Deluxe reverb reissue oscillating like mad!

[Getchellamps]

I've got a DRRI on the bench for repairs with what initially was thought to be either a bad power tube or rectifier by the owner.


However, it turns out that neither were the cause.

Here's whats going on so far:

When the amplifier is turned up past 4 on the reverb channel, the amplifier breaks into a horrible parasitic oscillation which causes distortion and a crackling/sputtering sound to the notes.

Turning the Reverb past 5 and knocking the tank causes a bad oscillation at about 5khz or so.

I've been through it with my gen and scope for the past week and can't figure it out!!

Here's what's been tried so far:
The obligitory changing tubes. I tested and changed out ALL of the tubes with known good tubes from my collection and that didn't help anything.
I've tried replacing the filter cap that feeds the preamp nodes.
I tried replacing the plate resistors for the mixer stage.
I tried replacing the cathode bypass cap for the mixer stage.
The amp does this with the reverb driver tube pulled, and the reverb recovery jack shorted or unshorted.

The power section seems to check out okay, tubes are biased properly, and I can inject a signal right at the input cap to the phase inverter and get full output very clean into a load or speaker and on the scope.


The problem seems to be coming from either the recovery mixer or the trem section. If I inject a signal on either side of the mixer (v4) it gives the same results that plugging a guitar in and playing does- nasty sputtering oscillating sound. I've re-flowed almost all the joints on the board, re-tensioned all the tube sockets...checked numerous resistors...

I am pulling my hair out! Any advice guys?

[Firestorm]

The problem seems to be coming from either the recovery mixer or the trem section. If I inject a signal on either side of the mixer (v4) it gives the same results that plugging a guitar in and playing does- nasty sputtering oscillating sound.

I think you're right about that. Exactly where are you injecting your test signal before and after V4?

Check R33 for value and continuity to ground; same for R31 (Reverb pot) and R48 (Intensity pot).

The other problem many current Fender builds have is with the three ribbon cables used: obviously if a lead is broken internally, it's a problem, but just the physical routing of the cable (how it's bent) can induce or stop oscillations.

[Getchellamps]

The problem seems to be coming from either the recovery mixer or the trem section. If I inject a signal on either side of the mixer (v4) it gives the same results that plugging a guitar in and playing does- nasty sputtering oscillating sound.

I think you're right about that. Exactly where are you injecting your test signal before and after V4?

Check R33 for value and continuity to ground; same for R31 (Reverb pot) and R48 (Intensity pot).

The other problem many current Fender builds have is with the three ribbon cables used: obviously if a lead is broken internally, it's a problem, but just the physical routing of the cable (how it's bent) can induce or stop oscillations.

Thanks, I forgot to check the reverb pot! Doh! I will try that this afternoon. I'm injecting my signal right at the grids for V4. Is the R33 220k grid leak for the recovery input of V4? If so, I remember it checking out okay.

[tubeswell]

reverb pan feeding back? (Put your finger on the back of the pan when it starts to howl in order to check)

I find that having the pan 'upside-down', with a thick piece of cardboard across the pan's open side, helps eliminate pan feedback

[Firestorm]

I'd isolate the problem by disconnecting the reverb pan and removing the footswitch; just look at what's happening at the reverb mix stage (I assume the Normal Channel behaves itself?)

R33 is the 220K grid leak/reverb balancing resistor that helps set the reverb/dry ratio and maximum signal into V4. If it's open or not grounded, signal into V4 will be too large. Reverb pot is part of that divider, but on the other side of the 470K so its impact is less. Trem/vibrato pot is a biggie, since it's designed to load down the signal to the PI.

I'd suggest injecting a test signal on the hot side of the Reverb Channel's 220K mix resistor (the side away from the PI input) to replicate output of V4 and then at the junction of C12 and R25 (the 3M3) to replicate input to it. Also check/replace C13 (the 10pF) -- if that's shorted or partially shorted, there will be a monster signal into V4.

[roknroll]

did you ever figure out the problem?

[industrial arts]

Here are some things to try:

First: clean all the push on connectors between the control panel pcb and the amp board, check these connectors for cold solder joints and for "crud" or contamination between pins. Reverse the input and output of the reverb pan and see what that does for you. I have seen some of the tanks mis-marked or incorrectly color coded. Swapping the in & out won't really hurt anything and could solve your problem.

If not here is what I did to fix an amp with horrible reverb oscillations

Lift all the legs of the components that tie to pin 7 of V4 and tie them together with a stiff piece of wire above the pc board (they are the legs that are toward the center of the board). Then take a short length of wire from this 'tie bus' you created and wire it to pin 7 of V4 (the green wire that goes from the tube to the pc board can be used if it reaches). Keep it short.

Install a 1800-2200 pf cap across R28, like in the original DR. Disconnect the wire that goes from pin 2 of V4 to the pc board. Get a new length of wire and attach it to R28 on the signal side (again, should be the leg that is toward the center of the board

At this point you may hear a big improvement in the oscillation problem. However, you may still be experiencing some oscillations, but it will sound more like a bad speaker. It's actually random hi freq oscillations on some low notes. If so, you may want to install some grid capacitors on the output tube, like Fender did in the old days (1600pf to 2200pf from pin 5 to pin 8 of V7 & V8). This may take some of the tone out of the amp, but it's better than the broken speaker sound.

If you have an oscilloscope and know how to work on guitar amps (ie, knowing how not to kill yourself with high voltage - if you have any doubts, don't go into the amp -you might die) watch the output and use a chop stick to move the wires around V4 to see if that clears up the oscillation.

Some things I have yet to try but may be helpful: grounding the output tube cathodes directly to the chassis, like old Fender amps. Gerald Weber has written about grounding problems with the screen filter cap ground being tied to the preamp filter cap ground in older Fender amps; I don't know how much of a difference this would make in the new amps. I have also seen reference made to the reverb pan input/output block grounding. Next time a DRRI is on my bench I will look at these.

Finally, I am not sure but I suspect that these problems are only with the cream colored boards. They seem problematic and Fender switched to green boards later.


Good Luck

Mark Springer
audio tech
Avenue N Guitars

[Stevem]

I do not know how old that RI is, but just had a 5 year old Hot rod deluxe that had two of its power supply filters leaking out there lead hole!

I did not even bother to measure the ESR as I sometimes do just for kicks.
I would remove the dog house cover and check out yours, as coupling between stages can make for problems like that also.

[industrial arts]

Your post about the filter caps was right on the money.

I had a Gibson Vanguard on the bench just after the DRRI that was oscillating in only one channel and I chased it for the longest time until a few things pointed me to the caps - desperation being one of them. I strapped a known good cap across one the "C+" caps (the filtering for the preamp and reverb tubes) and the oscillation disappeared. Aha!

Since the customer with the DRRI hadn't picked it up yet, I went back and opened it up, undid all my work and put the amp back to stock. Then I opened up the capacitor pan and one by one strapped the know good cap across the "C+" caps (the 22uf - C33, C34 & C35). Sure enough when I got to C35 the oscillation stopped. I replaced all 3 of the 22uf caps and the problem was gone. Plus the amp sounded better than it did with all the mods I had tried.

So good lesson learned here. Random oscillations in the reverb circuit may very well be filter caps. This is not the first time I have weird capacitor problems in the Fender reissue amps, so I am going to be looking at them much closer in the future.

Mark Springer
Avenue N Guitars