Help with repair of Fender reverb.

Keith Hilton · From: 248 Laurel Road Ozark, Missouri 65721

I don't think my reverb is working on my Fender Vibrosonic amp. I think it is the same reverb setup that is in a Fender Twin. I am NOT a amp repair person, even though I design electronics. Where should I start, and go with the repair?

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Do you have a signal injector or signal tracer? If you do, just go from the plate, then to the grid of the recovery triode, and then do the same from the driver. That will tell you where the trouble lies. Nine times out of ten, if it's not the tube, it's a broken wire in the reverb tank or feed lines.

I occasionally troubleshoot signal paths using just a screwdriver and my finger (using 60-cycle hum), and going from one tube to the next, but I don't advise others to do that. Shocked

Jerry Erickson · From: Atlanta,IL 61723

Keith, When the amp is on, if you shake the amp, can you hear the reverb springs making noise? You have to have the reverb pot turned up, but that will tell you if the return side is good. You might want to pull the tank out and take a look at the input and output wires on the tank. They're small and like to break.

Jay Ganz · From: Out Behind The Barn

Yeah, that's the case with almost every amp I've had here in the shop.
It's those pesky little fine wires that lead to the input & output jacks.
The spring unit bounces around & eventually one of the wires snaps.

Jim Sliff · From: Lawndale California, USA

Yep, if it stopped working suddenly and you get the "crashing surf" sound when you hit it, it's usually the input wire in the tank. They are almost impossible to repair and not worth the effort - a new tank is only 25-30 bucks and you can replace it yourself in 20 minutes with no soldering; all you need is a screwdriver.

I'd go about checking it this way:

1. Hit it. If it makes noise go to #2. If not, #3.
2. Check the cables running into the unit. If they are OK, try new tubes at V3 and V4. to do this easily if you don't have spares, pull the second preamp tube and first replace the third preamp tube with it (it's the wrong tube, but close enough for a test). If you get reverb, it's a bad reverb driver tube - replace it with a GOOD 12AT7 - a JJ would be OK, or new-old-stock US or european tube. NOT a Groove Tube, Sovtek, EH, or similar...you need a really strong tube here. If THAT doesn't work, put the tube back and do the same replacement with the next one - if THAT works, see replacement notes above. Note - pulling the second tube, by the way, doesn't hurt your amp; it just disables the normal channel.
3. If you get no noise from hitting the tank, go through all the procedures outlined in item #2, and if none of them work, replace the tank. If replacing the tank does not work, take the amp to a reputable tech for service; there's something more seriously wrong.

All the procedures outlined about will take 20-30 minutes, tops (unless you replace the tank - you can find them on eBay or any of the guitar/amp parts houses for under $30.00. Make sure you get an Accutronics tank that's the same size - other than that it's a no-brainer (you can get one with more springs that gives you even MORE reverb, but unless you play surf guitar it's not really worth it).

Hope that helps -

Jim
_________________
No chops, but great tone
1930's/40's Rickenbacher/Rickenbacker 6&8 string lap steels
1921 Weissenborn Style 2; Hilo&Schireson hollownecks
Appalachian, Regal & Dobro squarenecks
1959 Fender 400 9+2 B6;1960's Fender 800 3+3+2; 1948 Fender Dual-8 Professional

Keith Hilton · From: 248 Laurel Road Ozark, Missouri 65721

You guys have been a big help. Hope to work on the amp this next week.

Ken Fox · From: Nashville GA USA

I had a Super Reverb in yesterday with no reverb. Output from the tank was fine, tank was OK, too. Ended up an open primary on the reverb drive transformer. Fortunately I stock those and I had the amp re-tubed, biased and the reverb repaired in about 1.5 hours. Sounded as good as new!

Usually I trace with a scope but I could feel that tube #3 was cold. The tube tested OK. The open primary on that transformer is also the feed for the B+ to the plates of V3 (reverb drive tube). A quick ohm check of the primary revealed it was open.

Jim Sliff · From: Lawndale California, USA

That's a quick way to check it Ken - but out of Keith's league, I think. I was trying to give him the most basic do-it-yourself method I could come p with.
_________________
No chops, but great tone
1930's/40's Rickenbacher/Rickenbacker 6&8 string lap steels
1921 Weissenborn Style 2; Hilo&Schireson hollownecks
Appalachian, Regal & Dobro squarenecks
1959 Fender 400 9+2 B6;1960's Fender 800 3+3+2; 1948 Fender Dual-8 Professional

Rick Johnson · From: Wheelwright, Ky USA

Ken
I thought you could take a ohm meter?

If you turn the reverb up and "crash the tank" do you hear anything? If you do that tells you the reverb recovery part is working. The reverb driver tube may not be driving the tank for some reason. The tanks are very fragile mechanically and you could have a broken lead or basically a bad tank. A good tank usually reads about 2 ohms across the IN jack and about 200 ohms across the OUT jack. An ohm meter check at the jacks where they plug into the amp is a good way to check both the tank and the cables. Any open or infinity readings indicate a bad tank or cable

Rick

www.rickjohnsoncabs.com

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