Fender Hot Rod Deluxe hum

[fredcapo]

A Hot Rod Deluxe III came in with complaint of "Noise when no guitar plugged in" (I don't deal directly with customers). What I hear s a relatively loud hum when volume is at 0 then the hum gradually fades as volume is raised CW. Also hear it on hot channel at 0 only proportionately louder and the hum fades as gain is raised. Otherwise the amp seems to be fine. When I connect a cable to power in jack, there is no hum...thus clearly preamp issue.
I am unsure where to begin on this. Could this be problem with voltage supplying the heater...noise? Or a ground loop in the preamp? Thanks for any suggestions.

[Jazz P Bass]

THere may be something goofy going on with the input jack.

The tip (signal) & ring (ground) should be connected without a cable inserted.

You can try to run a cable into the jack, jumper the other end, tip to ring & then power on the amp.

If that takes care of the noise, you have a jack problem.

If not, then we move on.

[fredcapo]

I have tried it with plug in and out of jack...will revisit but... What is of interest is how the hum fades away as the volume pot is rotated CW... So when the vol pot is at 0 the grid is referenced at ground...and when pot is rotated CW the grid is pulled away from ground. So maybe the ground at the input that feeds the volume pots (2 channels that behave the same in this case)... After reading thru previous posts It's sounding more like a cold connection somewhere...

[Enzo]

Ground loop?

Don't over-think it. Hum can come from more than one source, and if they are out of phase, they can cancel.

Is the board firm in the chassis or out? Note on the face of those input jacks is a little point. It digs into the chassis when the jack is in its hole. That is an important ground connection. Has the input jack been replaced with another? The jacks generally available do not include this important feature.

I'm with JB, it sure sounds like a simple jack issue.

[Jazz P Bass]

[QUOTE=fredcapo;332434]I have tried it with plug in and out of jack...QUOTE]
That is not enough.
You have to jumper the tip & ring.

[fredcapo]

Enzo, JB - amp is still assembled. I use a 1/4 jack with 1k resistor tied tip to ring as a quiet instrument test. Should be above freezing in the shop later so will get back to it with your suggestions. I am going to bring a golden 12ax7 to try in v1 also...Thanks.

[Enzo]

Don't try to "fool" it, if the complaint is noise even without a guitar plugged in, then work on it with nothing plugged in. You do get noise that way right?

[g1]

Both input jack's switches must close when nothing is plugged in. You need 2 shorting plugs to test. They must be shorted, not 1K terminations.
Is the same hum problem there when the amp is in use with a guitar plugged in?

[Jazz P Bass]

Is the same hum problem there when the amp is in use with a guitar plugged in?

And the other input jumpered.

Actually, two minutes with a meter will tell if the jacks are grounding properly.

It can be a trick bending the tab back in, I will admit that much.
It helps to insert a plug 1/2 way to open everything up, as you attempt to rebend the contact that it connects to the ground tab when nothing is inserted.

[fredcapo]

I worked on this again today. I did not pay attention the first session but have determined This is a thermal problem. The hum starts after amp on about 5 minutes. Independent of all jacking possibilities, dropped the board off the chassis and probed and resoldered everything near the input, and tubes are ok. Tried freeze and heat to provoke with no positive results, yet... There is one tiny capacitor that becomes micro phonic so I suspect that, but have not yet looked at a schematic... Btw this is a HRDeluxe III 2009

FWIW I just repaired another HR Deluxe 2002 that had a jack soldering problem.

Thanks

[Alex R]

I worked on this again today. I did not pay attention the first session but have determined This is a thermal problem. The hum starts after amp on about 5 minutes. Independent of all jacking possibilities, dropped the board off the chassis and probed and resoldered everything near the input, and tubes are ok. Tried freeze and heat to provoke with no positive results, yet... There is one tiny capacitor that becomes micro phonic so I suspect that, but have not yet looked at a schematic... Btw this is a HRDeluxe III 2009

FWIW I just repaired another HR Deluxe 2002 that had a jack soldering problem.

Thanks

none of that rules out the non-grounding input jack. Unless you mean it now does it even with a guitar plugged in. If it still goes away when you plug a guitar in then it's likely going to be the jack itself, in fact I can't offhand think of anything else it might be.

[fredcapo]

the hum is 60 hz and it consistently appears about 5 minutes after amp is powered on, starting with a few clicks and pops, then steady hum whether guitar plugged in or not. The hum is before the channel switching and tone stack ( greatly amplified when the gain and drive boosted). I can see it on the wiper of the treble pot that feds the next stage, but not before maybe due to scope sensitivity. DC voltages look OK on the 12ax7s. What is a safe way to short the grid at each stage? What does 60 vs 120 hz imply?

The ambient temp in room is cool, so sounds like only the tubes could get hot so soon...thought I swapped them out with same result...Thanks.

[g1]

Something like a .1uf 600V cap should be fine for shorting things to ground.

[Jazz P Bass]

Get on R9 (V1B) & measure the Vac ripple.
If that decoupling capacitor is bad (C46) it can cause all sorts of grief.
http://www.blueguitar.org/new/schem/...ues_deluxe.gif

[guitician]

I would check how the ground for the power supply, which on that schematic, shows it going to the chassis, and that it needs to be a very low ohms connection to the pre-amps ground. I don't see a chassis to board ground connection on that schematic, but I'm sure that there is one. Maybe that R92 to the pots brackets.