Help with intermittent reverb diagnosis: '65 drri?

[Mango Moon]

Hello All,

I need some help diagnosing an incorrigible reverb in a ten-year old '65 DRRI, and the principal question I have is as follows:

Other than the 12AT7 reverb driver and the set of RCA connector wires, or the foot switch or reverb pan itself, is there any other electronic component in the reverb circuitry on this amp that could potentially go bad over time?

I ask this question because I've got a '65 DRRI that I've had here for years now that has functioned properly the entire time, but over the past 1-2 years, the reverb has started acting irregular at times.

It started-out a year or more ago with the reverb simply quitting right in the middle of operation, without any other symptoms. I'd pull the pan out to check the springs and maybe switch the two wires around or turn the foot switch off and back on again, and most of the time, bang, the reverb would resume again. Then, about a month ago, it simply quit altogether. No more of the rushing reverb "swoosh" when I powered-up or when I activated the reverb via the foot switch, nothing! Just dead quiet.

Since then I've checked, replaced or substituted every single component that I can possibly think of in an effort to diagnose the problem, with no success. For example:

1) I've swapped out the 12AT7 with a new one, no change;

2) I've swapped-out the stock Accutronics 4AB3C1B reverb pan with another (identical but new) 4AB3C1B, no change;

3) I've swapped-out the stock Accutronics 4AB3C1B with a comparable 3-spring unit (9AB3C1B), no change;

4) I've checked for proper function (open/closed continuity) on both sides of the Fender two-way footswitch, no apparent issues;

5) I've replaced the two-way Fender footswitch with a brand new one just in case; no change;

6) I've replaced the set of RCA reverb wires and even checked for continuity on both sets, no issues;

7) I've checked the inside connections (solder points) on the 1/4" female (Switchcraft) footswitch jack, no apparent issues;

I'm flat-out "stumped". Anybody got any ideas? What else could it possibly be?

NOTE: Also, just for the purposes of advance clarification, all testing was conducted using Channel Two (i.e., the dedicated Reverb/Vibrato channel).

[bkahuna]

Does it make the standard "reverb crash" when you thump it, or is it completely dead? If it still "crashes" it could be a bad reverb transformer. I've seen a few of those go bad over the years.

edit: Also, be aware there is half of a 12AX7 tube used for the reverb recovery amp. It's V4. I don't think I read that you changed that...

[Mango Moon]

Big Kahuna,

Many thanks for your response. To answer your question, no, it no longer makes that classic 'reverb crash' or 'reverb swoosh' sound that you're referring to at start-up. It used to make that sound whenever I powered it up, but that sound has now stopped, whether its at start-up with reverb pot completely off or even at start-up with reverb pot fully dimed. Nothing either way.

Also, your point regarding V4 is a good one. I replaced V3 on multiple occasions assuming that it could be the Reverb Driver (V3), but you are correct, I have not tried replacing (or substituting) V4. It is my understanding that V4 serves as a Channel 2 gain stage, but it may also serve as the Reverb Return, so I'll give that a try. Unlike the 12AT7 in V3, the V4 position is a 12AX7 (currently a Tung-Sol Gold), so I'll give that a rip and I'll post my results staright-away.

Again, many thanx.

[bkahuna]

It is my understanding that V4 serves as a Channel 2 gain stage,

Yep. Half of it is in the Channel 2 signal path, but the other half is the reverb recovery amp...

[Mango Moon]

Well, unfortunately, switching out V4 didn't change anything either. Shoots! I'm really puzzled by this one! You mentioned a "reverb transformer". I'm not familiar with these as I'm not a trained amp tech, but I'm beginning to wonder if maybe that's what I'm dealing with here?

Two or more tube swaps at positions V3 and V4, two or more different foot switches, two or more sets of reverb wires, and two or more different reverb pans and still nothing . . . what else could it possibly be? I accidentally plugged-in the foot switch once at about mid-volume and dam-near blew the windows out of the place with crackling (don't know if that's normal or not?), but I wonder if that fouled something up, like the On/Off capabilities? Or what about the whole female RCA assembly (i.e., the part that accepts the male RCA connectors up on the PCB board)? Could that have gotten corrupted somehow? I'm lost!

I should also note that the vibrato/tremolo works perfectly fine including being fully switchable with either one of the same two foot switches.

[Mango Moon]

Basically, this thing acts like its not even seeing or detecting the foot switch (i.e., the On/Off signal) when switched. Its like its always Off and isn't being turned back On when the foot switch is depressed, which makes me wonder just how that 'switchability' function actually works?

If I open-up the Fender foot switch and test the positive (+) and ground (-) terminals on each side of the foot switch (vibrato and reverb sides) with a multimeter set to resistance (ohms), it reads Infinite ohms (i.e., unity or open circuit) at one setting and the 0.0 ohms (i.e., closed circuit) when depressing the foot switch again. It does this for both sides of the pedal (vibrato and reverb). Thus, it seems to me that both sides of the pedal (vibrato and reverb) are working properly. However, it appears that even though I'm depressing the pedal to activate the Reverb function, and even though the pedal is indeed functioning properly, the signal to switch On or Off, is somehow, not being detected at the board. Put differently, its not clear to me just how that 0.0 ohm (closed) or Infinite ohm (open) circuit condition on the foot switch actually transmits an 'On/Off' function to the amplifier?

Presumably, an Infinite ohm (or open circuit) condition on the Reverb side of the foot switch should produce 'Reverb On' function, right?

Anybody?

[bkahuna]

You mentioned a "reverb transformer".

The transformer is on the "send" side. If it was bad you would still hear the crash when you thump it.

There should be a standard disclaimer somewhere we could copy and paste, but here goes: There are LETHAL voltages inside all tube amps. DO NOT open them unless you REALLY KNOW what you're doing!

With that out of the way, at this point, I would be looking for an open plate resistor on pin 1 of V4, bad solder on the wires going to V4, or bad reverb pot/solder connections to reverb pot. Also check the solder connections to the RCA jacks, if you haven't already.

[Mango Moon]

Big Kahuna,

It sounds like you're confident that the problem lies on the 'return' side of things (i.e., V4 not V3) and I certainly trust your judgment on it. No one has ever been inside this amp since its date of production ('99/2000), so there's no reason to believe that anything has been inadvertently broken or whatever, but the tubes have all been pulled in and out several times over the years, so who knows, tube connections could certainly have been damaged, as could the RCA connections, etc.

In any event, I'm aware of the risks associated with capacitors and the like (i.e., the death cap, etc.), but I'll open it back up tomorrow and carefully evaluate everything you've identified here, and I'll follow-up with my findings in another post afterwards. Thanks for all the guidance. I appreciate it.

[bkahuna]

It sounds like you're confident that the problem lies on the 'return' side of things

If it were a problem only on the send side, you would still hear the reverb pinging when you tap it. That's why it seems to be on the return side.

[52 Bill]

I know that you have tried different tanks, but have you actually read the resistance of the output coils in any of the tanks at the RCA jack?

And while you are testing the reverb leave the pedal unhooked for now, it may only confuse the issue.

Try this, turn down the reverb control and turn on the amp. Plug any working RCA cable into the return jack on the amp. Turn up the reverb control a little and touch the open end of the RCA cable with your finger. Do you hear a buzz?

[Mango Moon]

I know that you have tried different tanks, but have you actually read the resistance of the output coils in any of the tanks at the RCA jack?

And while you are testing the reverb leave the pedal unhooked for now, it may only confuse the issue.

Try this, turn down the reverb control and turn on the amp. Plug any working RCA cable into the return jack on the amp. Turn up the reverb control a little and touch the open end of the RCA cable with your finger. Do you hear a buzz?

Bill,

Wow! Thanks for your input on this! This is the kind of analysis that I was hoping to get from someone. I will gladly proceed with what you've suggested here immediately, and I will follow-up momentarily, so please watch for that, but let me first expound on a few things, all of which might prove to be helpful.

1) I have looked-over all of the various connections and wires and solder points per Big Kahuna's earlier suggestions, and unfortunately, I can find nothing that is overtly wrong, not anywhere (i.e., no cracks or broken solder points, no loose wires or any areas of excessive heat, etc.). Everything looks pristine.

2) Disconnecting the foot switch (as you've suggested) makes perfectly good sense to me, "if" I can do so and still be assured that the reverb is in the 'On' position, because its one of many variables that are potentially confusing the issue here (i.e., bad foot switch, bad reverb pan, bad RCA wires, reversed RCA wires, etc., etc.). But how can I be assured that I've actually left the reverb in the 'On' position when disconnecting the foot switch?

3) No, I'm afraid I have not yet "read the resistance of the output coils in any of the tanks at the RCA jack" (at least not with any degree of confidence), and this is precisely the kind of thing that I would like to address, but I'm afraid I don't know exactly how to conduct such a test (i.e., amp 'On' vs. 'Off', etc.). Please keep in mind that I'm not a trained amp tech, but are you saying to simply test the output at the female RCA jacks on the pans themselves without being connected to the amp, via multimeter? If so, what specific connections do I make with the multimeter (where on the pan)? And what units and magnitude of output (what multimeter setting) should I be looking for? I will say this much, I tried to obtain such readings at the Input and Output RCA connectors (internal green and black wires) with the reverb pan sitting here on the desktop (disconnected from the amp), and I obtained the following readings . . . Input = 0.0 ohms, Output = 0.212 ohms for original 4AB3C1B and Input = 0.0 ohms, Output = 0.214 ohms for similar 4AB3C1C.

4) I'd be happy to conduct the other test you've mentioned here (i.e., "turn down the reverb control, turn on the amp, plug any working RCA cable into the return jack on the amp, turn up the reverb control a little and touch the open end of the RCA cable with my finger . . . to see if I hear a buzz"), but its not clear to me which of the two female RCA jacks up under the amp itself is 'Input' vs. 'Output'? The reverb pans themselves all have 'Input' and 'Output' clearly marked on them, but this is not the case for the two RCA jacks up under the amp chassis, and I know from previous experience that reversing these connections relative to the pan itself can cause the reverb to either work or stop working. Can you help by clarifying which jack (up under the amp) is which? For example, when looking at the amp from the back panel, is the deepest RCA port (i.e., the one closest to the grill cloth) the Input or the Output? Similarly, when looking at the amp from the back panel, is the front-most RCA jack (the one closest to the back panel) the Input or Output? That alone would help me immensely.

Thanks for your willingness to help Bill! I really appreciate it.

[52 Bill]

Disconnecting the foot switch (as you've suggested) makes perfectly good sense to me, "if" I can do so and still be assured that the reverb is in the 'On' position, because its one of many variables that are potentially confusing the issue here (i.e., bad foot switch, bad reverb pan, bad RCA wires, reversed RCA wires, etc., etc.). But how can I be assured that I've actually left the reverb in the 'On' position when disconnecting the foot switch?

The reverb pedal grounds out the signal that comes back from the tank to turn off the reverb. Leaving it unplugged assures that the return signal from the tank is not grounded out during testing.

...but are you saying to simply test the output at the female RCA jacks on the pans themselves without being connected to the amp, via multimeter? If so, what specific connections do I make with the multimeter (where on the pan)? And what units and magnitude of output (what multimeter setting) should I be looking for? I will say this much, I tried to obtain such readings at the Input and Output RCA connectors (internal green and black wires) with the reverb pan sitting here on the desktop (disconnected from the amp), and I obtained the following readings . . . Input = 0.0 ohms, Output = 0.212 ohms for original 4AB3C1B and Input = 0.0 ohms, Output = 0.214 ohms for similar 4AB3C1C.

Yes, the input side of the tanks should read around 2 ohms and the output around 200 ohms. If your meter has a lower scale than the one that you used to get your readings, you may get a better read on the input coils. Your readings of 0.212 ohms, I assume are really 0.212 K (1000) ohms, which equals 212 ohms.

Can you help by clarifying which jack (up under the amp) is which? For example, when looking at the amp from the back panel, is the deepest RCA port (i.e., the one closest to the grill cloth) the Input or the Output? Similarly, when looking at the amp from the back panel, is the front-most RCA jack (the one closest to the back panel) the Input or Output?

I'll have to look that one up. In any case you can try the test with both of the jacks.

[Mango Moon]

Bill, I apologize for my ignorance here, but does this mean then that the reverb is always 'On' by default when the footswitch is disconnected, regardless of its setting at the time of disconnection?

My multimeter has a working Ohm range from 2M through 2K and 200 ohms to the lowest single setting labeled "+", and it was at the "+" setting that yielded 0.212 (212) ohms for the Output resistance on both pans. I've since retested both sets of 'Inputs' on both pans at that same (lowest) "+" setting, and if I wait patiently enough, they drop to a bottom reading of 0.003 (or 3) ohms. So, can I interpret this to mean that I conducted the test properly and that both pans are functioning correctly?

I don't really know how to read one, but if a schematic for the DRRI would help, I can certainly post one.[/COLOR][/QUOTE]

[Mango Moon]

Bill,

O.K., I conducted the 'buzz' test that you suggested on both female RCA jacks up under the amp itself using a single RCA wire, and here's what I found:

DEEPEST RCA JACK (the one closest to the grillcloth or furtherest from the rear panel): A loud buzzing sound occurs when touching the bare end of a single connected RCA wire, at any Reverb pot setting, even when set to zero

FRONT-MOST RCA JACK (the one furthest from the grillcloth or closest to the rear panel): Dead quiet when touching the bare end of a single connected RCA wire, regardless of Reverb pot setting (0-10)

Hope this helps.

[Mango Moon]

Bill,

I should also add that I've got about six other reverb pans here to experiment with if needed (i.e., Gibbs 2-spring, Accutronics 2- and 3-spring, both floor and wall mounted, even a Ruby or two), so let me know if I need to consider the use of any others.

Many thanx.