it for an AC supply would hopefully make the unit work again with no damage.

[Tom Phillips]

...were you saying that if I hooked up a DC supply that the pedal is broken beyond fixing?

Dave,
I'm sure it's fixable. The first order of business, as Steve said, is to verify that you have or procure a proper wall wart supply. Maybe that's all that is required to get it working. You need 12VAC at (I'm guessing based on the schematic) around 2 Amps capability (Maybe a little more). If you don't have the original that came with the VTwin you could email Mesa for support. Once that is hook up you can proceed to check all the internal voltages. (B+, DC heaters, +12V and -12V)
Regards,
Tom

[bulldogguitars]

I didnt see that it was an Ac supply, my apologies, I was assuming (not good) that it was a dc supply. And was trying to help from that stand point, again Im sorry for that over sight. Danny