71 Super Reverb Slow To Engage Vibrato

[keithb7]

Hi folks. My 1971 Super Reverb AA270 circuit, is slow to engage the vibrato. Only the first time I turn it on after the amp powers up. When I click the foot switch, it can take 45 seconds or so to start throbbing.
After that first initial engagement, it works fine. Immediately after I hit the switch it engages properly. Until I shut it down for a while, and caps maybe bleed down? Then it takes a while again to engage when
I fire the amp back up. I thought the tremolo was a little weak sounding compared to my 64 Bandmaster so I did change the roach. I also changed 2 of the small disc caps in the tremolo circuit. One was .01 , the other .02 uF.
The middle .01 sandwiched between those two I have not yet changed. Maybe I should? It was the hardest to reach, so I left it, just to see if the other cap changes made any improvement. The tremolo sounds the same,
as it was. Maybe just a little different design than in my 64 Bandmaster. It seems to work, probably like it should sound I suppose. Rate and depth knobs seem to work, maybe not as much depth adjustment as my 64 BM, but its
there. For example my 64 BM depth set at 5, sounds about the same as my SR on 8.5. SR vibrato is slow to engage initially as described. I had read somewhere that "slow to engage" meant
the caps in the circuit need replacing. Well two out of three caps replaced did not seem to fix anything. Maybe that one original .01 cap I left in there is the culprit? I also checked the resistors in the circuit with my DMM. They
seem to be in good range. I tested that remaining .01 cap with my DMM, it tested very close to spec.

Any other suggestions where to start next to fix the slow engaging vibrato? I am guessing a cap maybe, as it takes time to build up stored voltage? Maybe I should try another 12AX7 in V5. Have not done that yet.
Not sure. Still have lots to learn. I am no pro for sure. Just a hobby-ist with a genuine interest.

Thanks in advance.- Keith

[Enzo]

First, the numbers are just marks on the knob, these things are not precision circuits. If I have a useful range of function, I don't much care what the "setting: might be.

Of COURSE try a different tube, just swap with one of the others in the amp. Makes a difference or not.

Problems do not happen in order of convenience. There are three feedback caps in the circuit, if I am changing one, I change all three. The three caps are in a row, so even the middle one should be accessible.

You mention three caps in the trem circuit, but I see six. Certainly those cathode bypass caps can affect operation.

But find out what the problem is. Look at your schematic, there are two sections to the trem. The left triode is the low frequency oscillator or LFO. The right triode is the interface, in this case the optocoupler driver. The LFO generates the trem signal, then the other triode amplifies it to drive the roach bulb. You need to decide if the problem is the LFO not starting or if the drive is not coming up. Or both.

I would certainly consider new cathode caps, but note the schematic has voltages on it, so how do yours match up?

Use a clip wire across the pedal jack, in case the stomp sswitch has gone resistive.

Wwhat if that last cap, the 0.022 across the bulb, has gone leaky, it might prevent the bulb from firing until it manages to charge.

[keithb7]

I appreciate your response. I am learning as I go here. The 3 caps on the left triode are the ones I was referring to that I changed 2 of. I marked those in the photo below. Everything I have changed has a green "X" on it. I will go back in there and change the 3rd, which I should have while I was in there the first time. That cap I did not change has the yellow arrow pointed at it. The other cathode caps I changed are also marked with a green "X" as seen in the photo below. The roach I also changed has a green X, as I changed it. I am a rookie at reading schematics properly. I see a .02 cap shown in the schematic in the right triode circuit, just down and to the left of the roach in the schematic. I am not sure where that is in my layout image below. There is another brown disc style cap, up and to the right of the roach. Is this it? A little confusing to me as it appears tied to pin 2 of the phase inverter tube in V6. Your patience is appreciated. - Keith

[Pedro Vecino]

I would check the strength of the RCA contacts (reverb and vibrato) on the chassis and activate the latter with a sharp scissors from outside (I use a bridged male RCA to test it). It´s quite possible that the problem comes from the footswitch/amp chassis contacts.

[keithb7]

A couple of weeks ago all 4 RCA jacks were been replaced with new. They were sloppy and loose. I will have a close look the footswitch.

[Jazz P Bass]

As Enzo stated, the cathode bypass capacitors, if dried out, will retard the LFO signal.
Lift each one & see what gives.

[keithb7]

Thanks Jazz. However I put in new cathode caps about a month ago. All of them across the board. Below here I put a green checkmark on all the caps that I replaced on the board.
The .022 cap in the red circle, in the tremolo circuit I cannot find. Any help with the location of it on the board, is appreciated. Maybe it's not the cap causing me trouble, but
for the sake of learning, I would like to know where it is located. Thanks, Keith

[Justin Thomas]

That cap may have already been removed or never installed. As it is parallel with the 10M, it was advised to remove it if in place by the "official" Fender tech notes for ticking tremolo repair. I wouldn't worry about it. You could always try to add it back and see if anything changes, though others have worked fine for years without it...

Justin

[loudthud]

My money is on the tube. That circuit biases the oscillator tube off when it's not in use. This causes a chemical problem on the cathode and it doesn't want to emit electrons when it is ask to. Special tubes had to be made when they started using them in early computers because they could be biased off for long periods of time.

[keithb7]

Thx for the replies. Its making sense now. The cap is gone. I figured I was just too inexperienced to locate it. Will do a tube swap and check back in. Totally get the dusty cathode problem. Will also go back in and change out that last .01 cap I skipped.

[Enzo]

I suspect that third cap is not the problem, but I would change it. However, I might listen to the amp and see if the speed range is too fast or not. I find most old Fender amps have the slowest trem speed too fast for my taste. By changing the two 0.01 cps in that feedback loop to 0.02, I slow the whole range down to where the slowest speed is a nice sexy throb. The fast end is still way faster than anyone can use.

[Chuck H]

If the vibrato "works" but just starts slowly, and you've replaced the bypass caps, my money is on the tube. You might also check for drift on the cathode resistor for the oscillator gain triode. Pretty much anything that could squash gain. had an especially whompy trem in a build once. The first thing I tried was a lower gain tube. I ended up with a different fix but I did notice that with the lower gain tube the trem was slow to start up. This should also apply to LT's post about the cathode. Try another tube.

[keithb7]

Well it wasn't the tube. Tried a couple of different good 12AX7s in there. No time to work on it currently. I will take to band rehearsal tonight and use it as is. Maybe later this week I can open it back up and take some more resistance and voltage readings.

[überfuzz]

I haven't read any posts in this thread, but checking the tubes is kind of the first thing to do. Now some 10 posts later you know it's not a tube issue, + you got some spare tubes in your stash.

The oscillator can be really tricky. Either you swap components one by one and reinstall all parts that aren't causing it. A more brute way is to just replace all components in the oscillator.

[Jazz P Bass]

The .022 cap in the red circle, in the tremolo circuit I cannot find. Any help with the location of it on the board, is appreciated. Maybe it's not the cap causing me trouble, but
for the sake of learning, I would like to know where it is located.

The .022 cap is sometimes in series, sometimes in parallel with the 10Meg resistor to ground.
Some schematics show no .022 at all.
I think that is the 'anti ticking' cap.
Move on, as that has nothing to do with the present problem.