No Reverb on Fender Bullet Reverb amp

[sadarar]

I am new to the forum and would appreciate any experienced advice on fixing the reverb on this amp.

The amp is a Fender Bullet Reverb made around 1995 based on the date on the spring reverb pan.  All functions are perfect except for the reverb.  Turning up the reverb control, increases the background noise, and once I opened the box and tapped the spring reveb unit it made plenty of ringing noise.  The noise can be controlled with the "Reverb" pot.  Tell me if I am wrong but I believe this means the output of the reverb circuits are good, and the problem is in the input to the reverb. 

After a little research I tried switching the input and output connections on the spring reverb pan.  The idea was to see if the input side transducer worked as an output transducer.  It did and the pan continued to make spring noise when tapped.  I think this means that the spring reverb pan is probably good, and that the problem is in the circuits supplying a signal to  the reverb pan, not the pan itlself.

Looking over the schematic for the amp, there are only a few resistors and, I think, one capacitor, and one side of an IC chip between the signal source and the input to the spring reverb unit.  I believe the signal is good before that, because the same signal source is split between the reverb and the main amp tone controls, and they all work perfectly.  So I think the signal is good at that point.

I am wondering what is the best thing to do next.  I considered replacing all the possible components between the signal source and the pan, but if there is a simpler way to determine what is most likely the specific failed component, that would be a better way to go.  I have not yet pulled the circuit board, but when I do is it possible that a solder connection has failed?  Does it make sense to reheat all solder joints first?  Is the capacitor more likely to fail than the resistors?  I know that some caps made during that time period were prone to failure and caused a lot of problems in computers and tvs.

Thanks for any advice.           

 

[Enzo]

While the functional test of reversing the pan connections does seem to reveal the pan itself as OK< really, the thing to do is disconnect it and measure for continuity at each jack.  The output end will measure usually something like 100-200 ohms, the input end anywhere from under an ohm to a few ohms.  It won;t be "wrong," it will either be good or open.

If you can make spring crash noise out the speaker, then the return circuit is OK, and if the pan is OK, then the drive circuit is the problem.

Assuming the cables from the pan to the board are intact and have integrity, then I'd bet my lunch money you have a bad IC at U5.   Certainbly cracked solder or a open resistor is possible, but really not nearly as likely.

Reverb problems usually break down as reverb pan bad, connections/cables bad, drive IC bad, then anything else.  In that order.

But look at the schematic, input to the drive side is pin 5 of U5, yes?   Hold a piece of bare wire or a small screwdriver in your fingers and tap it on pin 5.  That should send little pops through the IC, which would come through the reverb as reverberated pops.  If that happens, well it demonstrates that the IC can amplify and signal may not be reaching it.   Just because there is a good signal coming out of U3 down there, that doesn;t mean the signal is getting allthe way to U5 pin 5.   And a more aggressive thing that touching with a wire?  Ground your meter, set it to ohms, and tap pin 5 with the red probe.  That will send a small voltage pulse into the IC for a louder pop.


But really, I'd just suspect a bad IC.

[sadarar]

Enzo, thanks for the advice.  I recently moved to a smaller place and can't find my old test meter yet.  You are right, I need to find or buy a meter, then test the pan for continuity, check the cables, and then go on to the circuit components.

Any recommendations for a good, cheap test meter?

By the way, I see that there are some slight modifications in later versions of the Bullet amp.  Do you think it makes any sense to make those modifications once the reverb is working? 

Thanks again.

[joecool85]

Any recommendations for a good, cheap test meter?

This is the meter I use: http://www.sears.com/craftsman-digital-multimeter-with-ac-voltage-detector/p-03482146000P?prdNo=1&blockNo=1&blockType=G1

It's cheap, very reliable and has removable leads which is great in case you ever need to replace them.  Also it can do amperage readings up to 10A which is very handy.  I never use the little AC tester thing though, just the multimeter.