Fender 68 Custom Deluxe Reverb noise issue

[glebert]

I've got a brand new 68 CDR in for warranty repair that exhibits noise issue after it warms up, seems like it is in the Custom channel. Noise is like muffled pops, and happens even at idle. Sometimes seems like something microphonic or a bad solder joint, but it isn't consistant. Seeing quite a few coomments online about others having similar problems. I've seen suggestions to basically replace all (or most) of the preamp tubes, but that seems a bit shotgun-like.

Fender doesn't seem to have much of a clue on fixing these. Anyone have a good permanent fix?

TIA,
Greg

[Jon Snell]

More info required!

Is it affected by the vol control on the Custom channel?
If yes, follow the signal back with your oscilloscope and determine exactly where it is comming from, Tp8 is a good place to start. Make sure your probe is X10 scale as a X1 probe will introduce parrasitic noise. It will be; anode load resistor, coupling capacitor, valve etc.
There is only one valve section involved so not complicated.

[glebert]

More info required!

Is it affected by the vol control on the Custom channel?
If yes, follow the signal back with your oscilloscope and determine exactly where it is comming from, Tp8 is a good place to start. Make sure your probe is X10 scale as a X1 probe will introduce parrasitic noise. It will be; anode load resistor, coupling capacitor, valve etc.
There is only one valve section involved so not complicated.

It's odd, because it seemed like sometimes it would go down with the vol control, but sometimes not. I need to spend some more time with it. will take a look at the spots you suggested.

[Helmholtz]

Make sure your probe is X10 scale as a X1 probe will introduce parrasitic noise.

How this?

Main difference is that the X10 probe has an additional ~9M series resistor for voltage dividing.
Drawback is reduced sensitivity which might cause small signals hide in the scope's noise floor.

[Chuck H]

I did a little reading on this and it seems that many techs are complaining that the coupling (to get reverb and trem on both channels) is an inherently noisy circuit. That may be, but it shouldn't cause pops and whooshes. It should be white noise only. Whether that's acceptable in this model is up to the user I suppose. My first consideration would be V1 as the culprit. Years ago I would order extras every time I bought preamp tubes for a build because I fully expected some to behave badly. I figured this was because the larger manufacturers were getting the top run product and it was no longer a given that DIYers could. Same with reverb tanks, but that's another topic and I digress. Anyway... Things have changed and I'm not even sure Fender can get guaranteed performance tubes right now. I wouldn't/can't know, but it occurs to me.

If you have a known good tube you should try it in the V1 socket. Nothing to lose. If that didn't stop the pops and any modulating noise I would chopstick the leads and components, tap on the tubes, etc. looking for bad contacts.

Just know that all reports indicate that these amps have a high white noise floor from the factory due to the design. Changing that would be a modification and probably beyond the scope of this repair.

[nosaj]

does anyone have a way to see if there are any service bulletins for this model?

nosaj

[glebert]

does anyone have a way to see if there are any service bulletins for this model?

nosaj

None for this according to the shop manager, who is our "authorized" tech.

[Jon Snell]

If you think it may be from the reverb circuit, removing V3 will stop any sound from that section or disconnect C17 to remove it completely to be sure.
A X10 probe does reduce not only parrasitic noise caused by loading on a X1 probe it also reduces the signal. If you use an industry standard oscilloscope, Tektronics, Multicomp, Fluke etc will not add noise to a X10 probes signal. Cheap junky scopes with poor Y amp capabilities will.
That is why I use a Tektronics scope.
Have you considered in these days of cost cutting, it may be mains borne interference. To cope with this phennomenon, place a https://uk.rs-online.com/web/p/power...ilters/2378678 in series with the mains lead. (Make up a short extention lead with the RFI filter in series, better filters are available but this is for example only). Ensure the case in connected to a known good earth/ground to return the interference to earth/ground. That may be the answer if it is mains borne; stop it getting into the amplifier in the first place.

And the last question ... Which volume control affects the noise?

[Helmholtz]

A X10 probe does reduce not only parrasitic noise caused by loading on a X1 probe it also reduces the signal. If you use an industry standard oscilloscope, Tektronics, Multicomp, Fluke etc will not add noise to a X10 probes signal. Cheap junky scopes with poor Y amp capabilities will.
That is why I use a Tektronics scope.

Even better DSOs (like my Tek 2430A) often have a noise floor around 0.5mVpp.
So with a X1 probe you might see a mV amp (noise) signal while with an X10 probe rather not.
As the scope's noise floor stays the same, a X10 probe lowers S/N ratio by 20dB.

Really wondering how and where a X1 probe might cause instability while when switched to X10 it does not.
Of course, connecting a X1 probe with DC coupling to a PI grid will severely offset operating conditions.
But that's not instability. Just use AC coupling here to avoid DC loading by the scope's 1M input resistance.

[glebert]

Update on this, but not super informative. Amp is super sensitive to tapping on the faceplate near the jacks. Inserting or removing plugs from jacks can also start or stop the noise. Noise happens even if both volumes are down. Pulling V1 is really the only thing that stops the noise (well, V4 too, but that is just because everything goes through that). Tapping on V1 does not start or stop the noise. Shop manager says he had one very similar and thinks eventually he just got Fender to replace, he is going to talk to them next week.

[Chuck H]

Did you try a known good and non microphonic tube in V1??? Admittedly I know such tubes are hard to come by. I've been dealing with sourcing good tubes for the V1 position for maybe twenty years!!! When I find a good one I tag it and it goes in the box so I know it's there when I need it. No extra noise and non microphonic. Very hard to come by. Well.. Some of the short plate ax7's from the last twenty years perform acceptably but don't sound as good. I use them in a pinch. And in todays market I don't know I that could even fall back on those regarding performance. But my point is just that it's possible the amp is snuffy but even Fender can't get tubes good enough for V1 all the time anymore. You really do need to try a good tube in that slot before giving up. Charge for that tube if you have to. They don't come easy.

[glebert]

I've never heard of a tube being more microphonic to a tap 6 inches away than a tap directly on the tube. That being said, since this almost seems more like an oscillation that starts and stops with the taps, I think it is something more than the tube. Changing to a new and different brand tube made no difference.

This amp has all the pots and jacks mounted to a pcb, I think I am going to take a closer look at that and see if there might be a broken solder joint in there somewhere. It looks like they used those bracket pots, and sometimes they used to use those to jump grounds over the other traces. Maybe one of those got flexed too hard.

[g1]

Do these have Bright switches/caps ?

[glebert]

Do these have Bright switches/caps ?

Negatory.

[Chuck H]

I think I see where g1 is going.?. Ceramic caps can also be microphonic. I don't know how many of the ceramic caps in that amp are accessible to tapping on but that's the next thing I'd look for.