Fender FM212R Reverb Failure

[Circadian]

So I noticed the other day that my FM 212R was no longer giving me any reverb. I've poured over the whole schematic, pulled the amp apart several times, checked every component and every solder joint. I've run it with the amp out of the case and checked to make sure all my DC voltages are where they need to be, and I've run signal through the reverb tank from another source to make sure it's still working correctly. I even put an oscilloscope on the output of the op-amp leading to the reverb send and verified that there's signal there (That's U4-A on the schematic), and yet the darn thing still won't give me reverb. Anyone else had this problem and managed to track it down? I'd be thrilled to get it working again!

Thanks!

[Enzo]

Well, THE #1 most likely problem is a bad reverb pan. Pull the cables from the pan jacks, remove the pan from the amp cab, and flip it over. Each jack on the pan has two little wires going to the transducer on the ends of the long springs. ANy broken off? Now measyre resustance across each jack. One should measure somehting low, up to maybe 10 ohms, and hte other maybe 100-200 ohms. the resustance doesnt much matter. it will either be an open transducer coil or it will be right. Either end open?

With the reverb cables danging, turn on th amp with the reverb up some. Touch the tip of each cable plug with your finger. One should hum. That one should be plugged into the OUTPUT jack of hte pan. Was it in the right hole? if it hums, then the recovery stage is working. If not, the return circuit needs work.

The other plug is the drive. it is all well and good that the IC makes signal, but is it getting to the tip of that plug?

And of course we should verify the cables themselves.

[Circadian]

Thanks for getting back to me!

I'm sorry to report that I've already checked the pan, and it seems to be working pretty well. To double check its function with the amp, I did the following:

-Run a signal to the pan and then to another amplifier. I got reverb.
-Pull the RCA cable from the board to the pan out and put it on a cable tester. Messed around with it to make sure I didn't have an intermittent fault in the cable.
-Send a low-level signal back down the return cable and test that I could control its volume with the reverb pot. This also worked.

Maybe I managed to miss a fault in the send cable; I'll go back and hit the end of that with the o-scope again to make sure. Thanks for your help and if you've got any other thoughts on the matter, I'd love to hear them.

[Enzo]

There is not much it can be. Circuits in the amp, the pan, and the cables. Oh, and getting the cables in the right holes. Isolate the problem, it is the drive or the return with the fault.

Make sure the pan is the right type, is it original? It is important the input jack not be grounded to the pan.

If bouncing the springs make a crash noise out the speaker, the return side works.. you tested that with a signal.

The drive puts a signal out the IC, yes, but the return side is not ground, it is through that 22ohm resistor and 22uf cap. Either open? With the pan connected and power off, what resistance do you get between U4 pins 1 and 2. It should be the same as the resistance of the INPUT side of the reverb pan, they are wired to it after all. That is as quick a test of the cable as I can think of.

And lastly, you have signal at the output of U4. Does it remain when the reverb pan is connected? perhaps it cannot handle the load, which would mean a bad IC.

[Circadian]

Good points - I did another couple tests and seem to be onto something, but still haven't completely nailed it down:

-The reverb pan is original and definitely works.
-The reverb return works quite nicely as well.

-I'm fairly certain the cable's good. I tested as you suggested, which confirmed what I saw before with the cable tester - it should be working properly.

However, there's no signal at the pan end of the send cable. I pulled the amp open again and checked the output from the jack on the amp end of the send cable. Watching the signal with the oscilloscope ground on the shield of the RCA cable, there's signal, but it's definitely no more than 2 or 3 mV. Placing the o-scope ground on the amp chassis, however, shows a very nice line-level signal coming from the jack. I double-checked the 22 ohm resistor (R53) and the capacitor (C32) between the outer RCA connector and ground; the resistor seems good, but I haven't got a great way to test the cap. To me, this seems to point to C32 as being my culprit.

Any suggestions? Does it look like I"m barking up the wrong tree before I whip out the soldering iron?

Thanks for your help.