Random snap

[Leevi]

I have an issue with a vibrato channel Super Reverb clone. There is a snap that occurs randomly and disappears after a while. The snap intervall is about 3 seconds. Volume and the other controls do not affect the snap. It might be easy to trouble shoot but the random occurrence makes that challenging.


 I have changed filter caps without success.


I changed the preamp tubes one by one, no affect.


Power tubes have been checked and are not causing that.


Any ideas?


https://el34world.com/charts/Schematics/files/Fender/Fender_superreverb_aa763.gif


/Leevi

[Esquirefreak]

Oscillation, maybe?

[bmccowan]

Power tubes have been checked

I'm wondering what that means? I would swap in a known good set of power tubes and see if that solves it. Power tubes are the most common source of something one might call a snap. Or it could possibly be dirty sockets and/or tube pins.

[Leevi]

Oscillation, maybe?

Yes, that's what I'm thinking  I have moved some critical wires without success.

I'm wondering what that means? I would swap in a known good set of power tubes and see if that solves it. Power tubes are the most common source of something one might call a snap. Or it could possibly be dirty sockets and/or tube pins.

The problem does not exist when running the amp without preamp tubes.

/Leevi

[tubeswell]

Cold/cracked solder joint somewhere. Examine all the joints for mechanical looseness by wriggling each wire with a pair of pliers with the power off, or chopstick test the live amp. reflow the bad joint with 60/40 leaded solder

[shooter]

what he said ^^^^^


easy check, set it up to be able to view both tubes n inner guts, make the room dark, play, watch for the snap

[Leevi]

Clip: https://rikstone.fi/Recording%20105.wav


/Leevi

[Leevi]

I think I have solved the issue. After some resoldering of the grounding contacts the snap does not exist any more.
I also have put lot of effort to other things like component changes all without success.
/Leevi

[Leevi]

I think I have solved the issue. After some resoldering of the grounding contacts the snap does not exist any more.

Unfortunately I  was wrong, the snap  occurs still now and then. I have to continue trouble shooting.

Could the choke be partially broken and cause that?

/Leevi

[tubeswell]

Have you tried chopsticking the components?

[Leevi]

Have you tried chopsticking the components?

Yes lot, you cannot get it snap with the chopstick. All the filter caps have been changed, also many resistors, rewirings, re-soldering...
I have never met this kind of issue before.


/Leevi

[tubenit]

The most difficult amp trouble shooting problem I've ever had was a brand new resistor broken inside the body under the enamel. 


Every time I tested it, it read perfect resistance.  However, a certain note on my guitar created some type of vibration that would cause this loose wire inside the resistor to open and close.


I spent weekend after weekend trying to figure out what was wrong? Chopsticking it didn't make the issue of the popping happen either.  At some point, I tugged the resistor lead with some needle nose pliers and noticed it moved inside the enamel body. Once I replaced the resistor all was good and no more amp issues with that amp.


Intermittent problems can be a bear to resolve.   


I am NOT saying this is your issue at all, just sharing my experience with something that perhaps had a similar popping type noise.


With respect, Tubenit

[Dave]

In continuation of Tubenit's post... I had a 120Hz hum problem I could not figure out. This had an effect on it, that had an effect on it, but nothing would make it go away. I checked everything. Made sure there was no ceiling fans anywhere around. Unplugged TV's, refrigerators, computers. Nothing had any effect and the buzz was way too loud to ignore. On the second day of troubleshooting, I got frustrated and decided to walk away from it and come back later. Reached over and unplugged the soldering iron and the noise disappeared completely.


Dave

[bmccowan]

What I find strange is that once it starts it is rhythmic - as you said, every three seconds. So I am wondering, what operates in the amp in some kind of rhythmic fashion? Caps in the tremolo circuit? Caps store and release power. You say it's a vibrato channel SR clone. I assume it has no normal channel? Can you jumper out the vibrato circuit and see if it still snaps? Grasping at straws, but
You could also probe with a listening amp. But that could be painfully slow with an intermittent issue.

[shooter]

what operates in the amp in some kind of rhythmic fashion

1st thought when I read ^^, an Ecap through much resistance.... part charge....more charge....SNAP, repeat
a distinctive SNAP should be visible on a dark room

[tubeswell]

Cracked jumpers/leads or cold soldered joints can be hard to find without a close inspection and a solid wiggle.

Or it could be a bad choke. I did have this issue once with a choke. If you disconnect the choke a sub in a 1k resistor, you should be able to tell.

[Leevi]

The most difficult amp trouble shooting problem I've ever had was a brand new resistor broken inside the body under the enamel.

Yes, they are difficult. In this case I have changed all the resistors between the nodes and even some plate resistors. Furthermore the resistors are about two years old 2W metal film resistors.

What I find strange is that once it starts it is rhythmic - as you said, every three seconds.

That is the best period I succeeded to record. Otherwise it is very random, sometimes you have to wait an hour in order to get it snap.

I assume it has no normal channel? Can you jumper out the vibrato circuit and see if it still snaps?

/Leevi

I have made several tricks by removing tubes and using jumpers and groundings but so far without success.

Or it could be a bad choke. I did have this issue once with a choke

That's the case I'm troubleshooting now.

There is an interesting history of the choke in this amp. The choke was wrongly connected i.e. the plate current flew through the choke before I corrected it.That has probably caused stress in it. The maximum current of the choke is 90mA and the amp was biased to 35mA/tube. Furtermore there is about 460V plate voltage.One thing is supporting this therory since the snaps occured often in bursts after you had switched from the standby to operation mode.And some times you can get it snap by playing hard.

/Leevi

[Leevi]

I'm pretty sure it is solved now. The choke was not the problem.The cause was oscillation/crosstalk. It would have been easier to solve if the problem had come clearly out but a snap now and then did not indicate to that kind of problem.All the critical wires were shielded but that was not enough to prevent the snap. Finally I routed the shieded wire from the volume pot to the v1 grid along sides of the chassis and added a 10K grid stopper. The preamp is very sensitive for this kind of phenomen since there are very high voltages everywhere. The plate voltage on V1 is 285V and on the node 420V. The process took a lot time and components, 2 rolls solder wick remover, added stress but  hopefully competence as well;)
Thank you for your replies!

/Leevi

[kagliostro]

Good you solved the problem

Thanks for sharing the solution

Franco