DRRI Headscratcher Solved

[Firestorm]

DRRI, maybe 5 or 6 years old, has crackling/popping noise. Not the snap-crackle-pop of old plate loads; more like the sound of an arcing film resistor. Intermittant and unpredictable. Happens more readily when everything is warmed up.

Goes away when you pull the PI tube: so it's in the preamps or the PI itself. Swapped out all the 12A_7 tubes. No dice.

Common to both channels: pull V1 (Normal Channel), it's still there. Put V1 back, pull V2 and V4 (Vibrato Channel) it's still there. So I think: it sounds like an arcing resistor and the channels share a dropping resistor, so let's swap that out. Nope.

Found a bad solder joint on one of the ground connections on a ribbon cable. Fixed that, but the crackling popping is still there.

After pulling and reinserting various tubes, it seems the problem is only there when V1 or V4 are installed; when there is a "signal" (hiss) being fed to the PI.

So before I undertake the ordeal of lifting the main PCB to look for naughty solder, I'm hoping there's someone out there who will say: "DRRI? Crackling sound? Yeah, that's always the framistan. No need to pull the board."

Anybody like that?

[Cliff Schecht]

It could be a bad connector or cable. Check that the RCA cables for the reverb are in good working order and Ohm out correctly. This amp is full of crimp lugs right? Make sure the wire is making good contact with the crimp and that the lugs are soldered securely. I repaired a JCM600 once that had a badly crimped heater wire. They didn't strip the wire properly and had only hit a few strands, the rest was PVC insulation under the crimp. I fixed this and the amp came to life again.

Are the shorting jacks throughout the amp making good contact? The plastic Cliff style jacks can eventually go intermittent with corrosion and such. If the amp was smoked around then this problem gets exacerbated and the pots of course go to shit from this as well.

With issues like this I get out the scope and look at the output of each channel until I figure out the stage where the noise is being introduced. Is there a time where it doesn't happen? When the normal channel volume is 0 and the vibrato channel is on it still does this popping? This makes me thing it's something in the reverb recovery circuit or maybe the mixing stage V4B. If the scope doesn't show any popping in the preamp stages before the reverb stage then start chopsticking around V4 maybe.

I've chased down issues like this in the past to loose hardware or a bad ground somewhere. Sometimes before ripping boards out of an amp I'll try to use a dental mirror to peek under the PCB (if this is possible, I think it is in new Fender's).

You said you swapped the preamp tubes but have you tried all known good tubes (especially the PA and rectifier)? Could be a mechanical problem, something that combo amps reveal very well.

[Milkmansound]

maybe look for things with bad solder somewhere along the B+ line - could be as simple as the sockets. I have seen that a lot with those new fenders - arcing on the circuit board at the sockets. A fresh flow usually fixes it for a few months :/

[Firestorm]

It could be a bad connector or cable. Check that the RCA cables for the reverb are in good working order and Ohm out correctly. This amp is full of crimp lugs right? Make sure the wire is making good contact with the crimp and that the lugs are soldered securely. I repaired a JCM600 once that had a badly crimped heater wire. They didn't strip the wire properly and had only hit a few strands, the rest was PVC insulation under the crimp. I fixed this and the amp came to life again.

Are the shorting jacks throughout the amp making good contact? The plastic Cliff style jacks can eventually go intermittent with corrosion and such. If the amp was smoked around then this problem gets exacerbated and the pots of course go to shit from this as well.

With issues like this I get out the scope and look at the output of each channel until I figure out the stage where the noise is being introduced. Is there a time where it doesn't happen? When the normal channel volume is 0 and the vibrato channel is on it still does this popping? This makes me thing it's something in the reverb recovery circuit or maybe the mixing stage V4B. If the scope doesn't show any popping in the preamp stages before the reverb stage then start chopsticking around V4 maybe.

I've chased down issues like this in the past to loose hardware or a bad ground somewhere. Sometimes before ripping boards out of an amp I'll try to use a dental mirror to peek under the PCB (if this is possible, I think it is in new Fender's).

You said you swapped the preamp tubes but have you tried all known good tubes (especially the PA and rectifier)? Could be a mechanical problem, something that combo amps reveal very well.

Thanks, Cliff.
It's got the problem with only V6 (PI) and V1 (Normal Ch. pre) installed. But it also has the problem with only V6 and V4 (Reverb Recovery/Reverb mix amp) installed. No problem with only V6. With V1 installed, fiddling the volume pot can trigger it. With V4 installed, fiddling the reverb pot can as well. I'm still thinking bad joint around the PI. Maybe even the coupling cap. The tubes really are all good. Rectifier's a NOS Mullard that'll be around long after the rest of the amp is in a landfill.

[Firestorm]

maybe look for things with bad solder somewhere along the B+ line - could be as simple as the sockets. I have seen that a lot with those new fenders - arcing on the circuit board at the sockets. A fresh flow usually fixes it for a few months :/

Thanks,

Guts of the B+ network are conveniently located under the doghouse. I've had that board up and everything is AOK. Come off of there to a batch of spade connectors. All solid (topside at least; still trying not to have to dismantle).

Sockets, refreshingly, are chassis mount, wired the old fashioned way.

[Firestorm]

New theory: something actually is arcing. Getting picked up in both channel preamps and the mix amp (RF?). Volume up in the channels or reverb up makes it louder, but it's still there when controls are zeroed out.

Hard to localize; when it "goes off" it's present everywhere. Reverb transformer?

[Cliff Schecht]

Try pulling the wire from the reverb tranny to B+, this would kill the reverb send circuit and any noise that xfmr is potentially causing. If that doesn't kill the noise then it's probably not that. You could try grounding the PI input (with the tube installed) to see if it still happens when no signal is feeding the phase inverter. If it's still there then it is probably somewhere in the PI or maybe a problem with the bias circuit??

[Cliff Schecht]

Google says this is a common issue in Fender RI amps, at least certain ones.

https://www.google.com/search?q=drri+cr ... =firefox-a

Found this (last post): http://www.tdpri.com/forum/amp-central- ... kling.html

Check that ribbon cable!

[Structo]

I know this may be obvious but did you power the amp on with the room lights off to see if you can spot any arcing?

Also, I think Cliff mentioned it but I had an amp that had a similar symptom and it turned out to be the FX loop jacks not making good contact on the shorting switches.

[Firestorm]

Google says this is a common issue in Fender RI amps, at least certain ones.

Lot's of problems, but no magic bullet solutions unfortunately.

It's not the reverb transformer, no tiny light shows watching it crackle in the dark and nothing suspect under the board (I touched up a couple of pads, but not really necessary). Gotta be an arcing component: cap or resistor. Resistors are mostly CFs and for them to arc the outside coating usually has to be damaged; caps are mostly metallized film, which are supposed to self heal. I'm going to start banging on things with chopsticks and maybe shotgun any ceramic caps blocking HT.

There's probably one or two specific parts Fender bought a little too cheaply...

Funny thing -- this one came in with a '67 Princeton Reverb; the Deluxe had this one little problem, the Princeton was nearly a basket case. Now, the Princeton sounds god-like and the damned reissue Deluxe still crackles.

[Firestorm]

Here's what it actually was: One of the plate connectors in the rectifier tube socket making contact only 99.99% of the time. Tube heats up, connector expands (apparently a little more than the tube pin) and every minute or so it would lose contact and arc for a fraction of a second. It would manifest itself in the preamp because the noise from the arc wasn't big enough to drive the power amp or PI audibly (probably would have been visible on the scope, tho), so it needs the preamp gain to make sound in the speaker.

These are the shiny ceramic sockets with the "U" shaped pin connectors, similar to what you see in Belton octal sockets. Incredibly hard to retension. Live and learn.

[martin manning]

Glad you got it sorted out. How did you find it?

[Firestorm]

Glad you got it sorted out. How did you find it?

Chopsticking. By moving the HT leads on the rectifier, I could trigger the crackle. Solder joints were good, so it was either the wire or the connector. Cleaning and retensioning the connectors in the socket fixed it.

I think the thing to watch out for with this type of socket is that the U-shaped connectors in the socket don't seem to be very flexible and won't spring back much if a tube with slightly larger pins is replaced by one with slightly smaller ones.

Related question: I have a couple of designs where one of the features is the ability to swap output tube types. Anybody have an opinion on which currently available sockets will tolerate this best?

[Cliff Schecht]

Belden seem the best for new manufacture. They're all I use in my amps anyways if I'm paying money. The Chinese stuff is shit IME.

[Firestorm]

Belden seem the best for new manufacture. They're all I use in my amps anyways if I'm paying money. The Chinese stuff is shit IME.

Beldens use the same style of connectors (but deeper in the socket, so even harder to retension). Is their metal more resilient? I agree with your opinion of the Chinese ones.