Fender SKX-65R non-functioning reverb, NOT the reverb tank

[fluffy]

Hey all, I've got a Fender SKX-65R amp with a dead reverb. The reverb tank is fine. It's an Accutronics Type 1 1EB3C1B 2-spring unit. I measure ~60ohm DC impedance on the input coil and ~200ohm on the output, which is typical, and the springs work. I can hear the springs through the amp when I tap on the tank, so the recovery amp is working, but I don't seem to get any signal into the tank, as I only measure VERY low signal going into it (around 0.005Vrms). The drive and recovery amps are a typical opamp design.

The problem started when I had to repair the effect loop jacks. It worked before, but not since. I wouldn't think the jacks have anything to do with the reverb circuit, but still.

I've got the schematic for it, if that helps anyone. Also, I'm knowledgeable enough about electronics to attempt any repair necessary, though I am kinda new to analog electronics..

Any ideas?

[Enzo]

The FX loop jacks may have nothing to do with this problem, however that doesn;t mean that your working on them didn;t cause the problem. You could have dropped a tiny solder bead between two points, you could have broken a wire connection, you could have caused a static discharge into the reverb wiring, etc etc. Many things can happen to unrelated circuits just from moving the boards around.

But the thing to do is troubleshoot the problem, not worry how we did it.

You measured 60 ohms on the reverb pan input. Turn off the amp, make sure the reverb pan cables are connected. and go to the drive IC, U4a, and measure resistance between pins 1 and 2. Still get about 60 ohms there? If you get more like 200 ohms, then your reverb pan cables are reversed.

Turn the amp on and touch pin 3 with a small screwdriver or a bent out paperclip. Your fingers touching the metal of the paperclip will introduce a hum signal into U4a, and that ought to go into the reverb unit. Do you hear any burst of noise going through? If you do, then the IC is working, if not, then it isn;t. You probably don;t have a 5218 op amp, but a plain old 4558 or something like it will work. Every Peavey reverb in the world is based on a 4558.

[fluffy]

The FX loop jacks may have nothing to do with this problem, however that doesn;t mean that your working on them didn;t cause the problem.

Yup, you're probably right there. I didn't bridge any connections or break any wires (not many to break that wouldn't affect the rest of the amp), but the IC certainly could be affected.

Still get about 60 ohms there?

Yup.

Turn the amp on and touch pin 3 with a small screwdriver or a bent out paperclip.

Great idea. I just tried that, and I sometimes got a small click, but not the hum I get when I touch pin 5. Presumably, that's what I should expect? Thankfully I do have a pile of RC4558's kicking around (great for TS9 mods!) so I can replace that and see how it goes.

I just did this test on U3 pin 6, which is where the signal into U4 comes from, and I got the expected hum. If only my oscilloscope worked...

[fluffy]

Ok, I got around to replacing U4 with a known good RC4558, which didn't fix my problem. I get the same lack of function as before. I still hear the springs when I tap the reverb tank or shut the amp off, but not while playing. As far as I can tell from the schematic, the signal comes into U4 from U3 through a 0.1uf cap. If U3 was not working, the amp itself wouldn't work, because it's part of the signal path for the guitar. The problem could be that 0.1uf cap. Also, I can determine that U4 is getting power, as I can hear the springs in other situations.

I can't think anything else to rule out, apart from that one cap between U3 and U4, unless the reverb tank is actually faulty. Maybe I could plug my ipod into the tank's input?

*edit*

I think I figured it out. After much looking at the schematic, I figured out that the voltage into the tank is dependent on the volume level (which I never had turned up much during testing), and after cranking the volume I measured ~2.5v rms at the tank, and still no reverb. I think I need to conclude that I have a dead tank, despite getting my 60 ohms on the tank input.

Am I correct on this? Is 2.5v rms correct for a reverb tank?