Fender Supersonic Bias...I smoked the R140

[Kyle Ashley]

I posted this over at the Fender forum as well, but maybe someone here can also offer a suggestion:

Attempting to adjust the bias in my brand new Supersonic, I smoked the R140 resistor resulting in blowing the amp fuse.

I couldn't find my bias probe so I stupidly attempted to do it with my multimeter. I had the black probe to chassis ground and just touched the resistor resulting in a reading of 26.8mA

There was a quick flash of light and that was it.....what went wrong?

The entire resistor didn't burn, just the end grayed a little bit. Will that need to be replaced as well?

Thanks for any ideas!

[Toggle]

I don't have a Supersonic schematic, and thus don't know the role of R140, but it sounds like you had your multimeter set to measure current (amperes), and created a path to ground for a whole bunch of current that wasn't intending to go there so quickly.

When set to measure voltage, the meter presents a very high resistance to current, and thus virtually none will flow through it. However, when measuring current, the path through the meter has practically no resistance. Current is measured within a path, such as when you break a circuit and insert a meter to both in and out points of that break. So, you can measure voltage with reference to ground (one lead at the test point, the other at ground), but not current--unless the current being measured terminates at ground right there.

You should probably replace the resistor.

-Tom

[rockon1]

Sorry to hear of your misfortune and welcome to the club. I slipped with a probe in my SS and did damage as well. I wont try it again without a probe! In your case it was probably due to attempting to check the bias improperly. The reading is supposed to be taken across the resistor(probe on both sides) not from resistor to ground. Sounds like you will need to at least replace the resistor as well as check over the rest of the circuit.

The upside of my screw up was I learned how to desolder! Downside is the amp blew up a couple months after I "repaired" it. Honestly it might have been from something entirely unrelated but it seemed like too much of a coincidence in my book. I sent it to a tech to get fixed and looked over. Bob

[rockon1]

Toggle's explaination makes more sense. The resistor goes to ground anyways. Bob

[Kyle Ashley]

Thanks for the info guys!

Thinking back on it, I've convinced myself that I allowed the tip of the probe to short against the adjacent component, but Tom's suggestion sounds right.

I've been in touch with a great amp tech who will get me straightened out....but I'll also pass your information along to him as well. Can't wait to hear the amp with great tubes and a good bias. And I'm going to have to find my real bias probe soon!

[rockon1]

Thanks for the info guys!

Thinking back on it, I've convinced myself that I allowed the tip of the probe to short against the adjacent component, but Tom's suggestion sounds right.



Well,thats how I fried mine. Pretty tight in there. Bob

[Dave C]

Fenders modern PC boards are pretty densely populated making it easy to short something...what's worse is the solder connections are on the underside of the board making repairing it more of a challenge than I'd like to see.

http://www.fender.com/support/amp_schematics/pdfs/Super-Sonic%20Combo%20(Updated).pdf

[rockon1]

Fenders modern PC boards are pretty densely populated making it easy to short something...what's worse is the solder connections are on the underside of the board making repairing it more of a challenge than I'd like to see.

http://www.fender.com/support/amp_schematics/pdfs/Super-Sonic%20Combo%20(Updated).pdf


Not too bad. I pulled the board up on mine in a few minutes. Of coarse a bit longer than the few seconds it took to fry it in the first place lol! Bob

[Kyle Ashley]

I put a new fuse in today and checked the amp today. It fired up OK and I carefully monitored the power tubes to make sure neither one was burning up or red-plating....The amp sounds incredible with the NOS Mullard CV4004s and Svet 6L6GCs. Very lush and smooth, but it did seem to be noticeably lower volume, so I only ran it for 2-3 minutes.

Since I only bought it yesterday, I'm not super familiar with the amp, but it seemed that with the volume on 2, it was blowing me out of my house, and I got the sense that my necessary stage volume would be no more than 3, but today I pushed it to 4,5,6 before I got to what I thought was my usual stage volume.

I shut it down rather quickly though, and will still have my tech check it before I gig out.

[dougb415]

Was this on the clean channel? Yes, 2 on clean is LOUD on my SS.

[rockon1]

Was this on the clean channel? Yes, 2 on clean is LOUD on my SS.

Yes 2 IS very loud on the clean channel ! FWIW I put a 12AY7 in my V2(clean channel only) and that helps a lot.Bob

[freakazoid]

++++ on the 12AY7. I use 12AT7, on most twin channel amps, it seems to make the transition between clean channel and "hot" channel overlap.