Fender Hot Rod Deville issue

[leydenjar]

There's a crackling, frying sound on notes that I initially thought was vibration but it does it through other cabs as well. It isn't as apparent using a signal tracer while hooked to a dummy load but I think it's there. Voltages check good on all tube pins. My scope just went out on me and I won't have a replacement for a few days. I've checked all the tubes with known good ones. I've reflowed almost all the solder on the power tube circuit board. I've jumped the filter caps with good ones as well.

The amp sound like a typical Deville to me. It's fine except for the crackling. The frying crackle is only present during notes/signal, particlularly low ones. I know I may be leaving something important out in my description of symptoms and what I've checked already but I've been up all night with this amp.

Any suggestions or directions would be greatly appreciated. I'll be back on it when I get up.

Thanks,
leydenjar

[gwright]

I would suggest we figure out if the issue is in the pre-amp or power-amp first. Try plugging your guitar directly into the power amp in jack and see if you hear the same issue. If you have a second amp around you can then plug a cable from your Deville pre-amp output jack into the input of another amp and see if you hear the sound. This will at least tell us what area of the amp is causing the issue... then we can continue

[Jazz P Bass]

Good call.

[leydenjar]

Thanks for the help.

It's not present from the preamp out jack. It is present going into the power amp in jack.

Also, it takes about 10 minutes for the amp to start producing the crackle.

After reading about a gajillion threads on Fender Hot Rod amps you'd think I'd know what to do with this one. Right now I'm inclined to touch up ALL the solder on both boards.


Thanks,
leydenjar

[Enzo]

SO it has to warm up to do this. That is what freeze spray is for. Once it starts we can spray chill small areas of the board, looking for somethng that stops the noise once chilled. we can also use a heat gun to warm up parts that only seem to fail when warm.

But my main suspect? That 82k plate load resistor in the phase inverter.

[leydenjar]

I just replaced both plate load resistors in the phase inverter.

No dice. It's still crackling/frying away.

[Enzo]

OK, does turning reverb to zero matter?

It is after the power amp jack? V3 is the phase inverter, pull it. Does that stop the noise? If not, swomething in the power tube stage is funny. If that stops it, then se just localized things back to that stage on back to the power amp jack.

If the noise remains without the PI, then pull both power tubes. If the noise remains, you either have one of the flyback diodes breaking down - and you can unsolder and lift them to see - or the transformer is breaking down. I think the transformer unlikely. A tube socket could be arcing, but that usually does more than crackle.

If pulling the tubes kills it, try installing just one tube. Noise? Now move that tube to the other socket. Noise? Now pull that and do the same two steps with the remaining tube. OH, I see you swapped tubes. DO this anyway at this point, because it could be something to do with the sockets, and this will help isolate it.

If pulling the PI stops it, we move backwards.

Ground a film cap to chassis. I use 0.047uf because I have lots of them around. A 0.1uf might be even better. Value not critical though, we are going to use it to shunt the signal path. ground it to chassis, and connect its free end to a clip wire. Now use that to shunt any signal - and noise is a signal - to ground. PIns 2 and 7 of the phase inverter are its grids. With the tube back in place, "ground" either grid. Either one affect the noise?

And use your shunt cap to ground TP23, just above the PI tube on the drawing.

All that stuff ought to isolate the problem

[leydenjar]

Thanks Enzo.

It crackles/fries when it's passing signal only. I said that in my first post. Because of that I don't see how doing the tube pulling thing will help. There's no noise to kill unless it's passing a signal and pulling the phase inverter tube will definitely kill my signal.

I did it anyway of course.

I'm starting to think it has something to do with bad solder on the main circuit board AND a component thermal issue. My reason for this is when I smack the amp on it's side with a rubber mallet it acts up even worse. But it also only starts to crackle fry AFTER it warms up.

Bad solder? Intermittent caused by bad solder and any slight vibration?

[Enzo]

Ah, I forgot that detail.

Needs a signal? You can still pull the PI for example and inject a signal into the 6L6 grids. I often do that during tests, even though I just use hum. I touch something to a grid pin and listen to see if hum results. If it does I know the tube amplifies. But using a cap to block DC, I can take a signal from a generator or just music from my shop stereo, and feed it into any circuit.

If the amp is mechanically sensitive - responds to whacking - then you have a connection bad somewhere. So does unplugging the amp's combo speaker and running a cord to a diferent cab make a difference? That removes the spaker vibration from the chassis. Also, get out the wooden chopstick and once it is warm enough, push on each and every part - any crackle? If you can play music through it and get a steady stream of this noise, you still can use ffreeze spray on section of the board to see if it tem,porarily stops it. That localizes the problem to a smaller area.

SOlder joints warm up just like parts do. Wires expand with heat - just like iron in a highway bridge - and can elbow their way out of a good solder conection as they warm up.

Those gray ribbon cables from socket boared to main board? They are bent into S shape usually. Try flexing them a little, or just push on them.


Sure it could be a part of course. Not a guarantee, but in general, I;d expect such behavior from something operating at voltage. SO plate resistors rather than cathode resistors. But that rules nothing out, just more of a trend.

[lowell]

First rule of thumb here is try to find the issure before this, which is what Enzo is suggesting, but if you exhaust ALL efforts at narrowing it down, I'd start with resoldering all the tube socket pins. Done this on who knows how many of these HR Fenders for this same reason.

[leydenjar]

Pulling the main circuit board and reflowing all the solder (with some extra) cured it.

Thanks to all who helped.

leydenjar

[Amp Kat]

I know this sounds basic but I had some Sovtek WXT's once intermitten doing this to me and after I replaced them it went away. Have also seen a semi loose ground from the main to power board cause this. Check all of the circuit board star grounds good and any rust or bad looking spots around pots and chassis.

[lowell]

So you soldered the main board NOT the tube socket board? Just so we're clear for anyone referencing this in the future.

[leydenjar]

@lowell

Yes. I reflowed ALL of the solder on the main circuit board and it corrected the problem.

This amp did not show visible signs of overheating. I had reflowed the solder on the power board first but that did not do anything.

The sound the amp made was a crackling / frying sound when being played. If you weren't playing you couldn't hear it and it didn't start doing it until it warmed up after 10 minutes or so.